5
III.1 - CLASSIFICATION AND SYMBOLS OF METEORS OTHER THAN CLOUDS
[EDITORIAL NOTE: Some symbols for the replacement tables and text of this section are
as yet unavailable in a visually appealing electronic format (they are denoted with yellow
highlighting). These will be inserted prior to publication of the revised ICA. None will be
changed from the previous (1975) version of the Volume I]
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III.1.1
7
Classification and symbols of meteors other than clouds
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The classification of meteors other than clouds, and basic symbols for these meteors, is shown in
the following table:
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Group
Designation of meteor
(a)
Symbol
Suspended
particles
Fog
Designation of meteor
(c) Particles raised by the
wind
Drifting and blowing snow
and
Fog
Drifting snow
Mist
Blowing snow
Ice fog
n/a
Steam devil
Falling particles
(precipitation)
n/a
Spray
Snow devil
(b)
Symbol
(d)
No
symbols
establish
ed
2
Hydrometeors
1
Deposits of particles
Rain
Deposit of fog droplets
n/a
Supercooled rain
Dew
n/a
Drizzle
Dew proper
Supercooled drizzle
Advection dew
n/a
Snow
White dew
n/a
Snow grains
Hoar frost
n/a
Snow pellets
n/a
Diamond dust
Advection hoar frost
Hail
Small hail
Ice pellets
Hoar frost proper
Rime
n/a
n/a
n/a
Soft rime
n/a
Hard rime
n/a
Group
Designation of meteor
Symbol
Designation of meteor
Clear ice
Glaze
Symbol
n/a
n/a
Tornado
Land spout
Cold air funnel
Waterspout
(a)
Suspension of
particles in the
atmosphere
(b)
Lithometeors
Haze
No symbols established
(e) Spout
Ensemble raised by
the wind
Drifting and blowing dust
or sand
Dust Haze
Drifting dust or sand
Smoke
Blowing dust or sand
n/a
Dust storm or sandstorm
Wall of dust or sand
Dust whirl or sand whirl
(dust devil)
Bishop’s ring
n/a
n/a
Solar
n/a
Mirage
Lunar
n/a
Shimmer
Corona:
Scintillation
Solar
Green Flash
Lunar
n/a
Irisation on cloud
n/a
Glory
n/a
Twilight colours
No symbols
established
Photometeors
Halo phenomena:
Crepuscular rays
Rainbow
Electro
meteo
rs
Fog bow
Thunderstorm
Lightning
n/a
Saint Elmo’s fire
n/a
Polar aurora
n/a
Group
Designation of meteor
Thunder
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Symbol
Designation of meteor
Symbol
n/a
As illustrated in the chart below, the character and intensity of precipitation (intermittent or
continuous; slight, moderate, or heavy) can be indicated with unique arrangements of the basic
symbols.
CHARACTER
INTERMITTENT
CONTINUOUS
INTENSITY
slight
moderate
heavy (dense)
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Combinations of the basic symbols may be used to indicate more complex situations. For example,
the symbol
or
denotes a mixture of falling raindrops and snowflakes; the symbol
a thunderstorm with rain at the place of observation.
indicates
As shown in the chart below, additional symbols have been established to indicate the nature of
showery precipitation, the trend over time, and location. These can be combined with one or
sometimes two basic symbols.
shower, slight
shower, moderate or heavy
|x
has increased (or formed) during the preceding hour
X|
has decreased during the preceding hour
X]
during the [past hour, but not at the time of observation
(x)
not at the station, but within sight [estimated within 5 km]
)x(
Not at the station, but within sight [estimated beyond 5 km]
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For example,
denotes fog which has become thinner during the preceding hour;
rain s h o w e r s during the preceding hour but not at the time of observation.
III.2 DEFINITIONS AND DESCRIPTIONS OF METEORS OTHER THAN CLOUDS
indicates
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III.2.1
Hydrometeors other than clouds
III.2.1.1
HYDROMETEORS CONSISTING OF A SUSPENSION OF PARTICLES IN THE ATMOSPHERE
(1)
FOG
Definition: A suspension of very small, usually microscopic water droplets in the air, reducing visibility
at the Earth’s surface.
The term ''fog" is used when microscopic droplets reduce horizontal visibility at the Earth's surface to
less than one kilometer, while the term "mist" is used when the droplets do not reduce horizontal
visibility to less than one kilometre. In practice mist is considered synonymous with “light fog”. The
term “smog” (from the words smoke and fog) is widely used for conditions where fog and heavy air
pollution are present, with chemical reactions between the fog droplets and various pollutants.
Reduction in visibility depends on the structure of the fog, and especially on the number density and
size distribution of the droplets. This structure may vary a great deal in time and space. The air in
"fog" usually feels damp, moist or wet. When illuminated, individual fog droplets are frequently
visible to the naked eye and are seen to be moving in a somewhat turbulent manner.
"Fog" forms a whitish veil which covers the landscape, while "mist" generally forms a fairly thin,
greyish veil. When mixed with dust or smoke, fog may take on a faint coloration.
“Freezing Fog” occurs at temperatures below 0°C when supercooled fog droplets freeze on impact
with the ground or other objects to deposit rime. In aviation observing and forecasting; any fog
consisting predominantly of water droplets at temperatures below 0°C is reported as freezing fog
(FZFG) whether it is depositing rime ice or not.
At temperatures below -10°C fog composed of ice crystals may form, usually from freezing of
droplets. Like diamond dust, this gives rise to optical phenomena.
Fog may form in a variety of meteorological situations, but in all cases its formation is as a result of
the air becoming saturated. The mechanism of formation can be indicated with these terms.
 Radiation fog is formed when the ground surface cools through radiation, usually at night, cooling
the air just above the ground to its saturation temperature. Radiation fog is known as ‘shallow
fog’ or ‘ground fog’ when it occurs in a shallow enough layer that it does not restrict horizontal
visibility when viewed from a height of about 2 metres on land or 10 metres at sea.
 Advection fog is formed when relatively warm, moist air moves (advects) over a cooler surface,
and the temperature of the air is cooled to saturation by contact with the cold surface. Examples
include relatively warm, moist air advected over a cool sea ('sea fog'), or over land when the
surface is frozen or snow-covered.
 Evaporation fog (or cold advection fog) is formed when cold, stable air moves over a much
warmer body of water. Evaporation from the warm body of water saturates the cold air above;
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


(2)
water vapour condenses in the cold air producing 'steam fog' or 'sea smoke', or in polar regions
'Arctic sea smoke'.
Upslope fog is formed when air flows upwards over rising terrain and is adiabatically cooled to its
saturation temperature. Upslope fog is a type of 'hill fog'. When viewed from beneath it will be
seen as Stratus. As one ascends into the cloud the visibility will be reduced to fog. When viewed
from clear air above, the observer will look down on the top surface of the cloud.
Hill fog is a term denoting low cloud covering high ground. Hill fog does not necessarily require
upslope motion, as in the case of the upslope type of hill fog. It can simply occur when the cloud
base is lower than the tops of the hills. When viewed from beneath it will be seen as Stratus
cloud.
Frontal fog is fog that forms in frontal zones when rain falls from warm air into cold, stable air
beneath. In conditions of light wind fog may form through evaporation of the raindrops saturating
air near the ground.
ICE FOG
Definition: A suspension of numerous minute ice particles in the air, reducing the visibility at the
Earth's surface.
Ice fog forms when water vapour, mainly resulting from human activities, is introduced into the
atmosphere. This vapour condenses forming droplets which freeze rapidly into ice particles having no
well-defined crystalline form. It is observed at high latitudes, usually in clear calm weather when the
temperature is below -30°C.
Horizontal visibility is usually very restricted in ice fog, often to less than 50 m. This is due to the small
diameter of the ice particles, typically between 2 and 30 μm with the smaller particles forming when
the temperature is between -40°C and -50°C.
Ice fog does not produce halo phenomena, although such phenomena may be present if the fog also
contains diamond dust.
III.2.1.2
HYDROMETEORS CONSISTING OF A FALL OF AN ENSEMBLE OF PARTICLES (PRECIPITATION)
(1)
RAIN
Definition: Precipitation of drops of water, which falls from a cloud.
The number density and size distribution of raindrops vary considerably with the intensity and nature
of the precipitation.
(2)
SUPERCOOLED RAIN (FREEZING RAIN)
Definition: Rain with the temperature of drops below 0°C.
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Drops of supercooled rain may freeze on impact with the ground, aircraft in flight, or other objects.
This is also known as Freezing Rain.
(3)
DRIZZLE
Definition: Fairly uniform precipitation in very fine drops of water very close to one another, which
falls from a cloud.
Drizzle drops have diameter usually less than 0.5 mm. The drops appear almost to float, and so make
visible even slight movements of the air.
Drizzle falls from a layer of Stratus, usually low, sometimes touching the ground (fog).
The amount of precipitation in the form of drizzle can be considerable (up to 1 mm per hour),
especially along coasts and in mountainous areas.
The drops falling on the edge of a rain zone or during light rainfall may be as small as drizzle drops,
owing to their partial evaporation. In this situation, raindrops are distinguished from drizzle drops in
that they are more scattered. Identification of the precipitating cloud as Stratus also distinguishes rain
from drizzle.
(4)
SUPERCOOLED DRIZZLE (FREEZING DRIZZLE)
Definition: Drizzle with the temperature of drops below 0°C.
Drops of supercooled drizzle may freeze on impact with the ground, aircraft in flight, or other objects.
This is called freezing drizzle.
(5)
SNOW
Definition: Precipitation of ice crystals, singly or stuck together, which falls from a cloud.
The form, size, and concentration of snow crystals differ considerably according to the temperature
and supersaturation at which they develop. A fall of snow usually includes various types of snow
crystals and almost all types of crystal may be observed during a single fall of snow.
Small droplets of frozen water are often attached to snow crystals. If present in great numbers, these
can obscure the crystalline structure of the snow.
At temperatures warmer than about -5°C, the crystals generally stick together into snowflakes.
(6)
SNOW GRAINS
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Definition: Precipitation of very small opaque white particles of ice which falls from a cloud. These
particles are fairly flat or elongated. Their diameter is generally less than 1 mm.
Although frozen and occurring when the temperature is between approximately 0°C and -10°C, the
other properties of this precipitation correspond to drizzle. When the grains hit hard ground, they do
not bounce.
Snow grains fall mostly from Stratus or from fog, and never in the form of a shower. Except in the
mountains, snow grains usually fall in small quantities.
(7)
SNOW PELLETS
Definition: Precipitation of white and opaque ice particles, which falls from a cloud. These particles are
generally conical or rounded, and their diameter may be as large as 5 mm.
Snow pellets are composed of a central nucleus covered with frozen cloud droplets. They form when
a particle of ice, usually a crystal, collects cloud droplets, which rapidly freeze. Their density is
generally low, less than 0.8 g cm-3, due to air gaps between the nucleus and frozen droplets.
Snow pellets are brittle and easily crushed. When they fall on hard ground they bounce and often
break. Showers of snow pellets fall from Cumulus or Cumulonimbus. The showers usually consist of
snow pellets and snowflakes together and normally occur when temperatures near the surface are
close to 0°C.
Crystals can be observed which are not completely surrounded by droplets; this is the intermediate
stage between a snow crystal and snow pellet.
(8)
DIAMOND DUST
Definition: Precipitation which falls from a clear sky in very small ice crystals, often so tiny that they
appear to be suspended in the air.
Diamond dust can be observed in polar regions and continental interiors especially in clear, calm, and
cold weather. It forms at temperatures less than -10°C in a rapidly cooling airmass. Diamond dust is
usually composed of well-developed crystals (often plates), with typical diameters of about 100 μm.
These crystals, which are visible mainly when they sparkle in the sunlight, can also produce halo
phenomena.
Visibility in diamond dust is varies considerably, the lower limit is greater than 1 km.
(9)
HAIL
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Definition: Precipitation of particles of ice (hailstones). These can be either transparent, or partly or
completely opaque. They are usually spheroidal, conical or irregular in form, and generally between 5
and 50 mm in diameter. The particles may fall from a cloud either separately or agglomerated into
irregular lumps.
Falls of hail always occur as showers. They are generally observed during heavy thunderstorms.
Hailstones usually form around a nucleus which may not be at their geometric centre. The nucleus
may be anywhere between a few millimetres and one centimetre in diameter. The nuclei are
spheroidal or conical and are composed of ice that is usually opaque, but sometimes transparent.
Hailstones can occur with a great variety of forms and dimensions, even within a single fall. An “onion
skin” formation, for example, consists of a nucleus surrounded by alternating layers of opaque and
transparent ice. There are usually not more than five layers, except in very large hailstones which
have been found to have twenty or more. Some other hailstones do not have any layers; they consist
of transparent or opaque ice only.
Hailstones typically have a density between 0.85 g cm-3 and 0.92 g cm-3, but may have lower density if
they have large cavities filled with air. Some hailstones are partly composed of spongy ice, which is a
mixture of ice, water and air.
In exceptional circumstances, large hailstones can stick together to form irregular lumps of giant hail.
Hailstones forms when a nucleus collects cloud droplets or drops of rain. There is no general
agreement on the nature of this nucleus; the tendency is however to admit that it is usually a particle
of small hail which has formed round a snow pellet.
(10) SMALL HAIL
Definition: Precipitation of translucent ice particles, which falls from a cloud. These particles are
almost always spherical and sometimes have conical tips. Their diameter may approach and even
exceed 5 mm.
Small hail always occurs in showers from Cumulonimbus.
Small hail consists of snow pellets totally or partially encased in a layer of ice. Gaps within the snow
pellets are filled with ice, or ice and water; a thin shell only may be frozen. The water may come from
cloud drops or partial melting of a snow pellet. The density of small hail is relatively high; it ranges
from 0.8 g cm-3 to in rare examples 0.99 g cm-3.
Usually, small hail is not easily crushable and when it falls on hard ground it bounces with an audible
sound on impact.
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Small hail is an intermediate stage between the snow pellet and the hailstone. It differs from the
snow pellet in its partially smooth surface and its higher density. It differs from the hailstone
particularly in its smaller size.
{11) ICE PELLETS
Definition: Precipitation of transparent ice particles, which falls from a cloud. These particles are
usually spheroidal or irregular, and rarely conical. Their diameter is less than 5 mm.
Ice pellets originate as raindrops or snowflakes (less common) that generally fall from Altostratus or
Nimbostratus. They fall into a sub-cloud layer of warm air where the snowflakes melt or partially
melt; and then fall into a cold layer of air (temperature below 0°C) where they freeze and reach the
ground as frozen precipitation.
Ice pellets of the form of frozen raindrops are transparent, the less common refrozen snowflakes are
in parts transparent and in parts opaque; dependent on whether the snowflake melted or only
partially melted.
Ice pellets are not easily crushable. When they fall on hard ground they generally bounce with an
audible sound on impact.
Ice pellets may be partly liquid. Their density is usually close to, or above, that of ice (0.92 g cm-3).
III.2.1.3
HYDROMETEORS CONSISTING OF ENSEMBLES OF PARTICLES RAISED BY THE WIND
(1)
DRIFTING SNOW AND BLOWING SNOW
Definition: An ensemble of snow particles raised from the ground by a sufficiently strong and
turbulent wind.
The occurrence of this hydrometeor depends on the wind conditions (speed and gustiness) and the
state and age of the surface snow. There are two sorts of phenomenon: drifting snow and blowing
snow.
(a)
DRIFTING SNOW
Definition: An ensemble of snow particles raised by the wind to small heights above the ground.
The motion of the snow particles is more or less parallel to the ground. Drifting snow may obscure
very low obstacles. Neither vertical visibility, nor horizontal visibility at eye level (1.80 metres above
the ground) is noticeably diminished.
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(b)
BLOWING SNOW
Definition: An ensemble of snow particles raised by the wind to moderate or great heights above the
ground.
The concentration of the snow particles is sometimes sufficient to veil the sky and even the sun. The
snow particles are nearly always violently stirred up by the wind.
Vertical visibility is diminished according to the intensity of the phenomenon. Horizontal visibility at
eye level is generally very poor, where eye level is defined as 1.80 metres above the ground. When
the phenomenon is severe, it is difficult to determine if precipitation in the form of snow is also
present at the same time.
(2)
SPRAY
Definition: An ensemble of water droplets torn by the wind from the surface of an extensive body of
water, generally from the crests of waves, and carried up a short distance into the air.
When the water surface is rough, the droplets may be accompanied by foam. With strong gales spray
may locally take the form of moving vortices
(3)
SNOW DEVIL
Definition: Snow raised from the ground in the form of a whirling column of varying height with a
small diameter and an approximately vertical axis.
This is a very rare phenomenon which occurs when surface wind shear acts to generate a vortex over
snow cover, resulting in a whirling column of snow particles being raised from the ground.
It is sometimes referred to as a snownado.
(4)
STEAM DEVIL
Definition: A small, gentle whirling column of saturated air of varying height, with a small diameter
and an approximately vertical axis, that forms when cold air is over a relatively much warmer body of
water or saturated surface.
Steam devils are typically seen in association with steam fog (arctic sea smoke). The vortices are of
the order of a metre in diameter and several metres high. They form over relatively warm bodies of
water (or exceptionally over a saturated land surface) when the air near the ground is very unstable
due to much colder air above. A steam devil has been observed over a frosty grass surface when the
sun is heating the ground.
III.2.1.4
HYDROMETEORS CONSISTING OF A DEPOSIT OF PARTICLES
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(1)
DEPOSIT OF FOG DROPLETS
Definition: Deposit of non-supercooled fog (or cloud) droplets on objects the surface temperature of
which is above 0°C.
