WBGT vs. heat index: which should athletics programs use?
Use wet-bulb globe temperature (WBGT) for outdoor practices and games. Heat index assumes shade and uses only temperature and humidity; WBGT is measured in direct sun and also accounts for wind, sun angle and cloud cover — the conditions athletes actually train in.
The short version
| Heat index | WBGT | |
|---|---|---|
| Inputs | Air temperature, relative humidity | Temperature, humidity, wind speed, sun angle, cloud cover |
| Assumes | Shade | Direct sunshine |
| Best for | Everyday conditions | Outdoor work and vigorous activity in full sun |
The National Weather Service puts it plainly: heat index assumes you are in the shade, and direct sunlight can add as much as 15°F to it. WBGT is measured in the sun and folds in wind and cloud cover, which is why athletic trainers and state associations base activity limits on it.
Why this matters for buying equipment
Because WBGT depends on the sun and wind at a specific spot, a reading from an airport or a phone app can be far off from conditions on your practice field. That is the case for an on-site WBGT sensor or station rather than a forecast alone.
Florida made this a legal requirement. Its 2020 Zachary Martin Act revised s. 1006.165, Florida Statutes, so that heat stress “must be determined by measuring the ambient temperature, humidity, wind speed, sun angle, and cloud cover at the site of the athletic activity,” with schools required to modify, suspend or move activities under FHSAA guidelines. The same law requires cold-water immersion or an equivalent cooling method to be immediately available.
What to ask a provider
- Does the device measure true WBGT (black globe, wet bulb, dry bulb), or estimate it from other readings?
- Can it be mounted permanently at the field, and does it alert staff automatically when a threshold is crossed?
- Does it log readings so you can document compliance with your association’s policy?
- Can alerts follow your association’s specific thresholds and activity-modification categories?
Common questions
- Is heat index good enough for football practice?
- The National Weather Service recommends WBGT, not heat index, for outdoor work or vigorous activity in full sun. Heat index assumes shade, and direct sunlight can add as much as 15°F to it.
- Does any state require on-site heat monitoring for high school sports?
- Yes. Florida's 2020 Zachary Martin Act amended s. 1006.165, F.S., to require member schools to monitor heat stress at the site of the activity and modify, suspend or move activities under guidelines set by the FHSAA.
- Can we use the WBGT forecast instead of measuring on site?
- A forecast helps with planning, but conditions differ field to field. Florida's statute, for example, requires heat stress to be determined from conditions measured at the site of the athletic activity.
Sources
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