Why did Delhi have only 13 days of heat wave from March till May officially? (Explainer)

Even when Delhi-NCR and entire northwest India reeled under scorching heat wave and severe heat waves, Delhi, officially, witnessed only 13 days of heat wave from March to May, IMD data showed on Friday.

“As per records for Safdarjung, there were 0, 9 and 4 days of heat waves for March, April, and May respectively,” an India Meteorological Department (IMD) meteorologist said.

Contrary to expectations, the number was reduced for May as the first 10 days and the last 10 days of May had relatively cooler due to the Western Disturbances (WDs).

“While the WDs brought in light to moderate rain in the first 10 days and relatively higher rainfall after May 21 to Delhi NCR, they brought in heavy to very heavy rainfall in Western Himalayan Region that has a direct impact on the temperatures of the northwest Indian plains,” an IMD scientist said.

Delhi-NCR and almost all areas of northwest India had experienced at least five spells of heat waves after the last significant rainfall on February 25. The latest round was from May 11 onwards till the thunderstorm with moderate rainfall cooled down the national capital.

Now, the question is why, even when the mercury rose several notches higher across many stations in Delhi for multiple days, is Delhi having only 13 days of heat waves officially?

One reason is data for which station is considered and another, what constitutes a heat wave or a severe heat wave?

Safdarjung is considered as the base station for Delhi even as the IMD carries out meteorological observations for about a dozen-odd stations across Delhi-NCR. So, for any and every official record for Delhi – be it temperature, rainfall, wind etc. – it is the data from Safdarjung that is considered.

Exactly why, when more than half a dozen stations recorded over 45 degrees Celsius maximum temperatures, Safdarjung had not and did not meet the criteria.

The IMD’s criterion for declaring a ‘heat wave’ or ‘severe heat wave’ too matters when it comes to counting the actual number of days that witnessed heat wave/severe heat wave conditions.

First and, of course, foremost is that if any station records 45 degrees Celsius or more maximum temperature or any station that records maximum temperature that shows a departure of plus 4.5 to 6.4 degrees Celsius from the normal temperature for that day, then it is called as a heat wave.

For severe heat wave condition, the maximum temperature should be at or above 47 degrees Celsius or the departure from normal should be more than 6.4 degrees Celsius (even when the actual temperature may or may not touch 45 degrees Celsius).

Concurrently, the maximum temperature needs to be at least 40 degrees Celsius for plains, 30 degrees Celsius for hills and 37 degrees Celsius for coastal areas.

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