WBBSE Class 7 Geography Chapter 3 Air Pressure Introduction
Air is a mixed gaseous substance. Like other substances, the air has its own mass. The weight of air is felt on the surface of the Earth due to gravity. For this weight, the value of the force exerted by the air per unit area on the surface is called the air pressure. There are mainly two types of air pressure high pressure and 2 low pressure. The pressure exerted at sea level by 76 cm of mercury column at 45° north latitude, is called the standard or average atmospheric pressure. It is equal to 1 kg per sq cm or 1013.25 Mb.
The unit that is used to measure air pressure is a millibar. 1 millibar is the pressure exerted by 0.02953 inches. column of mercury. In some definite regions on the Earth’s surface, high pressure or low pressure is observed throughout the year. These definite regions of atmospheric pressure surround the Earth in the form of belts. These are called atmospheric pressure belts.
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An imaginary line joining all the places on the Earth’s surface having equal atmospheric pressure is known as an isobar. It is shown in the weather map expressing its value in millibars. If the average air pressure is more than 1013.25 MB, it is known as high pressure.
The region where the atmospheric pressure is higher than the surrounding areas is known as the high-pressure area. It is denoted on the map as ‘H’. If the average air pressure is less than 986 MB, it is known as low pressure.
The region where the atmospheric pressure is lower than the surrounding areas is known as the low-pressure area. It is denoted on the map as ‘L’ The horizontal movement of air along the surface of the Earth from a high-pressure area to a low-pressure area is known as the wind.