BharatNotes What is Wet-Bulb Temperature?
Wet-bulb temperature (WBT) is the lowest temperature that air can reach through the evaporation of water alone, at constant pressure. It is measured using a thermometer whose bulb is wrapped in a water-soaked cloth and ventilated (classically in a sling psychrometer alongside a dry thermometer). As water evaporates from the wick, it cools the bulb until equilibrium is reached. Because a sweat-covered human body cools the same way, WBT is a direct measure of how well a person can shed heat in given conditions of temperature and humidity.
How it differs from related metrics
Crucially, WBT is never higher than the ordinary (dry-bulb) air temperature. The drier the air, the faster water evaporates and the lower the WBT falls below air temperature. At 100% relative humidity, evaporation stops and WBT equals the dry-bulb temperature, meaning sweating no longer cools the body.
| Metric | What it captures |
|---|---|
| Dry-bulb temperature | Ordinary air temperature only |
| Wet-bulb temperature (WBT) | Combined effect of heat and humidity (evaporative cooling potential) |
| Heat index | "Feels-like" temperature from heat and humidity (in shade) |
| Wet Bulb Globe Temperature (WBGT) | Adds the effect of sunlight and wind; used for outdoor workers, athletes |
Significance for human survivability
A WBT of 35°C was long treated as the theoretical limit of human survivability, the point at which neither convection nor evaporation of sweat can cool the body, leading to fatal hyperthermia after sustained exposure. However, recent physiological research (Penn State PSU HEAT Project, 2022; studies in PNAS and Nature Communications, 2023) found that real-world tolerance limits are markedly lower, often around 31°C in hot-humid conditions and even lower in hot-dry conditions, because the 35°C figure assumes idealised conditions. The takeaway: no single universal WBT threshold applies everywhere, and danger sets in well below 35°C.
Current status in India
WBT is increasingly central to India's heat-risk planning. IMD studies note rising wet-bulb temperatures along coastal areas, and projections suggest a large share of India's population could experience WBT of 32°C or more by the end of this century. India's heat governance combines several tools:
- The India Meteorological Department (IMD) issues colour-coded, impact-based heatwave warnings. A heatwave is declared in the plains when the maximum temperature reaches 40°C (30°C in hills) with a departure of 4.5–6.4°C from normal, or whenever it touches 45°C; a severe heatwave is declared at a departure above 6.4°C.
- Heat Action Plans (HAPs), developed by NDMA with IMD and state and local authorities, guide preparedness and response across heatwave-prone states.
UPSC angle
For Prelims, focus on the definition, the WBT-versus-dry-bulb relationship, and distinctions among heat index, WBGT and WBT. For Mains (GS3), link WBT to disaster management, occupational health of outdoor and informal workers, urban heat islands, and climate adaptation through early-warning systems and Heat Action Plans.
