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IMD Heatwave Warning Criteria: Many parts of India continued to swelter over the weekend, with the rising mercury showing no signs of a let-up. According to the India Meteorological Department (IMD), there is a high probability of heatwave conditions in isolated places in Odisha during March 13-17; Jharkhand during March 14-17, and Gangetic West Bengal on March 15-17. Interior Odisha, south
Rajasthan and north Gujarat are likely to see heatwave conditions in the March 21-17 week too.
On March 16, heatwave warnings and alerts were issued for various districts of Odisha, Chhattisgarh, Karnataka, and Maharashtra. In Jharkhand, temperatures crossed 40 degree Celsius in seven districts, while Karnataka health minister Dinesh Gundu Rao on March 15 urged people to stay indoors between noon and 3 pm.
If temperatures in your area are high, that does not automatically mean your region is experiencing a heatwave. How does the IMD define a heatwave? Why does it issue specific warnings for different places? We explain.
A heatwave is basically a period of unusually high temperatures over a place. Thus, the threshold to declare a heatwave depends on the temperatures normally seen in that area in that time of the year. So what is considered a heatwave in Kerala will not be called that in Odisha.
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 the normal temperature, 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, the IMD says, 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”, heatwave is declared on the second day, the IMD says.
In coastal areas, if the maximum temperature departure is 4.5 degree C or more from normal and the actual maximum temperature is 37 degree C or more, a heatwave can be declared.
Heatwaves in India occur mainly from March to June and in some cases, July.
Discomfort for people is worsened if along with temperatures, humidity is also high, as sweat is unable to evaporate, and if the night-time temperatures are high, because then the body is unable to recover from the day’s heat.
“IMD has a big network of surface observatories covering the entire country to measure various metrological parameters like Temperature, Relative humidity, pressure, wind speed & direction etc. Based on daily maximum temperature station data, climatology of maximum temperature is prepared for the period 1991-2020 to find out normal maximum temperature of the day for a particular station. Thereafter, IMD declares heat waves over the region as per its definition,” the weather body says.
