due to its leeward situation along the Western Ghats. Factors Related to Air Pressure and Wind To understand the differences in local climates of India, we need to understand the mechanism of the following three factors: Distribution of air pressure and winds on the surface of the earth. Upper air circulation caused by factors controlling global weather and the inflow of different air masses and jet streams. (iii) Inflow of western cyclones generally known as disturbances during the winter season and tropical depressions during the south-west monsoon period into India, creating weather conditions favourable to rainfall.
The mechanism of these three factors can be understood with reference to winter and summer seasons of the year separately. Mechanism of Weather in the Winter Season Surface Pressure and Winds : In winter months, the weather conditions over India are generally influenced by the distribution of pressure in Central and Western Asia. A high pressure centre in the region lying to the north of the Himalayas develops during winter. This centre of high pressure gives rise to the flow of air at the low level from the north towards the Indian subcontinent, south of the mountain range.
The surface winds blowing out of the high pressure centre over Central Asia reach India in the form of a dry continental air mass. These continental winds come in contact with trade winds over northwestern India. The position of this contact zone is not, however, stable. Occasionally, it may shift its position as far east as the middle Ganga valley with the result that the whole of the northwestern and northern India up to the middle Ganga valley comes under the influence of dry northwestern winds.
Jet Stream and Upper Air Circulation : The pattern of air circulation discussed above is witnessed only at the lower level of the atmosphere near the surface of the earth. Higher up in the lower troposphere, about three km above the surface of the earth, a different pattern of air circulation is observed. The variations in the atmospheric pressure closer to the surface of the earth have no role to