National capital plunged below 5°C mark today morning, first time in the season. This happens to be the coldest morning, as Safdarjung observatory records lowest of 4.9°C, closely followed by Meteorological office Lodhi Road at 5°C. Occurrence of low single digit minimum temperatures is infrequent, during first half of December. Earlier, the city recorded the coldest day yesterday only, by measuring 6.2°C.
Plunge of over 1°C without any major weather system around, is rather unusual. Minimum of 4.9°C means 4°C below the normal and literally missing cold wave condition by a whisker. Cold wave conditions are declared in the plains when the minimum temperature is less than 10°C and about 4.5°C or more, below the normal. Also, the cold wave in the plains is announced when the minimum temperature lowers to 4°C or less. And, severe cold wave occurs when the mercury drops to 2°C or less.
Delhi has not witnessed such low temperatures during first fortnight of December. Sudden plummeting of mercury is frequented in the last week of December. Invariably, it follows after simultaneous rain and snow in the hills and plains, around Christmas and New Year. In the last one decade or so, Delhi recorded low single digit temperature during the first fortnight, only once in 2020. A lowest temperature of 4.1°C was recorded on 15Dec2020. However, the minimum temperatures have dropped to low single digit frequently during last week of December.
National capital has registered nearly the lowest temperature in the plains of North India. Amritsar, otherwise supposed to coldest clocked 5.4°C and Churu in Rajasthan was way above at 7.4°C. Only, Hisar in Haryana was a shade lower at 4.2°C. Delhi is even colder than the hills like Shimla ( 6.8°C), Mussorie (6.2°C), Nainital( 5.7°C), Dharamshala (8.2°C), Dalhousie(6.8°C). Radiational cooling on account of clear sky, cold northwesterly winds, coupled with localized factor of water body and pocket of dense green trees could have triggered substantial drop in the night temperature. These may not drop any further. Rather, approach of another western disturbance tomorrow, albeit mild, may raise it marginally over the next two days.