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What's a wet bulb? Terms to know when dealing with extreme heat

With extreme heat getting more common and more likely, the way we measure and warn about heat is in the spotlight.
What's a wet bulb? Terms to know when dealing with extreme heat
Posted at 8:49 PM, Aug 25, 2023

This summer has been unusually hot for the U.S. and the world, setting records for the hottest day, week and month.

It's likely you felt the heat yourself — one analysis found 4 out of 5 people worldwide have already felt extreme heat driven by climate change this year.

On Friday more than 111 million people in the U.S. were under a heat advisory, watch or warning for some time.

With extreme heat becoming more common for so many of us, here's a guide to the tools and terms you may encounter in your forecasts.

Heat watches, advisories, and warnings

A heat watch warns that an extreme heat event may occur within the next 24 to 72 hours. At the time it's issued, forecasters may not yet have more details about exactly where it will occur, or how severe it might be.

Heat advisories warn that the heat index is expected to exceed 100 degrees Fahrenheit for at least two consecutive days, and that overnight temperatures won't get below 75 degrees.

A heat warning indicates that the heat index is expected to exceed 105 degrees for at least two consecutive days, and that overnight temperatures won't get below 75 degrees.

The National Weather Service sends these messages 12 hours before the heat conditions they deal with are forecast to occur. Watches and warnings may also be issued in places that don't meet the exact temperature criteria above, depending on other factors like whether a region is used to high temperatures.

SEE MORE: What's the best way to run your air conditioner in extreme heat?

The heat index

The heat index, sometimes called the apparent temperature, is a measure of how hot it feels in the shade. The National Weather Service calculates the heat index from the high temperature and the humidity, and charts them to show their relative risk.

Depending on the severity of the heat index, its value may call for caution when working or staying outside for long periods. Higher heat index values can present a danger or extreme danger to our health, up to and including heat stroke.

Wet bulb temperatures

The wet bulb temperature is an even more scientific measure of heat, which accounts even more specifically for the effects that heat and humidity have on human health.

It gets its name from how it may be measured: A wet cloth is placed over the bulb of a thermometer. The moisture evaporating from the cloth in direct sunlight mimics the cooling effect of sweating. The resulting thermometer reading is the minimum temperature to which a person can cool down using nothing but evaporative cooling based on the air temperature and humidity of that specific location.

Government groups and sporting organizations often use the wet bulb temperature to set guidelines for activities done outside. Depending on the wet bulb temperature, the time spent outside or exercising might be shortened, for example, or more water and shade breaks might be given.

Measurements like the heat index and wet bulb give us important guidance for staying healthy, especially as the frequency and likelihood of extreme heat events increase as a result of climate change.

The National Weather Service maintains a website specifically for heat, where it publishes daily conditions like the heat index and shares important messages for people who are under threat from extreme heat.

SEE MORE: NOAA, NASA agree that July 2023 was the hottest month on record

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