Mechanism of Weather in the Winter Season
During the winter season winds generally blow from north-east to south-west in the direction of trade winds.
These winds are dry, devoid of moisture and are characterized by lower temperature and higher pressure conditions over the country.
The term trade winds refers to the westward flowing currents of rising air that occur near tropical areas, such as over the Indian Ocean and Pacific Ocean.
Surface Pressure and Winds
During 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 with a ridge extending from it southeastwards and an adjacent trough extending north-northeastwards and northwards. Winds blow south-westerly to north-westerly and depressions generally move westwards.
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 north-western India.
Jet Stream and Upper Air Circulation
The next major type of upper air circulation phenomenons is the jet stream. As mentioned before, this large-scale surface feature is the dominate feature of the mid to upper troposphere.
It forms due to a vertical temperature gradient between warmer air to our north and cooler air at higher latitudes.
This vertical temperature gradient forces air molecules to move from a region of low pressure to a region of higher pressure to maintain balanced thermodynamic processes.
The variations in the atmospheric pressure closer to the surface of the earth have no role to play 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. These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan.