What is a ‘wet bulb temperature’ and why is it so important this week?

(NEXSTAR) – While eye-popping triple-digit heat forecasts may capture headlines, you might want to start paying attention to the “wet bulb” temperature, especially if you work outside.

The name comes from the thermometer covered in a wet cloth that is used to take the temperature reading, according to the National Weather Service. The measurement is designed to effectively mimic how the body tries to cool itself with sweat.

Unlike the heat index, which tells you how the human body would feel under shade, the wet bulb temperature indicates the expected stress on the human body when it is in direct sunlight by measuring the effect of temperature, humidity, wind speed and solar radiation. The NWS uses a black globe and dry bulb to detect solar radiation and temperature data.

When it’s hot, humans sweat to cool off, but if the humidity is also high enough there’s a point at which sweat loses its cooling effect.

The upper limit that humans could withstand was thought to be 95 F at 100% humidity, according to a 2010 study. New research out of Penn State University’s Noll Laboratory found that the critical limit is in fact even lower – 88 F at 100% humidity.

You can find your wet bulb temperature using this NOAA map.

Wet bulb readings reach extreme levels this week

On Tuesday, parts of Texas and northern Oklahoma, as well as nearly the entire state of Arkansas and eastern Michigan, had the highest NWS wet bulb reading – extreme.

By Wednesday afternoon, the NWS showed extreme wet bulb temperatures only across the southern tip of California and in southwest Arizona. Large swathes of “high” readings remained across Texas, Arkansas, Oklahoma, Louisiana, New York, Connecticut and Massachusetts.

The NWS says the tool, which it classifies as experimental, “is most useful for active, acclimatized people such as outdoor workers, athletes, and anyone else performing strenuous outdoor activities — and has been used for decades by military agencies, OSHA, and marathon organizers.”

The weather service warns that wet bulb readings can vary geographically and cites OSHA’s recommendations for outdoor workers:

• Acclimatize workers starting the first day working in the heat and after any extended absences
• Provide shade for outdoor work sites
• Schedule work earlier or later in the day
• Use work/rest schedules
• Limit strenuous work (e.g., carrying heavy loads)