On the basis of mode of genesis pressure belts are divided into four main categories: 1. Equatorial Low Pressure Belt 2. Sub-Tropical High Pressure Belt 3. Sub-Polar Low Pressure Belt 4. Polar High Pressure Belt.
Category # 1. Equatorial Low Pressure Belt:
The equatorial low pressure belt is located on either side of the geographical equator in a zone extending between 5°N and 5°S latitudes but this zone is not stationary because there is seasonal shift of this belt with the northward (summer solstice) and southward (winter solstice) migration of the sun.
During northern summer this belt extends up to 20°N in Africa and to the north of tropic of Cancer in Asia while during southern summer this low pressure belt shifts to 10° to 20°S latitude. The equatorial low pressure belt is thermally induced because the ground surface is intensely heated during the day due to almost vertical sun’s rays and thus the lowermost layers of air coming in contact with the heated ground surface also gets warmed.
Thus, warmed air expands, becomes light, and consequently rises upward causing low pressure. The equatorial low pressure belt represents the zone of convergence of north-east and south-east trade winds. There are light, feeble and variable winds with in this convergence belt. Because of frequent calm conditions this belt is called a belt of calm or doldrum.
Category # 2. Sub-Tropical High Pressure Belt:
Sub-tropical high pressure belt extends between the latitudes of 25°-35° in both the hemispheres. It is important to note that this high pressure belt is not thermally induced because this zone, besides two to three winter months, receives fairly high temperature throughout the year.
Thus, this belt owes its origin to the rotation of the earth and sinking and settling down of winds. It is, thus, apparent that the sub-tropical high pressure belt is dynamically induced. The convergence of winds at higher altitude above this zone results in the subsidence of air from higher altitudes. Thus, descent of winds results in the contraction of their volume and ultimately causes high pressure.
This is why this zone is characterized by anticyclonic conditions which cause atmospheric stability and aridity. This is one of the reasons for the presence of hot deserts of the world in the western parts of the continents in a zone extending between 25°-35° in both the hemispheres. This zone of high pressure is called ‘horse latitude’ because of prevalence of frequent calms.
In ancient times, the merchants carrying horses in their ships, had to throw out some of the horses while passing through this zone of calm in order to lighten their ships. This is why this zone is called horse latitude. It is interesting to note that this zone of high pressure is not a continuous belt but is broken into a number of high pressure centres or cells (fig. 35.1).
Category # 3. Sub-Polar Low Pressure Belt:
This belt of sub-polar low pressure is located between 60°-65° latitudes in both the hemispheres. The low pressure belt does not appear to be thermally induced because there is low temperature throughout the year and as such there should have been high pressure belt instead of low pressure belt. It is, thus, obvious that this low pressure belt is dynamically produced.
In fact, the surface air spreads outward from this zone due to rotation of the earth and low pressure is caused. It may be pointed out that this factor should be more effective at the poles but the effects of the rotation is negated or say overshadowed due to exceptionally low temperature prevailing throughout the year at the poles.
The sub-polar low pressure belt is more developed and regular in the southern hemisphere while it is broken in the northern hemisphere (fig. 35.1) because of over-dominance of water (oceans) in the former. Instead of regular and continuous belt there are well defined low pressure centres or cells over the oceans in the northern hemisphere e.g., in the neighbourhood of Aleutian Islands in the Pacific Ocean and between Greenland and Iceland in the Atlantic Ocean.
It may be noted that due to great contrasts of temperatures of the continents and oceans during northern summer the low pressure belt becomes discontinuous and is found in a few low pressure cells while the temperature contrast between the continents and oceans is much reduced during winter and hence low pressure belt becomes more or less regular and continuous in the northern hemisphere.
Category # 4. Polar High Pressure Belt:
High pressure persists at the poles throughout the year because of prevalence of very low temperature (below freezing point) all the year round. In fact, both the factors, thermal and dynamic, operate at the poles. There is thinning out of layers of air due to diurnal rotation of the earth as the air spreads outward due to this factor but this effect is overshadowed by thermal factor and hence high pressure is produced due to very low temperature.
The horizontal distribution of air pressure is represented and studied through isobars for the months of July (to represent pressure conditions during summer season) and January (to represent air pressure during winter season in the northern hemisphere). Figs, 35.3 and 35.4 portray the world distribution of air pressure through isobars in July and January respectively. The class interval of isobars is 3 mb.