California is the only state that spans all three national WBGT risk categories, from cool coastal cities to desert valleys that regularly clear 100 degrees. Every CIF member school has to monitor wet bulb globe temperature (WBGT), follow tiered activity limits based on which of the three categories they fall into, and keep cooling equipment on site at every practice and contest.
We brought together Ryan Lisk of Orange Lutheran High School, Dr. Christianne Eason of the Korey Stringer Institute, and Dr. David Martin of Perry Weather to walk through what that actually means for schools. Here is what they covered, plus the live Q&A from the session.
The History of Heat Illness in High School Athletics
Between 1980 and 2009, there were 159 heat stroke fatalities among youth, high school, and collegiate football players. A 2018 study looking at a sub-sample of 61 of those exertional heat stroke deaths found that 100 percent occurred during conditioning drills and 37 percent involved athletes being conditioned as punishment.
One position carries a disproportionate share of that risk. Dr. Eason, who has upcoming published research specifically on the topic, explained why:
“(Linemen’s) bodies heat up faster and cool off slower, and so if they’re being asked to do the same conditioning that a wide receiver is doing, their body’s going to respond differently to the heat stress.”
– Dr. Christianne Eason, President of Sport Safety & Education, Korey Stringer Institute
Body size changes everything about how long cooling takes, a point Dr. Martin illustrated with an anecdote from Kelci Stringer, founder of the Korey Stringer Institute and widow of NFL player Korey Stringer:
“Korey would sit in the locker room for 30 to 40 minutes after practice, and he was always the last one there. She would ask him, why do you hang out so long? And he just said, I need to cool down, takes me a while. In his contract, he couldn’t be less than 330 pounds, so all that heat he was generating during practice, it took him that much longer to cool down after.”
– Dr. David Martin, Director of Science, Perry Weather
Heat stroke isn’t confined to hot days, either. Dr. Eason confirmed she was on-site as a medical volunteer when five athletes suffered heat strokes at the Boston Marathon, an event held in 50-degree weather.
The Georgia Study That Started It All
The research that got states moving toward WBGT-based policy came from Georgia. Bud Cooper and Andrew Grundstein, both at the University of Georgia, looked at historical heat illness data and asked what would happen if schools modified activity, removed equipment, or added rest and hydration breaks once conditions crossed certain WBGT thresholds.
The result was a 30 to 100 percent decrease in heat illness across every modification category tested. That data is what got a wave of states, including California, to build WBGT into policy.
What is Wet Bulb Globe Temperature?
Ambient temperature, the number on a phone’s weather app, only accounts for air temperature. Heat index adds humidity, but it was built around a person standing in the shade in long clothing, not someone exercising. WBGT, developed by the military in the 1950s after roughly 600 heat illness cases in a single boot camp season, combines air temperature, humidity, wind speed, and solar radiation into one number that reflects what an exercising body actually experiences.
| Measurement | What it accounts for | Why it falls short for athletics |
|---|---|---|
|
Ambient temperature
|
Air temperature only. | Ignores humidity, sun, and wind. Two days at the same reading can carry very different risk. |
|
Heat index
|
Air temperature and humidity. | Built for a person walking in the shade in long clothing. Ignores solar load and wind, and athletes are not in the shade. |
|
Wet bulb globe temperature
Athletics standard
|
Air temperature, humidity, wind speed, and solar radiation. | It doesn’t. Measured on site, where the athletes actually are, which is why it’s the standard used by the military, OSHA guidance, and FHSAA. |
That number is not interchangeable with the temperature on a weather app, and mixing the two up is the single biggest source of confusion schools run into. Dr. Martin gave a concrete example:
“I was just in New Mexico, it was 106 degrees ambient temperature, but the humidity was much lower, so the wet bulb globe temperature was just going from 88 into 89.”
– Dr. David Martin, Director of Science, Perry Weather
The CIF WBGT Zones
California is split into three risk categories based on 14 years of hourly WBGT data collected from 217 airport weather stations across the country. Category 1 covers the cooler coastal and northern climates; Category 2 covers inland valley and central regions; Category 3 covers the desert and southern climates where athletes see the most consistent heat exposure. Ryan Lisk’s school, Orange Lutheran, falls in Category 1, the most restrictive tier despite being in Southern California, because it sits close enough to the coast to average cooler and more humid conditions than inland schools.
