There’s a difference between a dry hot day and a humid hot day—humidity can vastly impact the way we experience heat and our body’s ability to cool down. A hot, humid day feels “muggy,” unpleasantly warm: You’re sweating, but that perspiration isn’t cooling you down. 

The term scientists use to describe heat combined with humidity is wet-bulb temperature, often called simply “wet-bulb,” and understanding wet-bulb is fundamental to building climate literacy in a changing world.

What is wet-bulb temperature? 

Wet-bulb temperature is a combined measure of heat and humidity—like a “real feel” estimation of heat—intended to represent how effectively sweating can cool a person down. 

Wet-bulb temperature is measured with a thermometer covered by a wet cloth which can cool the thermometer the same way sweating cools down the human body: As moisture evaporates, the surface beneath it cools. If the air is fully saturated (100% humidity, which is often experienced as fog), no water will evaporate from the cloth, and the thermometer will read the same temperature as the air.

In high temperatures, if the air is dry, the moisture in the wet cloth will evaporate, cooling the thermometer and causing it to register a temperature significantly lower than the air temperature. If the air is humid (already filled with moisture) the water in the cloth will evaporate very slowly, and the thermometer will read a temperature only slightly lower than the air temperature.

Wet-bulb temperature vs. dry-bulb temperature 

Whereas wet-bulb temperature accounts for both heat and humidity, dry-bulb temperature only accounts for heat. It’s measured with an uncovered, dry thermometer. Dry-bulb temperature is the measure most people use when they refer to the temperature of the air: “It’s 75°F outside.”

Wet-bulb vs. wet-bulb globe temperature

Wet-bulb temperature only takes into account heat and humidity and is measured in the shade. Wet-bulb globe temperature combines that heat and humidity measure with sunlight and wind, to create a fuller picture of the conditions a person would experience. People can use this information to make decisions about how to conduct activities, such as outdoor labor or play, since the temperature in shady or windy spots could be cooler.

Why does wet-bulb temperature matter?

Wet-bulb temperature tells us more about how our bodies will experience heat than air temperature alone, which in turn helps us make decisions and plan our daily lives.

In dry heat, mechanisms like sweating can provide relief and cool the body down. Sweating works to cool the body by evaporating off the skin, releasing some of the excess heat from the blood right beneath the skin, and letting that blood circulate again at a lower temperature. In humid heat, the air is already full of moisture and sweat cannot evaporate at the same rate. 

Without the body’s self-cooling function working, our body temperature rises, like when we have a fever. If our bodies overheat, our organs become stressed, and ultimately begin to fail. High enough wet-bulb temperatures can completely restrict our ability to be anywhere that isn’t climate-controlled—scientists estimate that humans cannot survive in wet-bulb temperatures above 35°C for more than a few hours. There is some evidence that the survivability threshold may be even lower, around 30-31°C wet-bulb. Often, being indoors without air conditioning in high heat can be even more dangerous than being outside, since there is no air circulation to cool down the body.

Wet-bulb temperature and climate change

If the idea of temperatures too hot and humid to survive seems strange, like something that would happen on another planet, that’s because extremes like uninhabitable wet-bulb temperatures are unprecedented in human history. For the last twelve thousand years, humans have lived in a very stable, hospitable climate, where almost nowhere on Earth was too hot or too cold for us. 

But, warmer air holds more moisture than cool air. As greenhouse gas emissions warm the atmosphere, dangerous combinations of heat and humidity are becoming more likely. Using projections of future temperatures and humidity levels from climate models, scientists can anticipate higher wet-bulb temperatures occurring more frequently, and in places that never used to experience that kind of heat.  

Want to see how wet-bulb temperatures could change in places you care about? Read Heat in our climate handbook to learn about the future of high temperatures.