Number Theory: The March of summer - A cool start with a hot end
An HT analysis of India Meteorological Department (IMD) data shows that this March was an ‘unusual’ month.
March 2022 saw reports of a heatwave in various parts of India. It is only on March 20 – the spring equinox – that the sun crosses the equator to the northern hemisphere, making the northern hemisphere, where India lies, hotter. Does this mean it was the warmest March in India? While a simple reading of the average maximum temperatures does not support such a conclusion, an HT analysis of India Meteorological Department (IMD) data shows that this March was an unusual month, as far as the weather is concerned. Here are four charts which explain this in detail.
1) How hot was March 2022 on average for India?
According to IMD’s gridded dataset, the average maximum temperature for March was 32.85 degrees Celsius. It was 1.41 degrees, or 4.5%, above normal. Normal temperatures are the average of temperatures in 1981-2010. It made March this year the 9th hottest since 1951. While March was very hot historically, average maximum temperatures have been higher in the past two decades. The hottest and the second hottest March since 1951 was recorded in 2010 (33.91 degrees) and 2004 (33.78 degrees).
2) Some states faced more heat than others
India may not have experienced the hottest March on record this year, but four states in the northeast did, including Assam, Manipur, Nagaland and Tripura. Another four – Gujarat, Rajasthan, Arunachal Pradesh, and Mizoram – experienced their second hottest March. Kerala, Odisha, Haryana and Punjab experienced their third, fourth, and fifth (both Punjab and Haryana) hottest March since 1951. Delhi (the gridded dataset actually covers an area bigger than Delhi’s administrative boundaries) ended up with March 2022 becoming the sixth hottest on record by the last day of the month, with the temperature reaching 40 degrees for the second day in a row and marginally higher than on March 30. Among the 30 states and union territories for which this calculation is possible using the dataset, March 2022 was among the top 10 hottest in 2022 since 1951 in 19 jurisdictions.
3) A bigger weekly deviation above normal was visible in the middle of the month
One reason why this March was not the hottest ever for India on average was that in the first six days, the average maximum temperature was lower than normal. This has dragged down the monthly average. The first week of March was 1% cooler than normal and 28th coolest this year since 1951. The second week was 4.5% hotter than normal and 18th hottest since 1951. It is the third week that pushed the boundaries of normal the most. The average temperature in this week was 2.7 degrees, or 8.5%, above normal and the third hottest since 1951. This week also saw the only three consecutive days of more than 10% deviation from normal in this month: March 15 to 17. The week ending March 28 and the last three days of March were the 12th and 11th hottest since 1951, deviating 5.2% and 5.3%, respectively, above normal.
4) Not all states recorded similar deviations at the same time
Another reason for the average India temperature not being ranked even higher in March 2022 was that the deviations for different states did not move in tandem. For example, week one was among the top three hottest this year by maximum temperature in five states, week two in seven, week three in 20, week four in four, and the last three days in eight states. This is why week three of March was the third hottest for India on average as well. The maps show how the deviation was not synchronised across states. Northeastern states were far beyond normal in the first three weeks, but have been closer or even below it towards the month’s end, a trend somewhat opposite to central India, the plains and some southern states.