Deposits of fog droplets are observed especially in high areas where orographic clouds are frequent.
The extent or depth of the deposit depends on the duration of the fog, the size distribution and
number density of the fog (or cloud) droplets, and the speed of impact of the droplets. It is also a
function of properties of the object, often a leaf, on which the deposit forms. When a large deposit
occurs, the deposited droplets run together and drip on to the ground. In some circumstances, the
amount of water falling from trees in this way during a single night may as much as the rainfall from a
moderate shower.
(2)
DEW
Definition: Deposit on objects of water drops produced by the direct condensation of water vapour
from the surrounding air.
There are two sorts of dew: dew proper and advection dew.
(a) DEW PROPER
Definition: Deposit of water drops on objects whose surface is sufficiently cooled, generally by
nocturnal radiation, to cause the direct condensation from the surrounding air.
Dew proper is ordinarily deposited on objects at or near the ground, mainly on their horizontal
surfaces. Dew is observed especially when the air is calm and the sky is clear.
Dew should not be confused with the deposit of drops from low fog on expose surfaces, nor with
droplets of water exuded by plants - a phenomenon known as guttation which often takes place at
the same time as deposit of dew but which can also occur separately.
(b) ADVECTION DEW
Definition: Deposit of water drops on objects whose surface is sufficiently cold to cause direct
condensation of the water vapour coming into contact with this surface through advection.
Advection dew is deposited mainly on vertical, exposed surfaces. It is observed when relatively warm
damp air suddenly invades a region where the temperature of the exposed surfaces is lower than the
dewpoint of the advected air. Advection dew must not be confused with deposition of fog droplets,
nor with the pseudo-dew observed in humid weather on exposed surfaces (such as roads) when
covered by a thin film of hygroscopic substances.
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(3)
WHITE DEW
Definition: A deposit of white frozen dew drops.
White dew is dew drops that have frozen. White dew must not be confused with hoar frost.
(4)
HOAR FROST
Definition: A deposit of ice produced by the deposition of water vapour from the surrounding air. It is
generally crystalline in appearance.
There are two sorts of hoar frost: hoar frost proper and advection hoar frost.
(a) HOAR FROST PROPER
Definition: A deposit of ice, which generally assumes the form of scales, needles, features or fans and
which forms on objects the surface of which is sufficiently cooled, generally by nocturnal radiation, to
bring about the deposition of the water vapour contained in the ambient air.
Hoar frost proper generally assumes the form of scales, needles, features or fans. It is ordinarily
deposited on objects at or near the ground, mainly on their horizontal surfaces. Hoar frost is observed
especially during the cold part of the year when the air is calm and the sky clear.
(b) ADVECTION HOAR FROST
Definition: A deposit of ice which generally assumes crystalline form and which forms on objects,
the surface of which is sufficiently cold to bring about deposition of the water vapour contained in
the air coming into contact with this surface, usually through a process of advection
Advection hoar frost is deposited mainly on vertical exposed surfaces. It is observed when relatively
warm damp air suddenly invades a region where the temperature of the exposed surfaces is below
0°C and below the frost-point of the advected air.
(4) RIME
Definition: Deposit of ice generally formed by the freezing of supercooled fog or cloud droplets on
objects with surface temperature below or slightly above 0°C.
There are three sorts of rime: soft rime, hard rime and clear ice. The processes resulting in the
formation of the different kinds of rime may occur nearly simultaneously, or may occur consecutively
during a longer period, or even may alternate. Therefore, in some circumstances very heterogeneous
"overall deposits" can be observed, with transitional states within the deposit.
(a) SOFT RIME
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Definition: Fragile rime consisting mainly of thin needles or scales of ice.
Near the ground, soft rime is deposited under calm or low wind conditions on all sides of exposed
objects. Soft rime easily drops if an object is shaken. It mainly forms with ambient air temperatures air
lower than -8°C. At temperatures well below -8°C the formation of soft rime does not necessarily
require the presence of fog.
(b) HARD RIME
Definition: Granular rime, usually white, adorned with crystalline branches of grains of ice.
Hard rime is formed by the rapid freezing of supercooled water so that the droplets freeze more or
less individually, leaving air gaps. Near the ground, it is deposited mainly on objects exposed to at
least moderate wind. In the windward direction, the deposit may form a thick layer. In the free
atmosphere hard rime may form on the parts of aircraft exposed to the relative wind. Hard rime is
rather adhesive but can be scratched off the object. Hard rime mainly forms with temperatures
between -2°C and -10°C.
(c) CLEAR ICE
Definition: Smooth compact rime, usually transparent, without a clearly defined shape or form, with a
ragged surface, and morphologically resembling glaze.
At and near the ground, clear ice is deposited mainly on the surface of objects exposed to the wind. It
is observed specially in mountain regions. In the free atmosphere, it occurs chiefly on the part of the
aircraft exposed to the relative wind.
Clear ice is formed by the slow freezing of supercooled water and so penetrates the air gaps between
the grains of ice before freezing. Clear ice is very adhesive and can only be removed from the objects
by breaking or melting off. Clear ice is formed in almost all cases with ambient air temperatures
between 0 and -3°C.
(6) GLAZE
Definition: A smooth compact deposit of ice, generally transparent, formed by the freezing of
supercooled drizzle droplets or raindrops on objects the surface temperature of which is below or
slightly above 0°C.
Glaze covers all the parts of surfaces exposed to precipitation. It is generally fairly homogeneous and
morphologically resembles clear ice. At or near the ground, glaze forms when drizzle droplets or
raindrops become supercooled as they fall through a layer of sub-frostpoint temperature air. In the
free atmosphere, glaze is observed when aircraft are exposed to supercooled precipitation. Glaze
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forms by the slow freezing of supercooled liquid water and so penetrates the air gaps between the
particles of ice before freezing.
The deposit of ice formed by the freezing of fog or cloud droplets not supercooled at the time of
impact with objects the temperature of which is well below 0°C, is also known as glaze.
Glaze on the ground must not be confused with ground ice, which on a road surface is known as 'black
ice'. Ground ice is formed when water from a precipitation of non-supercooled drizzle droplets or
raindrops later freezes on the ground, or when snow on the ground refreezes after having completely
or partly melted, or when snow on the ground is made compact and hard by traffic.
III.2.1.5
HYDROMETEORS CONSISTING OF A VORTEX OF PARTICLES
(1)
SPOUT
Definition: A phenomenon consisting of an often violent whirlwind, revealed by the presence of a
cloud column or inverted cloud cone (funnel cloud), protruding from the base of a Cumulonimbus or
Cumulus cloud, and of a "bush" composed of water droplets raised from the surface of the sea or of
dust, sand or litter, raised from the ground.
The axis of the funnel cloud is vertical, inclined or sometimes sinuous. Sometimes the funnel merges
with the bush. The air in the whirlwind, or vortex, rotates rapidly, most often in a cyclonic sense.
Rapid rotation may also be observed outside the funnel and the "bush". Further away, the air is often
very calm. The diameter of the cloud column, which is normally of the order of ten metres, may
occasionally reach some hundreds of metres. Several spouts (multiple vortices) may sometimes be
observed connected with a single cloud. Some spouts (tornadoes) can be extremely destructive,
where they may leave a path of devastation up to 5 kilometres wide and several hundred kilometres
long. Weak spouts are occasionally observed under Cumulus clouds.
Spouts can be categorized as follows:
(a) TORNADO
Definition: A rotating column of air, extending from the base of a cumuliform cloud, and often visible
as a condensation funnel in contact with the ground, and/or attendant circulating dust or debris cloud
at the ground.
A large tornado in which the condensation funnel is at least as wide horizontally at the ground as it is
in height from the ground to the cloud base may be referred to as a wedge tornado.
During the dissipation stage of a tornado the condensation funnel will shrink and narrow in width,
becoming rope-like (a rope funnel), and may also become contorted.
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Some tornadoes may contain secondary vortices within the main circulation (suction vortices or sub
vortices).
Tornadoes can be classified into the following distinct formation groups:
Type I - in association with Supercells (with Mesocyclone)
• Ia - Classic supercells
• Ib - Low-top mini-Supercells
• Ic - Tropical Storm / Hurricane related mini-Supercells
• Id - Anticyclonic secondary vortices
Type II - in association with quasi-linear convective systems (Cold Pool, Shear, Mesocyclone)
• IIa - LEWPs (Line echo wave pattern)
• IIb - Bows (Bow echo produced by a cold pool)
• IIc - BEVs (Bookend vortex at the cyclonic end of the bow echo)
• IId - IJs (Inflow jets along sections of the QLCS)
• IIe - Mesovortices not associated with LEWPs, bows or inflow jets
• IIf - Tropical Storm / Hurricane spiral bands
Type III - Localised Convective and Shear Vortices
• IIIa - Landspouts
• IIIb - Waterspouts
• IIIc - Cold-air funnels
LANDSPOUT
Definition: A tornado that does not arise from organised storm-scale rotation and is therefore not
associated with a wall cloud (murus) or a mesocyclone.
Landspouts are typically observed beneath cumulonimbus or towering cumulus clouds (cumulus
congestus), often as no more than a dust whirl, with rotation typically originating from lines of
horizontal shear. They are essentially the land-based equivalent of waterspouts.
COLD-AIR FUNNEL
Definition: A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small
shower or thunderstorm when the air aloft is unusually cold.
Cold-air funnels typically do not reach the ground (funnel cloud), but when they do reach the ground
as tornadoes they are much less violent than other types.
Tornado intensity
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The strength of a tornado can be estimated from the degree of damage caused using the Enhanced
Fujita Scale (EF-Scale)
EF
number
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1
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3-second gust speed
Damage
105 - 137 km/h
65 - 85 mph
138 - 178 km/h
86 - 110 mph
Minor damage: shingles blown off or parts of a roof peeled off; damage to
gutters or sidings; branches broken off trees; shallow rooted trees toppled.
Moderate damage: more significant roof damage; windows broken;
exterior doors damaged or lost; mobile homes overturned or badly
damaged.
Considerable damage: roofs torn off well constructed homes; homes
shifted off their foundations; mobile homes completely destroyed; large
trees snapped or uprooted; cars can be tossed.
Severe damage: entire stories of well constructed homes destroyed;
significant damage done to large buildings; homes with weak foundations
can be blown away; trees begin to lose their bark.
Extreme damage: well constructed homes are leveled; cars are thrown
significant distances; top story exterior walls of masonry buildings would
likely collapse.
Massive/incredible damage: well constructed homes are swept away; steelreinforced concrete structures are critically damaged; high-rise buildings
sustain severe structural damage; trees are usually completely debarked,
stripped of branches and snapped.
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179 - 218 km/h
111 - 135 mph
3
219 - 266 km/h
136 - 165 mph
4
267 - 322 km/h
166 - 200 mph
5
>322 km/h
>200 mph
(b) WATERSPOUT
Definition: A tornado occurring over water. It is normally a relatively small, weak rotating column of
air over open water beneath a cumulonimbus or towering cumulus cloud.
Waterspouts are essentially the water-based equivalent of landspouts. They are most common over
tropical or subtropical waters and along horizontal shear boundaries.
III.2.2
Lithometeors
III.2.2.1
LITHOMETEORS CONSISTING OF A SUSPENSION OF PARTICLES IN THE ATMOSPHERE
(1)
HAZE
Definition: A suspension in the air of extremely small, dry particles invisible to the naked eye and
sufficiently numerous to give the air an opalescent appearance.
As light is scattered by haze particles, distant bright objects or lights seen through the haze appear
yellowish or reddish, while dark objects appear bluish. Haze particles may have a colour of their own
which also contributes to this effect.
(2)
DUST HAZE
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Definition: A suspension in the air of dust or small sand particles, raised from the ground prior to the
time of observation by a dust storm or sandstorm. The dust storm or sandstorm may have occurred
either at or near the station or far from it.
(3)
SMOKE
Definition: A suspension in the air of small particles produced by combustion.
This lithometeor may be present either near the Earth's surface or in the free atmosphere. Viewed
through smoke, the sun appears very red at sunrise and sunset, and shows an orange tinge when high
in the sky. Smoke from relatively nearby cities may be brown, dark grey or black. Smoke in extensive
layers originating from fairly near forest fires scatters the sunlight and gives the sky a greenish-yellow
hue. Evenly distributed smoke from very distant sources generally has a light grayish or bluish hue.
When smoke is present in large quantities, it may be distinguished by its smell.
When the lithometeor "smoke" is present in the free atmosphere, it is, by convention, distinguished
from clouds of smoke (clouds from fires or clouds resulting from industry) by its diffuse appearance
and by the absence of any discernible outlines.
III.2.2.2
LITHOMETEORS CONSISTING OF ENSEMBLES OF PARTICLES RAISED BY THE WIND
(1)
DRIFTING AND BLOWING DUST OR SAND
Definition: An ensemble of particles of dust or sand raised, at or near the station, from the ground to
small or moderate heights by a sufficiently strong and turbulent wind.
The wind conditions (speed and gustiness) necessary to produce these lithometeors depend on the
nature, the state and the degree of dryness of the ground.
(a)
DRIFTING DUST OR DRIFTING SAND
Definition: Dust or sand, raised by the wind to small heights above the ground. The visibility is not
sensibly diminished at eye level (eye level is defined as 1.80 metres above the ground).
Very low obstacles are veiled or hidden by the moving dust or sand. The motion of the particles of
dust or sand is more or less parallel to the ground.
(b)
BLOWING DUST OR BLOWING SAND
Definition: Dust or sand raised by the wind to moderate heights above the ground. The horizontal
visibility at eye level is sensibly reduced (eye level is defined as 1.80 metres above the ground).
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The concentration of the particles of dust or sand may sometimes be sufficient to veil the sky and
even the sun.
(2)
DUST STORM OR SANDSTORM
Definition: An ensemble of particles of dust or sand energetically lifted to great heights by a strong
and turbulent wind.
Dust storms or sandstorms generally occur in areas where the ground is covered with loose dust or
sand. Sometimes, after having travelled over great distances, they may be observed over areas where
no dust or sand covers the ground. The forward portion of a dust storm or sandstorm may have the
appearance of a wide and high wall which advances fairly rapidly. Walls of dust or sand often
accompany a Cumulonimbus which may be hidden by the dust or the sand particles. They may also
occur without any clouds along the forward edge of an advancing cold air mass.
(3) DUST WHIRL OR SAND WHIRL (DUST DEVIL)
Definition: An ensemble of particles of dust or sand, sometimes accompanied by small litter, raised
from the ground in the form of a whirling column of varying height with a small diameter and an
approximately vertical axis.
Dust whirls or sand whirls occur when the air near the ground is very unstable as, for instance, when
the soil is strongly heated by the sun (insolation). The rotation may be in either direction around the
centre. Heights are generally less than 30 metres but heights of up to a kilometre have been reported.
Gustnados are a very specific dustwhirl. They are generally a weak, short-lived, shallow dust whirl that
may occasionally be observed along the edge of a gust front (arcus) of a cumuliform cloud. This
feature, commonly known as a gustnado, is in the form of a whirling column of dust that visually
resembles the debris cloud of a tornado but without any condensation funnel. A gustnado is not
formed in the same manner as a dust devil (i.e. by strong heating of the ground), nor is this whirling
vertically orientated column (vortex) a true tornado. Gustnadoes are rarely strong enough to cause
much damage.
III.2.3
Photometeors
(1)
HALO PHENOMENA
Definition: A group of optical phenomena in the form of rings, arcs, pillars or bright spots, produced by
the refraction or reflection of light by ice crystals suspended in the atmosphere (cirriform clouds,
diamond dust, etc.).
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Halo phenomena formed by refraction of sunlight may show colours, while those produced by the
light of the moon are always white. On extremely rare occasions circular halos of other radii are
possible.
(a) Small halo: This is the most frequently observed halo phenomenon. The small halo appears as a
white or mostly white luminous ring of 22°s radius with the light source (sun or moon) at its centre. It
shows a faint red fringe on the inside and, in some rare cases, a violet fringe on the outside. The sky
inside the ring is conspicuously darker than the rest of the sky. The halo always has a 22° diameter
regardless of its position in the sky. Sometimes, depending on the amount of cirriform cloud, only a
part of the complete circle may be visible. The diameter can be verified by using your hand and
knowledge that the approximate angle between thumb and little finger, with outstretched arm and
fingers spread, is 20°. So with the thumb held over the sun, the 22° halo will be found near the tip of
the little finger.
(b) Large halo: A circular halo with a radius of 46° is sometimes observed. This halo is much less
common than the small halo and is always less bright. The large halo is easily confused with the
supralateral arc.
(c) Luminous pillar (light pillar): A white luminous pillar, appearing as a broken or continuous trail of
light and also known as a light pillar, may be observed vertically above and below a light source, such
as the sun or moon. Light pillars may occasionally be observed above terrestrial light sources, and on
rare occasions small light pillars may be seen extending above and/or below a bright planet, such as
Venus. When the sun is the light source the phenomenon is known as a sun pillar. A pillar above the
light source is an upper pillar, and below the light source is a lower pillar. Lower pillars are best seen
from a hill or mountain, or an aircraft, when there is ice cloud, freezing fog, or ice crystals such as
diamond dust, below.
