Wet-bulb temperature describes how far air can be cooled by water evaporation under particular conditions. It helps explain the combination of heat and moisture, but it is neither the ordinary air temperature nor a direct forecast of how every person will feel.
How it is measured
The classic method uses two ventilated thermometers. The dry-bulb thermometer measures air temperature. The second has its bulb wrapped in a wet wick. Evaporation removes heat and lowers its reading.
In dry air, evaporation is efficient and the difference between the readings is larger. In very humid air, less water evaporates and the values move closer together. At 100% relative humidity, under ideal measuring conditions, they are equal.
Modern stations and models can calculate wet-bulb temperature from temperature, humidity and pressure. Simple online calculators are approximations whose result depends on the formula; they do not replace a properly ventilated meteorological measurement.
What the value tells you
A wet-bulb reading well below air temperature indicates substantial potential for evaporative cooling. A reading close to dry-bulb temperature indicates humid air in which evaporation is less effective.
Two places may have the same air temperature but different wet-bulb values. The more humid location will generally have the higher value because water evaporates less readily.
Not the same as dew point
Dew point is the temperature to which air must cool to become saturated while essentially retaining its water-vapour content. Wet-bulb temperature results from evaporative cooling and the exchange of heat and moisture. Both relate to atmospheric water vapour, but they represent different processes.
Not the same as “feels like” temperature
Heat index attempts to estimate the combined effect of temperature and humidity on people under specified assumptions. Wet-bulb temperature is a physical property related to air and evaporation. The two values are not interchangeable scales.
Wet bulb and WBGT
Wet Bulb Globe Temperature (WBGT) is not simply wet-bulb temperature. The National Weather Service explains that WBGT incorporates temperature, humidity, wind, solar radiation and other weather parameters, making it useful for decisions about outdoor work and exercise.
Do not label a published WBGT number as “wet-bulb temperature”. Check the variable name, unit, sun or shade assumptions, and the guidance issued with it.
Reading it in an app or map
- Check time and location: the value can change quickly and over short distances.
- Compare air temperature: the relationship between both readings is more informative than wet bulb alone.
- Add wind and sunshine: ventilation and direct radiation affect real heat load but may not fit into one number.
- Consider duration: several hot, humid hours and warm nights matter more than a brief peak.
- Follow official alerts: never replace local advice with an isolated threshold found online.
When heat and humidity combine
Move strenuous tasks to cooler hours, reduce intensity, use shade or cooler spaces, take breaks and maintain appropriate hydration. NWS guidance also recommends gradual heat acclimatisation and work/rest schedules.
No single value determines personal risk by itself. Sunshine, wind, duration, activity, clothing, acclimatisation and health all matter. During serious symptoms or an emergency alert, follow local health and civil-protection advice.
In short
Wet-bulb temperature shows how much evaporative cooling the environment permits. It is useful for understanding humid heat, but it belongs alongside other variables and must not be confused with dew point, heat index or WBGT.