November 2020 was Delhi’s coldest in 71 years, with global weather phenomenon such as La Nina and the absence of western disturbances, as well as local ones such as less rain in September and the absence of cloud cover, contributing to a mean minimum temperature of 10.2 degrees Celsius.
Typically, mean minimum temperatures in November are around 12.9 degrees, according to the India Meteorological Department.
It wasn’t just November, though — Delhi also experienced its coldest October in 58 years with the mean minimum temperature dipping to 17.2 degrees Celsius. To find a lower October mean temperature, one will have to travel back in time to 1962, when it was 16.9 degrees Celsius, according to IMD data.
And it will likely get worse, with IMD’s seasonal forecast on Sunday predicting a colder winter (“below-normal minimum temperatures”) in New Delhi, and parts of north and north-west India. Monday was the coldest day in Delhi in November with the minimum temperature falling to 6.9 degrees
Kuldeep Srivastava, head of IMD’s regional weather forecasting centre (RWFC), attributed the colder November to “ a combination of factors including global conditions such as La Nina” and local ones such as “an absence of cloud cover …; whenever there are clear skies, the minimum temperature drops.”
He added that the minimum temperature will remain between 7 degrees and 8 degrees over the next few days, a degree or two degrees below normal — but that the maximum temperatures would be marginally higher than normal.
La Nina is a climate phenomenon that sees cooler ocean temperatures in the Pacific — which has a strong correlation with wetter monsoons and colder winters in India. It also has a significant correlation with the number and strength of cyclones in the Bay of Bengal. Cyclone Nivar crossed the Tamil Nadu coast last week, bringing widespread rain to the region and causing some damage. IMD has predicted that another cyclone is set to cross the Sri Lankan coast in early December, bringing more rain to Tamil Nadu.
The Capital has also experienced four cold waves this month — on November 3, 20, 23 and 24. The last time Delhi saw multiple cold waves during November was in 1964 when three such occurred. Significantly, the last time Delhi saw a cold wave in November was back in 2014. The all-time low mean minimum temperature in November was recorded in 1930, at 8.9 degrees Celsius.
DS Pai, climate head of climate research and services, IMD Pune, said that in addition to La Nina and other factors, the absence of western disturbances too resulted in a variation in temperature. He explained that these disturbances — storms originating in the Mediterranean — usually result in a rise in temperature.
IMD director-general M Mohaptra said that while global factors like La Nina have previously made their effect felt, this is magnified when local and regional factors also contribute, resulting in extreme weather such as what Delhi has seen this November.
He also pointed to the dry September the capital saw this year as another reason. “… means there was less moisture and the atmosphere was mostly dry resulting in clear sky conditions. Clear skies allow more radiative cooling, which means the heat dissipates fast making the night cooler, which leads to a dip in minimum temperature.”
Meanwhile, lower minimum temperatures and a slight reduction in wind speed resulted in air quality deteriorating to the “very poor” zone on Monday. The average air quality index (AQI) was 318, up from Sunday’s 256. There’s bad news in store on that front too. “The winds will turn calm from December 4-5 and the air quality may deteriorate further, as both low temperature and absence of winds will not allow dispersion of pollutants,” said Srivastava.