According to the India Meteorological Department (IMD), December 2020 marked the second lowest temperature for the national capital in the month of December for the past 15 years.
The data released by the IMD on Thursday showed the mean minimum temperature (MMT) for this December to be 7.1 degrees Celsius, down from 7.6 degrees Celsius last year.
The lowest MMT recorded in the past 15 years in Delhi stood at 6.7 degrees Celsius in 2018, also marking the only instance the mean temperature fell below 7 degrees Celsius.
The national capital also witnessed eight cold waves this winter in December, equalling that of December 2018.
The average MMT for December was 6 degrees Celsius in 2005 and 5.9 degrees Celsius in 1996, while the city had recorded the maximum of 9 cold wave days in 1965, mentioned the IMD data.
A cold wave is defined as the minimum temperature dips to 4 degrees in the plains. It is also declared if the minimum temperature is 10 degrees Celsius or lower, and at least 4.5 notches below normal temperature.
Kuldeep Srivastava, the head of the IMD's regional forecasting centre informed that the reasons for such low temperatures is attributed to clear skies over Delhi-NCR, multiple western disturbances affecting the Himalayan region, as well as the global impact of La Nina, reported PTI.
La Nina occurs due to shifting wind patterns in the atmosphere near the equator region, and is characterised by below-normal sea surface temperatures in the Pacific Ocean. It leads to colder-than-normal temperatures in the Northern Hemisphere, affecting the Southern Hemisphere in the opposite manner.
Srivastava added that "near-normal" were seen in Delhi until December 12 owing to cloudy weather and rainfall in the plains under the influence of western disturbances. Clouds help in maintaining the temperature by trapping and radiating back some of the outgoing infrared radiation to the ground.
"After December 12, western disturbances mostly affected the western Himalayan region, leading to significant snowfall and rain over Jammu and Kashmir and Himachal Pradesh," PTI quoted Srivastava as saying.
The north-westerly winds then blow southward from J&K and Himachal Pradesh, leading to temperatures falling in the national capital region, with "uplifted fog" over neighbouring states of Punjab and Haryana also contributing to colder winds, he explained.