Key Terms & Concepts — UPSC Mains
Wet-Bulb Temperature
"The lowest temperature achievable by evaporative cooling — a combined measure of heat and humidity that determines the human body's ability to cool itself; above 35°C wet-bulb, survival is impossible even in shade."
Wet-bulb temperature (WBT) is measured by wrapping a wet cloth around a thermometer bulb and allowing evaporation to cool it. It reflects both ambient temperature and relative humidity. The human body cools itself primarily through sweating — evaporation of sweat lowers skin temperature. When wet-bulb temperature is high (meaning ambient air is both hot and humid), sweat cannot evaporate effectively, and the body cannot regulate its core temperature. The critical threshold is 35°C wet-bulb temperature: above this level, even a healthy adult at rest in the shade cannot maintain core body temperature and will die within hours. This is considered the theoretical survivability limit for humans. In practice, health impacts begin much earlier: at 30–32°C WBT, outdoor physical labour becomes dangerous; at 33–35°C WBT, heat exhaustion and heat stroke become likely for outdoor workers. WBT is distinct from the more commonly reported dry-bulb temperature (standard air temperature). A temperature of 40°C with 50% humidity has a wet-bulb temperature of approximately 31°C — already dangerous for outdoor workers. A temperature of 35°C with 90% humidity approaches the survivability threshold. China's Yangtze River basin, South Asia (including the Indo-Gangetic Plain), and the Persian Gulf region are the areas most at risk of crossing the 35°C wet-bulb threshold under climate change projections. Some climate models project that parts of South Asia may regularly experience unsurvivable wet-bulb conditions by 2050–2100 under high-emission scenarios.
Increasingly tested in UPSC GS3 (Climate Change, Disaster Management, Science & Technology). The concept explains why humid coastal heatwaves (Mumbai, Odisha, Bangladesh) can be more lethal than drier but hotter conditions (Rajasthan, UP) despite lower dry-bulb temperatures. Wet-bulb temperature links climate science to public health and to the existential risk dimension of climate change — relevant for essays on climate justice and human adaptability limits.
- 1 Wet-bulb temperature combines heat + humidity — measures effective thermal stress on the body
- 2 35°C WBT = theoretical survivability limit — no healthy human can survive beyond this even at rest
- 3 Human body cools via sweat evaporation — high humidity prevents evaporation, raising WBT danger
- 4 Distinct from dry-bulb (standard air) temperature; WBT is always ≤ dry-bulb temperature
- 5 South Asia (Indo-Gangetic Plain), Persian Gulf, coastal China: highest at-risk regions by 2050
- 6 Heat Action Plans increasingly use WBT alongside dry-bulb temperature for risk assessment
- 7 Climate projections: parts of India may experience unsurvivable WBT conditions under high emissions by 2100
- 8 Wet-bulb globe temperature (WBGT) is used by the military and sports bodies for heat stress standards
During the 2015 Karachi heatwave, temperatures reached 44°C but with high humidity — resulting in a wet-bulb temperature of approximately 33°C, which contributed to over 1,200 deaths in two days as the body's sweating mechanism failed to provide adequate cooling. This was more lethal than drier heat events at similar or higher temperatures in Rajasthan.
