Indian Weather Mechanism in winter
Weather Mechanism in winter are as follows
Surface Pressure and winds
During winter, weather in India are generally influenced by the distribution of pressure in central and western Asia. A high pressure centre developed in the region North of the Himalayas which accounts for the movement of air from the North towards the South of the mountain range. These winds are responsible for making whole North-Western region dry during winter season.
Jet Stream and Upper Air Circulation
Higher up in the lower troposphere, about 3 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 in the making of upper air circulation.
All of Western and Central Asia remains under the influence of Westerly winds along the altitude of 9-13 km from West to east. Tibetan highlands act as barrier in the path of these jet streams. Jet streams get bifurcated into two branches, after striking with the Tibetan highlands.
The northern branch blows to the North of the Tibetan highlands while the Southern branch blows in an Eastward direction, south of the Himalayas. This Southern branch of the jet stream has great influence on the winter weather in India.
Western Cyclonic Disturbance and Tropical Cyclones
The Western cyclonic disturbances which enter the Indian sub-continent from the West and the North-West during the winter months brought by the Westerly jet stream in India. And the tropical cyclones originate over the Bay of Bengal and the India Ocean have very high wind velocity and heavy rainfall.
Weather Mechanism in summer season
Weather mechanisms in summer season are as follows
Surface Pressure and Winds
During the summer sets in and the sun shifts Northwards, the wind circulation over the sub-continent undergoes a complete reversal at both, the lower as well as the Upper levels. The Inter Tropical Convergence Zone (ITCZ) shifts Northward by the middle of July.
By this time, the Westerly jet stream withdraws from the Indian region. The ITCZ being a zone of low pressure, attracts inflow of winds from different directions. The Maritime Tropical airman (MT) from the Southern hemisphere, after crossing the equator, rushes to the low pressure area in the general South-Westerly direction and is popularly known as the South-West monsoon.
Jet streams and upper air circulation an Easterly jet stream flows over the Southern part of the peninsula in June. The Easterlies normally do not extend to the North of 30° N latitude in the upper atmosphere.
Easterly Jet Stream and Tropical Cyclones
The Easterly jet stream steer the tropical depressions into India. These depressions play an important role in the distribution of monsoon rainfall over the Indian sub-continent.
Inter Tropical Convergence Zone (ITCZ)
ITCZ is a low pressure zone located at the equator where trade winds coverage and so it is a zone where air trends to ascend. In July, ITCZ is located around 20° N-25° N latitudes (over the Gangatic plain), sometimes called the monsoon through encourages the development of thermal low over North and North-West India.
Due to the shift of ITCZ, the trade winds of the Southern hemisphere cross the equator between 40° E and 60° E longitude and start blowing from South-West to North-East due to the cariolis force and called as South-West monsoon. In winter, the ITCZ moves Southward and so the reversal of winds from North-East to South and South-West, takes place. They are called North-East Monsoons