(d) Tangent arc: This is the generic term for several types of luminous arcs that form tangent to other
halos. Tangent arcs are sometimes seen on the outside of the small or the large halo. These arcs
touch the circular halo at its highest or lowest points (upper tangent arc and lower tangent arc,
respectively). The arcs have a form which varies with the angular elevation of the light source (the sun
or moon). They are often short and may even appear as only a bright spot. When the light source is
just above the horizon the upper tangent arc appears as a narrow V-shape. As the sun or moon rises
in the sky the V shape of the upper tangent arc opens to a shape resembling the outstretched wings
of a large bird.
For the 22° halo, the lower tangent arc is normally below the horizon when the sun or moon is at
within 22° of the horizon, so it can only be seen from a high location, such as a mountain or aircraft. If
visible, the inverted V shape of the lower tangent arc narrows and then broadens as the elevation of
the light source increases.
When the sun or moon reaches 29° above the horizon the upper and lower tangent arcs link to form
what is known as a circumscribed halo. This roughly oval halo is outside the 22° halo, touching it at
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the highest and lowest points. Sometimes only the brighter upper and lower portions may actually be
visible. When the sun or moon rises higher than about 50° the circumscribed halo gradually becomes
circular, and at even higher elevations it merges with the 22° halo so that it may be difficult to tell
them apart. However, the circumscribed halo is generally brighter, with purer colours, and with red as
the innermost color.
(e) Circumzenithal and circumhorizontal arcs: Occasionally, circumzenithal and circumhorizontal arcs
(previously known as the upper and lower circumzenithal arcs) are observed displaying the 'colours of
the rainbow'.
The circumzenithal arc is a near-zenith arc with small radius. It has bright colours with red on the
lower, outside of the arc and violet on the upper inside. This arc occurs only when the elevation of the
light source (e.g. the sun) is less than 32°. When the angular elevation of the light source is about 22°
the circumzenithal arc touches the large (46°) halo, if visible. These two features are increasingly
separated with a light source further from 22°. The arc may be visible without the large halo also
being present.
The circumhorizontal arc is a near-horizon arc with a large radius (i.e. fairly flat). It shows a red colour
above and violet nearest the horizon. The circumhorizontal arc occurs only when the elevation of the
light source is more than 58°. When the angular elevation of the light source is about 68° the
circumhorizontal arc touches the large (46°) halo, if visible. These two features are increasingly
separated with a light source further from 46°. The arc may be visible without the large halo being
visible.
(f) Parhelic circle: The parhelic circle is a white, horizontal circle at the same angular elevation as the
sun. Bright spots may be observed at certain points of the parhelic circle. These spots occur most
commonly a little outside the small halo (parhelia, often brilliantly coloured). Occasionally, bright
spots (paranthelia) are seen at an azimuthal distance of 120° from the sun and, very rarely, opposite
the sun (anthelion). When the parhelia, paranthelia or the anthelion are particularly bright, they are
often called mock suns.
The corresponding phenomena produced by the moon are called: paraselenic circle, paraselenae,
parantiselenae and antiselene. When the paraselenae, parantiselenae or the antiselene are
particularly bright they are sometimes called mock moons. Parhelia and paraselenae are sometimes
connected with the small halo by obliquely oriented arcs of Lowitz (or Lowitz arcs). These pale
coloured arcs are a rare type of tangent arc only seen when the Sun's elevation is high.
(g) Subsun (or undersun): This halo phenomenon is produced by reflection of sunlight on ice crystals in
clouds. It appears vertically below the sun as a brilliant white spot, similar to the image of the sun on
a calm water surface. The subsun can only be viewed when looking downward, and so is only
observed from aircraft or mountains.
(h) Supralateral arc: This is a rare, faintly rainbow-coloured large arc which forms only when the light
source is below 32° elevation. It appears as a fragment of a large circle and always above the
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elevation of the parhelic circle. The supralateral arc may be mistaken for the 46° large halo, but can
be distinguished from it by the brighter and more coloured appearance of the supralateral arc. In
contrast to the large 46° halo, the shape of the supralateral arc varies with the elevation of the light
source. The apex of the arc is near, or touches the circumzenithal arc.
(i) Infralateral arc (or lower lateral tangent arc): These are a rare pair of coloured arcs just outside and
tangent to the 46° large halo, and are always found below the level of the parhelic circle. Their shape
changes with the elevation of the light source.
(j) Other halo phenomena: A number of other halo phenomena can be seen on rare occasions. These
include: Hastings arcs, Helic arcs, Kern arc, Lowitz arcs, Moilanen arc, Parry arcs, subhelic arc, Tricker
arcs, and the Wegener arc
(2)
CORONA
Definition: One or more sequences (seldom more than three) of small-diameter coloured rings centred
on the sun or moon.
In each sequence the inside ring is violet or blue, the outside ring is red, and other colours such as
green and yellow may occur in between. The innermost sequence, which is generally not more than
5° in diameter, usually has a central area that is intensely bright, and a distinct outer ring of reddish or
chestnut colour, called the "aureole".
Coronae are caused by the diffraction of light from the light source (the sun or moon), passing
through mist, fog or through a thin cloud composed of very small water or ice particles. Coronae may
also occur when light is diffracted by large amounts of pollen in the air. The radii of the aureole and of
the successive, approximately equidistant, red rings are greater for smaller particles. Except when the
particles are very uniform in size, the colours observed in coronae are less pure and fewer than the
colours of rainbows.
Sometimes coronae seen in clouds appear distorted because of particle size differences in various
parts of the clouds. Distorted coronae with small radius may also be observed around the moon
because, except when full, the moon is not a symmetric light source. The corona can be as much as
15° in diameter, and may shrink or swell as different clouds pass across the sun or moon.
(3)
IRISATION or IRIDESCENCE
Definition: Colours appearing on clouds, sometimes mingled, and sometimes in the form of bands
nearly parallel to the margin of the clouds. Green and pink occur most often, and in pastel shades.
The irisation or iridescent colours are often brilliant, and resemble the colours observed on motherof-pearl. Within about 10° from the sun, diffraction is the main cause of irisation or iridescence.
Beyond about 10°, however, interference is usually the predominant factor. Irisation or iridescence
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can extend to angles exceeding 40° from the sun, and even at this angular distance the colours may
be brilliant.
(4)
GLORY
Definition: One or more sequences of coloured rings, seen by an observer around his own shadow on a
cloud consisting mainly of numerous small water droplets, on fog or, very rarely, on dew.
The glory always occurs directly opposite the sun, centred on the antisolar point. Therefore, it is
below the horizon, except at sunrise and sunset. It is observed when the sun is behind you, and
looking down from an elevated location, such as a mountain, hillside or an aircraft, onto mist, fog or
cloud. The coloured rings are due to the diffraction of light, and their arrangement is the same as in a
corona.
The glory is nearly always accompanied by your shadow. Airborne observers may see a glory around
the shadow of the aircraft in which they are flying. When clouds or fog are near the observer and the
observer's shadow appears very large and distorted by perspective, it is called a "Brocken spectre",
whether or not a coloured glory is seen.
(5)
RAINBOW
Definition: A group of concentric arcs with colours ranging from violet to red, produced on a "screen"
of water drops (raindrops, droplets of drizzle or fog) in the atmosphere by light from the sun or moon.
This phenomenon is mainly due to refraction and reflection of light. When rainbows are produced by
the sun, their colours are usually brilliant, and when produced by the moon (moonbow) their colours
are much weaker or sometimes absent.
(a) Primary rainbow: The primary rainbow is a coloured bow or arc which appears on a "screen" of
water drops when light from a bright light source (the sun or moon) falls upon them. A thick "screen"
of raindrops will produce more vivid colours than a thin curtain of fewer drops. If the screen of drops
is only present for some of the arc, only a fragment of a bow is visible.
The size of the drops or droplets determines which colours are present and the width of the band
occupied by each of them. It is unusual to distinguish all of the so-called "colours of the rainbow" (red,
orange, yellow, green, blue, indigo and violet). When the sun is near the horizon, dust may cause a
red rainbow. This happens because the dust preferentially scatters the shorter blue and green
wavelengths of light.
The rainbow is seen in a part of the sky opposite the light source (sun or moon) and the arc is always
centred on a point relative to the observer directly opposite the light source (i.e. an extension of a line
from the light source through the observer, and typically into the ground. This point is known as the
antisolar point with the sun as the light source, and the antilunar point when the light source is the
moon. In all cases violet is on the inside (radius of the arc 40°) and the red on the outside (radius 42°).
The sky is darker outside the bow than inside, a phenomenon known as Alexander's dark band.
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The amount of arc seen by the observer also depends on the elevation of the light source. Over flat
terrain near sunrise or sunset, (or moonrise, moonset) the primary bow can be almost a semicircle. As
the sun or moon rises in elevation, the centre of the arc, and so of the primary rainbow, lowers below
the horizon. When the light source has an elevation approaching 40°, the arc of the primary bow (with
a radius of 42°) will almost sit on the horizon with only the top of the bow visible in the sky. If the sun
or moon rises higher than 42° the primary bow will be below the horizon.
From an elevated vantage point such as a hill an observer may see rainbow arcs greater than a
semicircle. From a very high vantage point such an aircraft, a rainbow forming a large part of a circle,
or even a complete ring, may be seen.
In exceptionally rare circumstances the primary bow appears to split into two equivalent bows, a
'twinned bow' rainbow. The inner bow does not at all resemble supernumerary bows.
(b) Secondary bow: In addition to the primary rainbow, there may be a secondary bow. A secondary
bow is much less bright than the primary and with a breadth almost twice as large. The colors are
reversed, with red on the inside (radius of the arc 51°) and violet on the outside (radius of the arc
54°). The discussion above on visibility of the primary bow applies similarly to the secondary bow, but
the angles of elevation in this case are those applicable to the secondary bow.
(c) Supernumerary bows: Rainbows bordered by narrow coloured bows (green, violet or orange) due
to interference of light waves are called supernumerary bows. They occur inside the primary rainbow
or on rare occasions outside the secondary rainbow. When raindrops are large (greater than 1 mm)
the diffraction pattern may have up to five supernumerary bows, the first merged with the primary
bow, and all repeating the vivid colour pattern. With smaller drops fewer supernumerary bows form.
(d) Fog bow: The fog bow is a primary rainbow caused by refraction and reflection, and to a minor
extent to diffraction, of sunlight or moonlight in very small water droplets. It appears on a "screen" of
fog or mist. The fog bow consists of a white band, usually fringed with a faint, thin red band on the
outside and a faint, thin blue band on the inside. The fogbow is also sometimes known as a cloudbow,
mistbow or white rainbow.
(e) Reflection rainbow: Reflection of sunlight from smooth water surfaces (such as lakes, or
occasionally a calm sea or calm coastal waters) sends reflected sunlight upwards. The reflected
sunlight can act as a light source for both primary and secondary 'reflection rainbows'.
The source of the reflected sunlight is usually a body of smooth water behind the observer, but it can
be in front, in which case only the base of the reflection bows will be visible.
The arc of a reflection rainbow is centred opposite the sun and at the same angular elevation, (the
anthelic point). This is the same elevation above the horizon as the centre of a normal rainbow is
below it. Reflection bows appear at a steeper angle in the sky than the corresponding normal bows,
which they intersect at the horizon.
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(f) Reflected rainbow: Sunlight reflected by a smooth water surface after having already passed
through raindrops may form primary and secondary reflected rainbows. Reflection inverts the
rainbow, which is seen below the horizon, and the centre of the arc is above the horizon at the
anthelic point. The reflected bow is not a reflection in the true sense, but is formed by different
raindrops.
(g) Sea spray bow: Rainbows may also be formed on a 'screen' of spray.
(6)
BISHOP'S RING
Definition: A whitish ring, centred on the sun or moon, with a slightly bluish tinge on the inside and
reddish brown on the outside.
Bishop's ring is caused by diffraction of light passing through a cloud of very fine volcanic dust in the
high atmosphere. The radius of the ring is about 22°. Exact dimensions are determined by the size of
the dust particles. The colours of a Bishop's ring are not very distinct. They are particularly faint in
rings observed around the moon, which usually shows only a pale red fringe. The Bishop's ring is
named after the Rev. S Bishop, who first described the phenomenon after the eruption of Krakatoa in
1883.
(7)
MIRAGE
Definition: An optical phenomenon consisting mainly of images of distant objects. These may be
steady or wavering, single or multiple, upright or inverted, vertically enlarged or reduced.
Objects seen in a mirage sometimes appear at a different angle above the horizon than they really
are. The difference may be as much as 10°. Objects located below the horizon or hidden by mountains
may become visible ("looming"), and objects that are visible under normal circumstances may
disappear during the occurrence of a mirage.
Mirages are due to the curving of light rays passing through layers of air with changing refractive
index because of differences in temperature and therefore density. Therefore, they are generally
observed when the temperature of the Earth's surface differs greatly from the air above.
A mirage may occur as a lower mirage (inferior mirage) over intensely heated water surfaces, soils,
beaches, roads, etc. or as an upper mirage (superior mirage) over snow fields, cold sea surfaces, etc.
The most common inferior mirage is that seen over a hot road surface or a desert surface, giving the
illusion of an area of water on the surface. It is caused by the upward refraction of light from the clear
sky towards the observer. In inferior mirage the image of a distant object is displaced downwards (e.g.
light from the sky appears as though it is water on the ground). Strong bending of light can create
several erect and inverted images.
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The superior mirage is seen above a flat surface of much lower temperature than the air above it. In
this case, light from an object is bent downwards towards the observer, making distant objects
appear higher than their true positions.
A Fata Morgana is a very complex superior mirage, usually seen in polar regions, and created by an
unusual density profile in which horizontal and vertical distortion, inversion and elevation of objects
occur in changing patterns.
The mirage of astronomical objects, such as the sun or moon and occasionally planets, bright stars
and bright comets, may produce distorted or multiple images of the object. The most commonly
observed are sunset and sunrise mirages.
In circumstances where a warm layer of air at the ground, usually over the sea, is overlayed by cold air
above, an inferior mirage may be formed. As the sun lowers towards the horizon, a miraged (inverted)
sun rises. Eventually they meet to form an 'omega'-shape [after the Greek letter]. This is also known
as the 'Etruscan vase'.
When warmer air overlays colder surface air, and when viewed from above the inversion, a Mockmirage may be seen. Multiple inversions may give rise to considerably distorted images of the sun.
The Novaya Zemlya effect is the premature visibility of the sun at the end of the polar winter night. It
is caused by strong refraction of sunlight through a large scale temperature inversion with sharp
thermocline (optical ducting). The light is bent along the Earth's curvature over great distance and a
distorted sun, in the form of a square or line(s) is seen above the horizon when the actual position of
the sun is still below the horizon.
(8)
SHIMMER
Definition: The apparent fluttering of objects at the Earth's surface, when viewed in the horizontal
direction.
Shimmer occurs chiefly over land when the surface is hot, usually when the sun is shining brightly. It is
caused by short period fluctuations of the refractive index in the surface layers of the atmosphere as a
result of the motion of small parcels of air. Shimmer may significantly reduce the visibility.
(9)
SCINTILLATION
Definition: Rapid variations, often in the form of pulsations, of the light from stars or terrestrial light
sources.
The apparent brilliance, colour and position of the stars or lights undergo variations ('twinkling'),
because of fluctuations of the refractive index in the portions of the atmosphere through which the
rays of light pass. This phenomenon is analogous to shimmer.
Other factors being equal, scintillation is more pronounced for longer paths traversed by the
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light through the atmosphere. So the scintillation, or 'twinkling' of stars is more pronounced near the
horizon than at the zenith.
(10)
GREEN FLASH
Definition: A predominantly green coloration of short duration, often in the form of a flash, seen at
the extreme upper edge of the sun, moon, or sometimes even a planet when disappearing below or
appearing above the horizon.
Flashes up to an altitude of several degrees have sometimes been observed. Although the colour of
the phenomenon is predominantly green, blue and violet may rarely be visible when the air is very
transparent. The phenomenon can only be seen if the horizon is clearly visible. It is more frequently
observed over sea than over land.
There are two main types of Green Flashes, each lasting for approximately 1 or 2 seconds.
1. Inferior mirage flash: This flash may occur as the last glimpse of the setting sun, or the first glimpse
of the rising sun. It may occur when the surface layer of air is markedly warmer than the overlying air,
and it is best seen from close to sea level.
2. Mock mirage flash: This flash occurs as an area detached from the upper limb of the sun shortly
before the sun has actually set, or shortly after it has risen. It occurs where warmer air is above cold
surface air and the observer is viewing from above the temperature inversion.
Other, very rare types of green flash are known. These include the sub-duct flash, where the upper
portion of an hourglass-shaped sun turns green for up to 15 seconds, and the green ray where a beam
of light extends upwards from a green flash, or is seen immediately after sunset. On very rare
occasions the green flash may be observed when the sun disappears behind mountains, the upper
edge of a cloud bank near the horizon, or coastal fog.
(11)
TWILIGHT COLOURS
Definition: Various colorations of the sky and of the peaks of mountains at sunset and at sunrise.
Twilight colours are produced by refraction, scattering or selective absorption of light rays from the
sun in the atmosphere. They can be observed in clear, cloudless air.
(a) Purple light: In the direction of the sun a glow called purple light is sometimes seen. This appears
as a segment of a large luminous disk and extends upwards from the horizon. The purple light rises
gradually, reaching a maximum both in size and luminance when the sun is 3 or 4° below the horizon.
It then descends and disappears when the sun is about 6° below the horizon (at the end of civil
twilight). Occasionally, when this first purple light has disappeared, the phenomenon repeats itself
with less intensity. The second patch of purple light appears at a slightly lower elevation than the first
but otherwise follows the same sequence.
