The India Meteorological Department (IMD) marked the onset of the first heatwave in parts of western Rajasthan on March 27. The current criteria employed by IMD to declare a heatwave primarily relies on temperature thresholds: 40°C in plains, 37°C in coastal areas, and 30°C in hills. Additionally, temperatures must exceed normal levels by 4.5°C for two consecutive days to qualify as a heatwave.
However, this approach overlooks the impact of relative humidity, a key factor influencing how living beings experience heat. Humid heatwaves, characterized by high moisture levels in the air, can lead to heightened temperatures felt by humans and other organisms, even if recorded temperatures remain below traditional thresholds.
Experts advocate for including wet bulb temperature or heat index measurements, which combine temperature and humidity, to provide a more accurate assessment of heatwave conditions. Wet bulb temperature, in particular, is crucial as it reflects the air’s evaporative cooling potential, directly impacting the body’s ability to regulate heat through sweating.
International standards suggest a safe wet bulb temperature limit below 30°C, with values above 35°C indicating extreme danger. However, the IMD’s current framework does not incorporate wet bulb temperature calculations, potentially overlooking critical heatwave conditions.
Recent incidents, such as high wet bulb temperatures recorded in Maharashtra despite not meeting traditional heatwave criteria, underscore the urgency of revisiting monitoring methods.
IMD’s introduction of terms like ‘warm night conditions’ and ‘hot humid weather’ acknowledges broader heat stress issues. Still, these terms’ clear definitions and specific thresholds are needed for effective monitoring and response strategies.
Scientific research underscores the need for nuanced approaches to heatwave monitoring, especially in tropical countries like India. A study published in a scientific journal highlighted the inadequacy of a universal 35°C wet bulb temperature threshold, emphasizing the need for context-specific thresholds tailored to local conditions.
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