Wet-Bulb Globe Temperature (WBGT)

Wet-Bulb Globe Temperature is a composite index of environmental heat stress that combines four separate physical measurements, ambient (dry-bulb) air temperature, humidity (via a naturally wetted wet-bulb thermometer, which reflects the cooling effect of evaporation), wind speed, and radiant heat from direct sunlight (via a black globe thermometer), into a single figure that more accurately represents the physiological heat load on a human body than air temperature alone. The index matters because the human body's primary cooling mechanism, evaporation of sweat, becomes progressively less effective as ambient humidity rises: a humid environment at a lower air temperature can impose a greater physiological heat burden, and therefore a higher mortality risk, than a hotter but drier environment, because sweat cannot evaporate efficiently into already-saturated air. India's official heatwave criteria, set by the India Meteorological Department, are currently based on dry-bulb temperature departures from the seasonal normal, a design that can under-detect dangerous humid-heat conditions, particularly in coastal regions, and over-detect danger in dry inland regions where the same dry-bulb reading is physiologically less severe. Qatar has adopted WBGT as the trigger for its outdoor-work stoppage rules under Ministerial Decision 17 of 2021: all work must halt, at any time of year, wherever WBGT exceeds 32.1 degrees, regardless of the calendar date, in contrast to most other Gulf states' fixed calendar-and-clock-based summer work bans (for example, a ban from 10:00 to 15:30 between 1 June and 15 September), which can miss dangerous heat occurring outside those hours or months. WBGT-based, risk-triggered thresholds are increasingly proposed as the technically superior standard for declaring heat emergencies and for occupational-safety regulation generally.

• 1 WBGT combines air temperature, humidity, wind speed and radiant (solar) heat into one composite heat-stress index.
• 2 Physiologically superior to dry-bulb air temperature alone, since it captures how effectively sweat can evaporate (the body's primary cooling mechanism).
• 3 Humid heat at a lower air temperature can be more dangerous than dry heat at a higher temperature, because evaporative cooling fails as humidity rises.
• 4 India's IMD heatwave criteria are currently based on dry-bulb temperature departures from normal, which can under-detect humid coastal heat risk.
• 5 Qatar uses a risk-based WBGT threshold (32.1°C, Ministerial Decision 17 of 2021) triggering an immediate, any-time-of-year work stoppage, unlike calendar/clock-based bans elsewhere in the Gulf.
• 6 Calendar-based heat bans can miss dangerous heat occurring outside their fixed hours or months, a documented gap Human Rights Watch flagged in 2026 Gulf reporting.
• 7 Proposed as the technically superior basis for declaring heatwave emergencies and triggering disaster-relief entitlements in India.

India's addition of heatwave to its notified-calamity list in 2026 was assessed as necessary but not sufficient, since the underlying IMD dry-bulb declaration threshold could still fail to trigger during a humid coastal heat emergency that a WBGT-based threshold would correctly flag.