October so far, the fifth coolest ever | Number Theory

Cloudy and rainy weather, but possibly also cold winds blowing from the mountains

Large parts of India appear to be experiencing cooler than normal temperatures this month. How cold is it? An HT analysis of the gridded data of the India Meteorological Department (IMD) suggests that while nights have also turned cooler in recent days, it is daytime temperatures which have seen a bigger plunge, averaging fifth lowest for October so far. What is the reason behind it? Cloudy and rainy weather, but possibly also cold winds blowing from the mountains.

India’s average maximum temperature up to October 9 (the gridded data for October 10 will only be published today) is 30.02°C.

• 
  This is the fifth coolest October by day time temperatures so far

  India’s average maximum temperature up to October 9 (the gridded data for October 10 will only be published today) is 30.02°C. This is the fifth lowest maximum temperature for this part of October since 1951, the earliest year for which IMD has published gridded data for temperature; and 1.73°C cooler than the 1981-2010 average, which IMD considers as the normal for temperature. On the other hand, minimum temperatures, which are representative of night-time temperatures, are somewhat warmer than normal. India’s average minimum temperature is 21.4°C so far this month, 0.24°C above normal, and ranked 30th highest since 1951.

• 
  Maximum has been moving up towards normal in the past three days, and minimum below normal

  To be sure, the average trend seen in the month so far was reversing in the past three days. While India’s average maximum temperature was still cooler than normal, it was moving up towards normal. On the other hand, minimum temperatures have turned cooler than normal in the past three days.

• 
  The geographical trends in maximum and minimum temperatures are near mirror images

  81% of the country’s area has experienced cooler than normal maximum temperatures on average this month. On the other hand, minimum temperatures were warmer than normal in 75% of the country. As this data suggests, trends in maximum and minimum temperatures would necessarily have to be opposite in large parts. This is indeed the case. As the accompanying maps show, most of northern India experienced maximum temperatures cooler than normal by at least 1°C, and north-western regions were cooler than normal by at least 2°C. But minimum temperatures saw an upward deviation from normal in northern India. To be sure, there are regions where both maximum and minimum temperatures were cooler than normal, such as the hilly states and UTs in the north. Similarly, large parts of peninsular India were warmer than normal by both maximum and minimum temperatures.

• 
  Rainy weather explains these opposing trends in maximum and minimum temperatures

  The accompanying map shows the departure in number of days of rain this October compared to the 1971-2020 average, which the IMD considers as the Long Period Average (LPA) for tracking rain’s performance. This map’s geographical trends largely fit in with that for temperature. The places where the days of rain increased compared to LPA saw cooler than normal maximum temperatures and warmer than normal minimum temperatures. This happens because clouds block incoming sunlight from reaching the surface, cooling day time temperatures. After sunset, however, when the earths’ surface loses heat through radiation, these clouds prevent it from leaving the atmosphere, increasing minimum temperatures.

• Snowfall possibly explains some cool weather persisting

  As the southwest monsoon retreats – it had retreated almost completely east of Uttar Pradesh, south of Gujarat and most of Madhya Pradesh on October 10 – winds have turned north-westerly in most of northern India. These winds could also be carrying some coolness to northern India from India’s hilly regions in the north, which experienced snowfall this month. Satellite derived data suggests the snowfall this month decreased deficit in the snow pack quickly. To be sure, the snow pack cycle just begins in October, and its big deficit makes it clear that it cannot have a prolonged impact without other factors or if there is no fresh snowfall.