Temperature and Air Pressure Measurement

Temperature is the measure of how hot or cold an object is, expressed against a chosen scale.

• High temperature causes air to rise.
• This lessens pressure acting on the surface. Hence;

a. High temperature areas are associated with low pressure (e.g. Equator belt). Regions of continuous low pressure belt are known as doldrums areas.

b. Low temperature areas are commonly associated with high pressure. This is because air is always descending.

Altitudes

• This is the height above sea level
• Air is compressible; Air which lies lowest is most greatly compressed, and is therefore densest.
• Upwards both density and pressure fall rapidly

Measurement of Air Pressure

Air pressure is measured in a unit known as millibar (mb) and instrument that measures atmospheric pressure is a barometer.

These are two types of barometers. These are:

1. The Mercury barometer

This is a very accurate although cumbersome instrument. In this instrument pressure is read in inches or in height of mercury in the glass tube balanced by the atmospheric pressure. At sea level this is 29.92 inches or 76 cm (760 mm) in metric units 

A Mercury Barometer

• A mercury barometer is an inverted tube which has a vacuum at the top, the open end is placed in mercury bath.
• The pressure of the air forces the mercury up the tube and the height of the mercury gives the air pressure, this is often quoted as ‘inches of mercury’.
• This varies with the weather. Low air pressure usually suggests changeable weather, whereas high pressure usually means stable weather.

2. The Aneroid Barometer

• This instrument consists of a hollow metal box which contains very little air.
• The top of the box is flexible so that it expands and contracts according to changes in atmosphere pressure outside the box.
• This movement operates a hand which is read against the graduated circular dial. 

• Wind is air in motion and has both direction and speed.
• The instrument widely used for measuring wind direction is a wind vane or weather cock.
• As wind direction is always deflected by trees and tall buildings weather cocks and wind vanes need to be created in an exposed position to get a true direction.

It is made up of top two parts. These are:

• One part is an arrow or vane on the top, which is free to move with the prevailing wind.
• The other part with the four compass points is stationery and shows in which direction the wind is moving
• Winds are always named from the direction they blow; an east wind is one that blows from east to west and a south -west wind is one that blows from the south to west.

Wind rose

• It is used for recording the direction of prevailing winds of a place over a period of a month.
• It consists of an octagon with the eight compass points.
• Each of the rectangles represents the date in which the wind comes from that direction (e.g.) on the fourth of the month, the wind is north east.
• These days which are without any wind is recorded in the box of the calms, and the number of calm days are indicated in the centre of the octagon e.g. 3 days in that month.

Anemometer

• It is used to measure the speed of wind.
• It consists of three or four semi – circular cups attached to the ends of horizontal spokes mounted on a high vertical shaft.
• Metal cups are fixed to the ends of the arms, and they do rotate when there is wind.
• The movement operates a meter which records the speed of the wind in km per hour.

• Since an anemometer is not easily available, a little practice of local wind observations will help us to assess the speed can be said about the strength of winds.
• The best guide is obtainable from the Beaufort wind scale

Sunshine

• The amount of sunshine a place receives depends on the seasons a factor determined by latitude and by the position of the earth in its revolution around the sun.
• In the tropics sunshine is abundant and at the poles there is less or no sunshine in the metrological station, sunshine duration is recorded by a sun dial, 102 mm (4inches in diameter, through which the sun’s rays are focused upon a sensitized card graduated in hours.
• An instrument which is used to measure sunshine is called sunshine recorder.
• On the maps places with equal sunshine duration are joined by isohels.

Clouds Cover

• When air rises, it is cooled by expansion.
• After dew point has been reached cooling leads to condensation of water vapourin the atmosphere.
• Tiny droplets of water vapour which are too small to fall as rain or snow will be suspended in the air and float as clouds.
• Their form shape, height and movements tell us a great deal about the sky conditions and the weather we are likely to experience.
• The amount of cloud cover in the sky is expressed in eights or oktas (e.g. 2k is quarter covered 4/g is half covered; 6k is three quarters obscured and x/g is completely overcast).
• On the maps places with an equal degree of cloudiness are joined by lines known as isonephs