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What Is “Wet Bulb Temperature”?

If you’ve ever said “it’s not the heat, it’s the humidity,” you already understand what goes into calculating wet bulb temperature.

Nick Keppler|
If you’re only measuring air temp, you’re only getting half the picture.
If you’re only measuring air temp, you’re only getting half the picture. | the_burtons/Moment/Getty Images

This year is on track to become Earth’s hottest on record, surpassing the previous record holder 2023. As the world grapples with its failure to act on climate change, temperatures are projected to continue to climb. In the U.S., the trend has coincided with a 43 percent increase in recorded heat-related deaths, from 1602 in 2021 to 2302 in 2023.

People aren’t used to these off-the-chart temps or the dangers of extreme heat. One way in which people commonly underestimate risk from heat is relying on a temperature readings that do not take into account humidity, which can negate the body’s primary cooling defense: sweat.

The average TV meteorologist or weather app reports dry-bulb temperature, a measurement of ambient outdoor air temp—the kind of measurement you get from a regular thermometer placed outdoors. A more accurate reading of what it actually feels like is wet-bulb temperature, which is a combination of air temperature and humidity, the saturation of water in air.

Wet bulb is assessed by covering a thermometer with a wet cloth or muslin. The drier the air, the quicker it will absorb moisture and dry the cloth; the more humid the air is, the slower it will dry the cloth. This action mirrors the effect of sweat evaporating on the body, and the evaporation has a cooling effect that gets measured by the difference between the dry-bulb and wet-bulb temperatures. (The dry-bulb temp is always lower than the wet-bulb temp unless humidity is 100 percent; then both readings will be the same.) 

A high wet-bulb temperature can indicate heat danger in a more holistic way, because it indicates how slowly sweat will cool your skin. Higher wet-bulb readings mean your body will have to work harder to cool itself, potentially leading to heat exhaustion or heat stroke. One study found that a wet-bulb temperature of 95°F could cause hyperthermia (overheating) within six hours. Climate change is predicted to increase humidity along with other weather hazards. Another hazard of humidity is that it blocks the natural dispersion of air pollution over an area, concentrating the heat and unhealthy air. 

If you want to keep track of the humidity risks through a wet-bulb temperature, there is an easy fix to a basic air thermometer [PDF]: Cover the bulb (the part that gathers temperature data) with a light wet cloth, or pick up a ready-made digital humidity meter for as little as $10. 

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