By Akshit Sangomla
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While usually there are three-five strong Western Disturbances (WD, extra tropical storm systems) in December, there was only one in last one. The feebly active WDs that did come in limited themselves to the hilly states of Jammu & Kashmir, Himachal Pradesh and Uttarakhand, and did not influence the weather in the northern plains which made the cold wave conditions persist for longer than usual, making it a continuously cold winter.
For instance, December 2018 turned out to be the third-coldest month in Delhi in the last 50 years. The average minimum temperature during the month was 6.7 degree Celsius — this happened last in 2005 when the average minimum temperature in December was 6 degree Celsius, preceded by 1996 when it was 5.9 degree Celsius.
“One of the reasons for less WD activity in the plains could be mid-0 and north latitude interactions forcing reduced southward propagation of WDs. Another factor which might be working against the WDs could be the lack of moisture coming in from the Indian Ocean and the mid-latitude Mediterranean region which generally feeds them,” AP Dimri, an expert on WDs at the School of Environmental Sciences, Jawaharlal Nehru University, had told Down to Earth.
However, the exact reasons for this phenomenon are still unknown. Dimri had further predicted that the WDs could be back with greater intensity in January. This proved to be true as in January the frequency of WDs increased and peaked in February with seven in that single month.
This enhanced activity of the WDs, which brought respite from the cold, was aided by the incursion of moisture by other easterly winds. But this too lasted only for a short time.
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