• The Tamil Nadu government has recently declared heatwaves as a state-specific disaster, signaling the seriousness of the situation. 

• This declaration allows for financial relief for those affected, compensation for families of victims, and the implementation of interim measures to manage heat-related crises, funded by the State Disaster Response Fund.

What are heatwaves?

• A heatwave is defined as a prolonged period of abnormally high temperatures occurring during the summer months in India, typically between March and June, and in rare cases extending into July.

IMD criteria for defining heatwaves

The India Meteorological Department (IMD) has established specific criteria to classify heatwaves based on different parameters, which are summarized as follows:

1) Based on Maximum Temperature:

• Plains: At least 40°C or more.

• Hilly Regions: At least 30°C or more.

• Coastal Areas: At least 37°C or more.

2) Based on Departure from Normal Maximum Temperature:

When the normal maximum temperature of a station is less than or equal to 40°C:

• Heatwave: Departure from normal is 5°C to 6°C.

• Severe Heatwave: Departure from normal is above 7°C.

When the normal maximum temperature of a station is more than 40°C:

• Heatwave: Departure from normal is 4°C to 5°C.

• Severe Heatwave: Departure from normal is 6°C or more.

3) Based on Actual Maximum Temperature:

• Heatwave: Actual maximum temperature is above 45°C.

• Severe Heatwave: Actual maximum temperature is above 47°C.

4) Declaration Criteria:

• The above conditions must be met in at least two stations within a meteorological sub-division. 

• The conditions must persist for at least two consecutive days. 

• The World Meteorological Organisation announced that 2023 was the hottest year recorded, reflecting the increasing frequency of heatwaves attributed to anthropogenic climate change, as detailed in the Intergovernmental Panel on Climate Change's Sixth Assessment Report. 

India has been experiencing the adverse effects of intense heatwaves, with historical data showing significant mortality linked to heat exposure:

• 1998: Severe heatwave with record temperatures. 

• 2003: Estimated over 3,000 deaths in Andhra Pradesh. 

• 2010: Approximately 1,300 deaths in Ahmedabad due to heat. 

• 2024: A heatwave resulted in temperatures exceeding 50.5°C in Rajasthan, with reported deaths and heat-related illnesses. 

What is wet bulb temperature?

Wet bulb temperature is crucial for understanding heat stress. It represents the lowest temperature a surface can reach through evaporation. Exceeding a wet bulb temperature of 35°C for extended periods can result in hyperthermia, making it vital for understanding the risks in humid climates like India.

(The author is a trainer for Civil Services aspirants.)