December is the month in which winters really kick off in northern India.Meanwhile, Northeast Monsoon retreats from southern portions of the country by second week. The major weather producing systemsareWestern Disturbances (WD) which move across the northern latitudes of the country.Rarely a tropical storm may form in the Bay of Bengal, but then it has to bein theearly part of December.This is becausesea surface temperatures are not favorable for cyclone formation after second week. The other prominent feature of this month isthewesterly jet stream which starts making its presence felt in North India.The jet stream flowswith mean speedsof 100-120 Kts ( 200- 240 Kmph) but at times it caneventouch 150 Kts (300 Kmph).
On an average 2-4 WDs move across India in this month, these systems cause fairly widespread (FWS)rainsin J&K, Himachal Pradesh,andUttarakhand. It also causes rainsin plains of North India. AsWDmoves from northwest India to northeast India, the rain belt shifts along with it.
When an intense WD moves across, it sometimes induces a low pressure area over Rajasthan whichthenmoves across Gangetic plains causing winter rains. WDs along with induced lows are the main rain producing systems of the countryduringthis season.
Besides causing rains, WDs arealsoresponsible for occurrence of fog in North India.Humidity goes up after rains and combineswiththecold&dry air which penetrates northern India & lowers night temperatures.Thusin wake of WD, conditions become favourable for radiation fog. If nights remain clear & temperatures continue to fall, the fog persists & becomes denser.
Persistence of cold dryairalso leads to minimums falling well below normal. If minimums fall more than five degrees below normal then this conditionisdescribed as ‘cold wave’.
Central Indiausuallyremains dry except when an induced low moves across and causes some rains. These spells are not prolongedthoughand generally last for a day.
In the south, Northeast monsoon remains active in the first fortnight, causing decent rains in Tamil Nadu, Kerala, South interior Karnataka, Rayalaseema and coastal Andhra Pradesh. However, by third week,thepeninsula hardly experiences any rain.
North east India remains dry and pleasant for most part ofthe month, except when remnants of WDs cause rain on few days.
