It's that time of year again: We need to talk about "Wet Bulb Globe Temperatures"

[WeatherBot]

Originally published at: It’s that time of year again: We need to talk about “Wet Bulb Globe Temperatures” – Space City Weather

In brief: In today’s post we talk about Wet Bulb Globe Temperatures, which are useful at this time of year to help differentiate “hot” weather from “extremely hot” weather you need to avoid being outdoors during. We’re going to see some of the latter this week.

WBGT, and what does it mean for me

There are many ways to talk about heat: high temperatures, the heat index, “extreme” heat warnings, and more. But the most practical (which also, I acknowledge, has the most confusing name) is something called Wet Bulb Globe Temperatures. It takes a lot of factors into account, including temperature, humidity, wind speed, Sun angle and cloud cover to determine the amount of heat stress. It is particularly helpful for determining whether outdoor exercise or work is safe. If you have kids in fall sports or marching band, these temperatures are often used to set practice times.

The confusion comes not just from the name (what the heck is a “wet bulb” anyway?) but also because there are other, similar names. For example, a dry-bulb temperature is the name for an air temperature measured by a standard thermometer in the shade. A wet-bulb temperature involves placing a wet cloth around a thermometer bulb, and it measures both temperature and humidity. Globe temperature measures radiant heat (i.e. sunlight and heat radiating from surfaces such as concrete).

All three of these are factored into a “Wet Bulb Globe Temperature.” So yes, it’s a bit complicated, but WBGTs are the very best way of differentiating between “hot” temperatures and really hot, borderline-unsafe temperatures.

This week we have a sustained stretch of “extreme” heat in terms of WBGTs. We’ve certainly seen hotter, but each day this week from about 1 to 3 pm CT we will experience extreme or very high WBGT conditions. It’s the time of year to take heat warnings safely. We typically see them in July, August, or early September.

Tuesday through Saturday

There is no mystery with our upcoming weather. High temperatures will be about 100 degrees in Houston for the next couple of days, and then push into the low-100s during the Thursday through Saturday period. It will be our hottest weather of the year. Along with the heat we’ll have plenty of humidity, sunshine, and generally light winds that will gust up to about 15 mph during the afternoon hours. Nights will remain warm and sultry. And that’s pretty much it—for most of us this is pretty brutal weather.

Sunday and beyond

High pressure begins to retreat toward the end of this week, and that will open up the possibility that we see some slightly different weather by the end of this weekend, and during the first half of this week. Best case? Daily high temperatures fall back into the mid-90s, and we see some decent daily rain chances. Worst case? We more or less continue the conditions of this week, and keep on trucking with very hot days and full sunshine. We’ll have to see how things play out.

[wd9get]

Yes! I first learned about WBGT serving a a Flight Surgeon in the USAF and use it extensively in managing outdoor activities. The NWS has greatly improved their heat tools this year which is fantastic for anyone engaged who has to work or play outside. A presentation Lisa Schmit of the NWS office Twin Cities/Chanhassen MN gave at the AMS in 2017 showed that emergency department visits for heat are much more closely correlated to WBGT than heat index. Thank you for helping get the word out about this somewhat confusing, but extremely helpful safety tool.

Keith B, MD

Alvin, TX

[Joseph]

Believe it or not, we used to go entire summers without the WBT reaching the extreme level. That has changed in recent years, but atleast this summer hasn’t been near as bad as the summer of 2023 in terms of WBT. We spent the majority of that summer in the “extreme” level.

[TxAg]

WBGT seems like it would be a great addition to the already great SCW app.

[Blackhawks_Fan]

I brought this up at work a year or two ago and it got shot down in flames because we would have to re-write our heat stress procedures, WBGT is not recognized by OSHA, and perhaps most critically, we would have to buy new stuff.

[RockinRobin]

Thank you for making me aware of the Wet Bulb Global Temperature several years ago. It is the most significant reality I took away from research for a presentation on Gardening with Weather Extremes for Harris County Master Gardeners. Not only am I able to keep myself safe, but I pass this on to others whenever I can. Y’all are doing an amazing job of keeping us informed about our weather!

[mbutera]

I’ve been reading about WBGT this past week because new UIL rules take effect on August 1, 2026, requiring Texas schools to monitor WBGT before and during outdoor athletic and marching-band practices. WBGT levels will determine the permitted length of practice, the frequency and duration of rest breaks, and when outdoor activity must stop altogether.

One thing I’ve learned from my reading is that wet-bulb temperature and Wet Bulb Globe Temperature (WBGT) are not the same thing. Some weather apps provide wet-bulb temperatures, which take into account temperature and humidity to gauge how cool the air can become through evaporation (i.e. the ability of sweat to cool the body). WBGT provides a more complete measure of heat stress at a specific location because it also accounts for direct sunlight, radiant heat from surrounding surfaces, and wind. An on-site WBGT reading, for example, could be lower on a natural grass practice field than on an asphalt parking lot. That’s not something a weather app would necessarily know.

The National Weather Service and other online tools can provide forecast or modeled WBGT estimates for a location. But an actual reading of conditions at a particular field or parking lot requires an instrument at the site. Things like sunlight, surface material (artificial turf, asphalt, or natural grass), shade, and wind can all make a significant difference in the reading. All of this is about to become much more relevant to Texas students and parents this year.