Instruments
Following instruments are used to know meteorological conditions.
1. Mercury Barometer.
2. Aneroid barometer
3. Precision aneroid barometer.
4. Barograph.
5. Air thermometer.
6. Sea thermometer
7. Hydrometer.
8. Whirling psychrometer.
9. Anemometer
10. Facsimile recorder.
Mercury Barometer
Mercury barometer is a widely used weather instrument, which measures
atmospheric pressure (air pressure or barometric pressure).
Principle: First fill up a glass tube (one end closed) of about 84 cm with mercury
and then invert it into a mercury-filled bowl (cistern). While inverting into mercury
of bowl, a finger should be placed over the open top, so that mercury from the
tube can not come out. After removing finger from the bowl, we will see that the
mercury at tube comes down to a certain level and then remains steady. Here
height of mercury in the tube is adjusted with the atmospheric pressure, exerted
on the mercury of bowl. High atmospheric pressure gives more force on the bowl,
forcing mercury higher in the column. Low pressure allows the mercury to drop to
a lower level. On this basis, we get the atmospheric pressure from the height of
mercury in the tube.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
1
Principle of mercury barometer
Description: When highest accuracy and reliability are required, then the
atmospheric pressure is measured with the Mercury Barometer. Here the glass
tube is filled with mercury under a vacuum and immersed in a mercury cistern. A
leather washer is fitted on top of cistern, so that mercury can not come out from
it. The length of the mercury column is measured against on an external scale.
This barometer is fixed-cistern type and must be mounted with a bracket. It is
normally supplied in wooden casing with all operating accessories.
Following errors to be considered, while using a mercury barometer
Altitude correction: All barometers are affected by altitude, because
atmospheric pressure reduces with vertical distance from the earth surface. If a
barometer is moved upwards from sea level, then its readings reduce roughly by
1 hPa (1 mbar) for every 10 meters increase in altitude.
Latitude correction: Earth’s polar radius is less than equatorial radius, so
gravitational force, as well as weight of mercury is grater at pole. Since
barometric reading’s uniform datum is for 45º latitude, so for more than 45º
latitude needs plus correction and less then 45º latitude needs minus correction,
which is about 1 millibar for every 12º of latitude.
Temperature correction: Barometer is build for 0º C (a standard temperature),
so if atmospheric temperature is not 0º C, then needs a correction, which is
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
2
approximate 1 milliibar for each 6º difference. The correction is additive if the
atmospheric temperature is less the 0º C and visa versa.
Index error: It is the deference between the corrected barometric pressure (after
correction for altitude, latitude and temperature) and the actual pressure. It can
be checked at any port, by representative from the local metrological authority
with the help of port agent or at sea, from some sources (weather bulletin,
metrological instruments etc), which are giving 100% correct atmospheric
pressure information. It should be corrected every three months.
Capillarity: Surface tension at tube causes the top surface of mercury, to
become upwards curvature shaped. So to avoid this error, readings should be
taken at the centre of curvature. Keeping a piece white paper behind barometer
scale, can help us to get easier readings.
Error of parallax: is caused due to location of observer’s eye, i.e. for higher
location, we get lower reading and vice versa. So to avoid this error, observer’s
eye should be such that front edge and rear edge of the bottom of curser must be
in line.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
3
Capacity: If the pressure rises, then the mercury column rises but the level in
the cistern falls and vice versa. So zero level changes with pressure change,
which causes the error of capacity. Since the quantity of mercury is fixed, so this
error can be eliminated by suitable adjustment in the distances between
graduations on the scale.
Pumping: It is the up and down oscillation of the top of mercury at tube, caused
by ship’s vertical movement due to rolling, pitching, heaving etc. To avoid this
error, many readings are taken at each time and then mean is calculated.
Kew pattern marine barometer is the modern format of a mercury barometer.
Here atmospheric pressure acts directly on the surface of the mercury in the
cistern, causing the mercury level in the column to rise or fall. Careful
construction of the tube and cistern ensures direct measurement from the scale,
with the help of a vernier.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
4
Operating Sequence of Kew pattern marine barometer:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Must be firmly supported at vertical condition.
