Atmospheric Pressure
Air pressure: The weight of a column of air of a unit area from mean sea level to the final limit of the atmosphere is called atmospheric pressure.
Under normal conditions the air pressure at sea level is 76 cm of mercury. or 760 mm. There is pressure exerted by a tall pillar. Air pressure is measured with a barometer. Atmospheric pressure is considered an important indicator for weather forecasting. The unit of atmospheric pressure is bar (1 bar = 105N/m²). Air pressure is also measured in millibar and Pascal. The widely used unit of pressure is called kilo Pascal (KPa). The average atmospheric pressure at sea level is 1,013.2 millibars or 1013.2 kPa. Air pressure in the lower part of the atmosphere decreases rapidly with height. This loss rate is every 10 m. At a height of 1 millibar. 10 km. At an altitude of 265 millibars there is air pressure and standard temperature is – 49.7°C. Isobar: The imaginary line that joins places with equal air pressure at sea level is called isobar. Air pressure is shown on the map by isobar lines. Pressure gradient is the decrease in pressure per unit distance.
(Pressure gradient). When isobars are close to each other, the pressure gradient is greater. But when isobars are far away from each other, the pressure gradient is less.
Note: The rate of pressure change with respect to distance is the pressure gradient.
There are four air pressure zones on the earth’s surface-
1. Equatorial low pressure: This belt is located between 10° north and 10° south latitudes from the equator. Here the sun’s rays fall perpendicularly throughout the year, due to which the temperature always remains high. In this zone, horizontal surface winds do not blow, but due to high temperature, the air becomes lighter and rises upwards and convection currents are born. Therefore this belt is called the Pacific Belt or Doldrum.
Note: The speed of rotation of the Earth is highest at the equator, due to which the centrifugal force is highest here, which pushes the air beyond the surface of the Earth. Due to this also the air pressure is less here.
2. Subtropical high pressure: High pressure belts are found from the Tropics of Cancer and Capricorn up to 35° latitude in the Northern and Southern Hemispheres respectively. There are two reasons for high pressure here
(a) The air rising from the equatorial zone after getting heated, becomes cold and heavy and descends between 35° latitude from the Tropics of Cancer and Capricorn and produces high air pressure.
(b) Due to the daily movement of the Earth, large amounts of air from the subpolar regions gather between the Tropics of Cancer and Capricorn between 35° latitude, due to which high air pressure is generated there.
Note: High pressure belts are present in both the hemispheres between 30°-35° latitude from the equator. This high pressure belt is called horse latitude. The reason for this is that in the Middle Ages, horses were loaded in sailing ships and sent to the Western Islands for farming in Europe. Often these ships found it difficult to move forward due to calm air between these latitudes. Therefore, to reduce the weight of ships, some horses were thrown into the sea.
3. Subpolar low pressure belts: Low air pressure belts are found between 45° north and south latitudes and the Arctic and Antarctic circles respectively, which are called subpolar low pressure belts.
4. Polar high pressure: High pressure belts are found from 80° north and south latitude to the North and South Pole.
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