Heatwaves in India

Why in News?

India Meteorological Department (IMD) warns that many parts of India continued to swelter for the upcoming days due to heatwaves.

A heatwave is basically a period of unusually high temperatures over a place.
The threshold to declare a heatwave depends on the temperatures normally seen in that area in that time of the year.
Criteria - According to the IMD, a heatwave is declared when the maximum temperature of a station reaches
- At least 40 degree C or more for plains and
- At least 30 degree C or more for hilly regions.
Based on Departure from Normal
- A heatwave is when the departure is 4.5 degree C to 6.4 degree C, and
- A severe heatwave is declared when the departure is more than 6.4 degree C.
Based on Actual Maximum Temperature
- A heatwave exists when the maximum temperature is greater than 45 degree C, and
- A severe heatwave when the temperature is over 47 degree C.
If above criteria met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it declared on the second day.
Supportive meteorological factors - High humidity, high wind speed, duration of heat wave events.
Prevalence - Heatwaves in India occur mainly from March to June and in some cases, July.
Heatwave-prone areas –
- Northwest plains- Rajasthan, Punjab, Haryana, Delhi,
- Parts of central India - Madhya Pradesh, Chhattisgarh), and
- The eastern and peninsular regions- Uttar Pradesh, Bihar, Jharkhand, West Bengal, Odisha, coastal Andhra Pradesh, Telangana, and Vidarbha in Maharashtra.

Heat Index – Combination of air temperature and relative humidity, it measures of how hot it really feels when relative humidity is factored in with the actual air temperature.