Give time to adopt ambient temperature.
Tap the barometer.
Set the vernier
Measure the barometer's temperature and add calibration correction.
Read the vernier with caution for error of parallax.
Apply the altitude correction.
Apply the temperature and gravity corrections.
Finally get barometer reading.
Transportation of Kew pattern marine barometer: It must be transported with
extreme care, with the prime considerations, so that air is prevented to enter the
vacuum space. Also to be careful so that the tube is safe from being broken and
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
5
mercury is not leaking. It is a better practice to turn it slowly until horizontal level.
The instrument should then be softy placed in its box-casing. While carrying, it
must be carried in horizontal position, preferably keeping the cistern a little higher
than the tube. Similarly during refitting the barometer, it should be gently
removed from the box, then attach it with the bulkhead, with the help of gimbals
ring. During the whole transportation operation, extreme care should be
maintained, so that there is no jerk and bumping.
Maintenances of the barometer:
1.
2.
3.
4.
5.
6.
Place it away from heat and vibrations.
Place graduated scale at eye level, so that no error of parallax.
No fingerprints on glass tube and gold slide (barometer correction slide).
No wear and tear with gimbals ring.
Gently wipe the instrument regularly with soft cloths.
A drop of clock oil helps to keep free-moving all the moving parts, i.e.
gimbals screw, gold slide etc.
Reasons of mercury use at instruments:
1.
2.
3.
4.
5.
6.
7.
It has high relative density, so it makes the instrument smaller.
It is easily visible through glass tube.
Due uniform coefficient of expansion, temperature corrections can be
applied more accurately.
It has low freezing point (- 39ºC), so does not easily frozen.
It has low boiling point (- 39ºC), so does not easily evaporate.
It can not escape through leather washers, so not easily leaking.
It does not wet the glass surface of tube.
Aneroid barometer
Principle: Aneroid barometer is the most common pressure measuring
instrument, where “aneroid" means "without liquid." A typical aneroid barometer
utilizes a bellows, made of thin metal that expands or contracts as the air
pressure changes. The movement of the bellows is communicated to an indicator
needle, via lever and spring. Then the indicator needle rotates on a dial and
gives us the readings of atmospheric pressure.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
6
Aneroid Barometer
Description: An aneroid barometer uses a small, flexible metal box (bellow),
which is called aneroid capsule. This aneroid capsule (cell) is made from an alloy
of beryllium and copper. A strong spring helps the evacuated capsules from
collapsing. Small changes in external air pressure cause the capsules to expand
or contract. A mechanical lever forwards the signals of the tiny movements of the
capsules, which is displayed on the face of the aneroid barometer. A manually
set needle is provided on the glass face of the instrument, which is used to
understand the barometric tendency. The barometer can be located anywhere
onboard but it is recommended that it is not positioned in direct sunlight or near
sources of other heat and vibrations. A clockwise movement of needle indicates
rising pressure and improving weather conditions. An anti-clockwise movement
indicates falling pressure and deteriorating conditions.
Operating Sequence of Aneroid barometer :
1. Tap the glass face of instrument lightly to release any sticking of internal
machinery.
2. Carefully read dial reading, so that there is no error of parallax.
3. After consideration of altitude correction, latitude correction and index
error, find our barometric reading.
Procedures for a new aneroid barometer setting:
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
7
1. From local source (i.e. local weather reports, local meteorological office or
another corrected barometer) get accurate aneroid barometer’s reading for
the area.
2. Make allowances for altitude correction with above reading.
3. Set the pointer on that reading by a small screwdriver, through the hole at the
back of the aneroid barometer.
