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Meteorology
Shipborne Meteorological Instruments
Shipborne Instruments
In order to get as complete a picture as possible of
the weather, careful observations should be made.
Many of the
observations are made visually; for example, the form of clouds, and direction
of the wind. Instruments make other
observations; for instance one cannot find the pressure or the relative
humidity although one may guess the air temperature.
Various instruments have been designed to observe the
different phenomena.
The principal
ones are pressure, temperature and wind velocity, whilst others have been
designed to measure sunshine hours, density of
the water and rainfall.
This is constructed by filling a tube, about 1 metre
long with mercury.
The end of the tube is temporarily closed and is
inverted and placed into a reservoir of mercury. When the closure is removed it will be seen
that the level of mercury fills in the tube.
The space above the mercury at the top of the tube is
known as a Torricellian vacuum. If in air bubble were to get into this space
it would depress the mercury (as the vacuum would no longer be complete) and an
incorrect reading would result.
To prevent this, an air trap is incorporated in the
tube.
A capillary tube is connects the reservoir and the
tube.
The mercurial barometer is liable to error on account
of the following: -
CAPILLARITY
The surface tension of the mercury forms a MENISCUS
and readings should always be taken at the top of this.
CAPACITY
The height of the barometer should be taken from the
top of the mercury in the cistern to the top of the mercury in the marine
tube. If the pressure increases, the
level of the mercury in the cistern falls, so that the new measurements cannot
be taken from a fixed point. Adjusting
the distance between the graduations compensates this error.
PUMPING
Due to the constant change in height above mean sea
level of a barometer on a vessel in a seaway, the there will tend to be a
continual change of reading. This
movement of the mercury will make it difficult to get in accurate
HEIGHT
All readings should be corrected to sea level. Increase of height means a decrease of
pressure by approximately 1 millibar for every 10 m.
This correction can be made by tables or by the Gold
Slide
LATITUDE
Due to the earth being somewhat ‘flattened’ at the
poles, mercury weighs more at the poles than at the equator. For equal atmospheric pressures the barometer
would appear to read less at the poles and more at the equator. Readings should all be corrected for mean
latitude of 45°. The correction can be made by tables or by
the gold slide.
TEMPERATURE
The column of
mercury will expand, with an increase of temperature and contract with a
decrease of temperature in exactly the same way as does a thermometer.
All readings must be reduced to a standard
temperature, which is 285°K in the
case of most millibar barometers. The
correction can be made by tables or by the Gold Slide
N.B. The
attached thermometer should always be read before the barometer as otherwise
heat from the observers body may’ give a false reading.
The temperature at which a barometer reads correctly
is known as the FIDUCIAL TEMPERATURE.
In lat. 45° at sea level this is the same as the standard temperature
but at sea level in lat. 57° the
FIDUCIAL temperature would be 291°K, and in latitude 21° at sea level it, would be 273°K. Whereas at 20m above sea level in 45°it would be 297°K
OBSERVATIONAL
ERRORS
The barometer should always be upright; it is the
vertical height of the column of mercury that balances the column of air. If
the barometer is not upright, too high reading is obtained.
The back and front of the vernier must be on the same
level as the observer’s eye, otherwise the reading will be too high.
GOLD SLIDE
Is not made of gold but since its inventor was Lt.
Col. Gold, so it is called such. This gives a rapid means of getting the
latitude, height and temperature correction.
To use the slide set the height of the barometer above sea level, against the latitude and read off the correction opposite the top of the mercury in the thermometer.
ANEROID
BAROMETER
This is a very much more robust and compact instrument
than the mercurial barometer.
The main component is a vacuum box, which is partially
exhausted of air. An increase of
atmospheric pressure compresses this box causing the pointer on the dial (via
the lever system) to register a higher pressure. Converse occurs with a
decrease of pressure.
As there is no mercury in this barometer there are no
corrections for latitude or temperature, but a height correction must be
applied.
There is an adjustment screw on the back of the
instrument to take out any index error.
The greater the area of the vacuum box the greater the
accuracy of the instrument. It is
usual to give the barometer a light tap before reading this helps to free the
fine chain, which may stick if pressure changes are only small.
It is simpler to transport and to read while
temperature correction is unnecessary.
