Heat continues to build across the Deep South and Texas Gulf Coast, where the hot temperatures have combined with high humidity. (SSEC/RealEarth/University of Wisconsin at Madison)

This story has been updated.

The heat index hit 100 degrees at 6 a.m. Thursday in Galveston, Tex. And, it didn’t drop below that until 10 p.m. Friday night. It’s part of a larger heat wave across the Deep South and Southern Plains that shows no signs of letting up through at least next week.

Heat advisories stretched over 1,000 miles from the U.S.-Mexico border to Georgia, encompassing more than 30 million in the forecast for “dangerous heat.”

Galveston failed to drop below 86 degrees Thursday, marking its warmest all-time low temperature on record. Records there date back to 1874. Galveston also set a daily record high Thursday, hitting 96 degrees. The heat index rose as high as 117 and hasn’t fallen below the upper 90s in days.

It’s not just Galveston. Much of the Lone Star State is baking, and that heat is set to expand this weekend. Houston peaked at 101 degrees Thursday; SpaceCityWeather.com reports it’s the first time the mercury has soared that high in 380 days. Houston averages three or four such days per year.

With the heat comes sweltering humidity. Dew points close to 80 degrees will make even overnight lows virtually unbearable. At a dew point of 80 degrees, there’s roughly a shot glass’s worth of water vapor hanging in the air just inside your car. Combined with the heat, it comes as no surprise the National Weather Service is warning folks about the dangers of extreme heat.

Nikki Hathaway, a meteorologist at the National Weather Service in Houston, said that weather like this she advises looking before you lock your car. It’s never safe to leave children or pets in a vehicle — even briefly — in summertime temperatures.

“We recommend people don’t spend long times outdoors between noon and 6 p.m.,” Hathaway said. “That’s during peak heating.”

The heat builds this weekend. Dallas could hit 100 degrees Saturday, Sunday and Monday. The temperature isn’t forecast to drop below 80 degrees until at least Wednesday. New Orleans, meanwhile, will see afternoon highs in the 90s each day with brutal humidity.

Sure, it’s summer, and it’s supposed to be hot. What makes this heat wave noteworthy is the exceptional humidity accompanying it. It’s much more difficult to get high temperatures when there’s copious water in the air. It’s because of water’s inherent “thermal inertia,” or the tendency for water to put the brakes on big temperature swings.

Gulf of Mexico water temperatures are running between 1 and 2 degrees above average. That adds even more water vapor to the air. Atop an environment that’s already warming because of climate change, these oppressive heat waves are becoming and will continue to be increasingly common in the years ahead.

The greater atmospheric moisture, characteristic of a warmer world, has a major role in warming overnight lows faster than daytime highs. Consider this: Between 1950 and 1990, Hobby Airport, just south of downtown Houston, saw 22 days total with nighttime lows at or greater than 80 degrees. That averages to about one every two years.

Since 1990 — 29 years — the same airport has seen 130 days with nighttime lows at or above 80 degrees. That averages to nine every two years.


Nights are sizzling in Houston. The number of nights with lows at or above 80 degrees has skyrocketed 900 percent since 1990. (Matthew Cappucci/The Washington Post)

In North Houston, at George Bush Intercontinental Airport, there has been a fourfold increase in nights failing to bottom out below 80 degrees since 2000.

In addition to climate change, urbanization and an increase in heat-absorbing surfaces in these areas are contributing to warmer nights.

Models hint the heat currently plaguing Texas and the Deep South will temper toward the middle of next week, but a hotter-than-average pattern looks to stick around for the foreseeable future.