4. While above settings, always move the indicator pointer clockwise.
Precision aneroid barometer
The precision aneroid barometer is designed to accurately indicate atmospheric
pressure. Here vacuum capsule consists of self-stable set of three to five sealed,
bellows-shaped, corrosion proof copper-beryllium-alloy chambers that expand
and compress with changes in air pressure. There is a filter fitted small hole,
through which air is allowed to pass to give pressure on capsule. The expansion
and compression of capsule is sensed by an electrical contact, which is lined to a
neon light. This neon light illuminates, when with the help of a small wheel, a
lever is arranged to contact lightly on a pivoted bar. This bar is linked with the top
of the capsule. Again the light stops, when contact is broken. The wheel moving
signals is linked with digital display, from which we can measure the 0.1mb of
pressure. A battery is provided with neon light; to prolong the life of battery, a
spring loaded switch is provided. This way by measuring the expansion and
compression of capsule, can find out the atmospheric pressure.
The instrument compensates temperature error, by a bimetallic strip or by
enclosing a small pocket of air in the capsule. Precision aneroid barometers must
be calibrated twice a year, when used aboard ship and once a year, when used
at shore stations.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
8
Simplified sketch of Precision aneroid barometer
Operating Sequence of Precision aneroid barometer:
1.
2.
3.
4.
Depress spring loaded switch and hold it there.
Slowly rotate the wheel until the light illuminates.
Immediately stop rotation of wheel and release the spring loaded switch.
Read the pressure in millibars and decimal of a millibar.
Corrections of Precision aneroid barometer:
Normally needs only below two corrections,
1.
2.
Index error correction
Height correction.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
9
Barograph
A barograph is a pressure measurement instrument, where a paper or foil chart
(which called a barogram) is used to record the barometric pressure over time.
Barograph
Barograph has a revolving dram, where a pen is placed with the help of a pen
arm. This pen arm is oscillated up and down with vacuum chamber. The drum is
rotated slowly by clockwork. Commonly, the drum makes one revolution per day,
per week, or per month, as per the selection by the user.
The pen contains slow drying wink, which needs to replenished normally once in
a week. The vertical lines of barogram indicate UTC, normally with two hours
intervals. These vertical lines are curved, having the same radius of the curvature
of pen arm.
Thermometer
Thermometer is a device, which measures the temperature. Two types of
thermometer are widely available. The common one is the liquid-in-glass-type,
where expansion and compression of liquid (mercury or spirit) is used and the
second type uses a bimetallic strip, which winds and unwinds. Both expansions compression of liquid and winding-unwinding of bimetallic strip occur with
temperature change.
Onboard ship normally liquid-in-glass-type thermometer is commonly used. As
per items to be measured, it is again two types.
1.
2.
Air thermometer, to measure atmospheric temperature.
Sea thermometer, to measure temperature of sea water.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
10
Description of liquid-in-glass-type thermometer: A capillary tube (whose top
end is closed and bottom end is fitted with a bulb) is evacuated of air and partially
filled with mercury. When temperature increases, then the mercury in bulb
expands and the length of mercury column increases at capillary tube. Opposite
happens, with temperature decreases. So from the length of mercury, we can
understand the amount of temperature. This thermometers can be calibrated
either by comparing them with other certified thermometers or by checking them
against known fixed points. The best known fixed points are the melting and
boiling points of pure water, keeping in mind that the boiling point of water varies
with pressure.
The traditional method of calibrating thermometer scale as follows,
1. Immerse the sensing portion in a mixture of pure ice and water and mark
the mercury’s highest point on tube.
2. Immerse the sensing portion in a steam bath at 1 standard atmosphere
(101.3 kPa; 760.0 mmHg) and again mark the point indicated.
3. Divide the distance between these marks into equal portions according to
the temperature scale being used.
There are 3 types of temperature scale,
1. Celsius scale thermometer, where 0ºC is the freezing point and 100ºC is
the boiling point.
2. Kelvin scale thermometer, where 273ºK is the freezing point and 373ºK is
the boiling point.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
11
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
3.
Fahrenheit scale thermometer, where 32ºF is the freezing point and
212ºF is the boiling point.