Height corrections can be ‘built in’ by resetting the datum on the
instrument. A pressure choke can be
attached if rapid height variations, leading to rapid pressure variations, are
expected; this smoothes the variations to negligible amounts.
BAROGRAPH
This is a recording aneroid barometer. A lever system
connects the vacuum pile to a pen arm which makes a mark on the chart on the
drum which is driven by clockwork.
The drum is wound and the chart changed weekly.
The prime purpose of the barograph is to record the
pressure tendency.
TEMPERATURE AND ITS MEASUREMENT
The forecasting of weather depends as much on
knowledge of temperature as of pressure.
The instruments for measuring temperature all depend
on the expansion and contraction of liquids or metals when heated and cooled.
The thermometer in its simplest form consists of a
capillary tube on the end of which is a bulb filled with mercury.
This thermometer is graduated by placing it in pure
melting ice and marking the position of the mercury, then placing it in boiling
distilled water and again marking the position of the mercury.
The barometric pressure in each case should be 760mm
of mercury.
These two points are known as the fixed points of the
thermometer.
The part of the tube between these points is then
divided into a number of equal divisions.
The number of divisions depends on the scale to be
used. The various scales are:
Conversions from one scale to another can be made by:
Celsius: -
(Fahrenheit ° - 32°x 5/9
Fahrenheit: -
(Celsius ° x 9/5) +
32°
Absolute: -
Celsius ° + 273°
Reamur: - Celsius ° x 4/5
At (-) 40° both the Celsius and the Fahrenheit scale readings are the
same.
PSYCHROMETER OR HYGROMETER
Mason’s Hygrometer consists of two thermometers
mounted side by side in a Stevenson’s screen.
One is a dry bulb thermometer, the other a wet bulb
thermometer.
Cambric or muslin is wrapped round the bulb of the wet
bulb and it is kept moist by means of a piece of cotton wick leading to a
container of distilled water.
The evaporation of water requires heat and this is
taken from round the wet bulb, which, unless the air is saturated, shows a
lower reading than the dry bulb.
Ensure that no finger touches the muslin; the oil from
the finger would prevent the muslin from acting like a capillary tube and
drawing the water up.
The screen and thermometers should be hung up to
windward away from local draughts or warm air currents and away from direct
sunlight. It definitely should not be placed within an enclosed room.
WHIRLING PSYCHROMETER.
More accurate readings can be obtained by using
a whirling psychrometer. This looks
rather like a rattle. The whirling ensures
a steady flow of air over the two bulbs
By entering tables with the dry bulb temperature and
the difference between the wet and dry bulbs as, arguments the dew point and
relative humidity can be found.
The number of whirls to be given should be adequate to
cool down the wet bulb thermometer. Ensure that no finger touches the muslin;
the oil from the finger would prevent the muslin from acting like a capillary
tube and drawing the water up.
WIND MEASURING INSTRUMENTS
The
Robinson Cup Anemometer consists of four hemispherical cups fixed to the
ends of rod set 90° from each other in a horizontal plane.
The spindle, to which the rods are attached, is connected to a
tachometer and from the number of revolutions made in a given time the ‘run’ of
the wind can be calculated.
The
anemometer assembly is generally fitted on top of a mast or a place without any
obstruction to the wind, the signals from this is led to the wheelhouse
repeaters by cables.
Different
calibration may be used to read the wind speed.
Note that
the wind as observed is the relative wind and not the true wind, which has to
be calculated.
Equipment
fitted with a gyro feed can however give true wind also, the calculations being
done by microprocessors within the equipment.
HYDROMETER
The hydrometer works on Archimedes’ principle that a
floating body displaces its own weight of the liquid in which it floats. It
consists of a float chamber through which passes a stem, the lower end of
Which is weighted so that it floats upright. The upper end is graduated to read the density/
specific gravity of water. These are generally made of nickel plated brass and
should be kept clean.
The
hydrometer is used to measure the density of the water and is one of the most
important measuring instruments on board a vessel. As soon as the vessel
arrives in port the density of the water is noted.
If the dock
water is connected to a river or is a river port then the water density should
be taken at two levels surface and half the draft, and the full draft an
average of the densities will give the correct draft.
All bulk
loaders should be aware of the necessity of taking the density especially at
the 3 levels if the river is experiencing any heavy rainfall.