Number Theory: Why November went from being abnormally hot to abnormally cold

The average maximum temperature for India for the month (up to November 28) was 28.90°C, only 0.14°C warmer than the 1981-2010 average for this period.

The India Meteorological Department (IMD) treats December as the beginning of the winter season. In large parts of north India, the winter chill is usually expected to set in much earlier and this is what was missing for a large part of November. Was November warmer than usual? Not the month as a whole, but definitely in the first half, an HT analysis of IMD’s gridded dataset shows.

A man sits next to a bonfire on a cold winter morning in Noida on November 21. (Photo by Sunil Ghosh / Hindustan Times)

Why November went from being abnormally hot to abnormally cold

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  Overall, maximum temperatures were nearly normal while minimums were higher

  The average maximum temperature for India for the month (up to November 28) was 28.90°C, only 0.14°C warmer than the 1981-2010 average for this period, considered the normal for temperature by the IMD. Average maximum for the month is ranked only 32nd highest since 1951, the earliest year for which IMD has this data. Minimum temperatures, however, were warmer than normal. The average minimum for the month up to November 29 (data for minimum temperatures becomes available earlier because it is recorded in the early hours of the day) was 0.83°C warmer than normal and ranked 17th highest since 1951.
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  But the first half of the month was extreme for both maximum and minimum temperatures

  For example, in the first ten days of the month, the maximum temperature on each day was ranked among the top ten highest for the day. The same was true for minimum temperatures up to November 14. The reason the month’s average appears moderate in comparison is because the second half was cooler than normal. This change can also be seen from individual places. For example, the maximum temperature in Delhi (IMD’s grids go somewhat beyond the administrative boundaries of Delhi) averaged a warming of 0.5°C up to November 15 and averaged a cooling of 1.4°C in the rest of the month.
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  ...continued

  To be sure, as the accompanying maps show, there were a few regional exceptions to the country-level trends, such as parts of the west coast and hilly states in the north experiencing cooler than normal maximum in the first half of the month too. Similarly, Delhi’s minimum temperatures averaged warmer than normal in both halves, although they came close to normal in the second half.
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  Snow and rain were drivers of these trends in northern and southern India

  This can be seen in the rain/precipitation (IMD does not make a distinction between rain and snow, but satellite-based datasets capture them differently) maps for the two halves of the month. This was a completely dry November for almost the entire northern half of India. While parts of Jammu and Kashmir, Ladakh, and Uttarakhand were an exception to this, the precipitation in the two UTs had big deficits compared to the 1971-2020 average, considered the normal for rain by the IMD. Because these regions received some rain or snow, the temperatures there were close to or cooler than normal. But this precipitation was not sufficient enough for the winds blowing from the region to cool down the rest of northern India. This changed in the second half of November. Almost all of Jammu and Kashmir and Ladakh received some precipitation and parts of the two UTs received even excess (surplus from normal of 20%-59%) and large excess (surplus of 60% or more) of precipitation. This cooled down most of northern India to temperatures close to or below normal. A similar trend was observed in peninsular India, particularly the part not on the west coast. The region east of the Western Ghats is expected to rainy at this time of the year because of the north-east monsoon. Most of Tamil Nadu, Andhra Pradesh, Telangana, and interior parts of Karnataka, had big deficits in the first half of the month. This changed in the second half for at least the coastal areas with the development of a storm in the Bay of Bengal in the last week of the month.