The important factors in winter season are:
(i) The Dry Continental Winds from Central and Western Asia.
A high pressure area develops in the north of the Himalayas in Central and West Asia. From this area wind blows towards north-western plains.
This is a dry continental airmass. Due to the influence of this airmass the north-western Indian Sub-continent becomes dry and cold.
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(ii) Zone of Contact.
The Indian Trade Winds come in contact with the northwest continental airmass. This creates a Zone of Contact between the Indian Trade Winds and the north-west Continental Winds.
The situation of this zone of contact is not stationary and often reaches the middle Ganga Valley. This results in the extension of cold and dry winds in the Ganga Valley. This makes the season cold and dry.
When this zone of contact shifts to north, the cold and dry wind situation is confined to the north only.
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(iii) Jet Streams.
Around 3 km above the earth’s surface, the pattern of winds rotation is different. The main characteristics are:
(a) The pressure changes in the lower air have virtually no effect on the upper air.
(b) The upper air of western and Central Asia is under the control of the westerlies. These winds blow parallel to the highland area of Tibet in the north of the Himalayas.
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The speed of these winds is almost constant. This wind current is called Jet Stream.
Due to the obstacle of Tibet, the jet stream is divided into two branches:
(a) The Northern Jet Stream.
This blows to the north of Tibet from the west to the east.
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(b) The Southern Jet Stream.
This blows to the south of Tibet from the cast to the west.
The Southern Jet Stream plays an important role in winter season. This attracts the low pressure areas from the Mediterranean Sea and the West Asia (mainly from the Oman Gulf).
These low pressure areas arc called Western Depressions and are responsible for the rainfall in the winter season in northern India.
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(c) The Shift of the Jet Stream towards South.
A Chinese scholar T.C. Yeh is of the opinion that the southward shift of the jet stream brings about incursion of cold and dry winds in north India. This makes this part of India very cold especially in the Ganga Valley.