Heat Waves: Official IMD Criteria, Wet-Bulb Physics & Disaster Management
In climatology, environmental disaster management, and public health, a Heat Wave represents an extreme meteorological hazard characterized by a prolonged period of abnormally elevated ambient surface air temperatures that pose severe physiological, economic, and ecological risks. Across South Asia, heat waves typically occur during the pre-monsoon summer months between March and June (occasionally extending into July), driven by strong solar insolation, cloudless skies, dry thermal advection from continental deserts, and atmospheric subsidence patterns. As anthropogenic global warming accelerates, the frequency, duration, and intensity of heat waves have escalated rapidly, converting what was once an annual seasonal discomfort into a lethal natural disaster threatening agricultural yields, power grids, and human survivability.
In India, the official meteorological criteria governing heat waves are formulated and monitored by the India Meteorological Department (IMD), functioning under the Ministry of Earth Sciences. According to official IMD standards, a heat wave is not declared arbitrarily; it must satisfy strict quantitative temperature thresholds based upon geographical terrain. Qualitatively, a heat wave is recognized only when the maximum surface air temperature of a meteorological station reaches at least 40 degrees Celsius for Plains, at least 37 degrees Celsius for Coastal stations, and at least 30 degrees Celsius for Hilly regions.
Once these baseline threshold criteria are crossed, the IMD deploys two distinct classification frameworks to formally declare a Heat Wave or a Severe Heat Wave. Under the Departure from Normal framework (applied when normal maximum temperatures are historically below forty degrees), a Heat Wave is declared when the departure from normal maximum temperature is between 4.5 and 6.4 degrees Celsius, while a Severe Heat Wave is declared when the departure is 6.5 degrees Celsius or higher. Under the Actual Maximum Temperature framework (applied regardless of historical normal baselines), a Heat Wave is declared when the actual maximum temperature reaches 45 degrees Celsius, and a Severe Heat Wave is declared when temperatures touch or exceed 47 degrees Celsius. To qualify as an official event, these temperature conditions must be recorded at least at two stations in a meteorological subdivision for two consecutive days.