Number Theory | Extreme temperature variations are becoming the norm in Delhi

Updated on Jun 08, 2022 05:07 AM IST
An HT analysis of long-term temperature data shows that such inter-day and inter-week variations in temperature are becoming more common in Delhi. Here are four charts which explain this.
Just after a week of this change from hot to cold weather, Delhi’s weather somersaulted again and the heatwave returned with a vengeance. (Raj K Raj/Hindustan Times)
Just after a week of this change from hot to cold weather, Delhi’s weather somersaulted again and the heatwave returned with a vengeance. (Raj K Raj/Hindustan Times)
By, New Delhi

On the morning of May 23, people in Delhi had to switch off their air conditioners and switch on water heaters. The reason for this shift from using what is a typical summer appliance to a winter one was a sharp fall in temperature. This was a result of sudden rainfall and strong winds on 23rd morning. Such was the change in weather between May 22 and May 23 that the inter-day drop in maximum temperature is the highest ever recorded for Delhi since 1951, the earliest period for which this data is available in the Indian Meteorological Department’s (IMD) gridded database. Just after a week of this change from hot to cold weather, Delhi’s weather somersaulted again and the heatwave returned with a vengeance.

An HT analysis of long-term temperature data shows that such inter-day and inter-week variations in temperature are becoming more common in Delhi. Here are four charts which explain this in detail.

Two sudden rainfall events in the last week of May led to a sharp fall in temperatures in Delhi

The abnormally and consistently hot weather in Delhi this summer changed suddenly on May 23, when 14.7mm of rain fell, accompanied by strong winds reaching up to 75km per hour around 7 am. Maximum and minimum temperatures dropped by 10.69 degrees Celsius (°C) and 6.69°C, the highest and 15th highest single-day variations respectively since 1951, the earliest year for which this data is available.

While the numbers above are derived from IMD’s gridded dataset, which goes beyond the administrative boundaries of Delhi for temperature, it is unlikely that the trends will be very different for the area within those boundaries. In the gridded dataset, half of the daily changes in maximum temperature (irrespective of whether it was an increase or decrease) Delhi has seen since 1951 are under 0.76°C and 99% are under 4.51°C. These thresholds for minimum temperature are 0.71°C and 3.96°C. Clearly, even for the area within Delhi’s boundaries, a drop of May 23rd’s magnitude would be rare.

To be sure, Delhi witnessed an even worse storm on May 30, when wind speeds reached up to 100km per hour at the Safdarjung station and there was a significant 9.5mm of rainfall. But because this storm took place early evening, it did not significantly affect the maximum and minimum temperatures, which are usually recorded during the afternoon and late-night or early morning, respectively. However, as HT reported earlier at the Safdarjung station, the temperature did change rapidly: from 40.6°C to 25°C between 4.20 pm and 5.40 pm. Because such granular data on temperature is not publicly available for historical analysis, it is not possible to check how rare this event was.

But the heat wave is back once again now…

Barring the rainfall driven relief in the last week of May, Delhi has been having a warmer than usual summer this year. From March 7 to May 22, the maximum temperature was above normal — it is defined as the average temperature on that day from 1981 to 2010 — on all days but three: April 21, May 5, and May 6.

The minimum temperature was below normal only on May 5. For a nine-day period after May 22 — from May 23 to May 31 — the maximum temperature was below normal on all days. Temperatures have risen once again over the last week in the national capital and were above normal. The average weekly maximum was 36.12°C in the week ending May 29 and 42.32 in the week ending June 5. This inter-week change in maximum of 6.2°C is ranked 257 among 26,076 such inter-week changes since 1951. In other words, this week witnessed a jump in maximum which is among the top one percent changes between consecutive weeks.

Large inter-day or even intra-week variations in temperature have been the norm of the past decade in Delhi

An HT analysis of inter-day changes from 1951 onwards shows that on 99% of the days, the maximum temperature rose or fell by 4.51°C or less. This means that if it changed by more than 4.51°C between two days, it would be among the top 1% inter-day shifts in maximum. Even in May, an inter-day change of 4.75°C or more puts that in the top 1%. On both counts — whether one looks at the whole year or just the month of May — the last 10 years are outliers. The average proportion of days (in either the whole year or just in May) when the inter-day shift in maximum was among the top 1% is the highest in the 2012-2021 decade for any decade since 1951-1960. Such abrupt changes have also become the norm for the changes in average weekly temperature, with the 2012-2021 decade recording the highest proportion of days when inter-week temperature change has been in the top 1%.

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  • ABOUT THE AUTHOR

    Abhishek Jha is a data journalist. He analyses public data for finding news, with a focus on the environment, Indian politics and economy, and Covid-19.

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