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.
