From UPSC perspective, the following things are important :
Prelims level : Heatwaves
Mains level : Not Much
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.