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Shipborne Meteorological 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: -
The surface tension of the mercury forms a MENISCUS and readings should always be taken at the top of this.
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.
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
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
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.
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
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.
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.
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.
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.
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.
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.