Why The Threshold Of A ‘Heat Advisory’ Depends On Where You Live

It is hot in Texas right now. We’re breaking electricity usage records to keep cool, squirrels are lying belly-down on the concrete, people are baking cookies in their cars. It’s the kind of heat that merits an official warning to the public. But what merits a ‘heat advisory?’

By Michael MarksJuly 24, 2018 12:24 pm| ,

You may have heard that term ‘heat advisory’ quite a bit during this latest heat wave. It’s a notice the National Weather Service sends out to tell people that they need to take precautions to stay safe in the heat – especially people who work outside.

In the Dallas Fort Worth Area, a heat index of 105 degrees triggers a heat advisory. And the reason it’s 105 degrees has to do with geography and, well, you.

Alex Lyster is a senior forecaster with the National Weather Service in Midland.

“Some places, you know, get hotter. So people are probably a little bit more acclimated to 105 degrees than other locations in the state, and that’s why their threshold is 110,” Lyster says.

When it comes to declaring a heat advisory, there’s a sliding scale in Texas – not a finish line. In Midland, Lyster and his team have issued heat advisories each of the past four days, since either the temperature or the heat index hit 105 degrees. But there are other parts of the state where that probably would not merit a heat advisory.

“And actually up toward the Red River near Wichita Falls and the eastern Texas Panhandle, that is also 110 degrees for the threshold for a heat advisory,” Lyster says.

This isn’t just guesswork, either. Craig Crandall is a professor of internal medicine at the University of Texas Southwestern Medical Center and the director of the thermal and vascular laboratory at Texas Health Presbyterian Hospital.

“The body has a very important capability of adapting to the heat,” Crandall says. “That’s very clear, lots of data. Even the high school football coaches know that. You never work the kids hard on the first day.”

Generally speaking, the more often and more intense our exposure to heat, the better we can acclimate to it. So since it tends to be more hot and humid in Houston than, say, the Davis Mountains, you’d expect Houstonians to handle the heat better than those west Texans. It makes sense then that the heat advisory threshold in the mountains is a heat index of 100 degrees, versus 108 in Houston. There is a condition for this to be true though: You actually have to go outside.

“But if the person in Houston spends all their time in the air conditioning, and the person in the Davis Mountains spends all their time out hiking, the person in the Davis Mountains is going to be more acclimated than the person in Houston,” Crandall says. “And the person in Houston is going to be at greater risk than the person in the Davis Mountains.”

In other words – although there may be theoretical differences in how Texans handle heat – air conditioning is the great equalizer.