Number Theory: Has winter arrived early in Delhi this year?

There is no temperature threshold, which defines winter officially. While IMD calls the December-February period as winter, these are not the coldest months (at least in terms of maximum temperature) in all of India.

The answer to this question will change depending on whether one is talking about temperatures during the day or night. While nights are still not cold enough, Delhi has seen colder than normal days five days into the official winter season, according to an analysis of India Meteorological Department (IMD) data. Here are four charts that explain this argument in detail.

Although it might have felt like an early winter during the day in Delhi, that is not really the case at nights when minimum temperature is recorded. (Hindustan Times)

Defining winter, statistically speaking

There is no temperature threshold, which defines winter officially. While IMD calls the December-February period as winter, these are not the coldest months (at least in terms of maximum temperature) in all of India. For instance, the 1981-2010 average (considered the normal for temperature) shows that in Karnataka and Kerala January is only the 7th and 8th coldest month, respectively. To be sure, minimum temperature is the lowest during December-February in all states except Telangana (where February was 0.2 degrees Celsius warmer than November in 1981-2010). Delhi is not part of such exceptions either by maximum or minimum temperature, both being the lowest in the city-state during the December-February period.

To be sure, minimum temperature is the lowest during December-February in all states except Telangana (where February was 0.2 degrees Celsius warmer than November in 1981-2010).

This year, Delhi’s maximum temperature reached December levels in mid-November itself

Given December-February period is indeed Delhi’s coldest, one can say that winter has arrived early in Delhi if it starts hitting December temperatures earlier. This has indeed been the case this year for maximum or daytime temperature. The normal maximum temperature for December 1 (it falls steadily through November) is 25.61 degrees Celsius. Delhi’s maximum this year has been below that mark on most days since November 17 (November 2022 was the 5th coldest since 1951). On 13 of 18 days up to December 4 to be precise. This descent to December temperatures was also faster than normal. On the normal chart it takes 12 days for the temperature to go from 28 degrees to 25 degrees Celsius (from November 19 to December 1). This year, this change happened in six days (from the week ending November 15 to the week ending November 21) if one considers rolling seven-day averages to paper over small daily fluctuations. In daily experience, the change likely felt even sharper, as the maximum decreased from 28.2 degrees on November 15 to 24.7 degrees on November 17.

The normal maximum temperature for December 1 (it falls steadily through November) is 25.61 degrees Celsius.

Minimum temperature is still not at winter-levels, but it has also fallen fast

Although it might have felt like an early winter during the day in Delhi, that is not really the case at nights when minimum temperature is recorded. Delhi’s normal minimum for December 1 is 9.05 degrees. Delhi recorded a minimum below that only on one day in November (November 29) and has been warmer than that on all December days until December 5, almost a week into the official winter season. To be sure, minimum temperatures are now much closer to normal than they were in early November. This means that although warmer than normal, night temperatures have also descended more rapidly than normal. On the normal chart, Delhi goes from 13.41 degrees to 10.24 degrees in 16 days (from November 10 to November 26). Delhi underwent this change in just a week this year: from 13.34 degrees in the week ending November 19 to 10.22 degrees in the week ending November 26.

Delhi’s normal minimum for December 1 is 9.05 degrees.

Delhi is an outlier of sorts in its colder day temperatures

To be sure, Delhi’s current weather seems to be more a local phenomenon than a widespread one. At the state level, for example, only Madhya Pradesh, Haryana, and Sikkim have displayed the same pattern as Delhi – lower than normal maximum and higher than normal minimum – in the past week on average. While Jammu and Kashmir and Ladakh were colder than normal in the past week by both maximum and minimum, most states are warmer on both counts.

At the state level, for example, only Madhya Pradesh, Haryana, and Sikkim have displayed the same pattern as Delhi – lower than normal maximum and higher than normal minimum – in the past week on average.

Although the trend is geographically unique to Delhi currently, it is not new for Delhi. Almost all of Delhi’s last winter was like the one it has experienced since mid-November. Both November and December in 2021 were about six degrees colder than normal by maximum temperature and one of the top 10 coldest since 1951. This was followed by the coldest ever January since 1951 (15.9 degrees colder than normal), until temperatures came within three degrees of normal in February. Minimum temperature on the other hand was the 7th highest, 2nd highest, and 11th highest for December, January, and February in the 2021-2022 winter.