Heatwaves
Scientific Definitions and the Wet-Bulb Temperature Threshold
A heatwave is a period of abnormally high temperatures, generally exceeding the historical average by a defined threshold for a minimum number of days. Definitions vary by country and context. India's IMD defines a heatwave when the maximum temperature exceeds 40°C (plains) or 30°C (hills) and deviates 4.5°C or more from normal. A severe cold day/cold wave definition follows similar deviation logic (4.5–6.4°C below normal for cold day; above 6.4°C for severe cold day). The critical physiological threshold is the wet-bulb temperature of 35°C — the point at which human thermoregulation through sweating becomes impossible, making prolonged outdoor exposure fatal even for healthy individuals in shade with airflow. Recent studies suggest the effective upper limit may be even lower (~31°C wet-bulb across high-humidity environments).
- Wet-bulb temperature (Tw): combines heat and humidity; a Tw of 35°C means sweat cannot evaporate, body cannot cool
- IPCC projections: limiting warming to 1.5°C prevents most tropical regions from exceeding the 35°C wet-bulb threshold; at 2°C, large portions of South Asia and Africa breach it seasonally
- WBGT (Wet Bulb Globe Temperature): includes radiation and wind; used by military and occupational health agencies to set work-rest cycles
- Person-day: a compound exposure metric — 1 person experiencing 1 day of heatwave = 1 person-day; allows comparison of scale across regions
- Urban Heat Island (UHI) effect: cities are typically 1–3°C warmer than surrounding rural areas due to concrete, reduced vegetation, and anthropogenic heat; the new study shows this does not necessarily translate into greater population exposure if urban populations are smaller
● Tracked since March 27, 2026 · last seen June 25, 2026 · updates as the daily brief publishes