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(b) Twilight arch: In the direction opposite to the sun, the Earth's shadow and the twilight arch are
sometimes seen. The Earth's shadow gradually rises above the horizon opposite the sun after sunset.
It appears as a segment of a disk, appearing dark blue-grey and sometimes with a violet tinge. The
twilight arch often appears as a rose violet ribbon bordering the upper edge of the shadow. Above
this arch a faint purple or yellow colour is sometimes seen.
(c) Alpenglühen: Near sunset, the sun may be hidden for a low-level observer, while the mountain
tops may still be in the sun's direct rays. The mountaintops then assume a rosy, pink or yellow tint.
This phenomenon is known as "Alpenglühen" (Alpenglow or Alpine Glow). It disappears after a short
period of blue coloration, when the shadow of the Earth reaches the mountaintops. Sometimes a
second or even a third Alpenglühen may be seen resulting from illumination of snowfields by the first
or second purple light. The Alpenglühen can also be observed near sunrise.
(d) Crepuscular rays: Sometimes dark bluish streaks and light beams are observed radiating from the
sun across the purple light. The streaks are the shadows of clouds at or below the horizon and are
called crepuscular rays. Occasionally, the beams and shadows may cross the sky, becoming visible
again at the antisolar point (anticrepuscular rays). The name "crepuscular rays" is also used to indicate
shadowed bands cast by clouds at any time of day. Pale blue or whitish beams are sometimes seen
diverging from the sun hidden behind cloud (usually cumulus or cumulonimbus). The darker streaks
are the shadows of parts of the irregular cloud. Sunbeams penetrating through gaps in a layer of low
cloud, and rendered visible by water or dust particles in the air, may also be referred to as crepuscular
rays.
III.2.4
Electrometeors
(1)
THUNDERSTORM
Definition: One or more sudden electrical discharges, manifested by a flash of light (lightning) and a
sharp or rumbling sound (thunder).
Thunderstorms are associated with cumulonimbus clouds and are most often accompanied by
precipitation which, when it reaches the ground, is in the form of a shower of rain, snow, snow
pellets, ice pellets or hail.
LIGHTNING
Definition: A luminous manifestation accompanying a sudden electrical discharge which takes place
from or inside a cloud or, less often, from high structures on the ground or from mountains.
Three main types of lightning can be distinguished. In addition there are other forms of luminous
electrical manifestations. Note also that lightning may sometimes be observed in association with
volcanic ash plumes during eruptions.
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(a) Ground discharges (popularly called thunderbolts or cloud-to-ground lightning).This type of
lightning occurs between cloud and ground. It typically appears to follow a tortuous course and is
usually branched downward from a distinct main channel (streak, forked, or ribbon lightning).
A ground discharge is typically initiated when a downward moving, negatively charged 'stepped
leader' connects with a streamer of positive charge reaching upwards. Once this electrically
conductive channel is established a massive electrical discharge follows. This is the 'return stroke' and
is the most luminous and noticeable part of the lightning discharge. Most cloud-to-ground lightning
flashes are made of several strokes thus causing a flickering or strobe-light effect.
Although much less common, cloud-to-ground lightning can also be initiated by a downward moving,
positively charged streamer. This typically originates from high in a thundercloud rather than the
lower part of the cloud. A positive cloud-to-ground discharge is typically very bright in comparison to
other lightning. It may also travel horizontally many kilometres to strike the ground as the, so called,
'bolt from the blue' (clear-air lightning, or anvil lightning).
Ground-to-cloud discharges initiated by an upward moving leader can sometimes originate from an
object on the ground, such as tall towers and skyscrapers.
(b) Cloud discharges (popularly called sheet lightning because it lights up the sky with a sheet of light).
This type of lightning occurs within the thundercloud (intra-cloud lightning) or from one cloud to
another (cloud-to-cloud lightning, or inter-cloud lightning). It typically creates a diffuse illumination
without a distinct channel being seen. This type of lightning includes the so-called heat lightning,
consisting of diffuse light flashed from distant thunderstorms seen at the horizon. Sometimes
lightning discharges originating beneath or within the anvil can be seen to move horizontally for some
distance, generating multiple tree-like branches. These are known as 'anvil crawlers'.
(c) Air discharges (also sometimes called “streak lightning”). This type of lightning occurs as sinuous
discharges passing from a thundercloud to the air and not striking the ground. The discharge often
divides into branches but with a distinct main channel. It frequently includes a long quasi-horizontal
part.
Other forms of luminous electrical discharges
(d) Transient Luminous Events (TLEs)
Short-lived luminous electrical manifestations formed in the upper regions of the atmosphere above
large thunderstorms.
Large thunderclouds are capable of producing electrical phenomena high in the atmosphere. They are
rarely seen visually and are mostly observed with sensitive photographic equipment and, because of
their weakness, only at night. TLEs include Sprites, Jets and ELVES.
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Sprites: These are large-scale electrical discharges which occur high in the atmosphere at about 50 to
90 kilometres above large thunderstorm systems. They usually occur at the same time as powerful
positive cloud-to-ground discharges. They appear as a large, but weak flash usually of red colour.
Sprites last no more than a few seconds. They are rarely seen with the human eye and only at night
above distant, large thunderstorms
Jets: These include blue jets, blue starters and gigantic jets. Blue jets are narrow, cone-shaped bluecoloured flares emerging from the top of a thundercloud and rising to around 40 to 50 kilometres.
Blue jets last only a fraction of a second but have been witnessed by aircraft pilots. Blue starters are
fainter and shorter than blue jets, reaching altitudes of about 20 kilometres. Gigantic jets have been
photographed to around 70 kilometres.
ELVES (Emission of Light and Very Low frequency perturbations due to Electromagnetic pulse
Sources): These appear on highly sensitive low-light video as a dim, flattened, expanding glow to
around 400 kilometres in diameter and lasting typically for less than a thousandth of a second.
(e) Ball lightning: A luminous globe has occasionally been observed, soon after a discharge to ground.
This globe, the dimension of which has been reported to be generally between 10 and 20 cm, but is
said sometimes to reach one metre, is known as ball lightning. It moves slowly in the air or on the
ground and usually disappears with a violent explosion.
THUNDER
Definition: A sharp or rumbling sound which accompanies lightning.
At a short distance the sound of thunder is brief, sharp and violent. When a lightning ground stroke is
very close, a sound of short duration like that of tearing paper followed by a second sound like "vit"
can often be distinguished before the sharp final crack. From a distant discharge, the thunder is heard
as a dull rumbling or a prolonged roll which varies in strength. The duration of a roll of thunder,
except in mountainous regions or cities, rarely exceeds 30 to 40 seconds.
Owing to the difference between the speed of light and the speed of sound, lightning is seen before
the associated thunder is heard. The time interval increases with increasing distance between place of
lightning discharges and observer. When the distance exceeds twenty kilometres thunder is usually
not heard. Sometimes thunder is not audible even when the discharge occurs at appreciably shorter
distances. This is due to the refraction of sound waves in the lower layers of the atmosphere.
(2)
SAINT ELMO’S FIRE
Definition: A more or less continuous, luminous electrical discharge of weak or moderate intensity in
the atmosphere, emanating from elevated objects at the Earth's surface (lightning conductors, windvanes, masts of ships) or from aircraft in flight (e.g. wing tips).
This phenomenon may be observed when the electrical field near the surface of objects becomes
strong. It often appears in the form of violet or greenish plumes or egrets, clearly visible at night.
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(3)
POLAR AURORA
Definition: A luminous phenomenon which appears in the high atmosphere in the form of arcs, bands,
draperies, or curtains.
Auroral activity is the result of the interaction of the solar wind with the Earth's magnetic field. The
solar wind tends to be greater around the height of solar activity which occurs with a cycle between
10 and 12 years. During the increasing phase of a solar cycle, sunspots indicate areas of solar
magnetic activity associated with solar flares and coronal mass ejections (CME). During the decrease
from solar maximum, increased solar wind speed is often associated with coronal holes which allow
charged particles to escape along open magnetic field lines.
Polar aurorae are due to electrically charged particles ejected from the sun (the solar wind) acting on
the rarefied gases of the higher atmosphere. The particles, mainly electrons and protons, are
channeled by the Earth's magnetic field to collide with atoms and molecules of gases in the high
atmosphere (thermosphere/exosphere). The collisions cause the electrons of nitrogen and oxygen
atoms to jump temporarily to a higher, 'excited' energy state. Some energy released on return to
normal energy levels is emitted in the form of photons of light of different wavelengths. Polar aurorae
are most frequently observed in arcs around the magnetic poles; the 'auroral ovals'.
Coronal mass ejections or solar flares, may temporarily enhance the solar wind reaching the Earth's
magnetosphere, perhaps creating a geomagnetic storm. During such events the auroral oval
temporarily enlarges, allowing aurorae to be seen from lower latitudes. In the northern hemisphere
the aurora is known as 'aurora borealis', or the northern lights. In the southern hemisphere the
aurora is called 'Aurora Australis' or the Southern Lights. Most auroral light is produced at heights
between about 90 and 150 kilometres, but can originate as low as 60 kilometres and occasionally as
high as 1000 kilometres, or more.
The luminance or brightness of polar aurorae is very variable. It may be faint and barely visible, or it
may be comparable with that of clouds illuminated by the full moon, and occasionally it may be much
greater. The brightness can be estimated by using the following scale:
1.
2.
3.
4.
Faint or weak. Barely visible. About the brightness of the Milky Way, with minimal colour present.
Similar brightness to moonlit cirrus cloud and may have a slight greenish colour.
Similar to the brightness of moonlit low-altitude clouds, with obvious colour.
Strong. Bright enough to read by and to cast shadows.
The colour of the polar aurorae is dependent on the particular atmospheric gas that emits the light,
its electrical state, and the energy of the solar particles. The brightest and most common auroral light
is white with greenish or greenish-yellow tinge, emitted by atomic oxygen at around 100 kilometres
above the ground. At an altitude of about 150 kilometres single oxygen atoms produce a diffuse red
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glow. Pinkish light on the lower fringes of arcs and bands comes from atomic Nitrogen as low as about
60 kilometres. Molecular Nitrogen emits bluish-purple light at the highest levels.
Polar aurorae display several distinct forms and structure. These can be classified as follows:
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Glow: A dawn-like glow along the horizon.
Homogeneous Patches: Diffuse patches of auroral light without distinct boundaries and of no
particular shape.
Veil: A general auroral light covering most of the sky, but with little structure.
Homogeneous Arc: An arc extending across the sky as a uniformly curved arch of light, without any
vertical ray structure. There is a relatively well-defined lower edge which is more clearly defined
than the upper edge.
Homogeneous Band: A band across the sky without the regular shape of an arc and without any
vertical ray structure. A twisted arc. The lower edge will be more irregular and less well defined
than that of an arc.
Rays: Rays or shafts of vertical auroral light, like a searchlight beam stretching upwards into the
sky. Rays may occur singly, scattered or in bundles.
Rayed Arc: An arc with vertical ray structure. It usually exhibits moderate activity of small
movements and irregular brightness variations.
Rayed Band: A band with vertical ray structure. If the rays are long it may show kinks and folds and
resemble a curtain or drapery waving in the sky. In a strong display multiple bands may be present.
Corona: Rays or other forms overhead converging to a point to form an auroral crown or corona.
When there is little change in form and brightness during a display the aurora is said to be 'quiet'.
However, any of the auroral forms may by ‘pulsating’, indicated by fading and brightening, or
disappearing and reappearing rhythmically. An aurora is ‘flickering’ when it displays subtle but
irregular rapid changes. An aurora is ‘flaming’ when waves of light move rapidly upwards, lighting up
existing forms. An aurora is ‘streaming’ when it displays irregular horizontal variations, such as a
brighter area rippling horizontally along a band.
III.3 OBSERVATION OF METEORS OTHER THAN CLOUDS FROM THE EARTH'S SURFACE
III.3.1
Introduction
The observation of meteors other than clouds should include the identification of the meteors, the
measurement, whenever possible, of their characteristic elements and, for certain meteors, the
identification of the clouds with which they are associated. The record should include the location,
intensity, form, and times of appearance and disappearance and of any marked changes during the
period of occurrence. As with observation of clouds, continuous observation is important.
III.3 .2
Observation of hydrometeors other than clouds
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These hydrometeors may occur in the form of particles suspended in the atmosphere (e.g., fog), of
precipitations (e.g., rain, drizzle, snow, hail), of particles raised by the wind (e.g., drifting or blowing
snow, spray) or else in the form of deposits (e.g., dew, hoar-frost, rime, glaze). In the case of
precipitation, mention should be made as to whether it is uniform (intermittent or continuous) or of
the showery type. For special studies, samples of rain water may be kept for analysis. Exceptionally
big hailstones should be weighed and measured and, if possible, photographed whole and in cross
sections. Photographs of hydrometeors in the form of deposits of dew, hoar-frost, rime etc. may be of
value. The thickness of layers of rime or glaze should be measured. When a spout is observed, the
height, diameter, sense of rotation and path of the cloud funnel (tuba) should be noted. Photographs,
time-lapse images or video should be taken if possible. It is also important to obtain information
about any damage done, including taking photographs.
III.3.3
Observation of Lithometeors
Lithometeors may occur as particles raised from the ground (e.g. drifting or blowing dust or sand, dust
storm or sandstorm), or as particles almost suspended in the atmosphere (haze, dust haze or smoke).
Whenever possible, records should include information regarding the height to which the meteor
extends and any abnormal colouring.
III.3.4
Observation of photometeors
Detailed descriptions, accompanied if possible by photographs, should be given of important or
exceptional photometeors. As already stated, photometeors associated with clouds should be
recorded with the cloud observations. The use of correctly orientated polarizing glasses will reveal
some photometeors that are almost invisible to the human eye. Polarizing glasses will also show
detail that would otherwise be missed.
III.3.5
Observation of electrometeors
Records of lightning should include information about the type, intensity, frequency of flashes and the
range of azimuths over which discharges are observed. Time between lightning and thunder should
also be noted. Care should be taken to distinguish between lightning and its possible reflection on
clouds or haze. In the case of Saint Elmo's fire, it should be stated whether the phenomenon appears
in a cloud, in precipitation or in clear air, etc. Exceptional polar aurorae should be described in detail,
accompanied if possible by photographs, time-lapse or video.
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APPENDIX 1
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ETYMOLOGY OF LATIN NAMES OF CLOUDS
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1. GENERA
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CIRRUS
- From the Latin cirrus, which means a lock of hair, a tuft of horsehair, a
bird's tuft.
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CIRROCUMULUS
- From the Latin cirrus and cumulus.
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CIRROSTRATUS
- From the Latin cirrus and stratus.
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ALTOCUMULUS
- From the Latin altum, which means height, upper air, and cumulus.
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ALTOSTRATUS
- From the Latin altum and stratus.
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NIMBOSTRATUS
- From the Latin nimbus; which means rainy cloud, and stratus.
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STRATOCUMULUS
- From the Latin stratus and cumulus.
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STRATUS
- From the Latin stratus, past participle of the verb sternere, which means to
extend, to spread out, to flatten out, to cover with a layer.
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CUMULUS
- From the Latin cumulus, which means an accumulation, a heap, a pile.
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CUMULONIMBUS
- From the Latin cumulus and nimbus.
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2. SPECIES
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FIBRATUS
- From the Latin fibratus, which means fibrous, possessing fibres, filaments.
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UNCINUS
- From the Latin uncinus, which means hooked.
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SPISSATUS
- From the Latin spissatus, past participle of the verb spissare, which means
to make thick, to condense.
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CASTELLANUS
- From the Latin castellanus, derived from castellum, which means a castle
or the enceinte of a fortified town.
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FLOCCUS
- From the Latin foccus, which means tuft of wool, fluff or nap of a cloth.
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STRATIFORMIS
- From the Latin stratus, past participle of the verb sternere, which means to
extend, to spread out, to flatten out, to cover with a layer, and forma,
which means form, appearance.
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NEBULOSUS
- From the Latin nebulosus, which means full of mist, covered with fog,
nebulous.
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LENTICULARIS - From the Latin lenticularis, derived from lenticula, diminutive of lens meaning a
lentil.
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FRACTUS
- From the Latin fractus, past participle of the verb frangere, which means to shatter,
to break, to snap, to fracture.
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HUMILIS
- From the Latin humilis, which means near the ground, low, of small size.
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MEDIOCRIS
- From the Latin mediocris, which means medium, keeping to the middle.
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CONGESTUS
- From the Latin congestus, past participle of the verb congerere, which means to
pile up, to heap up, to accumulate.
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1321
CALVUS
- From the Latin calvus, which means bald, and, in a wider sense, is applied to
something stripped or bared.
1322
1323
CAPILLATUS
- From the Latin capillatus, which means having hair, derived from capillus which
means hair.
1324
VOLUTUS
- From the Latin volutus, which means rolled.
1325
1326
3. VARIETIES
1327
1328
INTORTUS
1329
1330
VERTEBRATUS - From the Latin vertebratus, which means having vertebrae, in the form of
vertebrae.
1331
1332
UNDULATUS
- From the Latin undulatus, which means having waves, waved; derived from undula,
diminutive of unda, which means wave.
1333
1334
RADIATUS
- From the Latin radiatus, derived from the verb radiare, which expresses the idea of
having rays, being radiant.
1335
LACUNOSUS
- From the Latin lacunosus, which means having holes or furrows.
1336
1337
DUPLICATUS
- From the Latin duplicatus, past participle of the verb duplicare, and expressing the
idea of doubled, repeated, something double.
1338
TRANSLUCIDUS - From the Latin translucidus, which means transparent, diaphanous.