Maximum Thermometer is a mercury-in-glass thermometer, having a special
constriction in the neck close to the bulb, where the glass tube is narrower. When
the temperature of air rises, the mercury in the thermometer expands and goes
up through the narrower way. But when the temperature cools, due to
construction of thermometer, mercury can not come down trough that narrow
way. So the maximum length of the mercury, which occurs due to maximum
temperature for a certain period, remains at that level. The observer can then
read the maximum temperature directly from the scale. To reset the thermometer
it must be swung sharply, which is similar to the medical thermometer.
Minimum thermometer measures the lowest temperature. It is also a mercuryin-glass thermometer, which contains a barbell-shaped marker within the
mercury. As the temperature decreases, the mercury and marker move down
through the thermometer tube. At the minimum temperature level, these
movements of mercury and marker are stopped. Again when the temperature
starts to rise, the liquid moves back trough the tube, but the marker remains
stationary. This level of the marker represents the minimum temperature. This
type of minimum thermometer must be held horizontally for proper operation. To
reset, the thermometer needs to simply turn upside down.
Six's thermometer is a thermometer, which can measure the maximum and
minimum temperature together, during any given time. It was invented by James
Six in 1782 and the instrument was named as per his name.
It consists of a U-shaped capillary tube with two separate temperature readings,
one for the maximum temperature and another for the minimum temperature.
There are two bulbs at the top of each arm. Here the minimum reading scale
tube contains alcohol, the other tube contains a vacuum or low pressure alcohol
vapour and the bend of U contains mercury. On each arm, there is a coloured
glass index containing a thin iron pin in the center.
During the temperature rise, the alcohol of minimum tube pushes the coloured
glass index of maximum scale tube, along with mercury, towards maximum
scale, until the achievement of maximum temperature. During decreasing of
temperature the mercury goes down, but the index remains stationary at that
highest point and gives the maximum temperature for that period.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
12
During temperature fall the alcohol of minimum tube compress, so the coloured
glass index of minimum temperature tube, along with mercury moves towards
minimum scale until the achievement of minimum temperature. During increasing
of temperature the mercury goes down but the index remains stationary at that
point and gives the minimum temperature for that period.
The coloured glass indexes are reset by using a small magnet, which can drag
these along the tube, so that they again rest on the surface of the mercury, to
start new measurement.
Hygrometer
Hygrometer is an instrument used for measuring humidity and dew point and
consists of two thermometers, one is dry-bulb and the other one is wet-bulb
thermometer, mounted vertically in a ventilated case or box. Dry bulb
thermometer is exposed directly to the air. The bulb of wet bulb thermometer is
covered with muslin (or starch free cotton) tightly. An extra length of wick of this
muslin is immersed in a small cup, filled with water. This way the muslin is kept
thoroughly moist at all times.
The dry-bulb thermometer records the temperature of the free air. The wet-bulb
thermometer records the temperature of evaporation, which is always less than
the temperature of free air. As the water evaporates and absorbs heat, causing
this wet -bulb thermometer reading to decrease.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
13
When the wet- and dry-bulb temperatures are known, then the relative humidity
and dew point of the atmosphere can be found by referring a table. Examples of
such tables are given on chapter 4.
Hygrometer
Stevenson screen is a screen to shield meteorological instruments
(thermometers, hygrometer, barometer, thermograph etc) against precipitation
and direct sunlight, but air is allowed come in contact with instruments freely. It is
normally a wooden box-shaped unit, which is supported by four metal or wooden
legs or a wooden post.
This screen is to be placed at least twice the distance of the height of any object
(e.g. 10 m from any tree, which is 5 m high). Door to be placed north, in the
northern hemisphere, to prevent direct sunlight on the thermometers.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
14
Stevenson screen
Whirling Psychrometer
Whirling Psychrometer is an instrument for the measurement of the temperature,
dew point and humidity of the air. It consists of two thermometers exposed side
by side. One is an ordinary dry bulb thermometer and the other wet bulb
thermometer, whose bulb is surrounded by a piece of thin muslin, which is kept
moist with distilled water. The instrument can be rotated with a handle, to ensure
passing of air across the thermometer bulbs. From the knowledge of dry bulb
and wet bulb readings, we can easily find out temperature, dew point and
humidity of the air.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
15
Whirling Psychrometer
Anemometer
An anemometer is a device, which is used for measuring wind speed and
direction. Some instrument can also measure the wind's pressure, as there is a
close connection between the pressure and the velocity. When a ship is moving,
anemometer shows apparent wind, which is a combination of true wind and
ship’s speed. So before using, apparent wind must be calculated to true wind.
