The pressure exerted by the atmosphere as a result of its weight, above a unit area of the earth’s surface is called atmospheric pressure. It is expressed in millibars ( mb ) and measured. The following are the major pressure belts operating in the earth’s atmosphere.
Atmospheric pressure is expressed in several different systems of units : millimetres ( or inches ) of mercury, pounds per square inch ( psi ), dynes per square centimetre, millibars ( mb ), standard atmospheres, or kilopascals. Standard sea – level pressure, by definition, equals 760 mm ( 29.92 inches ) of mercury, 14.70 pounds per square inch, 1,013.25 × 103 dynes per square centimetre, 1,013.25 millibars, one standard atmosphere, or 101.35 kilopascals. Variations about these values are quite small; for example, the highest and lowest sea – level pressures ever recorded are 32.01 inches ( in the middle of Siberia ) and 25.90 inches ( in a typhoon in the South Pacific ). The small variations in pressure that do exist largely determine the wind and storm patterns of the Earth.
Near the Earth’s surface the pressure decreases with height at a rate of about 3.5 millibars for every 30 metres ( 100 feet ). However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air. The pressure at 270,000 metres ( 10 − 6 mb ) is comparable to that in the best man – made vacuum ever attained. At heights above 1,500 to 3,000 metres ( 5,000 to 10,000 feet ), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken.
1. Equatorial Low Pressure Belt : It extends from the equator to about 10°N and S. It is characterised by extremely low pressure with calm conditions, and due to calm air movement it is also known as doldrums.
2. The Sub Tropical High Pressure Belt : It extends from near the tropics to about 35°N and S. A calm condition with variable and feeble winds is created in these high pressure belts, called Horse Latitudes.
3. Sub Polar Low Pressure Belt : This belt is located between 45°N and S to the Arctic and Antarctic circles. Cyclonic storms or ‘Lows’ are produced due to great contrast between the temperature of the winds from sub-tropical and polar source regions.
4. Polar High : These belts surround the poles. Due to high intensity of cold at the poles, the air becomes very cold and develops high pressure belts around the poles.
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