Wet-Bulb Temperature Calculator

Wet-bulb temperature is the lowest temperature a thermometer with a wet wick can reach by evaporative cooling under the current air conditions. It is always at or below the air (dry-bulb) temperature and matches it only when the air is fully saturated at 100 percent humidity. Because sweat cools the body the same way the wick cools the thermometer, wet-bulb is a better gauge of heat-stress danger than air temperature alone: when wet-bulb is high, sweat does not evaporate, and the body cannot shed metabolic heat.

Roland Stull's 2011 formula (Journal of Applied Meteorology and Climatology) reproduces full psychrometric-chart wet-bulb values to within about 0.3 deg C for dry-bulb temperatures from -20 C to 50 C and relative humidities from 5 to 99 percent. It is the standard analytic shortcut for back-of-the-envelope heat-stress work and is what this calculator uses. Outside that envelope (very dry, very cold, or extremely humid air), use a psychrometric chart or a sling psychrometer for higher accuracy.

No. A 35 C wet-bulb value is a theoretical upper physiological limit under idealized assumptions, not a safety threshold. Dangerous heat illness can occur at much lower values depending on exertion, sun, wind, clothing, health, acclimatization, and exposure time. Follow public-health heat warnings rather than waiting for this number.

Wet-bulb is one measurement, taken with a wet thermometer in still or moving air. WBGT (Wet-Bulb Globe Temperature) is a composite heat-stress index that adds a dry-bulb reading and a black-globe radiant temperature. For outdoor work in direct sun, the standard formula is WBGT = 0.7 * Tw + 0.2 * Tg + 0.1 * T. This calculator shows a simplified shade-only WBGT (0.7 * Tw + 0.3 * T), which is conservative for indoor or shaded environments but underestimates risk in full sun. OSHA, NIOSH, and the military all use full WBGT for work-rest cycle planning.

Dew point is the temperature at which the air would have to cool, at constant pressure and moisture content, before water vapor condenses into liquid. Wet-bulb is the temperature an evaporating surface reaches under current conditions. Dew point depends only on absolute humidity, while wet-bulb also accounts for how much evaporative cooling is still available. Dew point is always at or below wet-bulb, and the two converge as the air approaches saturation. In practical terms, dew point tells you how sticky outdoor air feels; wet-bulb tells you how hard it is for your body to cool itself.