When maximum is minimum: Explaining Delhi’s winter chill
Parts of the National Capital Region recorded cold wave conditions for the first time this winter on the intervening night of Saturday and Sunday. While a wind chill factor is also taken into account when declaring a cold wave, the minimum temperatures were low in parts of Delhi on Sunday night – 3.3 degrees Celsius at Jafarpur and 3.6 degree at Lodhi Road – even without accounting for it. An HT analysis, however, suggests it is the daytime maximum temperatures that have fallen to historical lows in December and not the minimum temperatures recorded at night. Here are five charts that explain this trend and the reasons behind it.
Average minimum temperature is 6th highest since 1951, but falling
Historical data on temperatures is publicly available from the India Meteorological Department (IMD) only for grids and not individual stations or cities. These grids are of 1 degree resolution: boxes of latitudes and longitudes 1 degree apart. Since Delhi is too small to be captured by them, this analysis includes all districts neighbouring Delhi in its analysis of temperatures. It shows average minimum temperature in Delhi in December is 9.74 degrees up to December 20. It is the 6th highest average minimum temperature for this period since 1951. This is not because of any particular day. On 13 of 20 days in December, the minimum temperature was ranked among the top 20 highest values for that day since 1951. However, with almost each passing day, both absolute minimum temperatures and the rank of 2021 for that day has fallen. December 18 this year was the 33rd coldest December 18 since 1951, December 19 was the 27th coldest, and December 20 was the 11th coldest.
Both average and daily maximum temperatures are very low this December
Unlike minimum temperatures in Delhi, maximum temperatures have kept historically low throughout the month. 2021 was among the top 20 coldest years since 1951 on 12 of 19 days of December. December 20 data will become available only on December 21. Moreover, just like minimum temperature, maximum temperatures progressed to greater lows through the month. As expected, the average of 22.2 degree is the 4th lowest since 1951.
The reason for these trends is unlikely to be local
A minimum temperature of under 4 degrees can hardly be called a high minimum temperature. However, Delhi reached this level only this weekend; it has not witnessed lower minimums than is expected for this time of the year on average. The opposite is true for maximum temperatures. Such trends are typically associated with rainfall. Clouds prevent all sunlight from reaching the earth, bringing down day temperatures. At night, these same clouds prevent the surface from releasing heat it absorbed during the day, and raise minimum temperatures. This was likely the guiding factor in early Decembe. It rained on December 3 and 6 in Delhi as well as other parts of north India.
Overall, however, rainfall in Delhi this month is at least 60% lower than average. This suggests local factors are not a likely cause of temperature trends in Delhi in the second half of December. Other northern states also have a deficit in rainfall so far in December that can be similarly categorized.
Wind rather than rainfall is the determining factor this December
If not rainfall, what is causing the peculiar trend in Delhi temperature? Advection, or transfer of heat through wind, is a likely cause, said D S Pai, head of climate research at India Meteorological Department. In the absence of rainfall, advection is likely the determining factor, and cold winds blowing from the north are responsible for the falling temperatures, he explained. Part of this factor can be seen in temperature trends across north India. The trend in the average temperature – lower than normal maximum temperature and higher than normal minimum temperature – is shared by all northern states.
How have weather conditions affected air quality?
Weather conditions have had conflicting impact on air quality this month. The early December rainfall helped clean up the air, but with a delay. This is because it was not enough to completely wash away pollutants until it was also helped by the wind. In the first 9 days of December, average air quality index (AQI) was 320.8, in the ‘very poor’ category, but better than in 2018 and 2020. With winds responsible for the drop in temperatures more recently, low temperature, which traps pollutants, and wind, which carries them away, have again been in conflict. On December 18 and 19, air quality improved, only to worsen on December 20.