Large parts of northern and eastern India are expected to be rainy this week, according to the forecast issued by the India Meteorological Department (IMD) on Monday. This follows similar weather patterns in April and May so far. This suggests that this summer season is likely to be one of the cooler ones. How cool exactly? The gridded data of the IMD shows that this is the 18th coolest summer since 1951, the first year for which IMD has published gridded data. However, it appears even cooler if one excludes the early part of the summer months, when temperatures are usually low in absolute terms.
Before discussing summer temperature trends further, however, it is important to note that the IMD has not clearly defined a summer season. It calls January and February “winter”, which is followed by the “pre-monsoon” season from March to May, the “southwest monsoon” from June to September (sometimes also called the summer monsoon), and the post-monsoon season in October-December. Even these definitions are not without contention. For example, IMD issues a forecast for the “cold weather season” – December to February – around the first day of December, which one can only interpret as a winter forecast.
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For the sake of simplicity – and because this is generally among the warmest periods of the year – HT considered the March-May period as the summer season. This shows that the March 1-May 10 period this year averaged a maximum temperature of 33.08°C, the 18th lowest average since 1951. This was also 0.51°C cooler than the average for these days in the 1981-2010 period, considered the normal. This leaves little doubt that this has been a cooler than normal summer.
To be sure, this summer would rank even cooler if one excluded the first 15 days of March from the analysis. This is because only 30 days have recorded warmer than normal maximum temperatures in the 71-day period from March 1 to May 10; and 14 of them are among the first 15 days of May. Another 14-day stretch occurred in the second half of April, but with a much smaller upward deviation than in the first half of March. In other words, the biggest reason why this summer does not appear among the coolest is the trend in the first half of March. If this period is excluded from the analysis, the average maximum for the March 16-May 10 period is 33.31°C, eighth lowest since 1951 and 1.16°C below normal.
Chart 1
India experiencing one of its coolest summer seasons, however, does not mean that all parts of the country have. Even in the March 16-May 10 period, only around two-thirds of the country averaged lower than normal maximum temperatures. Only 44% of the country has experienced an average maximum at least 0.5°C cooler than normal and only 31% of the country has experienced an average maximum at least 1°C cooler than normal. As the accompanying map shows, a relatively small part of the country – northern and north-western regions, the Indo-Gangetic plains, and the north-eastern regions – is making this summer look cooler than average for India. Most of central, western, and peninsular regions have been somewhat warmer than normal.
Map 1
As expected in summer, these trends are driven by trends in rainfall. The more extra days of rain a place got, the more likely it was to have experienced a cooler than normal temperatures on average. For example, the places that have experienced a cooler than normal summer so far have seen at least seven days of rain more than normal in the period after March 15.
Map 2