Number Theory: How bad is the current heatwave?

The short answer: Very bad, and perhaps too soon in the season, at least in Delhi. Now for the long answer.

At least one weather station in Delhi recorded a maximum temperature of over 50°C on May 29, an extreme threshold that some stations in Rajasthan crossed on May 28. While this is an extreme event, the more important question to ask is how bad is the ongoing heatwave in large parts of northwestern India? HT has looked at the India Meteorological Department’s (IMD) gridded database to answer this question. These grids cover an area bound by two latitudes and longitudes 1 degree apart, or over 100km, and the data can used to calculate temperatures for 29 states in India. Here is what it tells us about the current heatwave.

An elderly person out on the streets on a sultry summer day in Mungeshpur, Delhi. (HT Photo)

How bad is the current heatwave?

• 
  How hot have the past two weeks been?

  Even small parts of a state experiencing cooler temperatures – this is possible in geographically diverse states -- can pull the state average down. Therefore, to find the heat extremes experienced in the past two weeks, HT calculated the average maximum of all grids in the IMD gridded database. Their averages show that in six states -- Haryana, Punjab, Rajasthan, Gujarat, Madhya Pradesh and Uttar Pradesh -- more than half the area had an average maximum temperature of more than 40°C from May 15 to May 28. HT analyed the average for the past two weeks because aggregated data shows that the heat shifted to northwestern states only from the middle of the month. Of the six states where the past two weeks were very hot in large parts, all of Haryana, Punjab, and Rajasthan averaged more than 40°C. The data also shows that 68% of Rajasthan and small parts of Madhya Pradesh and Uttar Pradesh averaged more than 45°C. To be sure, as the accompanying map shows, some area in other states also averaged a maximum of more than 40°C. While an area-wise calculation is not possible for Delhi because of its small size, the city and its adjoining districts taken together also averaged 43.8°C in the past two weeks.
• 
  Was it abnormally hot in the last two weeks?

  The entire area in 11 of 29 states for which this calculation is possible has been warmer than normal (the average temperature in 1981-2010 period) in the past two weeks in terms of maximum temperature. As the accompanying map shows, some of these are northeastern states where the normal is low in absolute terms at this time of the year. However, five of them are states that were very hot over large parts even in absolute terms. For example, all of Punjab and Haryana averaged a warming of more than 3°C in the past two weeks, while such areas covered 84% of Rajasthan. In around a quarter of Rajasthan and Punjab, this deviation was more than 5°C. In Delhi, the deviation of the past two weeks (as seen in the gridded database) was 3.2°C. It is important to note that Delhi has been becoming warmer every day in the past five days with the deviation from normal increasing from 1.2°C on May 24 to 6.3°C on May 28.
• 
  Was it very hot every day in the past two weeks?

  To answer this question, HT counted the number of days each grid has recorded maximum of 40°C or higher. The number of such days were grouped as “no day”, “up to 3 days”, “3-7 days”, “7-10 days”, and “10-14 days”. In Punjab and Rajasthan, the entire area of the state was in the “10-14 days” category. This means that all of the area in these states experienced a maximum of 40°C or higher on at least 10 days in the past two weeks. In Madhya Pradesh, Haryana, Uttar Pradesh, and Gujarat, such areas were 50%-80% of the state’s total area. On how many days were 45°C and higher maximum temperatures experienced? As expected, Rajasthan was the hottest on this count, too, with 37% of the state’s area breaching the 45°C threshold on at least 10 days and 75% of the area , on at least three days. However, even in Haryana and Punjab 50% and 25% of the area breached this threshold on 3-7 days. Small parts of Madhya Pradesh (6.6% of total area) and Uttar Pradesh (3.9% of total area) breached this threshold on 7-10 days while another 9.8% and 11.6% of these state’s area did so on 3-7 days in the past two weeks. In Delhi, where such area calculation is not possible using IMD’s gridded data, each of the past 14 days crossed the 40°C threshold and the past three days crossed the 45°C threshold.
• 
  Is this duration of extreme maximum temperatures normal?

  No, not even in Rajasthan. In all of Haryana and Punjab, the duration of 40°C or higher maximums was at least three days longer than normal, with the deviation more than seven days in half of Punjab. In Rajasthan, the deviation was of more than three days in 90% of the area; and of more than seven days in 10% of the area. In Uttar Pradesh and Gujarat, the deviation was of at least three days in around two-thirds of their area and of more than seven days in 8% and 50% of their area. In Delhi, just crossing the 40°C threshold on all 14 days is completely normal. However, the city crossed the 45°C on three days longer than normal, as a 45°C maximum does not appear on the normal chart up to May 28.