PRESSURE BELTS Two factors leading to the formation of high and low pressure are thermal and dynamic.
(i) Thennal Factor Low pressure is caused by heating. Heating results in expansion of air, resulting in low density and-I thus 'leading to low pressure, e.g., equatorial lows in North America and North India during summer. High pressure is the result of contraction of cooled air and increase in density, e.g., polar highs which occur in North Asia and North America during w~nters. The main processes in the formation of thermal highs and lows are (a) latent heat of condensation, (b) conduction and radiation, and (c) advection of air masses (advection: horizontal movement of air or liquid transferring heat).
(ii) Dynamic Factor It operates through a fractional drag and centrifugal force. The centrifugal force is very high
along the equator, where the velocity of rotation is high. Hence, the air masses tend to be thrown out, resulting in low pressure. Examples of dynamically produced pressure systems are sub-tropical highs and sub-polar lows.
Atmospheric pressure decreases with height. The distribution of pressure is characterised by its zonal or belted nature. Each zone or belt constitutes elongated or circular cells of high or low pressure. There are seven pressure belts on the earth's surface.
The equatorial belt separates the three pairs of belts in northern and southern hemisphere, namely, the polar high, the sub-polar low and the subtropical high. Location of belts is based on the annual average. Pressure belts shift seasonally as the sun moves apparently from one hemisphere to the other, due to the relationship between insolation, heating, expansion, density and air pressure.
The pressure belts comprise cells in the northern hemisphere, where all belts shift a little north of their annual average locatio!) during summer and a little south of the annual average location in winters. In the southern hemisphere, the belts comprise isobaric bonds and opposite conditions prevail seasonally.
In both high and low pressure belts, the winds are light, blowing in all directions. As the air bodies of different properties meet, they rise up and become unstable, resulting in rainfall. The air in high pressure belts sinks and spreads and is therefore stable and dry while in the low pressure belts, the air converges and is humid. The diagram: gives a representation of the planetary winds and pressurE:! belts.