Atmospheric pressure varies from place to place and from time to time. This variation depends on the following factors:
i. Altitude: At sea level, the air layers are dense, so it exerts greater pressure. Hence, the pressure is higher at sea level and it decreases with the altitude because the air becomes thinner and exerts lesser pressure. The loss of pressure is one centimetre for every ascent of 110 metres.
ii. Temperature: The temperature also affects the pressure. The hot air expands and is lighter. Hence, it exerts less pressure. On the other hand, cold air contracts and is heavier so exerts high pressure.
Thus, a low-pressure belt lies near the equator because of high temperature and a high-pressure belt lies on the poles because of low temperature. The following relation holds between temperature and pressure:
Formula: High-Temperature Low Pressure
Low Temperature ↔ High Pressure.
iii. Water vapour: The moist air is lighter and exerts lesser pressure than that of dry air because the water vapour is lighter than air.
iv. Rotation of the earth: The rotation of the earth also affects the pressure distribution on the earth’s surface. A low-pressure belt near the 60°N and 60°S is developed by the rotation of the earth. Due to the rotation of the earth, the air near 60°N and 60°S moves towards the sub-tropical regions. Thus, the low-pressure belt develops near 60°N and 60°S and high-pressure belt near 30° and 30°S.
