Seasonal Shifting of Wind Belts | Climatology | Geography

In this article we will discuss about the seasonal changes in the relative positions of the pressure and wind belts.

In the absence of the revolution of the earth around the sun in about 365 days the global pressure belts would have been permanent and stationary at their places but the relative position of the earth with the sun changes within a year due to earth’s revolution and thus the position of all the pressure belts except the polar high pressure belts changes with the northward and southward migration of the sun.

At the time of summer solstice the sun is vertical over the tropic of Cancer (June 21) and therefore all the pressure belts except the northern polar high pressure belt shift north­ward (fig. 35.11). The equatorial low pressure belt prevails between 0° latitude (equator) and 10° N latitude, subtropical high pressure belt extends be­tween 30°- 40°N latitudes. Thus, all the wind belts associated with the said pressure belts also shift north­ward.

The sun becomes vertical over the equator at the time of autumnal equinox (23 September) and hence all the pressure belts which shifted to the north occupy their normal positions (fig. 35.11). After this there is southward migration of the sun which becomes verti­cal over the tropic of Capricorn at the time of winter solstice (23 December) and hence the pressure and wind belts shift southward except the southern polar high pressure belt.

Thereafter the sun again becomes vertical over the equator at the time of vernal equinox (21 March) and hence all the pressure and wind belts occupy their normal positions (fig. 35.11), thus, there is shifting in the positions of the pressure and wind belts due to seasonal changes of position of the earth in relation to the sun.

These seasonal changes in the relative positions of the pressure and wind belts intro­duce the following typical climatic conditions:

(i) The Mediterranean climatic regions are found in the western parts of the continents within the latitu­dinal zone of 30°-45° in both the hemispheres. The subtropical high pressure belts extending between 30°- 35° latitudes are characterized by dry trade winds during summer season because mostly they come from over the land. Moreover, this zone is characterized by anticyclonic conditions due to descent of air currents.

This belt extends upto 40° latitudes in the northern hemisphere at the time of summer solstice and in the southern hemisphere at the time of winter solstice. Thus, the western parts of the continents within the zone of 30°-40° latitudes do not receive rainfall during summer season.

On the other hand, the subtropical belt shifts towards the equator at the time of winter solstice in the northern hemisphere and at the time of summer solstice in the southern hemisphere, consequently, the zone between 30°-40° latitude is characterized by westerlies which give much precipitation during winter season because they come from over the oceans. This is why the Mediterranean regions are characterized by dry summers and wet winters and a typical Mediterranean type of climate is produced.

(ii) The regions lying between 60°-70° latitudes are characterized by two types of winds in a year because of shifting of pressure and wind belts. With the northward migration of the sun at the time of summer solstice the polar easterlies are weakened during north­ern summer because the westerlies extend over these areas due to northward (poleward) shifting of sub-polar low pressure belt while the situation is quite opposite in the southern hemisphere because the polar easterlies extend over much of the areas of the westerlies due to equator-ward shifting of sub-polar low pressure belt.

The situation is reversed at the time of winter solstice when there is southward migration of the sun. Thus, the polar easterlies are re-established over the areas lying between 60°-70° latitudes in the northern hemisphere because of the shifting of the belt of westerlies south­ward. Consequently, a typical climate characterized by wet summers through westerlies and associated cy­clones and dry winters due to polar eastelies is pro­duced.

(iii) Monsoon climate is the result of the shifting of pressure and wind belts. Due to northward migration of the sun in the northern hemisphere at the time of summer solstice the north intertropical convergence (NITC) is extended upto 30°N latitude over Indian subcontinent, south-east Asia and parts of Africa. Thus, the equatorial westerlies are also extended over the aforesaid regions.

These equatorial westerlies, in fact, become the south-west or summer monsoons. These south-west monsoon winds bring much rains because they come from over the ocean and are asso­ciated with tropical atmospheric storms (cyclones). The NITC is withdrawn from over the Indian subcon­tinent and south-east Asia because of southward shift­ing of pressure and wind belts due to southward migra­tion of the sun at the time of winter solstice.

Thus, north-east trades are re-established over the aforesaid areas. These north-east trades, in fact, are north-east or winter monsoons. Since they come from over the lands, and hence they are dry.