The cold weather season begins with November in north India and by the beginning of December the whole country comes under its grip. In this season the southerly branch of the jet stream occupies position south of the Himalayas indicating that oil more the Northern Hemisphere polar dynamics in command of the situation.
The return of the westerly jet stream is accompanied with the restriction of light north-east trade winds (monsoon) toll surface, withdrawal of the ITC, formation of antic clinic cell over north-western India and dry wealth conditions prevailing over most of the part of the country.
Sources Wrested, F., World Climatic Data, Climate Data Press 1972, for Jaisalmer.
US Dept of Commerce, World Weather Records, 1951 -60, Vol. 4 Asia 1967 for Ludhiana and Kodaikanal.
GreatBritain, Meteorological Office, Tables of Temperature, Relative Humidity and Precipitation for the World, Part V: Asia, 1966, for the remainder.
Temperature-during winter season there is general increase of temperature from north to south and the isotherms run almost parallel to the latitudes. The 21° isotherm for the month of January runs east- west through the middle of the country connecting Tapi estuary in the west and the Mahanadi delta in the east. In January the north-west India- Punjab, Haryana, west Uttar Pradesh and north Rajasthan experience less than 15° C temperature while the average temperature is less than 10°C over Jammu and Kashmir and northern U.P. South India the isotherms tend to bend to the and run parallel to the coast.
The western cold warmer than the eastern coast by about 1.7° (J mean daily minimum temperature varies front in the north-western part of India to 24°C Peninsula. The night temperature in the plains Punjab and Haryana sometimes goes below freezing point producing ground frost conic such period of unusually cold weather is gem described as ‘cold wave’. The Peninsular region of the country, however, does not have well defined cold weather season.
The mean maximum temperature for the month of January at Thiru vananthapuram is as high as 31°C (cf. 29.5°C in June). The diurnal range of temperature is 14°C to 17°C in north-west India, with decreasing tendency towards east and the south. December and January are the coldest months of the winter season.
Pressure and Winds-the distribution of temperature has a direct bearing on atmospheric pressure which decreases from land to sea. The isobars of January month exhibit the general condition of pressure during this season. The isobar of 1013 my surrounding the southern tipoff the Peninsula (Kerala coast) depict the lowest pressure. It goes on increasing towards the north and the west.
The isobar of 1019 my, occupying the north-western part of the country, exhibits the high pressure cell. The average pressure of January month for Thiruvananthapuram was 1005.4 mb.Chennai 1013.6 my. Bangalore 913.1 my, Hyderabad 954.3 my.
Nagpur 981.1 my, Lucknow 1004. 5 my, Patna
1011.5 My and Jaipur 972.2 my.
During winter season anticyclone conditions Ii are found over north-western part of the country (from where winds move toward the oceanic low of the south. These winds blow from north-west in north-west India, from west to east in the middle Ganga plain, from north-east to south-west in the n Bay of Bengal, Arabian Sea and Peninsula. A charts act eristic feature of the cold weather season is the s inflow of depression from the west and the northwest. These low pressure systems, called the western disturbances, originate in West Asia and near” Mediterranean Sea. They travel eastward across I and Pakistan and reach India during the wanly season.
The westerly jet stream plays key role steering these disturbances into India. Their averred frequency is 4 to 5 depressions per month we greater intensity between December and February Rainfall-According to C.W.B. Normand if weather, clear skies, low humidity and, thereof® absence of rainfall, low temperature and a lair diurnal variation in it are the usual features of winey season in India. However, slight rainfall
(About 5 cm) is caused in the northern India (Punjab, Haryana, northern Rajasthan, Jammu and Kashmir and western Uttar Pradesh) by the western disturbances which are active over this area between December and February. This rainfall, although small, is very useful for rabi crops of wheat and gram. Also there is snow fall over the hills of Jammu and Kashmir, Himachal Pradesh and Uttaranchal.
The north-eastern part of India also gets some rainfall during winter season. Arunachal Pradesh and Assam may get as much as 50 mm of rainfall during these months.
During October a low pressure area occupies the northern parts of the Bay of Bengal which moves further southwards by the beginning of November. The land winds attracted by this low pressure cell are deflected towards the Coromandel Coast and produce copious rains during October and November. The easterly depression and the presence of Inter Tropical Convergence are responsible for this rain. Some cyclones which originate west of 90°E and travel towards the coast also bring rains to Tamil Nadu coastal plains.