Wet Bulb Calculator
A wet bulb temperature is the reading a thermometer gives when its bulb is wrapped in a wet cloth and air moves past it. The water on the cloth cools the bulb as it evaporates, so the reading sits below the air temperature unless the air is nearly saturated.
This page estimates that reading from the air temperature and the relative humidity.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
The estimate uses a published approximation by Roland Stull, from 2011. It takes the air temperature in Celsius and the relative humidity in percent, then returns the wet bulb temperature in the units you pick.
The approximation is meant for air from about 5 to 99 percent humidity and ordinary temperatures. At 100 percent humidity the wet bulb and the air temperature are the same.
Worked example
At 86 °F air temperature and 50% relative humidity, the wet bulb temperature comes out near 72.134 °F. Drier air pulls that reading further below the air temperature.
Questions
Why is the wet bulb lower than the air temperature?
Water on the cloth evaporates, and evaporation takes heat away from the bulb. The drier the air, the faster the water leaves, and the bigger the drop.
Is this the same as a heat index?
No. A heat index is a separate measure that combines temperature and humidity in its own way. A wet bulb reading comes from evaporation.
How accurate is the estimate?
It is an approximation, usually within a fraction of a degree across the ordinary range. A real wet bulb thermometer gives a direct reading to check it against.
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