1339
PERLUCIDUS
- From the Latin perlucidus, which means allowing light to pass through it.
1340
OPACUS
- From the Latin opacus, which means shady, shadowy, thick, bushy.
1341
1342
- From the Latin intortus, past participle of the verb intorquere, which means to
twist, to turn, to entangle.
1343
4. SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
1344
INCUS
- From the Latin incus, which means anvil.
1345
MAMMA
- From the Latin mamma, which means udder or breast.
1346
VIRGA
- From the Latin virga, which means rod, stick, branch.
1347
PRAECIPITATIO - From the Latin praecipitatio, which means a fall (down a precipice).
1348
ARCUS
- From the Latin arcus, which means bow, arch, arcade, vault.
1349
TUBA
- From the Latin tuba, which means trumpet, and, in a wider sense, tube, conduit.
1350
PILEUS
- From the Latin pileus, which means a cap.
1351
VELUM
- From the Latin velum, which means sail of a ship, flap of a tent.
1352
PANNUS
- From the Latin pannus, which means piece of cloth, piece, shred, rag, tatter.
1353
CAVUM
- From the Latin cavum, which means cavity, hole or hollow.
1354
FLUCTUS
- From the Latin fluctus, which means wave or billow.
1355
ASPERITAS
- From the Latin asperitas, which means roughness.
1356
MURUS
-From the Latin murus, which means wall.
1357
CAUDA
- From the Latin cauda, which means tail.
1358
FLUMEN
- From the Latin flumen, which means to flow, flowing, a current, a river or stream.
1359
1360
5. SPECIAL CLOUDS
1361
1362
FLAMMAGENITUS
- From the Latin flamma, meaning fire, and genitus, meaning generated or
made.
1363
1364
HOMOGENITUS
- From the Latin homo, meaning man, and genitus, meaning generated or
made.
1365
1366
1367
1368
1369
CATARACTAGENITUS
- from the Latin cataracta, meaning waterfall, and genitus, meaning
generated or made.
SILVAGENITUS
- from the Latin silva, meaning forest, and genitus, meaning generated or
made.
1370
1371
HOMOMUTATUS
- From the Latin homo, meaning man, and mutatus, meaning changed or
mutated.
1372
.....................................
1373
APPENDIX II
1374
HISTORICAL BIBLIOGRAPHY OF CLOUD CLASSIFICATION
1375
1802
LAMARK, J.B
-
Sur la forme des nuages. Annuaire Météorologique pour l'an X de la République
Francaise, no. 3, Paris, 1802, pp. 149-164.
1803
LAMARK, J.B
-
HOWARD, L
-
Tableau des divisions de la région des météores. Annuaire Météorologique pour l'an XI de
la République Francaise, No. 4, Paris, 1803, p. 122.
On the modifications of clouds. Philosophical Magazine, 1803; reprinted in Neudrücke von
Schriften und Karren über Meteorologie und Erdmagnetismus, no. 3, Berlin, 1894.
1805
LAMARK, J.B
Nouvelle définition des termes que j'emploie pour exprimer certaines formes de nuages qu'il
importe de distinguer dans l'annotation de l'état du ciel. Annuaire Meteorologique pour l'an
XIII de Ia République Francaise, No. 6, Paris, 1805, pp. 112-133.
1806
LAMPADIUS, W. A.
-
Systematischer Grundriss der Atmosphärologie. Freyberg, 1806, 392 p.
1815
FORSTER, Th.
-
Researches about atmospheric phaenomena. 2d ed., London, 1815, 271 p.
1831
KAEMTZ, L.F.
-
Lehrbuch der Meteorologie. 1. Bd., Halle, 1831, pp. 377-405.
1840
KAEMTZ, L.F.
-
Vorlesungen über Meteorologie. Halle, 1840, pp. 144-152.
1846
FRITSCH, K.
-
Ueber die periodischen Erscheinungen am Wolkenhimmel. Abhandlungen der Königl.
Böhm.Gesellschaft der Wissensclzaftell, 5. Folge, Bd. 4, Prag, 1846, 72 p.
1855
RENOU, E.
-
Instructions météorologiques. Annuaire de la Société Météorologique de France, tome 3,
Paris, 1855, pp. 142-146.
1863
POEY, A.
-
Sur deux nouveaux types de nuages observés à La Havane, dénommés Pallium
(Pallio-Cirrus et Pallio-Cumulus) et Fracto-Cumulus. Comptes rendus hebdomadaires des
séances de l'Académie des Sciences, tome 56, Paris, 1863, pp. 361-364.
1865
POEY, A.
-
POEY, A.
-
Instructions pour servir á l'observation des nuages, des courants inférieurs et supérieurs de
l'atmosphère. Annuaire de la Société Météorologique de France, tome 13, Paris, 1865, pp.
85-100.
Considérations synthétiques sur la nature, la constitution et la forme des. Annuaire de la
Société Méteorologique de France, tome 13, Paris, 1865, pp. 100-112.
1870
POEY, A.
-
New classification of clouds. Annual report of the Board of Regents of the Smithsonian
Institution, Washington, 1870, pp. 432-456.
1871
FRITSCH, K.
-
Ueber Poey's neue Eintheilung der Wolken. Zeitschrift der Österreichischen Gesellschaft
für Meteorologie, Bd. 6, Wien, 1871, pp. 321-327.
1872
POEY, A.
-
Nouvelle classification des nuages, suivie d'instructions pour servir à l'observation des
nuages et des courants atmosphériques. Annales Hydrographiques, 1e série, tome 35, Paris,
1872, PP 615-715.
1873
METEOROLOGICAL
CONGRESS, Vienna, 216 September 1873.
Protocols and Appendices. London, 1874, pp. 11-49.
1874
MUHRY, A
-
Entwurf eines allgemeinen Wolkensystems. Zeitsclzrift der Österreichisclzen Gesellschaft
für Meteorologie, Bd. 9, Wien, 1874, pp. 70-71.
1878
NEUMAYER, G
-
Instruktion zur Führung des meteorologischen Journal der Deutschen Seewarte. Hamburg,
1878, pp. 12-13.
1879
LEY, W.C.
POEY, A
HILDEBRANDSSON, H
-
Clouds and weather signs. Modern Meteorology, Lecture IV, London, 1879, pp. 102-136.
Comment on observe les nuages pour prévoir le temps. 3e édition, Paris, 1879, 172 p.
Sur la classification des nuages employée a l'Observatoire Météorologique d'Upsala.
Upsala, 1879, 9 p.
1880
WEILBACH, Ph.
-
RITTER, C.
-
Formes des nuages en Europe septentrionale. Annales du Bureau Central Météorologique
de France, tome 1, partie B, Paris, 1880, pp. 11-40.
Essai d'une théorie provisoire des hydrométéores. Annuaire de la Société Meteorologique
de France, tome 28, Paris, 1880, pp. 105-144.
1881
WEILBACH, Ph.
-
Nordeuropas Skyformer og deres Inddeling fremstillet til vejledning ved iagttagelsen af
skyhimlen. Kobenhavn, 1881, 15 p.
1887
KÖPPEN, W
-
ABERCROMBY, R
-
HILDEBRANDSSON, H
-
ABERCROMBY, R.
-
ABERCROMBY, R.
-
Einiges über Wolkenformen. Meteorologisclze Zeitschrift, Bd. 4 (22), Berlin, 1887, pp.
203-214, 252-261.
On the identity of cloud forms all over the world. Quarterly Journal of the Royal
Meteorological Society, vol. 13, London, 1887, pp. 140-146.
Remarks concerning the nomenclature of clouds for ordinary use. Quarterly Journal of the
Royal Meteorological Society, vol. 13, London, 1887, pp. 148-154.
Suggestions for an international nomenclature of clouds. Quarterly Journal of the Royal
Meteorological Society, vol. 13, London, 1887, pp. 154-166.
Weather. London, 1887, pp. 70-122.
1889
HILDEBRANDSSON,
H.
MAZE, Abbé
CONGRÈS
MÉTÉOROLOGIQUE
INTERNATIONAL,
CLAYTON, H.
-
Rapport sur la classification des nuages. Congrès Météorologique International, Paris, 1926 septembre 1889, tome 2, Mémoires, Paris, 1889, pp. 12-24.
Sur la classification des nuages. Congrès Météororogique International, Paris, 19-26
septembre 1889, tome 2; Mémoires, Paris, 1889, pp. 25-37.
Paris, 19-26 septembre 1889. Procès-verbaux sommaires. Paris, 1889, pp. 6-7
-
Cloud observations. Annals of the Astronomical Observatory of Harvard College.
Observations made at the Blue Hill Observatory in the year 1887, vol. 20, part 1,
Cambridge, 1889, pp. 50-57.
-
Wolken-Atlas. Hamburg, 1890.
-
Protocols with Appendices and Supplements. London, 1893.
1892
SINGER, K.
-
Wolkentafeln. Müinchen, 1892.
1893
GASTER, F.
-
MANUCCI, F
-
Suggestions, from a practical point of view, for a new classification of cloud forms.
Quarterly Journal of the Royal Meteorological Society, vol. 19, London, 1893, pp. 218-229.
Classificazione delle nubi. Publicazioni della Specola Vatlcana, tome 3, Roma, 1893, pp.
165-169.
1890
HILDEBRANDSSON,
H., KÖPPEN, W. und
NEUMAYER, G.
1891
INTERNATIONAL
METEOROLOGICAL
CONFERENCE, Munich,
1891.
1894
LEY, W.C.
COMITÉ
MÉTÉOROLOGIQUE
INTERNATIONAL et
COMMISSION
lNTERNATIONALE
POUR L'ÉTUDE DES
NUAGES, Upsala, 1894.
-
Cloudland. A study on the structure and characters of clouds. London, 1894, 208 p.
Rapport sur la réunion. Paris, 1895, pp. 38-41.
-
-
Discussion of the cloud observations made at the Blue Hill Observatory. Annals of the
Astronomical Observatory of Harvard College, vol. 30, part 4, Cambridge, 1896, pp. 273500.
Atlas International des Nuages. Paris, 1896 (reprinted in 1910).
1899
POLIS, P.
-
Wolkentafeln. Karlsruhe, 1899.
1900
BESSON, L
-
Classification des nuages. Congrès International de Météorologie, Paris, 1900. Procèsverbaux des séances et Mémoires, Paris, 1901, pp. 61-64.
1903
VINCENT, J
-
VINCENT, J.
-
Notes bibliographiques sur les nuages (Classification et nomenclature). Observatoire Royal
de Belgique, Annuaire Météorologique pour 1903, tome 70, Bruxelles, 1903, pp. 430-449.
Étude sur les nuages: I. Les nuages lacunaires; II. Les faux cirrus de l'alto-cumulus; III. Les
variétés de l'alto-cumulus. Annates de l'Observatoire Royal de Belgique, nouvelle série,
Annales Météorologiques, tome 6, Bruxelles, 1903, 48 p.
1905
CLAYDEN, A
OSTHOFF, H.
-
Cloud studies. London, 1905.
Die Formen der Cirruswolken. Meteorologische Zeitschrift, 22. Jahrg., Berlin, 1905, pp.
337- 343, 385-398, 439-455.
1907
VINCENT, J.
-
Atlas des Nuages. Bruxelles, 1907. Also published in Annales de l'Observatoire Royal de
Belgique, nouvelle série, Annates Météorologiques, tome 20, Bruxelles, 1909, 29 p.
1908
DE QUERVAIN, A.
-
Beiträge zur Wolkenkunde. Meteorologische Zeitschrift, 25. Jahrg., Berlin, 1908, pp. 433453.
1909
VINCENT, J.
-
Notes bibliographiques sur les nuages (Classification et nomenclature). Observatoire Royal
de Belgique, Annuaire Meteoro/ogique pour 1909, tome 76, Bruxelles, 1909, pp. 126-128.
1921
BESSON, L.
-
La classification détaillée des nuages en usage à 1'Observatoire de Montsouris. Annales des
Services Techniques d'Hygiène de Ia Ville de Paris, tome 1, Paris, 1921, pp. 297-318.
-
Circulaires C.E.N. et Procès-verbaux des sessions de la Commission, de 1921 à 1937.
-
Apercu historique sur Ia classification des nuages. Mémorial de l'Office National
Météorologique de France, n° 2, Paris, 1923.
-
International Atlas of Clouds and States of the Sky, Abridged Edition for the use of
Observers, Paris, 1930, 50 p. Reprinted in 1939, under the title: International Atlas of
Clouds and of Types of Skies, Abridged Edition for the use of Observers.
1896
CLAYTON, H.
HILDEBRANDSSON,
H., RIGGENBACH, A.
et TEISSERENC DE
BORT, L.
1921-1937
COMMISSION
INTERNATIONALE
POUR L'ÉTUDE DES
NUAGES.
1923
BESSON, L.
1930
INTERNATIONAL
COMMISSION FOR
THE STUDY OF
CLOUDS.
1932
INTERNATIONAL
COMMISSION FOR
THE STUDY OF
CLOUDS.
INTERNATIONAL
COMMISSION FOR
THE STUDY OF
CLOUDS.
1949-1953
COMMITTEE FOR THE
STUDY OF CLOUDS
AND
HYDROMETEORS.
2013-2016
COMMISSION FOR
INSTRUMENT AND
METHODS OF
OBSERVATION
-
International Atlas of Clouds and States of the Sky, tome I, General Atlas, Paris, 1932, 106
p. Reprinted in 1939, under the title: International Atlas of Clouds and of Types of Skies,
tome 1, General Atlas.
-
International Atlas of Clouds and States of the Sky, tome 2, Atlas of Tropical Clouds, Paris,
1932, 27 p.
-
Final Reports and Reports of sessions of the Committee, from 1949 to 1953.
-
Final Reports of Sessions of the WMO Task Team for Revision of the International Cloud
Atlas, from 2013-2017.
1376
1377
APPENDIX III
1378
BIBLIOGRAPHY OF CLOUD NOMENCLATURE
1379
1380
1381
The following bibliography gives brief references to books, publications or documents in which, as far as is known, the various types of
cloud are mentioned, described or studied for the first time.
1382
1. Cirrus
CIRRUS
-
HOWARD 1803.
On the modifications of clouds, etc., see Appendix II.
CIRRUS FIBRATUS
-
This type of cloud was initially called Cirrus filosus by CLAYTON in 1896
(Discussion of the cloud observations, etc., see Appendix 11).
-
The use of the term filosus was extented to Cirrocumulus and to Altocumulus by
BESSON in 1921 (La classification détaillée des nuages, etc., see Appendix II).
-
The term filosus was applied to Cirrostratus by the INTERNATIONAL COMMISSION
FOR THE STUDY OF CLOUDS (C.E.N.) in 1930 (International Atlas of Clouds and
States of the Sky, Abridged Edition for the use of Observers, etc., see Appendix II).
-
In 1951 the COMMITTEE FOR THE STUDY OF CLOUDS AND
HYDROMETEORS (C.C.H.) replaced the term filosus by the term fibratus which is
etymologically preferable. Furthermore, the use of this term was limited to Cirrus and to
Cirrostratus (Reports of the third session, Paris, January 1951).
CIRRUS UNCINUS
-
MAZE 1889.
Sur la classification des nuages, etc., see Appendix II.
CIRRUS SPISSATUS
-
The appellation Cirrus spissatus was introduced by C.C.H. in 1949 (Final Report of the
first session, Paris, August 1949). It replaced both the following two appellations:
-
Cirrus densus, introduced by BESSON in 1921 (La classification détaillée des nuages
en nusage à l'Observatoire de Montsouris, etc., see Appendix II);
-
Cirrus nothus, introduced by C.E.N. in 1926 (Reports of the session in Paris, April
1926, published in Circular 47 of C.E.N.).
-
The term castellatus (subsequently replaced by the term castellanus) was introduced by
LEY in 1879, in the appellation Stratus castellatus (Clouds and weather signs, etc., see
Appendix II).
-
In 1903, VINCENT described in detail Altocumulus castellatus (Études sur les nuages:
III. Les variétés de l'alto-cumulus, etc., see Appendix II).
-
In 1951, C.C.H. replaced the term castellatus by the term castellanus which is
etymologically preferable to it. The use of this term was extended to Cirrus,
Cirrocumulus and Stratocumulus (Reports of the third session, Paris, January 1951).
-
The term floccus was introduced by VINCENT in 1903, in the appellation Altocumulus
floccus (Études sur les nuages: III. Les variétés de l'alto-cumulus, etc., see Appendix II).
-
In 1930, C.E.N. extended the use of this term to Cirrus (International Atlas of Clouds
and States of the Sky, Abridged Edition, etc., see Appendix II).
-
The term floccus was later applied to Cirrocumulus by C.C.H. (Final Report of the
second session, Paris, June 1950).
-
The WMO Task Team for the revision of the International Cloud Atlas extended the use
of the species floccus to Stratocumulus in the 2017 edition of the Atlas.
CIRRUS INTORTUS
-
The term intortus, applied to Cirrus, was introduced by C.C.H. in 1951 (Reports of the
third session, Paris, January 1951).
CIRRUS RADIATUS
-
The term radiatus was introduced by C.E.N. in 1926, and it was applied to Cirrus,
Altocumulus and to Stratocumulus (Reports of the session in Paris, April 1926,
published in Circular 47 of C.E.N.).
-
In 1949, C.C.H. extended the use of this term to Altostratus (Final Report of the first
session, Paris, August 1949).
