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 systems are Western 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 be in the early part of December. This is because sea surface temperatures are not favorable for cyclone formation after second week. The other prominent feature of this month is the westerly jet stream which starts making its presence felt in North India. The jet stream flows with mean speeds of 100-120 Kts ( 200- 240 Kmph) but at times it can even touch 150 Kts (300 Kmph).
On an average 2-4 WDs move across India in this month, these systems cause fairly widespread (FWS) rains in J&K, Himachal Pradesh, and Uttarakhand. It also causes rains in plains of North India. As WD moves 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 which then moves across Gangetic plains causing winter rains. WDs along with induced lows are the main rain producing systems of the country during this season.
Besides causing rains, WDs are also responsible for occurrence of fog in North India. Humidity goes up after rains and combines with the cold & dry air which penetrates northern India & lowers night temperatures. Thus in 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 dry air also leads to minimums falling well below normal. If minimums fall more than five degrees below normal then this condition is described as ‘cold wave’.
Central India usually remains dry except when an induced low moves across and causes some rains. These spells are not prolonged though and 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, the peninsula hardly experiences any rain.
North east India remains dry and pleasant for most part of the month, except when remnants of WDs cause rain on few days.
