What wet-bulb temperature actually measures
Evaporation links moisture to heat stress
Wet-bulb temperature is the lowest temperature that air can reach through evaporation alone. A traditional instrument wraps a thermometer bulb in wet cloth and moves air across it. Evaporation removes heat, so the wet bulb reads below the ordinary “dry-bulb” air temperature unless the air is already saturated.
The gap between the two readings is the useful part. A large gap means dry air and strong evaporative cooling; a small gap means humid air with little capacity to accept more water vapor. At 100% relative humidity, no net evaporation is possible and wet-bulb temperature equals air temperature. This is why a hot, humid afternoon can strain the body more than a hotter but drier day. Our dew point and humidity guide explains the moisture side of that relationship.
Wet bulb, heat index and WBGT are not interchangeable
| Measure | Main inputs | Represents | Best use |
|---|---|---|---|
| Wet-bulb temperature | Air temperature, moisture, airflow | Evaporative cooling limit | Understanding humidity and physiological cooling |
| Heat index | Air temperature, relative humidity | How heat feels in shade | Public forecasts and everyday decisions |
| WBGT | Wet bulb, dry bulb, black globe | Heat stress including sun and wind | Work, sport and military activity |
Why WBGT is the number used for work and sport
Wet-bulb globe temperature (WBGT) is a separate index, despite the similar name. Outdoor WBGT combines a natural wet-bulb sensor, a shaded dry-bulb sensor and a black-globe sensor that responds to solar radiation. It therefore accounts for humidity, wind, sunshine and radiant heat around a person.
The heat index assumes shade and light wind; WBGT is designed for activity in the actual environment. A coach, worksite or race may use locally defined WBGT action levels for extra water, longer recovery, reduced equipment or cancellation. Those limits differ by climate, workload, clothing and acclimatization, so a single universal cancellation number is misleading.
What a high wet-bulb reading means for the body
Sweat cools only when it evaporates. As wet-bulb temperature rises, each gram of sweat removes less heat before dripping away, while the body continues producing heat through metabolism. Risk therefore depends on more than one reading: duration, direct sun, exertion, age, health, clothing, hydration and access to cooling all matter.
Do not use the much-discussed 35°C (95°F) wet-bulb value as a personal “safe until” line. Heat illness can occur far below it, especially during activity. Confusion, fainting, seizures or hot skin with altered behavior can signal heat stroke and require emergency help. For routine planning, follow local alerts, use WBGT where available and compare the forecast with the site’s heat index calculator.
Can you estimate wet bulb from a weather report?
A psychrometric calculation can estimate wet-bulb temperature from air temperature, humidity and pressure, but shortcuts lose accuracy near extremes. A weather app’s temperature and humidity may also come from a station miles away, while sunshine and wind at your location alter real exposure.
For casual comparison, remember that wet bulb must lie between the dew point and the air temperature. For safety-critical work or athletics, use a properly positioned WBGT instrument or an official local WBGT forecast rather than a damp household thermometer. Check readings in the location where activity happens, not beside an air-conditioned wall or reflective surface.
Use wet bulb with activity, sun and local guidance
A wet-bulb value describes evaporative cooling under the measurement conditions. It does not include every source of heat around a person. Direct solar radiation, radiant heat from equipment, clothing and metabolic heat from work can make exposure more demanding than a simple psychrometric estimate suggests.
That is why work and sport programs often use WBGT rather than wet-bulb temperature alone. The black-globe part responds to radiant heat, while the program can connect the reading with workload, clothing and rest schedules. Use the locally specified procedure because action levels can depend on acclimatization and the activity.
For public weather planning, wet bulb is most useful as an explanation of the evaporative limit. Follow official heat alerts, reduce exertion, find effective cooling and act on symptoms well before any theoretical survivability threshold. A calculated value is not a personal medical boundary.