Throughout the summer, we’ve talked about temperatures and the heat index, but there’s another measurement that’s been used to talk more specifically about heat stress in direct sunlight.
The Wet-Bulb Globe Temperature (WBGT) was originally introduced for the safety of workers outside. OSHA and the International Organization for Standardization recognize this measurement as a standard for occupational environments.
The University Interscholastic League, or UIL, is also recommending coaches use the WBGT to protect their athletes from heat-related illnesses when exercising outdoors.
Here’s why this measurement is used to keep you safe in direct heat:
What does the WBGT measurement take into account?
According to the National Weather Service, the WBGT measures the heat stress on the body based on four parameters:
- Temperature
- Relative humidity
- Wind
- Sunlight
The measurement’s output is a number in degrees (much like the heat index) and is categorized into four danger thresholds by the U.I.L: low, moderate, high, and extreme. Here’s a chart of the risk level, WBGT, and U.I.L Guidelines.
| RISK LEVEL | WBGT | U.I.L GUIDELINES |
|---|---|---|
| EXTREME | Above 92° | No outdoor practice. Delay practices until a cooler WBGT is reached. |
| HIGH | 90° to 92° | Maximum practice time: 1 hour; FOR FOOTBALL: No protective equipment may be worn during practice, and there may be no conditioning activities. FOR ALL SPORTS: There must be 20 min of rest breaks distributed throughout the hour of practice. |
| MODERATE | 87° to 89° | Maximum practice time is 2 hours; FOR FOOTBALL: players are restricted to helmets, shoulder pads, and shorts during practice. If the WBGT rises to this level during practice, players may continue to work out wearing football pants without changing to shorts. FOR ALL SPORTS: Provide at least four separate rest breaks each hour with a minimum duration of 4 min. each. |
| LOW | 82° to 86° | Use discretion for intense or prolonged exercise; Provide at least three separate rest breaks each hour with a minimum duration of 4 min. each. |
How is it different from the heat index?
The heat index is a calculation of what it feels like to our bodies based on the humidity and the temperature in the shade (i.e., not in direct sunlight).
On the other hand, the WBGT takes into account those two parameters while also including solar radiation and wind speed. The output is then based on how all four parameters affect individuals outside.
We know that hotter temperatures can be more dangerous for those working or exercising outside, but here’s why the other three parameters are also important to consider:
- Relative humidity is important to the measurement because the greater the humidity, the harder it is for the body to cool down.
- Wind also helps the body cool down; if there’s no wind, you immediately feel hotter.
- Solar radiation takes into account how much direct sunlight is present. If clouds are out when measuring the WBGT, the output will be lower because the clouds reflect a portion of the sun’s radiation back into space.
KSAT contacted the Texas Education Agency in Austin to learn if it has a specific directive on this particular tenet of the school day.
In a statement sent to KSAT, TEA shared that it leaves this up to the state’s individual districts to decide.
However, the University of Interscholastic League or UIL does have rules in place when it comes to temperatures during outdoor practices.
Guidelines for San Antonio-area schools
KSAT contacted five of the biggest school districts in the San Antonio area to find out what guidelines they have adopted to regulate outdoor activities on their respective campuses during heat temperature events.
In a statement from North East Independent School District, it was explained that the district’s principals have been advised and given guidance that outdoor activities will be limited and recess will be moved indoors during the hottest parts of the day.
The Northside Independent School District revealed that elementary school campuses are also advised to limit or move recess indoors when the heat index hits or passes 105°.
For its secondary-level campuses, NISD’s hot temperature day plan utilizes the WBGT guidelines.
The hot weather precautions shared by the San Antonio Independent School District set a caution level of 90° to 104°, when vigorous outdoor activities are limited to 10 minutes.
When the heat index reaches 105°F or higher, students remain indoors at all times, according to SAISD.
Judson Independent School District told KSAT that it follows UIL rules for extracurricular activities. If the temperature exceeds 105° or the heat index exceeds 99°, students cannot go outside for PE or recess.
“Campus administrators can also make that decision if the temperature levels are high but don’t necessarily meet that threshold,” an email from Judson read. “On warm days, water must be accessible to students and considerations or alternatives should be considered for those students with health concerns.”
As a general practice within the Alamo Heights Independent School District outdoor exposure is limited when the heat index goes above 90 degrees.
According to a statement AHISD sent to KSAT, the campus nurses share weather guidelines regarding temperatures with the staff at their respective school.
The statement goes on to read that after being advised, campus leadership is able to determine alternative activities if necessary.
When students go outside heat safety steps are engaged.
“There are shaded areas for students and they are also encouraged to bring water bottles with them,” according to AHISD’s statement. “Teachers are monitoring students closely during this time.”
WBGT Forecast
To find the forecast WBGT for your area, go to this website. Change the category from “Heat Risk Experimental” to “Wet Bulb Globe Temperature” and use the time bar at the top to scroll through time.
- Read more from the meteorologists on the Whatever the Weather page
- Download KSAT’s weather app for customized, accurate forecasts in San Antonio, South Texas or wherever you are
- Find the latest forecasts, radar and alerts on the KSAT Weather Authority page
