The Konkan region, including Mumbai, has been experiencing sweltering heat in recent days, with the maximum temperatures touching the 40 degrees mark.

What is a Heatwave and when is it declared?

• Heatwaves occur over India between March and June.

• IMD declares a heatwave event when the maximum (day) temperature for a location in the plains crosses 40 degrees Celsius.
• Over the hills, the threshold temperature is 30 degrees Celsius.

Following criteria are used to declare heatwave:

To declare heatwave, the below criteria should be met at least in 2 stations in a Meteorological subdivision for at least two consecutive days and it will be declared on the second day.

a) Based on Departure from Normal

• Heat Wave: Departure from normal is 4.5°C to 6.4°C
• Severe Heat Wave: Departure from normal is >6.4°C

b) Based on Actual Maximum Temperature (for plains only)

• Heat Wave: When actual maximum temperature ≥ 45°C
• Severe Heat Wave: When actual maximum temperature ≥47°C

How long can a heatwave spell last?

• A heatwave spell generally lasts for a minimum of four days. On some occasions, it can extend up to seven or ten days.
• The longest recorded heatwave spell, in recent years, was between 18 – 31 May 2015.

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