Ch 6
Ch 6
Atmospheric Moisture
Sec A
• I. Measuring Relative Humidity
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A. A Psychrometer is an instrument for measuring relative humidity
B. A common psychrometer uses two thermometers with a wet
gauze wrapped over the bulb of one. Air is blown
over both thermometers and the difference in temperature
between the two thermometers can be used to calculate
the Relative Humidity.
C. For Example the dry-bulb thermometer reads 25 degrees C
and the wet-bulb reads 20 degrees C,
the relative humidity is 63%
63%. This is commonly found in a table
that comes with the psychrometer.
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Ch 6
Sec A
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Atmospheric Moisture:Clouds
2. Stratus clouds (STratus, AS:AltoStratus, CS:CirroStratus)
a. Large flat clouds often covering whole sky
b. Form at about 2.5 kilometers
c Can bring light rain or drizzle
c.
d. Nimbostratus are stratus clouds that are raining
e. Stratus clouds close to the ground are called fog.
f. Our fog in SJ and Santa Cruz is called Pacific Stratus
g. Ground fog forms when the ground cools quickly
at night causing the temperature of the air
in contact with ground to go below the dew point.
Fog is usually as stratus cloud
cloud.
Ch 6
Atmospheric Moisture:Clouds
Sec B
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3. Cirrus Clouds (CI, CS, CC)
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a. light feathery clouds that form at very
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high altitudes between 6 and 12 Km.
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b made of ice crystals
b.
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c. usually indicate rain or snow is coming
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Atmospheric Moisture:Clouds
Sec B
• II. A. Clouds form when the air becomes colder than the dew
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point and the water vapor in the air condenses onto
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microscopic dust particles. Need 1. water vapor,
2 condensation
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ti surface,
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and
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3. cooling.
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B. Clouds are classified by their shape and altitude.
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C. There are three main types of clouds:
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cumulus, stratus, and cirrus
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1. Cumulus Clouds (SC,AC,CC,CU,CB)
piles of white cotton balls.
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a. Look like p
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b. Usually have flat bottoms
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c. Form at 2.4 to 13.5 kilometers.
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c. Cumulonimbus clouds are large cumulus
clouds that are grey and usually bring rain
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d. The suffix ...nimbus means rain.
4. Naming of clouds
a. The three types cumulus, stratus, and cirrus
b. add nimbus after the word or put nimbo before
the cloud type
yp to indicate it is raining
g or
threatening rain.
c. add alto before the word to indicate high
d. add cirro before the cloud type for very high
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
Ch 6
Sec B
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
Ch 6
Sec B
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3.
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Station Plot Cloud Types
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Atmospheric Moisture:Clouds
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The Bottom of clouds are called their bases
The Base of Cumulus Clouds( CU) can be
approximated by: H ~= 1000ft x (T –Tdp)/4.4 Fahrenheit Degs
Wh
Where
H iis th
the h
height
i ht iin ffeett
of the Base of the CU above the surface.
So Altitude of base of CU is:
Alt ~= Elevation + 1000ft x (T –Tdp)/4.4 Fahrenheit Degs
T is the OAT(Outside Air Temperature)
Tdp is the dew point temperature
See:
http://cimss.ssec.wisc.edu/satmet/modules/clouds/makeCU.html
And
http://www.csgnetwork.com/estcloudbasecalc.html
Ch 6
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Prevailing Visibility: Greatest horizontal visibility which
bright lights or large objects can be seen
seen. Statute Miles
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Runway Visibility: Horizontal Visibility of a bright objects
down a given runway. Given in Hundreds of feet.
• Ceiling: the height above the Earth’s surface of the lowest
cloud layer that has 5/8 or greater sky coverage.
• Vertical
V ti l Visibility:
Vi ibilit
Th vertical
The
ti l di
distance
t
you can see iinto
t a
ground based obsuration: fog, smoke, ash, or haze.
Reported in Hundreds of feet eg. VV005
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Ch 6
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Ch 6
Flight Rules
Sec B
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VFR: Visual Flight Rules
– Ceiling > 3000 feet AGL
– Visibility > 5 Statute Miles
• MVFR: Marginal Visual Flight Rules
– Ceiling 1000 to 3000 feet AGL
– Visibility 3 to 5 SM
Atmospheric Moisture:Precipitation
Sec B
Precipitation
• A. Water vapor that has condensed and fallen to Earth
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as rain, sleet, snow or hail.
• B.
B Start as very small droplets and then hit and combine
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with other droplets to become larger.
• C. A rain drop contains about 1 million times the water as
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a droplet in a cloud
• D. Frozen rain is called sleet
• E. Snow is a six pointed crystal
• F. Snow forms when water vapor changes directly to a crystal.
• G. Hail is like large sleet ranging in size from .5 to 7.5 cm.
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1. Hail is like an onion: it has several layers from
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raindrops hitting it and freezing on it.
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2. Can be up to 3 inches in size.
• IFR: Instrument Flight Rules
– Ceiling: 500 to 1000 Feet AGL
– Visibility: 1 to 3 SM
• LIFR: Low Instrument Flight Rules
– Ceiling < 500 Feet AGL
and/or
– Visibility < 1 SM
Atmospheric Moisture:Visibility
Sec B
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Tower Visibility: Horizontal Visibility from the tower.
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Ch 6
Atmospheric Moisture:Precipitation
Sec B
Ch 6
From Fed. Met. Handbook 1
Sec B
• Intensity of Precipitation
Light: Indicated with a –’ve sign
Moderate: No intensity symbol
Heavy: Indicated with a + sign
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Ch 6
Atmospheric Moisture:Precipitation
Sec B
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Lab 11
• Answer the all of the questions questions at the
Wisconsin Website.
• Make a table for questions 1,2,&3 using data from that
web site, the cumulus calculator(2nd web site) and the
equation given in these notes. A row for each question
and a col for each method of calculating the cloud base.
Elevation=1ft
• Table of 3 rows and 3 columns
• http://cimss.ssec.wisc.edu/satmet/modules/clouds/makeCU.html
htt // i
i
d / t t/ d l / l d / k CU ht l
• And
• http://www.csgnetwork.com/estcloudbasecalc.html
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