Wet Bulb Calculator

What the wet bulb calculator does

The wet-bulb temperature is the lowest temperature that evaporation can cool something down to in the current air. This calculator finds it from the air temperature and the relative humidity, and in advanced mode also reports the wet-bulb depression and the wet-bulb globe temperature used in heat-safety work.

Below is what wet-bulb temperature means, how evaporation produces it, the equation behind it, and why it matters for heat stress.

How to use it

1. Enter the air temperature and the relative humidity.
2. For heat-safety figures, switch to advanced mode and add the globe thermometer temperature, which captures radiant heat from the sun.
3. Press Calculate for the wet-bulb temperature and, in advanced mode, the related heat-stress measures, or Reset to clear it.

What wet-bulb temperature is

The wet-bulb temperature is what a thermometer reads when its bulb is wrapped in a wet cloth and air is blown across it. As water evaporates from the cloth, it carries away heat and cools the thermometer below the ordinary air temperature. The reading settles at the wet-bulb temperature, the coolest that evaporation can achieve under those conditions.

How far below the air temperature it falls depends entirely on the humidity. In dry air, water evaporates readily and the cooling is strong, so the wet-bulb temperature sits well below the air temperature. In humid air, the air is already nearly full of moisture, evaporation is sluggish, and the wet-bulb temperature stays close to the air temperature. When the air is fully saturated at 100 percent humidity, no evaporation is possible and the wet-bulb temperature equals the air temperature. This calculator works out where that balance lands.

Cooling by evaporation

The wet-bulb temperature is really a measure of evaporative cooling, the same effect that cools you when you sweat. Turning liquid water into vapour takes energy, and that energy is drawn as heat from whatever the water is sitting on, leaving it cooler. This is why stepping out of a swimming pool feels cold, why a breeze on damp skin chills you, and why a wet cloth on the forehead cools a fever.

Because the effect depends on water actually evaporating, it is throttled by how much moisture the air can still accept. Dry air is thirsty and pulls water off quickly, giving strong cooling. Humid air is nearly full and accepts little more, giving weak cooling. The wet-bulb temperature captures exactly how much cooling power the air has left, which is what makes it so useful for understanding both comfort and danger in hot weather.

The equation it uses

The calculator uses Stull's formula, an empirical relationship published in 2011 that gives the wet-bulb temperature directly from the air temperature and relative humidity without the repeated trial-and-error that older methods needed:

T_w = T · atan[0.151977 · √(RH + 8.313659)] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 · RH^1.5 · atan(0.023101 · RH) − 4.686035

Here T is the air temperature in Celsius, RH is the relative humidity in percent, and the result is the wet-bulb temperature. The formula looks forbidding, but it was carefully fitted to match the true psychrometric behaviour of moist air and is accurate to within a fraction of a degree across normal conditions, which is why so many tools rely on it.

Wet bulb and the limits of human heat tolerance

The wet-bulb temperature carries a sobering significance for human survival, because our bodies cool themselves by sweating, which is evaporative cooling. As long as the wet-bulb temperature stays below body temperature, sweat can evaporate and shed heat. But as the wet-bulb temperature climbs, that escape route narrows, and there comes a point where the body can no longer cool itself at all, no matter how much it sweats or how much shade and water are available.

Researchers put that theoretical limit at a wet-bulb temperature of around 35 degrees Celsius. At that point the air is so hot and so saturated that evaporation from the skin stops working, and even a healthy person at rest can overheat dangerously within hours. This is very different from ordinary air temperature: 35 degrees of wet-bulb requires both high heat and high humidity together. It is why humid heat waves are far more deadly than dry ones at the same thermometer reading, and why the wet-bulb temperature has become a key measure in studying the habitability of a warming world. This calculator lets you see how close a given combination of heat and humidity comes to that threshold.

Wet-bulb globe temperature

In advanced mode the calculator also reports the wet-bulb globe temperature, a composite heat-stress index used by the military, athletic organisations, and workplaces to decide when conditions become unsafe for exertion. It blends together the cooling power of the air, the plain air temperature, and, outdoors, the radiant heat of the sun captured by a globe thermometer.

The indoor version weights the wet-bulb temperature most heavily alongside the air temperature, while the outdoor version adds the globe temperature to account for direct sunshine beating down. Because it folds all these together into a single number, the wet-bulb globe temperature gives a fuller picture of heat stress than any one reading alone, which is why it underpins official guidelines on when to limit activity, mandate rest breaks, or call off events in the heat.

Working range and precision

The calculator works where Stull's formula is reliable, for temperatures between −20 and 50 degrees Celsius and relative humidity between 5 and 99 percent, which covers the conditions that matter for heat stress, and it accepts temperatures in Celsius, Fahrenheit, or kelvin. The formula is an accurate empirical fit, with errors of well under a degree across its range. Results carry several significant figures.

A worked example

Suppose the air temperature is 30 degrees Celsius and the relative humidity is 50 percent.

Stull's formula gives a wet-bulb temperature of about 22.3 degrees Celsius, nearly 8 degrees below the air temperature, showing that this moderately humid air still has plenty of evaporative cooling power. The indoor wet-bulb globe temperature works out to about 24.6 degrees. Both sit comfortably below the danger thresholds. Push the humidity much higher at the same heat, though, and the wet-bulb temperature would climb toward the air temperature as the cooling margin shrank.

Questions people ask

What is wet-bulb temperature?

The lowest temperature that evaporation can cool something to in the current air. It is what a wet, ventilated thermometer reads, and it depends on both the air temperature and the humidity.

Why does it depend on humidity?

Because it measures evaporative cooling, which only works when the air can accept more moisture. Dry air gives strong cooling and a low wet-bulb temperature; humid air gives weak cooling and a wet-bulb temperature near the air temperature.

Why is a wet-bulb temperature of 35 degrees dangerous?

Because the body cools itself by sweating, which is evaporative cooling. At around 35 degrees wet-bulb, evaporation from the skin can no longer shed heat, so even a healthy person at rest can overheat. It marks a theoretical limit of human heat tolerance.

What is wet-bulb globe temperature?

A heat-stress index combining the wet-bulb temperature, the air temperature, and outdoors the radiant heat of the sun. It is used to set safe limits for exercise and work in hot conditions.

References

A quick note on where the science comes from. The wet-bulb temperature formula is from Roland Stull's 2011 paper in the Journal of Applied Meteorology and Climatology, and the concept and its heat-stress significance are described in the Wikipedia article on wet-bulb temperature. The wet-bulb globe temperature index is documented by the US National Weather Service.

1. Stull, R. (2011), Wet-Bulb Temperature from Relative Humidity and Air Temperature, Journal of Applied Meteorology and Climatology, 50, 2267-2269. https://journals.ametsoc.org/view/journals/apme/50/11/jamc-d-11-0143.1.xml
2. Wikipedia, Wet-bulb temperature. https://en.wikipedia.org/wiki/Wet-bulb_temperature
3. National Weather Service (NOAA), Wet Bulb Globe Temperature. https://www.weather.gov/tsa/wbgt