CIRRUS CASTELLANUS
CIRRUS FLOCCUS
CIRRUS VERTEBRATUS
CIRRUS DUPLICATUS
-
During the final editing of the Atlas, the use of the term radiatus was extended to
Cumulus.
-
The cloud form vertebratus was described by MAZE in 1889, under the appellation
striga pennata which means: (cloud) band having the appearance of a bird feather, fern
leaf, skeleton of a fish, etc. (Sur la classification des nuages, etc., see Appendix II).
-
The appellation Cirrus vertebratus was introduced by OSTHOFF, in 1905 (Die Formen
der Cirruswolken, etc., see Appendix II).
-
The appellation Cirrus duplicatus was introduced by MAZE in 1889 (Sur la
classification des nuages, etc., see Appendix II).
-
In 1908, DE QUERVAIN described in detail Altostratus duplicatus (Beiträge zur
Wolkenkunde, etc., see Appendix II).
-
The use of the term duplicatus was later extended to Cirrostratus and to Altocumulus by
C.C.H. (Final Report of the second session, Paris, June 1950).
-
During the final editing of the Atlas, the use of the term duplicatus was extended to
Stratocumulus.
2. Cirrocumulus
CIRROCUMULUS
CIRROCUMULUS STRATIFORMIS
-
HOWARD 1803.
On the modifications of clouds, etc., see Appendix II.
-
RENOU 1855.
Instructions météorologiques, etc., see Appendix II .
-
The term stratiformis was introduced by C.C.H. in 1949, and it was applied to
Cirrocumulus, Altocumulus and Stratocumulus (Final Report of the first session,
Paris, August, 1949).
-
The term lenticularis was introduced by LEY in 1894, in the appellation Stratus
lenticularis (Cloudland, etc., see Appendix II).
-
In 1930, C.E.N. extended the use of this term to Cirrocumulus, Cirrostratus,
Altocumulus and to Stratocumulus (International Atlas of Clouds and States of the
Sky, Abridged Edition, etc., see Appendix II).
-
C.C.H. subsequently limited the use of the term lenticularis to Cirrocumulus,
Altocumulus and Stratocumulus (Final Report of the first session, Paris, August
1949).
CIRROCUMULUS CASTELLANUS
-
LEY 1879 and C.C.H. 1951.
Same references as for Cirrus castellanus.
CIRROCUMULUS FLOCCUS
-
VINCENT 1903 and C.C.H. 1950.
Same references as for Cirrus floccus.
CIRROCUMULUS UNDULATUS
-
The appellation Cirrocumulus undulatus was introduced by CLAYTON in 1896.
Although the author has not mentioned other genera, he has stressed the fact that "this
characteristic of clouds is found at every altitude" (Discussion of the cloud
observations, etc., see Appendix II).
-
The use of the term undulatus was explicitly extended to Cirrus, Altocumulus,
Altostratus, Stratocumulus, Stratus and Cumulus, by C.E.N. in 1930 (International
Atlas of Clouds and States of the Sky, Abridged Edition, etc., see Appendix II).
-
C.C.H. subsequently limited the use of this term to Cirrocumulus, Altocumulus,
Altostratus, Stratocumulus and Stratus (Final Report of the second session, Paris,
June 1950).
-
During the final editing of the Atlas, the use of the term undulatus was extended to
Cirrostratus.
-
The cloud form lacunar was described by VINCENT in 1903, and this term was
applied to Cirrus, Cirrocumulus and Cirrostratus (Études sur les nuages: I. Les
nuages lacunaires, etc., see Appendix II).
-
The term lacunaris was introduced by C.E.N: in 1930, and only applied to
Cirrocumulus and to Altocumulus (International Atlas of Clouds and States of the
Sky, Abridged Edition, etc., see Appendix II).
-
In 1951, C.C.H. replaced the term lacunaris by the term lacunosus which is
etymologically preferable to it (Reports of the third session, Paris, January 1951).
-
During the final editing of the Atlas, the use of the term lacunosus was extended to
Stratocumulus.
CIRROCUMULUS LENTICULARIS
CIRROCUMULUS LACUNOSUS
3. Cirrostratus
CIRROSTRATUS
-
HOWARD 1803 and RENOU 1855.
Same references as for Cirrocumulus.
CIRROSTRATUS FIBRATUS
-
CLAYTON 1896, C.E.N. 1930 and C.C.H. 1951.
Same references as for Cirrus fibratus.
CIRROSTRATUS NEBULOSUS
-
The appellation Cirrostratus nebulosus was introduced by CLAYDEN in 1905
(Cloud studies, etc., see Appendix II).
-
The term nebulosus was later applied to Stratus by C.C.H. (Final Report of the second
session, Paris, June 1950).
CIRROSTRATUS DUPLICATUS
-
MAZE 1889, DE QUERVAIN 1908 and C.C.H. 1950.
Same references as for Cirrus duplicatus.
CIRROSTRATUS UNDULATUS
-
CLAYTON 1896 and C.C.H. 1953
Same references as for Cirrostratus undulatus.
4. Altocumulus
-
RENOU 1870.
Bulletin de l'Observatoire de Montsouris, Paris, 1870.
-
HILDEBRANDSSON 1889.
Rapport sur la classification des nuages, etc., see Appendix II.
ALTOCUMULUS STRATIFORMIS
-
C.C.H. 1949.
Final Report of the first session, Paris, August 1949.
ALTOCUMULUS LENTICULARIS
-
LEY 1894, C.E.N. 1930 and C.C.H. 1949.
Same references as for Cirrocumulus lenticularis.
ALTOCUMULUS CASTELLANUS
-
LEY 1879, VINCENT 1903 and C.C.H. 1951.
Same references as for Cirrus castellanus.
ALTOCUMULUS FLOCCUS
-
VINCENT 1903.
Étude sur 1es nuages: III. Les variétés de l'alto-cumulus, etc., see Appendix II.
The WMO Task Team for the revision of the International Cloud Atlas extended the
use of the species floccus to Stratocumulus in the 2017 edition of the Atlas.
ALTOCUMULUS
-
The term translucidus was introduced by C.E.N. in 1926, in the appellation
Altostratus translucidus (Reports of the session in Paris, April 1926, published in
Circular 47 of C.E.N.).
-
In 1930, C.E.N. extended the use of this term to Altocumulus and to Stratocumulus
(International Atlas of Clouds and States of the Sky, Abridged Edition, etc., see
Appendix II).
-
The term translucidus was later applied to Stratus by C.C.H. (Final Report of the
second session, Paris, June 1950).
ALTOCUMULUS PERLUCIDUS
-
The term perlucidus, applied to Altocumulus and to Stratocumulus, was introduced
by C.C.H. in 1951 (Reports of the third session, Paris, January 1951).
ALTOCUMULUS OPACUS
-
The term opacus was introduced by BESSON in 1921, in the appellation Altostratus
opacus (La classification détaillée des nuages en usage a l'Observatoire de
Montsouris, etc., see Appendix II).
-
In 1930, C.E.N. extended the use of this term to Altocumulus and to Stratocumulus
(International Atlas of Clouds and States of the Sky, Abridged Edition, etc., see
Appendix II).
-
The term opacus was later applied to Stratus by C.C.H. (Final Report of the
session, Paris, June 1950).
ALTOCUMULUS DUPLICATUS
-
MAZE 1889, DE QUERYAIN 1908 and C.C.H. 1950.
Same references as for Cirrus duplicatus.
ALTOCUMULUS UNDULATUS
-
CLAYTON 1896, C.E.N. 1930 and C.C.H. 1950.
Same references as for Cirrocumulus undulatus.
ALTOCUMULUS RADIATUS
-
C.E.N. 1926.
Reports of the session in Paris, April 1926, published in Circular 47 of C.E.N.
ALTOCUMULUS LACUNOSUS
-
VINCENT 1903, C.E.N. 1930 and C.C.H. 1951.
Same references as for Cirrocumulus lacunosus.
ALTOCUMULUS TRANSLUCIDUS
second
ALTOCUMULUS VOLUTUS
-
The term volutus was recommended by the WMO Task Team for the revision of the
International Cloud Atlas to identify detached roll clouds of Altocumulus and
Stratocumulus as separate from the supplementary feature arcus. (Final Report of the
first session, Geneva, November 2013); introduced in the 2017 edition of the Atlas.
5. Altostratus
-
RENOU 1877.
Bulletin de l'Observatoire. de Montsouris, Paris, 1877.
-
HILDEBRANDSSON 1889.
Rapport sur la classification des nuages, etc., see Appendix II.
ALTOSTRATUS TRANSLUCIDUS
-
C.E.N. 1926.
Reports of the session in Paris, April 1926, published in Circular 47 of C.E.N.
ALTOSTRATUS OPACUS
-
BESSON 1921.
La classification détaillée des nuages en usage a l'Observatoire de Montsouris, etc.,
see Appendix II.
ALTOSTRATUS DUPLICATUS
-
MAZE 1889 and DE QUERVAIN 1908.
Same references as for Cirrus duplicatus.
ALTOSTRATUS UNDULATUS
-
CLAYTON 1896, C.E.N. 1930 and C.C.H. 1950.
Same references as for Cirrocumulus undulatus.
ALTOSTRATUS RADIATUS
-
C.E.N. 1926 and C.C.H. 1949.
Same references as for Cirrus radiatus.
ALTOSTRATUS
6. Nimbostratus
NIMBOSTRATUS
-
C.E.N. 1930.
International Atlas of "Clouds and States of the Sky, Abridged Edition, etc.,
I.c. Appendix II.
7. Stratocumulus
STRATOCUMULUS
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KAEMTZ 1840.
Vorlesungen iiber Meteoroiogie, etc., I.c. Appendix II
STRATOCUMULUS STRATIFORMIS
-
C.C.H. 1949.
Final Report of the first session, Paris, August 1949.
STRATOCUMULUS LENTICULARlS
-
LEY 1894, C.E.N: 1930 and C.C.H. 1949.
Same references as for Cirrocumulus lenticularis.
STRATOCUMULUS CASTELLANUS
-
LEY 1879 and C.C.H. 1951.
Same references as for Cirrus castellanus.
STRATOCUMULUS TRANSLUCIDUS
-
C.E.N. 1926 and C.E.N. 1930.
Same references as for Altocumulus translucidus.
STRATOCUMULUS FLOCCUS
-
The WMO Task Team for the revision of the International Cloud Atlas extended the
use of the species floccus to Stratocumulus in the 2017 edition of the Atlas.
STRATOCUMULUS VOLUTUS
-
The term volutus was recommended by the WMO Task Team for the revision of the
International Cloud Atlas to identify detached roll clouds of Stratocumulus and
Altocumulus as separate from the supplementary feature arcus.
(Final Report of the first session, Geneva, November 2013); introduced in the 2017
Atlas.
STRATOCUMULUS PERLUCIDUS
-
C.C.H. 1951.
Reports of the third session, Paris, January 1951.
STRATOCUMULUS OPACUS
-
BESSON 1921 and C.E.N. 1930.
Same references as for Altocumulus opacus.
STRATOCUMULUS DUPLICATUS
-
MAZE 1889, DE QUERVAIN 1908 and C.C.H. 1953.
Same references as for Cirrus duplicatus.
STRATOCUMULUS UNDULATUS
-
CLAYTON 1896, C.E.N. 1930 and C.C.H. 1950.
Same references as for Cirrocumulus undulatus.
STRATOCUMULUS RADIATUS
-
C.E.N. 1926.
Reports of the session in Paris, April 1926, published in Circular 47 of C.E.N.
STRATOCUMULUS LACUNOSUS
-
VINCENT 1903, C.C.H. 1951 and C.C.H. 1953.
Same references as for Cirrocumulus lacunosus.
8. Stratus
-
HOWARD 1803.
On the modifications of clouds, etc., see Appendix II.
-
HILDEBRANDSSON 1887.
Remarks concerning the nomenclature of clouds for ordinary use, etc., I.c. Appendix
II.
-
ABERCROMBY 1887.
Suggestions for an international nomenclature of clouds, etc., see Appendix II.
STRATUS NEBULOSUS
-
CLAYDEN 1905 and C.C.H. 1950.
Same references as for Cirrostratus nebulosus.
STRATUS FRACTUS
-
This type of cloud was initially called Fractostratus by C.E.N. in 1930 (International
Atlas of Clouds and States of the Sky, Abridged Edition, etc., see Appendix II).
-
In 1949, C.C.H. replaced this appellation by Stratus fractus, which is more in
accordance with other appellations of species (Final Report of the first session, Paris,
August 1949).
STRATUS OPACUS
-
BESSON 1921 and C.C.H. 1950.
Same references as for Altocumulus opacus.
STRATUS TRANSLUCIDUS
-
C.E.N. 1926 and C.C.H. 1950.
Same references as for Altocumulus translucidus.
STRATUS UNDULATUS
-
CLAYTON 1896, C.E.N. 1930 and C.C.H. 1950.
Same references as for Cirrocumulus undulatus.
STRATUS
9. Cumulus
CUMULUS
-
HOWARD 1803.
On the modifications of clouds, etc., see Appendix II.
CUMULUS HUMILIS
-
VINCENT 1907.
Atlas des Nuages, etc., see Appendix II
CUMULUS MEDIOCRIS
-
The term mediocris, applied to Cumulus, was introduced by C.C.H. in 1951 (Reports
of the third session, Paris, January 1951).
CUMULUS CONGESTUS
-
MAZE 1889.
Sur la classification des nuages, etc., see Appendix II.
CUMULUS FRACTUS
-
This type of cloud was initially called Fracto-Cumulus by POEY in 1863 (Sur deux
nouveaux types de nuages observes a La Havane, etc., see Appendix II).
-
In 1949, C.C.H. replaced this appellation by Cumulus fractus, which is more in
accordance with other appellations of species (Final Report of the first session, Paris,
August 1949).
-
C.E.N. 1926 and C.C.H. 1953.
Same references as for Cirrus radiatus.
CUMULUS RADIATUS
10. Cumulonimbus
CUMULONIMBUS
-
WEILBACH 1880.
Formes des nuages en Europe septentrionale, etc., see Appendix II.
CUMULONIMBUS CALVUS
-
C.E.N. 1926.
Reports of the session in Paris, April 1926, published in Circular 47 of C.E.N.
CUMULONIMBUS CAPILLATUS
-
C.E.N. 1926.
Reports of the session in Paris, April 1926, published in Circular 47 of C.E.N.
11. Upper Atmospheric clouds
NACREOUS CLOUDS
-
MOHN, H. 1893.
Perlemorskyer, Videnskabselskab, no. 10, Christiania, 1893.
Also published under the title "Irisierende Wolken" in Meteorologische Zeitschrift,
10. Jahrg., Berlin, 1893, pp, 80-97, 460
NOCTILUCENT CLOUDS
-
JESSE, 0. 1890,
Untersuchungen über die sogenannten leuchtenden Nachtwolken,
Silzungsberichte der Kgl. Preussischen Akad. der Wissenschaften, Koenigsberg,
1890, 1891.
-
STORMER, C. 1932.
Height and velocity of luminous night clouds observed in Norway, 1932.
Observatory of University of Oslo, no. 6, Oslo, 1932.
-
VESTINE, E. H. 1934.
Noctilucent clouds. Journal of the Royal Astronomical Society of Canada, Ottawa,
July-August, September 1934, pp. 249-272, 303-317 (including extensive
bibliography).
-
A summary of the articles mentioned above was published in the Bulletin of the
American Meteorological Society, vol. 16, Washington, February 1935, pp. 49-50.
1383
ICA GLOSSARY
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Accretion - Growth of a cloud or precipitation particle by the collision and union of a frozen particle
(ice crystal or snowflake) with a supercooled liquid droplet which freezes on impact.
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Anvil Zits - [Colloq], frequent (often continuous or nearly continuous), localized lightning discharges
occurring from within a thunderstorm
(WMO No. 182)
Advection -Transport of water or air along with its properties (e.g., temperature, chemical tracers) by
the motion of the fluid. Regarding the general distinction between advection and convection, the
former describes the predominantly horizontal, large-scale motions of the atmosphere or ocean,
while convection describes the predominantly vertical, locally induced motions.
(WMO No. 182)
Agglomeration - A term used in cloud physics for any of the processes by which precipitation grows
through the collision and amalgamation of various cloud or precipitation particles.
(http://oxfordindex.oup.com)
Aggregation - The process in which solid precipitation particles combine in the atmosphere to
produce larger particles, e.g. hailstones.
(WMO No. 182)
Anvil – See II.2.3.4 (a) Supplementary feature Incus
A cirriform cloud with an anvil shape, which forms the upper part of a well developed Cumulonimbus.
Its glaciated top spreads out horizontally upon reaching the tropopause or by the action of the winds
aloft.
(WMO No. 182)
Anvil Crawler - [Colloq], a lightning discharge occurring within the anvil of a thunderstorm,
characterized by one or more channels that appear to crawl along the underside of the anvil. They
typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an
active MCS (mesoscale convective system). (See III.2.4.(1)(b))
(Glossary for storm spotters)
Anvil Dome – A domelike protrusion above a Cumulonimbus anvil, representing the intrusion of an
updraft through its equilibrium level (level of neutral buoyancy).
It is usually a transient feature because the rising parcel's momentum acquired during its buoyant
ascent carries it past the point where it is in equilibrium; the air within it rapidly becomes negatively
buoyant and descends. Tall and persistent overshooting tops are frequently observed with strong or
severe thunderstorms in which there is a nearly continuous stream of buoyant updrafts.
(AMS Glossary)
Anvil Rollover - [Colloq], a circular or semicircular lip of clouds along the underside of the upwind part
of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles,
mushroom.