Wind direction is measured in degrees from the bow of the ship and speed is
measured in knots.
Anemometer has to place onboard ship, at any place, where it remains out of any
obstructions, so that wind can come in contact with instrument directly, without
any collision with any ship’s structure. Best place is the top of X-mas tree, at
monkey Island.
Cup anemometers : It is the simplest form, consisted of three to four
hemispherical cups, which are mounted on a vertical shaft, with horizontal arms,
placed at equal angles to each other. Wind force turns the cups. Since the
rotations of caps are proportional to wind speed, so by counting the rotations can
find out wind speed.
Propeller anemometer : It measures the speed of a wind by powered turbine in
the form of a propeller or wind vane. Here the axis of propeller is parallel to the
direction of the wind. It combines a propeller and a tail on the same axis, to
obtain accurate and precise wind speed and as well as wind direction.
Hot wire anemometer : It uses a very fine wire of several micrometers, which is
heated up to some level. When air is flowing near the wire, then there will a
cooling effect on the wire. An electrical resistance is build up on the wire, which is
proportional to cooling effect. So from this resistance of the wire, wind velocity
can be measured.
Laser Doppler anemometer : It uses a beam of light from a laser, which is split
into two beams. One beam comes out from the anemometer and hits airborne
particles and then reflects or backscatters. When this reflected or backscattered
light comes in contact with detector, then it is compared with other laser beam.
Movements of air born particles due to wind speed, cause a Doppler shift effect
with the beams, from which we can calculate the speed of wind.
Sonic anemometer : It uses ultrasonic sound waves to measure wind speed and
direction. Its function is based on the time of flight of sonic pulses, between pairs
of transducers.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
16
Ping-pong ball anemometer: It uses a ping-pong ball attached to a string.
When the wind blows horizontally, then the ping-pong ball moves easily with
wind. From the angles created by the string-ball apparatus, we can get wind
speed.
Plate anemometers : It is an old fashioned instrument, where a flat plate is
suspended, so that the wind deflects the plate. Later versions of this instrument
consist of a flat plate, either square or circular in size. A spring is fitted to balance
the pressure of the wind on the plate. From the compression of this spring can
find out the wind speed.
Tube anemometers : It uses a glass J or U tube containing liquid, with one end
bent, in a horizontal direction to face the wind. If the wind blows into the mouth of
the tube, then pressure increases on the liquid, from which we can find out the
wind speed. Small departure of the instrument, from the direction of the wind
causes large variations in the result of wind speed.
Facsimile recorder
Facsimile recorder is a weather reception instrument, which reproduces an exact
copy of weather map, which is basically drawn at weather forecasting station. It is
capable of receiving black and white graphic weather maps.
Meteorologists use weather satellites, land stations, weather ships etc to
measure ozone layer, water vapor, pollution levels etc to plot storms, jet streams,
fronts, fog, snow, ice cover etc at weather map. Then transmit those weather
maps via various metrological stations. More than 1,000 stations are located in
80 countries throughout the world, for this purpose.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
17
Facsimile recorder produces this weather map by using white, moist electrosensitive paper, which is drawn on stainless steel writing surface. An electronic
stylus moves horizontally at a suitable rate. Then chemical reaction discolours
the paper as per the current’s signals, which then becomes black and white
weather map.
It is very simple to operation, only needs to be switched on or off. Only items,
which require special care that is periodic change of roll of paper, tip of stylus
and stainless steel writing surface.
Weather maps (in other term facsimile charts) are two types,
1. Analysis chats: It represents actual weather situations at a given time.
2. Prognosis charts: It represents predicted weather situation at specific
future time.
Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE.
For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations.
For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury
18