The invisible Atmosphere, which extends upto 1600 km from the earth’s surface, has enormous mss and substance with a total weight of at least 100,000 newtons per square meter. This weight of air exerts a pressure or atmospheric pressure. The air pressure at any place is exactly the weight of air above that place. It is measured with an instrument called a barometer. Air pressure is determined by measuring the force atmosphere exerts on a column of mercury. A more convenience instrument sued popularly is an aneroid barometer (it works without a mercury_. Air press is measured in a unit called millibars (mb). Normal air pressure at sea level is 1013 mb.
Changing Atmospheric Pressure
Atmospheric pressure is not uniform over the earth’s surface. Inequalities between pressures in different locations help to drive the weather machine.
Air pressure varies due to:
- Changing temperature
- Change in altitude
World Pressure Belts
Air pressure varies from place to place and between one season and another, even at the same altitude or latitude. This is greatly due to the unequal heating of the air around.
- Near the equator high temperatures throughout the year cause the warm air to expand and rise thus producing a low pressure belt on the ground. This area is called the Equatorial Low Pressure Belt or Doldrums.
- Around the two poles, very cold temperatures result in cold dense air setting no the earth’s surface and creating high pressure areas called Polar Highs. The subsiding air reaches the earth’s surface and starts flowing towards the lower latitudes.
- Air rising at the equator cools on reaching the upper layers and eventually sinks at 30˚ N and 30˚ S latitudes, to pile up there. In this manner the Subtropical High Pressure Belts, called ‘Horse-latitudes are produced. (This air descending at 30˚ N and S is relatively dry because it has shed most of its moisture at the equator and s the world’s tropical deserts lie in the this belt.
- At 60˚ N and 60˚ S is areas of comparatively lower pressure called the sub polar Lower Pressure Belts. They are formed due to the convergence of cold polar air with warm air moving out from the subtropics which causes the warm air to rise and hence creates this low pressure belt.
Thus we have alternate bands of high pressure and low pressure on either side of the equator.
Differences in pressure set up circulation in air as convectional currents from the equator to the poles and vice versa. On the earth’s surface air moves horizontally it is called Wind. When air descends or ascends vertically it is called an air current.