(Glossary for storm spotters)
(Glossary for storm spotters)
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Arctic sea smoke (Frost smoke, Steam Fog) - See III.2.1.1
A type of advection fog that forms primarily over water when cold air passes across warmer waters.
(http://download.weather.com/glossary)
Back-building Thunderstorm - A thunderstorm in which new development takes place on the upwind
side (usually the west or southwest side), such that the storm seems to remain stationary or
propagate in a backward direction.
(Glossary for storm spotters)
Back-sheared Anvil - [Colloq], a thunderstorm anvil which spreads upwind, against the flow aloft. A
back-sheared anvil often implies a very strong updraft and a high severe weather potential.
(Glossary for storm spotters)
Banner Cloud – Stationary orographic cloud which forms in the neighbourhood of a mountain crest or
peak and takes the shape of a banner streaming downwind from the mountain peak. This type of
cloud must not be confused with snow which is blown off a mountain summit and carried downwind.
(WMO No. 182)
Barber Pole - [Colloq], a thunderstorm updraft with a visual appearance including cloud striations that
are curved in a manner similar to the stripes of a barber pole. The structure typically is most
pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often
erodes the clouds on the trailing side of the updraft.
(Glossary for storm spotters)
Barrage cloud - A band of thick orographic cloud that arises when a moist airstream rises over a
mountain barrier. This generally produces heavy, long-lasting precipitation on the windward slopes,
and may create föhn conditions to leeward.
(oxfordindex.oup.com)
Bear's Cage - [Colloq], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy
precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one
associated with an HP storm.
(Glossary for storm spotters)
Beaver('s) Tail - [Colloq], a particular type of inflow band with a relatively broad, flat appearance
suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly
parallel to the pseudo-warm front,. As with any inflow band, cloud elements move toward the
updraft. Its size and shape change as the strength of the inflow changes. See also inflow stinger.
(Glossary for storm spotters)
Bergeron-Findeisen process – The process by which precipitation is initiated in a mixed cloud with a
temperature below freezing. Because the equilibrium vapour pressure of water vapour with respect
to ice is less than that with respect to liquid water, ice crystals grow at the expense of supercooled
water droplets. Eventually the crystals become sufficiently heavy to begin to fall, and may
subsequently be modified by other processes such as accretion, melting, coalescence, and
evaporation.
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(oxfordindex.oup.com)
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Cloud bank - A fairly well-defined mass of cloud observed at a distance; it covers an appreciable
Black Ice – (1) Sometimes called clear ice, refers to a thin coating of glaze ice on a surface, especially
on roads. (2) A mariner's term for a dreaded form of icing sometimes sufficiently heavy to capsize a
small ship.
(WMO No. 182)
BKN - The cloud amount shall be reported (observation) or given (forecast) as broken (5 to 7 oktas)
using the three-letter abbreviation BKN
(WMO No.306)
Cap (or Capping Inversion) – The stable layer or inversion that caps the Planetary Boundary Layer.
The cap can limit or delay the vertical development of Cumulus clouds.
Cap cloud – Stationary cloud on or above an isolated mountain peak.
(WMO No. 182)
Cell - Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically
seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm
consists of several cells (see multi-cellular thunderstorm).
(Glossary for storm spotters)
Cirriform clouds – Composed of small particles, mostly ice crystals. Because the particles are fairly
widely dispersed, this usually results in relative transparency and whiteness, often producing halo
phenomena. These clouds generally have bases above 20,000 feet in the mid-latitudes, and are
classified as high clouds. They include all varieties of Cirrus, Cirrocumulus, and Cirrostratus clouds.
(http://download.weather.com/glossary)
Clear Slot - A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air;
often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A
clear slot is believed to be a visual indication of a rear flank downdraft.
(Glossary for storm spotters)
portion of the horizon sky but does not extend overhead.
(WMO No. 182)
Cloudburst - Sudden, heavy rainfall of a showery nature
(http://download.weather.com/glossary)
Cloud étage - A meteorological term used in the 1956 and 1975 editions of the International
Cloud Atlas (ICA) to define the range of levels within which clouds of certain genera occur more
frequently. Étage has been replaced with “level”, primarily because it can be easily translated to
many different languages.
The use of étage in this context was proposed by J.B. Lamarck in the first published classification of
clouds (1802).
(ICA Vol I, 1956 edition).
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Cloud Streets - Clouds arranged in lines roughly parallel to the wind direction and appearing, on
account of perspective, to converge towards a point or two opposite points on the horizon called
the radiation point(s). The cloud most frequently appearing in cloud streets is Cumulus mediocris.
(WMO No. 182)
Cloud Tags - Ragged, detached cloud fragments; fractus or scud.
(Glossary for storm spotters)
Coalescence – Process of formation of a single liquid water drop by the union of two or more colliding
drops.
(WMO No. 182)
Collar Cloud - A generally circular ring of cloud that may be observed on rare occasions, surrounding
the upper part of a wall cloud. Distinct to the orographic collar cloud sometimes observed
surrounding the upper part of an isolated mountain
(Glossary for storm spotters)
Comma Cloud - A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on
satellite photographs associated with large and intense low-pressure systems.
(Glossary for storm spotters)
Condensation - (1) The transition from the gaseous to the liquid state. (2) The physical process by
which water vapour is transformed into dew, fog or cloud droplets.
(WMO No. 182)
Condensation Funnel – see II.2.3.4 (a) Tuba
Condensation nucleus - Nucleus on which water vapour can condense.
(WMO No. 182)
Condensation trail (contrail) – Cloud which form in the wake of an aircraft when the atmosphere at
flying level is sufficiently cold and humid. (Mother–cloud “ homogenitus”)
(WMO No. 182)
Convection - Organized motions within a layer of air leading to the vertical transport of heat,
momentum, etc.
(WMO No. 182)
Convective clouds – Cumuliform cloud that forms in an atmospheric layer made unstable by
heating at the base or cooling at the top.
(WMO No. 182)
Condensation Level – Level at which the air subjected to a lifting process becomes saturated.
(WMO No. 182)
Cumuliform Anvil - A thunderstorm anvil with visual characteristics resembling cumulus-type clouds
(rather than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises
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from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil
rollover, knuckles, mushroom.
(Glossary for storm spotters)
Cumuliform Cloud - Cloud with the bulging appearance of a Cumulus. When such clouds, arranged in
lines and joined by a common base, possess protuberances giving them a turreted appearance, they
are classed in the species castellanus (of the genera Stratocumulus, Altocumulus, Cirrus and
Cirrocumulus). When they constitute elements separated into tufts they are classed in the species
floccus (of the genera Stratocumulus, Altocumulus, Cirrus and Cirrocumulus).
(WMO No. 182)
Debris Cloud - A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a
condensation funnel and surrounding the base of a tornado.
(Glossary for storm spotters)
Deposition - The formation of ice on a surface directly from water vapour, without passing through a
liquid phase.
(oxfordindex.oup.com)
Dew Point (or Dew-point Temperature) – Temperature to which a volume of air must be cooled at
constant pressure and constant moisture in order to reach saturation.
Dissipating trail (Distrail) – A clearly delineated lane forming behind an aircraft flying in a thin cloud
layer; the opposite of a condensation trail.
The heat of combustion of the aircraft fuel, released into the swept path by the exhaust stacks of the
aircraft, can, under certain conditions, evaporate existing clouds (if not too dense) and yield a distrail.
Clouds of low liquid water content and relatively high temperature are susceptible to distrail
formation but the phenomenon is comparatively rare.
(Mother–cloud “homogenitus”).
Downburst - Violent and damaging downdraught reaching the surface, associated with a severe
thunderstorm.
(WMO No. 182)
Dry line - A boundary separating moist and dry air masses, often an important factor in severe
weather development. An example is the dry-line in the Great Plains of the United States. It typically
lies north-south across the central and southern high Plains states during the spring and early
summer, where it separates moist tropical air from the Gulf of Mexico (to the east) and dry desert
(continental) air from the southwestern states (to the west). The dry line typically advances eastward
during the afternoon and retreats westward at night.
(edited from Glossary for storm spotters)
Electrical storm – (1) Popular public term for thunderstorm (2) Sometimes applied to a relatively
rare condition of disturbed atmospheric electric field in the lower atmosphere that arises when
strong winds are blowing and much dust is in the air, but there is no thunderstorm activity.
Blowing dust may charge fences and other metallic objects to such an extent that slight shocks
are felt upon touch.
(AMS Glossary)
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Elevated Convection – Convection that originates in an unstable layer above the planetary
boundary layer.
With elevated convection, stability indices based on near-surface measurements (such as the Lifted
Index) typically underestimate the amount of instability present. Severe weather is possible from
elevated convection, but is less likely than it is with surface-based convection.
(edited - Glossary for storm spotters)
Entrainment - The mixing of environmental air into a cloud or an air current so that the
environmental air becomes part of the current or the cloud.
(WMO No. 182)
Evaporation - Water changing from liquid to vapour at a temperature below boiling point.
(WMO No. 182)
Eye wall - An organized band of convection surrounding the eye, or center, of a tropical cyclone. It
contains cumulonimbus clouds, intense rainfall and very strong winds.
(http://download.weather.com/glossary)
Fair-weather cumulus – A popular term for Cumulus of the humilis or mediocris species, used
because these forms are normally accompanied by significant amounts of sunshine but do not
produce any rain.
(oxfordindex.oup.com)
Fallstreak – see II.2.3.4 (a) Supplementary feature - virga
Fallstreak hole – see II.2.3.4 (a) Supplementary feature - cavum
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Also known as a hole punch cloud, punch hole cloud, sky punch, canal cloud or cloud hole, is a large
circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. Such holes are formed
when the water temperature in the clouds is below freezing but the water has not frozen yet due to
the lack of ice nucleation.
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Few - The cloud amount shall be reported (observation) or given (forecast) as few (1 to 2 oktas) using
(http://w1.weather.gov/glossary/ NOAA)
Fast ice – Sea ice which forms and remains fast along the coast, where it is attached to the shore, to
an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be
observed during changes of sea-level. Fast ice may be formed in situ from sea water or by freezing of
floating ice of any age to the shore, and it may extend a few metres or several hundred kilometres
from the coast.
(WMO No. 259)
Feeder Bands – See II.2.3.4 (b) Accessary cloud - flumen
Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually
parallel to the inflow. Same as inflow bands.
(Glossary for storm spotters)
Fern frost - see Window Frost
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the three-letter abbreviation FEW
(WMO No.306)
Flanking Line - A line of cumulus or towering cumulus clouds connected to and extending outward
from the most active part of a supercell. The line normally has a stair-step appearance, with the
tallest clouds closest to the main storm.
(Glossary for storm spotters)
Flurries (snow) - Are an intermittent light snowfall of short duration (generally light snow showers)
with no measurable accumulation (trace category).
(http://w1.weather.gov/glossary/ NOAA)
Foehn Wall (Foehn Bank) - Cloud formation which, during a foehn episode, lies over and along a
mountain ridge and which presents, to an observer downwind from the ridge, the appearance of a
vertical wall.
(WMO No. 182)
Forward Flank Downdraft - The main region of downdraft in the forward, or leading, part of a
supercell, where most of the heavy precipitation is.
(Glossary for storm spotters)
Frazil ice - Accumulation of primary ice crystals in water and/or at the bottom of a water body,
formed by supercooled turbulent waters.
International Glossary of Hydrology
Freezing - The phase change in a substance from a liquid to a solid state that occurs with cooling.
(oxfordindex.oup.com)
Front - A boundary or transition zone between two air masses of different density, and thus (usually)
of different temperature. A moving front is named according to the advancing air mass, e.g., cold
front if colder air is advancing
(Glossary for storm spotters)
Frost flower – Individual ice crystals, sometimes over 200 mm in height and usually occurring in small
clumps, that grow on fresh sea ice under calm conditions, when humidity above the ice is high. Frost
flowers act as wicks, producing a higher concentration of dissolved ions than found in the subjacent
sea water. They appear to be a significant source of atmospheric bromine monoxide (BrO) in polar
regions in spring.
(oxfordindex.oup.com)
Frost point – Temperature to which a volume of air must be cooled at constant pressure and moisture
in order that it shall be saturated with respect to ice
Frost smoke - Fog-like clouds due to contact of cold air with relatively warm water, which can appear
over openings in the ice, or leeward of the ice edge, and which may persist while ice is forming.
(WMO No. 259)
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Fumulus – [Colloq] is a Cumulus cloud caused by the moisture rising from industrial sources such as
smokestacks and cooling towers. This cloud is classified as Cumulus homogenitus.
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A gust front is often associated with a pressure jump, wind shift, temperature drop, and sometimes
with heavy precipitation.
(https://books.google.com)
Granular cloudlets – Cloud with elements resembling small grains or particles. (e.g. a Cirrocumulus
cloud).
(http://www.bing.com)
Granulometry - Is the measurement of the size distribution in a collection of grains
(https://www.google.com)
Graupel – see III.2.1.1 (7) Snow pellets
German word for “soft hail”; soft hail was officially renamed to “snow pellets” in 1956.
Ground icing - Structural icing that occurs on an aircraft on the ground, usually produced by snow or
frost.
(http://aircrafticing.grc.nasa.gov/resources/glossary.html)
Gunge - [Colloq], anything in the atmosphere that restricts visibility for storm spotting, such as fog,
haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc.
(Glossary for storm spotters)
Gust front - The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes
associated with a shelf cloud (Supplementary feature arcus) or roll cloud (Supplementary feature
volutus). See also downburst, gustnado, outflow boundary.
(Glossary for storm spotters)
(AMS Glossary)
Guttation - A process in which water in liquid form is given off by plants.
(http://www.dictionary.com)
Haboob ( habub "blasting/drafting") - Strong wind, producing a dust storm or sandstorm.
(WMO No. 182)
Heat lightning - Lightning that occurs at a distance such that thunder is no longer audible.
(http://w1.weather.gov/glossary/ NOAA)
Horse shoe vortex cloud - is a very rare, short lived cloud formation (resembling a horse shoe)
originating from within regions of rotating air caused by a swift updraft coming in contact with
perpendicular horizontal winds
(https://horseshoevortex.wordpress.com)
HP Storm or HP Supercell - High Precipitation storm or High Precipitation supercell. A supercell
thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the
mesocyclone . Precipitation often totally envelops the region of rotation, making visual identification
of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of
rotation in many HP storms develops in the front-flank region of the storm. HP storms often produce
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extreme and prolonged downburst events, serious flash flooding, and very large damaging hail
events.
(Glossary for storm spotters)
Hygroscopic – A substance which tends to absorb moisture by accelerating the condensation of water
vapour
(Meteorological Glossary – HMSO)
Ice flowers – See window frost.
Ice Prisms – See III.2.1.1 (8) Diamond dust
Ice storm - Intense formation of ice on objects by the freezing, on impact, of rain or drizzle.
(WMO No. 182)
Inflow Bands – See Feeder bands.
Inflow Stinger - A beaver tail cloud with a stinger-like shape.
(http://w1.weather.gov/glossary/ NOAA)
Isentropic lift - Lifting of air that is traveling along an upward-sloping isentropic surface. Situations
involving isentropic lift often are characterized by widespread stratiform clouds and precipitation,
but may include elevated convection in the form of embedded thunderstorms.
(Glossary for storm spotters)
Intercycle ice - Ice that consists of residual Ice, plus ice accreted between boot cycles of an aircraft
engine.
(http://aircrafticing.grc.nasa.gov/resources/glossary.html)
Inversion – An increase of temperature with height.
(WMO No. 182)
Inversions and stable layers inhibit vertical motion, often limiting the vertical extent of cumuliform
clouds. If the air below an inversion is relatively moist, the inversion often caps a layer of stratiform
cloud.
Killing frost - A sharp fall in temperature that damages a plant so severely as to cause its death, or
that prevents the reproduction of an annual, biennial, or ephemeral plant. As the falling temperature
approaches freezing-point, some water is lost from the vacuole into the intercellular spaces, where a
further drop in temperature causes it to form small crystals of ice. If the temperature then rises
slowly, the water will be reabsorbed by the cell as the ice melts and the cell will recover, but if the
thaw is rapid the water will be lost and the cell will die from dehydration.
(oxfordindex.oup.com)
Knuckles - [Colloq], lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm
anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of
the anvil due to the presence of a very strong updraft. They are not the Supplementary feature
mamma. See also cumuliform anvil, anvil rollover.
(Glossary for storm spotters)
Lapse rate - The rate of change of any meteorological element with height. e.g. A steep (temperature)
lapse rate is a rapid decrease of (temperature) with height
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Layer – A term of limited meaning used within the technical regulations of the International Cloud
Atlas (1956 and 1975 and its predecessor of 1939).
Layer is used to describe a sheet of considerable horizontal extent; often covering the whole sky.
Levanter cloud - The cap cloud that forms in moist stable easterly winds over the Rock of Gibraltar.
(http://www.bing.com)
LP Storm (or LP Supercell) – Low Precipitation storm (or Low Precipitation supercell). A supercell
thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic
supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual
appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation.
They are capable of producing tornadoes and very large hail. Radar identification often is difficult
relative to other types of supercells, so visual reports are very important. LP storms almost always
occur on or near the dry line, and thus are sometimes referred to as dry line storms.
(Glossary for storm spotters)
Luminance – The quality of being luminous; emitting or reflecting light
(Webster-Dictionary.org)
Luminosity - The attribute of an object or colour enabling the extent to which an object emits light to
be observed (formerly known as brightness)
(British Dictionary – Dictionary.com)
Mares’ tails - Irregularly curved wisps. Cirrus uncinus.
