The layer of invisible gases that envelops the Earth and held by the gravitational pull is called atmosphere or the sphere of air. Apart from air, the atmosphere is composed of other particles like dust and water vapour. Like all other materials, it has weight and exerts pressure on the surface of the Earth. This pressure is called atmospheric pressure or air pressure. It is the force exerted on a surface by the air above it, as the gravity pulls it to the Earth. The atmosphere exerts a pressure of more than 2 kg per sq cm at sea level.
This would make the column of mercury in a barometer rise to 760 mm or 1013 millibars. Air pressure is measured with the help of a Barometer, which contains a glass tube with a column of mercury, which rises or falls as the weight of the atmosphere changes. The Meteorologists describe the atmospheric pressure by how high the mercury rises.
Air pressure is not uniform over the surface of the Earth and it varies from place to place, mainly due to temperature, altitude and rotation of the Earth.
Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind and precipitation. High-pressure systems usually lead to fair and calm weather. When the temperature rises, air expands, as it becomes less dense and exerts less pressure. On the other hand, when temperature falls, air becomes denser and its pressure increases.
Hot air expands and becomes lighter and less dense. Since it has nowhere to go, it goes up forming a low pressure area. It is called low pressure, since the air is less dense and exerts less pressure. The upward movement of the air causes condensation of water vapour, leading to the development of cloud and rain. Hence, a low pressure area is usually associated with bad weather.
On the other hand, cold air is heavier and denser. So, it exerts more pressure and settles down on the ground forming a high pressure area, which is favourable for evaporation. Hence, a high pressure area usually related with fine and dry weather.
As air is compressible, it is denser in the lower parts of the atmosphere and becomes lighter or rarefied with increase in altitude. Therefore, air pressure will be higher over a lowland area and lower over a mountainous region. In other words, atmospheric pressure drops as altitude increases and that is why the mountaineers need oxygen at a high altitude.
As we travel by an airplane or go uphill and downhill in a car, often our ears pop. The reason is, the atmospheric pressure becomes lower than the pressure of the air inside our ears. The ears pop because they are trying to equalize, or match the pressure. Those pops equalizes the pressure in our ears, so they will not burst when the pressure increases or decreases.
The influence of the Earth’s rotation on air pressure is apparent in Polar Regions where winds are greatly deflected.