# Wet Bulb Calculator

Created by Julia Żuławińska
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Daniel J. Vecellio, S. Tony Wolf, Rachel M. Cottle, and W. Larry Kenney Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project) Journal of Applied Physiology (28 Jan 2022)
Last updated: Aug 05, 2022

With many countries reported their highest ever temperatures, such as reaching 49.6 C, it's important that you understand the different factors affecting heat and the body's ability to regulate it. What many people might fail to take into account is the combined result of the heat and humidity - the wet bulb effect. This tool will teach you about the wet bulb, and lets you estimate it's scale.

The wet bulb calculator operates on a simple principle. You can use it to work out the wet bulb temperature from just two numbers: temperature and relative humidity. Keep reading if you want to discover some wet-bulb applications, what the military benefits of the Wet-Bulb Globe Temperature are, and what it has to do with our health - especially within the context of our scorching summers! If you want to find out more about our atmosphere, check out our heat index calculator and our air density calculator.

## What is the wet-bulb temperature?

Despite what you might think at first, wet-bulb temperature has nothing to do with light bulbs. It is instead the temperature read by a special thermometer that is wrapped in water-soaked fabric and ventilated. This thermometer is part of a device called a psychrometer. It includes a dry-bulb thermometer, a wet-bulb thermometer and a psychrometric chart - a graph that plots the relationships between the dry and wet-bulb temperature, relative humidity, and dew point at constant pressure.

By definition, wet-bulb temperature is the lowest temperature a portion of air can acquire by evaporative cooling only. When air is at its maximum (100 %) humidity, the wet-bulb temperature is equal to the normal air temperature (dry-bulb temperature). As the humidity decreases, the wet-bulb temperature becomes lower than the normal air temperature.

Data about the wet-bulb temperature is essential when it comes to preventing our body from overheating. Our bodies sweat to cool off, but, because water evaporates slower in more humid conditions, we cool down a lot slower in humid conditions. This causes our internal body temperature to rise. If the wet-bulb temperature exceeds 35 °C (95 °F) for an extended period of time then people in the surrounding area are at risk of hyperthermia.

## How to calculate the wet-bulb temperature?

Although many equations have been created over the years our calculator uses the Stull formula, which is accurate for relative humidities between 5% and 99% and temperatures between -20°C and 50°C. It loses its accuracy in situations where both moisture and heat are low in value, but even then the error range is only between -1°C to +0.65°C.

The wet-bulb calculator is based on the following formula:

$\footnotesize T_w = T\arctan(0.151977(RH\% + 8.313659)^{1/2}) \\\ + \arctan(T + RH\%) - \arctan(RH\% - 1.676331) \\\ + 0.00391838(RH\%)^{3/2}\arctan(0.023101RH\%) \\\ - 4.686035$

It might look intimidating, but don't worry - we do all the calculations for you. Just input two numbers:

• $\footnotesize T$ Temperature - air temperature or dry-bulb temperature is the temperature given by a thermometer not exposed to direct sunlight.

• $\footnotesize RH$% Relative humidity - a ratio of how much water vapor is in the air to how much it could contain at a given temperature.

Remember that both temperature and wet bulb temperature in this formula are expressed in °C! If you would like to use other units, you need to convert them to the Celcius scale before you start calculations.

If you would like to know more about the relative humidity formula, check out our dew point calculator. You can also use our RH calculator to calculate the relative humidity if you know the dew point temperature. Alternatively, you can also find it from the mixing ratio of air. Feel free to check it out!

## Wet-bulb calculator applications

The wet-bulb temperature might not be a widely known measure, but it has some valuable functions:

• Construction - different materials react differently to different humidities, so this temperature is needed when designing a building in different climates.

• Snowmaking - snow production needs low temperatures and when the humidity decreases the temperature rises.

• Meteorology - forecasters use wet-bulb temperature to predict rain, snow, or freezing rain.

## Wet-Bulb Globe Temperature

Wet-Bulb Globe Temperature is a kind of an apparent temperature - the temperature perceived by humans - used to estimate the effect of temperature, humidity, wind speed, and sunlight on humans. Athletes, industrial hygienists and the military use it to prevent heat stroke by following guidelines for physical activity and water intake.

Wet-Bulb Globe Temperature is determined by the following equation:

$\footnotesize WBGT = 0.7 T_w + 0.2 T_g + 0.1 T$

where $\footnotesize T_g$ - the globe thermometer temperature - is measured by a thermometer situated in a black globe. It allows for the estimation of direct solar radiation.

Measured indoors, without direct solar radiation/sunlight, the Wet-Bulb Globe Temperature uses a shortened formula:

$\footnotesize WBGT = 0.7 T_w + 0.3 T$

Wet-Bulb Globe Temperature gives us vital information if we want to be safe on warmer days.

Julia Żuławińska
This wet bulb calculator is designed to help you estimate the wet bulb temperature based on only two pieces of data - the temperature 🌡️ and the relative humidity 💧.
Temperature
°F
Relative humidity
%
Wet-bulb temperature
°F
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