MCC - Mesoscale Convective Complex - A large MCS, generally round or oval-shaped, which normally
reaches peak intensity at night. The formal definition includes specific minimum criteria for size,
duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite
photographs:
 Size: cloud top temperature of -32° C or colder covers 100,000 km2 or more (size of Republic
of Korea), and cloud top temperature of -52°C or colder covers 50,000 km2 or more.
 Duration: size criteria must be met for at least 6 hours.
 Eccentricity: Minor/major axis at least 0.7.
(Glossary for storm spotters)
MCS - Mesoscale Convective System - A complex of thunderstorms which becomes organized on a
scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs
may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs
(among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the
size, shape, or duration criteria of an MCC.
(Glossary for storm spotters)
Melting - Phase change from a solid to liquid.
(oxfordindex.oup.com)
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Mesocyclone - A storm-scale region of rotation, typically around 3 -10 km in diameter and often
found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The
circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.
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An elongated cloud band, visually similar to a roll cloud, usually they appear in the morning hours,
when the atmosphere is relatively stable. Morning glories result from perturbations related to
gravitational waves in a stable boundary layer. They are similar to ripples on a water surface; several
parallel morning glories often can be seen propagating in the same direction. (Species “Volutus”).
(Glossary for storm spotters)
Microburst - An intense, localized downdraft of air that spreads on the ground, usually below a
Cumulonimbus, causing rapid changes in wind direction and speed. (Diameter 4 kilometers or less)
(WMO No. 182)
Macroburst – See Microburst (diameter greater than 4 kilometers)
Mixed cloud - Cloud in which ice particles are mixed with supercooled droplets of water
(WMO No. 182)
Mixed icing conditions - An atmospheric environment where supercooled liquid water and ice crystals
coexist.
(http://aircrafticing.grc.nasa.gov/resources/glossary.html)
Morning Glory – see II.2.2.2 Species – volutus
(Glossary for storm spotters)
Multi-vortex tornado - A tornado in which two or more condensation funnels or debris clouds are
present at the same time, often rotating about a common center or about each other. Multiple-vortex
tornadoes can be especially damaging.
(Glossary for storm spotters)
Mushroom - [Colloq], a thunderstorm with a well-defined anvil rollover, and thus having a visual
appearance resembling a mushroom.
(Glossary for storm spotters)
Needle ice – A phenomenon that occurs, when the temperature of the soil is above 0°C and the
surface air temperature is below 0°C. The sub-surface liquid water is brought to the surface by
capillary action where it freezes and contributes to a growing needle-like ice column.
(https://en.wikipedia.org)
Opaque - Not transparent or translucent; impenetrable to light; not allowing light to pass through.
See II.2.3.4 Varieties – opacus
Orphan Anvil - [Colloq], an anvil from a dissipated thunderstorm, below which no other clouds
remain.
(Glossary for storm spotters)
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OVC - The cloud amount shall be reported (observation) or given (forecast) as overcast (8 oktas) using
the three-letter abbreviation OVC
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Used to describe swellings, turrets and towers seen in many clouds; e.g. the small towers seen in
Cumulis mediocris, the sometimes complex mass of towers in Cumulus congestus or the very small
turrets of Cirrus castellanus
(WMO No.306)
Overshooting Top (Penetrating Top) - A dome-like protrusion above a thunderstorm anvil,
representing a very strong updraft and hence a higher potential for severe weather with that storm. A
persistent and/or large overshooting top ( see anvil dome) often is present on a supercell. A shortlived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a
pulse storm or a cyclic storm.
(Glossary for storm spotters)
Permafrost - A soil layer below the surface of tundra regions that remains frozen permanently.
(http://download.weather.com/glossary)
Pile d’assiettes (Pile of plates)- A stack of lenticular clouds. Technical classification is Stratocumulus
lenticularis duplicatus or AltocumuIus lenticularis duplicatus (height of cloud base dependent)
Planetary Boundary Layer (PBL) –The part of the troposphere that is in contact with the Earth’s
surface.
The PBL is often turbulent and is capped by a stable layer or a temperature inversion. The depth
of the PBL is variable in time and space, ranging from tens of meters in strongly stable situations,
to several kilometers in convective conditions over deserts. During fair weather over land, the
PBL has a marked diurnal cycle. By day the PBL grows in depth due to turbulence and convection.
At night the base of the PBL cools due to radiational cooling at the Earth’s surface; this
transforms the PBL into a stable, shallow boundary. Cumulus and Stratocumulus clouds can form
near the top of the PBL (moisture dependent), while fog can form at the bottom of a stable
boundary layer.
Polar vortex - The large-scale cyclonic circulation in the middle and upper troposphere centered
generally in the polar regions.
(WMO No. 182)
Popcorn convection – [Colloq] Clouds, showers and thundershowers that form on a scattered basis
with little or no apparent organization, usually during the afternoon in response to diurnal heating.
(Glossary for storm spotters)
Precipitation – Hydrometeor consisting of a fall of an ensemble of particles. The forms of
precipitation are: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail and ice pellets.
(WMO No. 182)
Protuberance – A projection or a bulge
(dictionary.com)
Pulse Storm - A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and
immediately after which the storm produces a short episode of severe weather. These storms
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generally are not tornado producers, but often produce large hail and/or damaging winds. See
overshooting top, cyclic storm.
(Glossary for storm spotters)
Pyrocumulus - A Cumulus cloud caused by hot air rising from a volcano or large fire.
(Mother-cloud “Flammagenitus”)
(https://books.google.com)
Rain Foot -[Colloq], a horizontal bulging near the surface in a precipitation shaft, forming a footshaped prominence. It is a visual indication of a wet microburst.
(Glossary for storm spotters)
Rain-free Base - A dark, horizontal cloud base with no visible precipitation beneath it. It typically
marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to
the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south
or southwest side of the main precipitation area.
(Glossary for storm spotters)
Rear Flank Downdraft (or RFD) - A region of dry air subsiding at the rear of, and wrapping around, a
mesocyclone. It often is visible as a clear slot wrapping around the wall cloud. Scattered large
precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show
up on radar as a hook or pendant.
(Glossary for storm spotters)
Refrangibility – (1) Ability to be refracted. (2) Ability to be bent
(http://www.babylon-software.com/definition)
Relative humidity - The ratio (%) of the observed vapour pressure to the saturation vapour
pressure with respect to water at the same temperature and pressure
(WMO No.8)
Roll Cloud - See II.2.2.2 Species – volutus
Rotor cloud – Turbulent cloud formation in the lee of large mountain barriers; the air in the cloud
rotates around an axis parallel to the range.
(WMO No. 182)
Scarf cloud – See Cap cloud
SCT - The cloud amount shall be reported (observation) or given (forecast) as scattered (3 to 4 oktas)
using the three-letter abbreviation SCT
(WMO No.306)
Scud - See II.2.8.2 Species - Stratus fractus and II.3.8.5 - Clouds from which Stratus may form
A mainly nautical term for ragged fragments of low cloud, often moving rapidly in a strong wind
below rain clouds. The meteorological term is Stratus fractus
(The Meteorological Glossary (HMSO)
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Sea-of-cloud -The appearance of the upper surface of a layer of cloud which shows undulations of
very different lengths. The whole aspect then suggests waves on the sea.
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Sheet is used to describe a relatively thin layer that covers less than the whole sky. See layer
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A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or
occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf
cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud
motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often
appears turbulent, boiling, and wind-torn.
(WMO No. 182)
Shallow fog - Shallow fog or ice fog at the station, whether on land or sea, not deeper than about 2
metres on land or 10 metres at sea
(WMO No.306 Table 4677)
Sheet – A term of limited meaning used within the technical regulations of the International Cloud
Atlas (1956 and 1975 and its predecessor of 1939).
Sheet ice - Ice formed by the freezing of liquid precipitation or the freezing of melted solid
precipitation.
(http://w1.weather.gov/glossary/ NOAA)
Shelf Cloud – see II.2.3.4 (a) Supplementary feature - Arcus
(Glossary for storm spotters)
Showers – Cumulus and Cumulonimbus clouds produce Showers. They are characterized by their
abrupt beginning and end and by the generally rapid and sometimes great variations in the
intensity of the precipitation. Drops and solid particles falling in a shower are generally larger
than those falling in non-showery precipitation. Between showers gaps may be observed unless
stratiform clouds fill the gaps between the cumuliform clouds
Sleet - Depending on the region, precipitation of rain and snow mixed, or rain and hail, or rain
and ice pellets, or melting snow, or sudden and brief rainfall with wind and hail.
(WMO No. 182)
Slush - Snow that has water content exceeding its freely drained condition such that it takes on fluid
properties (e.g., flowing and splashing).
(http://aircrafticing.grc.nasa.gov/resources/glossary.html)
Smoking mountain – see Banner cloud.
Snow burst - Very intense shower of snow, often of short duration, that greatly restricts visibility and
produces periods of rapid snow accumulation.
(http://www.timeanddate.com/weather/glossary.html)
Snowflake - Agglomeration of snow crystals.
(WMO No. 182)
Snow rollers – Cylindrical masses of snow formed, more often in hilly regions, when moist snow is
driven by the wind and then rolled onward.
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(WMO No. 182)
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An ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is
analogous to evaporation of a liquid. Colloquially the terms are used interchangeably for the solid–
vapor transition (evaporation). For growth, the term sublimation has been replaced by deposition
since the 1970s. There is evidence that deposition nucleation does occur, although there may be an
adsorbed layer prior to nucleation. It appears that most nuclei in the atmosphere require near–water
saturation before they initiate ice.
Snow squalls – are intense, but of limited duration, periods of moderate to heavy snowfall,
accompanied by strong, gusty surface winds and possible lightning. In Canada, snow squall are said to
bring strong winds, flurries and poor visibility.
(http://www.timeanddate.com/weather/glossary.html)
Snowdrift –Mass of snow heaped up by the wind and deposited along an obstruction or an
irregularity of the terrain.
(WMO No. 182)
Sprouting - The process whereby seeds or spores sprout and begin to grow
(http://www.webster-dictionary.org)
Sprouting is used to describe the appearance of upper parts of clouds such as Cumulus mediocris and
congestus, and Cumulonimbus calvus
Squall Line - A non-frontal line or narrow band of thunderstorms (with or without squalls).
(WMO No. 182)
Stable layer – A layer in the troposphere where the temperature lapse rate slows, or there is no
change of temperature with height, or the temperature increases with height (inversion). Stable
layers restrict up motion; a shallow stable layer may momentarily delay up motion; a deep stable
layer may stop up motion. Stable layers often determine the type and vertical extent of cloud.
Steam fog – An alternative name for Arctic sea smoke, or frost smoke.
Stratiform clouds – Low and middle level clouds that have predominantly horizontal development.
The clouds are Nimbostratus, Altostratus, Altocumulus and Stratocumulus of the variety stratiformis,
and Stratus other than of the species fractus. See Cirriform clouds and Cumuliform clouds.
Stratosphere - The region of the atmosphere above the troposphere extending (from about 8 km
at the poles and about 18 km at the equator) to about 50 km altitude. The temperature generally
increases with height through the stratosphere.
Sublimation -The process of phase transition from solid directly to vapor in the absence of melting.
(AMS Glossary)
Supercell - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible
for a remarkably high percentage of severe weather events - especially tornadoes, extremely large
hail and damaging straight-line winds. They frequently travel to the right of the main environmental
winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or
pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual
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characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line,
overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear
or southwest part of the storm.
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Used in describing the Cloud Variety duplicatus where cloud patches, sheets or layers may be at
slightly different levels or partly merged.
(Glossary for storm spotters)
Superposed - To place above or upon something else, or one upon another.
(dictionary.com)
Surface-based Convection- Convection occurring within a surface-based layer, i.e., a layer in which
the lowest portion is based at or very near the earth's surface. Compare with elevated convection.
(Glossary for storm spotters)
Table cloth – A name given to a cloud that forms over the top of Table Mountain, Cape Town, South
Africa., due to orographic lift.
(https://www.google.com)
Tail Cloud - A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the
precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending
from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as
that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the
wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds.
(Glossary for storm spotters)
Tilted storm or tilted updraft - A thunderstorm or cloud tower which is not completely vertical but
instead has a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for
severe storm development.
(Glossary for storm spotters)
Towering Cumulus (TCU) – Cumulus congestus of great vertical extent
Trace Icing - Ice becomes perceptible. Rate of accumulation is slightly greater than the rate of
sublimation. Deicing/anti-icing equipment is not utilized unless encountered for an extended period
of time (over 1 hour).
(http://aircrafticing.grc.nasa.gov/resources/glossary.html)
Translucent - Permitting light to pass through but diffusing it so that persons, objects, etc., on the
opposite side are not clearly visible.
See II.2.3.3 Varieties - translucidus
Transparent - having the property of transmitting rays of light through its substance so that bodies
situated beyond or behind can be distinctly seen.
Tropopause – The boundary between the troposphere and the stratosphere, where an abrupt
change in lapse rate usually occurs. It is defined as the lowest level at which the lapse rate
decreases to 2˚C km-1 or less, provided that the average lapse rate between this level and all
higher levels within 2 km does not exceed 2˚C km-1.
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(WMO No.182)
Troposphere - Lower part of the Earth’s atmosphere, extending from the surface up to a height
varying from about 8 km at the poles to about 18 km at the equator, in which the temperature
decreases fairly uniformly with height.
Twister – [Colloq], tornado.
Turkey Tower- [Colloq], a narrow, individual cloud tower that develops and falls apart rapidly. The
sudden development of turkey towers from small cumulus clouds may signify the breaking of a
capping inversion.
(Glossary for storm spotters)
Updraft Base - Alternate term for a rain-free base.
(Glossary for storm spotters)
Wall cloud – See II.2.3.4 Supplementary feature - Murus.
Window frost- (also called fern frost or ice flowers) forms when a glass pane is exposed to very cold
air on the outside and warmer, moderately moist air on the inside.
(https://en.wikipedia.org)
Wind shear -Space variation of wind speed in a given direction (horizontal or vertical).
(RA IV/TD-No. 494 (1995))
Glossary terms and their sources
(Glossary for storm spotters)
Anvil Crawler, Anvil Dome, Anvil Rollover, Anvil Zits, Back-building Thunderstorm,
Back-sheared Anvil, Barber Pole, Bear's Cage, Beaver ('s) Tail, Cap (or Capping Inversion),
Cell, Clear Slot, Cloud Streets, Collar Cloud, Comma Cloud, Cumuliform Anvil, Debris Cloud, Dry-line,
Elevated Convection, Feeder Bands, Flanking Line, Forward, Flank Downdraft, Front, Gunge, Gust front,
HP Storm or HP Supercell, Isentropic lift, Knuckles, Mesocyclone, Morning Glory, Multivortex tornado,
Mushroom, Orphan Anvil, Overshooting Top, Popcorn convection, Pulse Storm, Rain Foot,
Rain-free Base, Rear Flank Downdraft (or RFD), Scud (or Fractus), Shelf Cloud, Supercell,
Surface-based Convection, Tail Cloud, Tilted storm or tilted updraft, Turkey Tower, Updraft Base,
Waterspout, Wedge
International Met Vocabulary (WMO No. 182)
Accretion, Advection, Aggregation, Anvil, Banner Cloud, Black Ice, Cap cloud, Cloud bank,
Cloud streets, Coalescence, Condensation, Condensation level, Condensation nucleus,
Condensation trail (contrail), Convection, Convective clouds, Cumuliform cloud,
Dew Point (or Dew-point Temperature), Downburst, Entrainment, Evaporation,
Foehn Wall(Foehn Bank), Haboob( habub "blasting/drafting"), Ice Prisms, Ice storm, Microburst,
Polar vortex, Rotor cloud, Scud, Sea –of-cloud, Sleet, Snow rollers, Squall line, Snowdrift,
Sublimation.
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International Glossary of Hydrology
Frazil ice,
WMO No. 259
Fast ice, Frost smoke
(WMO No.306)
BKN, SCT, OVC, Shallow fog
(ICA Task Team 2013)
Cap (or Capping Inversion), Dew Point (or Dew-point Temperature), Dissipating trail (Distrail),
Frost point, Lapse rate, Layer, Planetary Boundary Layer (PBL), Showers, Stratiform clouds,
Stratosphere, Towering Cumulus (TCU), Translucent, Transparent, Troposphere
(RA IV/TD-No. 494 (1995))
Wind shear
(ICA Vol I, 1956 edition).
Cloud étage
(http://download.weather.com/glossary)
Arctic sea smoke (Frost smoke, Steam Fog), Cirriform cloud, Cloudburst, Cumuliform cloud, Eye wall,
(http://aircrafticing.grc.nasa.gov/resources/glossary.html)
Ground icing, Intercycle ice, Mixed icing conditions, Slush, Trace Icing
(http://w1.weather.gov/glossary/ NOAA)
Fallstreak hole
(oxfordindex.oup.com)
Barrage Cloud, Bergeron-Findeisen process, Deposition, Fair-weather cumulus , Freezing,
Frost flower, Killing Frost, Melting,
(Meteorological Glossary – HMSO)
Hygroscopic
(https://en.wikipedia.org)
Needle ice
(https://horseshoevortex.wordpress.com)
Horse shoe vortex cloud
(http://www.bing.com)
Granular cloudlets,
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(https://www.google.com)
Granulometry
(http://www.webster-dictionary.org)
Sprouting
(AMS Glossary)
Gust front
(dictionary.com)
Protuberance