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Wet Bulb Calculator: Wet-Bulb Temperature from Humidity

Enter the air temperature and relative humidity to get the wet-bulb temperature: how far evaporating water can cool the air.

Temperature unit

Wet-bulb temperature

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Wet-bulb depression (air minus wet bulb)
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Dew point (for comparison)
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Stull (2011): Tw = T·atan[0.151977·(RH + 8.313659)½] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH3/2·atan(0.023101·RH) − 4.686035, with T in °C, RH in %, atan in radians. Sea-level pressure; valid for 5–99% RH and −20 to 50 °C (−4 to 122 °F) except where low humidity and cold combine; error −1 to +0.65 °C.

The wet-bulb temperature is the lowest temperature you can cool air to by evaporating water into it. It’s what a thermometer reads when its bulb is wrapped in a wet wick and air blows across it. In dry air the water evaporates fast and the reading drops well below the air temperature; in saturated air nothing evaporates and the two readings are the same. Enter the air temperature and relative humidity above to get it in °F or °C.

How the calculator works

Working out the wet-bulb temperature exactly takes an iterative calculation. Roland Stull (University of British Columbia) published a single equation instead, found as an empirical fit, in the Journal of Applied Meteorology and Climatology (2011):

Tw = T·atan[0.151977·(RH + 8.313659)^½] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH^(3/2)·atan(0.023101·RH) − 4.686035

with T the air temperature in °C, RH the relative humidity in percent (65.8 for 65.8%) and the arctangent in radians. °F inputs are converted to °C first. The calculator also shows the wet-bulb depression (air temperature minus wet-bulb temperature) and the dew point, from the same Magnus formula as our dew point calculator, so you can see where the wet bulb sits between the two.

Valid range and error

Stull’s paper states the equation is valid at standard sea-level pressure for:

  • relative humidity between 5% and 99%,
  • air temperatures between −20 °C and 50 °C (−4 °F to 122 °F),
  • except where low humidity and cold temperature combine.

Over that range the errors run from −1 °C to +0.65 °C (−1.8 °F to +1.2 °F), with a mean absolute error under 0.3 °C (0.5 °F). The calculator warns you when your inputs fall outside it. At high altitude, where pressure is lower, the true wet-bulb temperature is a little lower than this sea-level estimate.

Worked examples

The first row is Stull’s own textbook example. The last column comes from the National Weather Service’s iterative psychrometric calculator (NWS El Paso) at sea-level pressure, 1013.25 hPa:

Air temperatureRelative humidityWet bulb (Stull)NWS psychrometric
20 °C (68 °F)50%13.7 °C (56.7 °F)13.9 °C (57.0 °F)
70 °F40%55.8 °F (13.2 °C)56.0 °F
75 °F50%62.7 °F (17.0 °C)62.7 °F
90 °F50%75.6 °F (24.2 °C)75.1 °F
95 °F60%83.3 °F (28.5 °C)82.9 °F
100 °F20%70.1 °F (21.2 °C)69.6 °F

All the differences are within Stull’s stated error band. Look at the last row: very hot, dry air at 100 °F has a wet bulb of about 70 °F, a 30 °F wet-bulb depression. At 90 °F and 50% the depression is under 15 °F.

What wet-bulb temperature means at home

Evaporative humidifiers and coolers

An evaporative humidifier blows room air through a wet wick, the same thing that happens on a wet-bulb thermometer. As the water evaporates, the air passing through the wick picks up moisture and loses a little heat, and evaporation alone can’t cool it below its wet-bulb temperature.

Evaporative (“swamp”) coolers rely on the same physics: the bigger the gap between air temperature and wet-bulb temperature, the more they can cool. In humid weather the gap shrinks and so does the cooling.

Comfort and heat

Your skin cools itself the same way, by evaporating sweat. When the wet-bulb temperature climbs close to the air temperature, evaporation slows and heat feels harder to shed. For the National Weather Service’s feels-like number, use the heat index calculator. The NWS’s wet bulb globe temperature (WBGT), used for outdoor work and sports, is a different measure that adds wind and sunshine.

Measuring humidity with a psychrometer

A psychrometer measures humidity with two thermometers, one dry and one with a wet wick; the sling version is whirled through the air. The difference between the two readings, the wet-bulb depression, gives the relative humidity. A digital hygrometer is simpler for home use; see how to check humidity in your house.

Frequently asked questions

What is wet-bulb temperature?

It is the temperature a thermometer reads when its bulb is wrapped in a wet wick and air blows past it. Water evaporating from the wick cools it until it balances with the air. The drier the air, the more it cools.

How do you calculate wet-bulb temperature from relative humidity?

This calculator uses Stull's (2011) formula: Tw = T·atan[0.151977·(RH + 8.313659)^½] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH^(3/2)·atan(0.023101·RH) − 4.686035, with T in °C, RH in percent and atan in radians.

What is the wet-bulb temperature at 90 °F and 50% humidity?

About 75.6 °F (24.2 °C) with the Stull formula. The NWS psychrometric calculator gives 75.1 °F at sea-level pressure.

How accurate is the Stull formula?

Stull reports errors from −1 °C to +0.65 °C, with a mean absolute error under 0.3 °C, for 5% to 99% relative humidity and −20 °C to 50 °C at sea-level pressure. It is less reliable where low humidity and cold temperatures combine.

Is wet-bulb temperature the same as the heat index or WBGT?

No. The heat index is a feels-like temperature from temperature and humidity. The wet bulb globe temperature (WBGT) also takes wind and sunshine into account. Wet-bulb temperature is a physical measurement of evaporative cooling.

Is the wet-bulb temperature always between the dew point and the air temperature?

Yes. It equals the air temperature at 100% humidity and drops toward the dew point as the air gets drier, but it never goes below the dew point.

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