In this article · 5 sections
Athletic directors are now asked to own two WBGT numbers. One comes from the instrument on the sideline. The other comes from a forecast. They are often treated as rivals — which one is "right" — when they answer different questions at different times of day.
The short answer: the on-site reading makes the compliance call, every time, in every state with a WBGT policy we have published. The forecast makes the planning call — the one at 1 pm about whether practice should start at 4 or at 6:30 — which no reading can make, because at 1 pm the 4 pm reading does not exist yet.
What WBGT measures, and why it is read on site
Wet-bulb globe temperature is a heat-stress index. Outdoors it combines three readings: a natural wet-bulb temperature, weighted 70%; a black globe temperature, weighted 20%; and the air temperature, weighted 10%. The National Weather Service points out the difference that matters most for a practice field: the heat index assumes you are in the shade, while WBGT is taken in direct sun and responds to wind, sun angle and cloud cover as well as temperature and humidity.
Those inputs change over short distances — a breeze along one side of a campus, an open field against one ringed by buildings — which is why OSHA's technical guidance says WBGT is most reliable when measured at, or as close as possible to, where people are working. State associations write their rules the same way: the number that counts is the one taken where the athletes are.
What Georgia, Texas and Florida require of the reading
| Attribute | Georgia (GHSA) | Texas (UIL) | Florida (FHSAA) |
|---|---|---|---|
| The device | A scientifically approved WBGT monitor, calibrated at least every two years. Phone apps are not accepted. | A scientifically approved WBGT instrument, or another scientifically proven method such as internet-based weather station software or an application. | A WBGT device with a black globe; a fan is recommended. |
| When to read | First reading 30 minutes before practice; hourly by the by-law, at least every 30 minutes per GHSA’s reminder. | Instrument set up 30 minutes before practice and read 15 minutes before it; then every 30 minutes. | Whenever WBGT is reasonably expected to exceed 82: set up 15–20 minutes ahead, then about every 30 minutes. |
| Where | At each practice | At the practice site | In the sun, three feet above the surface, not under shade |
Georgia (GHSA)
- The device
- A scientifically approved WBGT monitor, calibrated at least every two years. Phone apps are not accepted.
- When to read
- First reading 30 minutes before practice; hourly by the by-law, at least every 30 minutes per GHSA’s reminder.
- Where
- At each practice
Texas (UIL)
- The device
- A scientifically approved WBGT instrument, or another scientifically proven method such as internet-based weather station software or an application.
- When to read
- Instrument set up 30 minutes before practice and read 15 minutes before it; then every 30 minutes.
- Where
- At the practice site
Florida (FHSAA)
- The device
- A WBGT device with a black globe; a fan is recommended.
- When to read
- Whenever WBGT is reasonably expected to exceed 82: set up 15–20 minutes ahead, then about every 30 minutes.
- Where
- In the sun, three feet above the surface, not under shade
Texas is the one to read carefully. UIL allows an internet-based method in place of an on-site instrument, checked within 15 minutes of the start. That is still a reading of current conditions taken at practice time, on whatever method your school has adopted for compliance. A forecast issued at noon for 4 pm is not that reading, and Flash does not present its outlook as one.
Which number makes which call
| Decision | On-site reading | WBGT forecast (Flash) |
|---|---|---|
| Stop or modify practice right now | Decides. Compliance reads the meter on the field. | Supports: shows whether the next hour is rising or falling. |
| Plan today’s practice time | Cannot. It reads the present only. | Decides: hourly outlook six hours ahead. |
| Compare fields before assigning sessions | Only by walking the device between them. | Per 1×1 km cell, so fields in different cells get their own numbers. |
| Warn trainers before the band changes | Alarms as it crosses; the threshold is already behind you. | Shows the hour each threshold is forecast to be crossed. |
On-site reading
- Stop or modify practice right now
- Decides. Compliance reads the meter on the field.
- Plan today’s practice time
- Cannot. It reads the present only.
- Compare fields before assigning sessions
- Only by walking the device between them.
- Warn trainers before the band changes
- Alarms as it crosses; the threshold is already behind you.
WBGT forecast (Flash)
- Stop or modify practice right now
- Supports: shows whether the next hour is rising or falling.
- Plan today’s practice time
- Decides: hourly outlook six hours ahead.
- Compare fields before assigning sessions
- Per 1×1 km cell, so fields in different cells get their own numbers.
- Warn trainers before the band changes
- Shows the hour each threshold is forecast to be crossed.
The band rules make the planning call matter more than it looks. In Georgia, once a reading has stayed in a band for 15 consecutive minutes, that band holds for the rest of practice — you can move up, never back down. A session that starts at 4 pm into a rising afternoon can lock itself into the two-hour cap by 4:30. Moving the start is a decision you can only make before the reading exists.
A day with both numbers
- Morning: the 6-hour outlook for each field shows when the afternoon will cross your thresholds — 82, 87, 90 and 92 under GHSA by default, or your own state’s or district’s bands set per field.
- Early afternoon: if the outlook puts practice time in the two-hour band, decide then whether to shorten, move to the evening or go indoors, and tell the bus, the trainers and the parents while there is time.
- Before practice: take the on-site reading on the schedule your association sets. This is the compliance record, and it decides the band you start in.
- During practice: keep reading on schedule. Use the outlook to see whether the next hour is heading up or down, so a pause is planned rather than improvised.
- Afterwards: the Flash decision log holds the sensor reading and the forecast side by side, timestamped, with who made the call — next to the readings your association asks you to keep.
Morning · the planning call
The forecast moves practice. The reading at practice time decides the band.
Frequently asked
No. GHSA, UIL and FHSAA compliance is decided by the reading taken at practice time on the method the association accepts. A forecast is a planning tool that tells you where that reading is heading.
UIL allows a scientifically approved instrument or another scientifically proven method, such as internet-based weather station software or an application, checked within 15 minutes of practice. Georgia does not accept phone apps.
The heat index assumes shade and uses only temperature and humidity. WBGT is taken in direct sun and also reflects wind, sun angle and cloud cover, which is why state associations use it for practice limits.
Six hours, hourly, per 1×1 km cell, alongside heat index, air temperature and dew point.
Founder & Chief Meteorologist, Flash Weather
Founder of Flash Weather, a 20-year National Weather Service veteran and former Chief Meteorologist at Atlanta Hartsfield-Jackson International Airport.