(NEXSTAR) — Readings of a key indicator of heat exposure danger have reached the highest possible threat level in multiple southern states, as well as in a few states in the Midwest.

When it comes to the “wet bulb temperature,” nearly all of Louisiana, Mississippi, and Arkansas are under “extreme threat.” The wet bulb temperature measures multiple stressors on the human body in direct sunlight, not in the shade, as the heat index does.

Thursday’s soaring temperatures marked yet another day under a deadly, oppressive heatwave across the South. A heat dome over Texas that shows no signs of abating has been tied to 13 deaths in the state, including another in Louisiana, according to The Associated Press.

Forecasters said temperatures could rocket up to 20 degrees above average in parts of the region as a heat dome that has taxed the Texas power grid spread eastward. Officials urged people to stay hydrated, shelter in air-conditioned rooms and check on each other as humidity combined with high temperatures to make conditions even more dangerous.

The National Weather Service issued an excessive heat warning for parts of Arkansas, Louisiana, Mississippi, and Tennessee for Thursday and Friday. The heat index, which indicates how hot it feels outdoors based on the temperature and relative humidity, was expected to reach 115 degrees (46 Celsius) in several cities. The heat dome was expected to be centered over the mid-South by the weekend.

What is the ‘web bulb temperature’?

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 (NWS). 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.

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)

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

The Associated Press contributed to this report.