Dr. Eason pointed out that the modifications in each WBGT category are based on real data from each region:
“It might seem that these are just very arbitrary numbers, but they’re designed based off the normal temperatures in that area and the level of heat exposure the body’s already used to.”
– Dr. Christianne Eason, President of Sport Safety & Education, Korey Stringer Institute
Heat Acclimatization
Every fall sport athlete goes through a mandatory 5-day acclimatization period, and it applies to every athlete, including anyone who joins after day one, with no more than one on-field practice per day and each session capped at 2 hours. Ryan Lisk walked through how it phases in protective equipment:
“The first three days, helmets only, then helmets plus shoulder pads, as the shoulder pads are going to keep in some of the heat, and then progressing into full pads as permitted. Until that day comes, there’s no other protective equipment allowed.”
– Ryan Lisk, Director of Sports Medicine, Orange Lutheran High School
Dr. Martin tied the schedule back to the physiology behind it: heat-chamber research shows sweat rate increases starting around day five and continuing to improve past day 14, while core temperature and heart rate at a given intensity both drop over that same window. Acclimatization is not just a safety measure, either. Better sweat efficiency and a lower heart rate at the same workload also translate into better performance once the season starts.
That decay works in reverse, too. An athlete out sick for a week, or sidelined by an injury and only passively exposed to the heat on the sideline, loses those adaptations and needs to work back into full activity rather than jumping straight back into a normal practice.
Cooling, Hydration, and Emergency Action Plans
Whole-body cooling equipment, an ice tub, a “taco tarp,” or ice towels, has to be accessible and ready for immediate use at every practice and contest venue. Ryan Lisk described what that looks like day to day at Orange Lutheran:
“We utilize a lot of those portable water units stationed all over the field, because typically we’re running multiple practices at one time. We have cooling stations, a water taco, and an ice bath or cold plunge that’s constantly on, so we can get an athlete into it quickly, or do it on the field if we’re not able to get them transported into the athletic training room.”
– Ryan Lisk, Director of Sports Medicine, Orange Lutheran High School
Hydration has no on-off schedule; athletes need access to water at all times, not just during a designated break. Dr. Martin added the physiology behind why that matters: for every 1 percent an athlete is dehydrated, core temperature rises 0.15 to 0.2 degrees Celsius, and more than half of athletes likely show up to practice already dehydrated.
Large athletes in full pads can lose up to 4 liters of sweat an hour, more than the stomach can realistically absorb during a single practice, which is why pushing hydration before practice matters as much as water breaks during it.
An emergency action plan is required at every venue, and Dr. Eason was direct about why a generic plan doesn’t hold up:
“You want to make sure your emergency action plan is designed for the specific venue you’re practicing at. If your varsity football team is practicing on one turf field but your soccer team is on a different grass field, each of those should have a plan specific to that location.”
– Dr. Christianne Eason, President of Sport Safety & Education, Korey Stringer Institute
She also flagged a persistent misconception worth correcting directly: a sweating athlete can still be having an exertional heat stroke. The body doesn’t stop sweating just because it’s failing to keep up with the heat it’s generating; the giveaway to watch for is central nervous system dysfunction, confusion, unusual aggression, forgetting plays, not the presence or absence of sweat.
Air Quality: California’s Added Layer
California schools deal with a heat-safety factor most states don’t have to plan around at the same scale: air quality, driven by both smog and wildfire smoke. Dr. Eason explained why the two risks compound each other:
“When AQI levels increase, it also increases the risk of heat-related illnesses. Part of this is because when there’s a lot of particulate density in the air, sweat is not able to evaporate as efficiently. There are serious health risks related to air quality in terms of respiratory issues, but we’re also seeing heat illness risk increase as air quality declines.”
– Dr. Christianne Eason, President of Sport Safety & Education, Korey Stringer Institute
CIF has its own air quality policy alongside the heat policy, and schools are expected to monitor local AQI, especially during wildfire season, with that responsibility delegated to a staff member who knows how to read it.
How Perry Weather Helps California Schools Stay Compliant
A physical weather station on campus, on a press box, a concession stand, or another centralized building, feeds live WBGT and air quality readings continuously, with both a black bulb sensor and an AQI sensor built in.
Every reading is logged automatically for as long as a school is a customer, which matters as much for answering a parent’s question about a specific day as it does for compliance. Policies are configured to a school’s specific CIF category, so a threshold crossing sends a notification with the zone and the specific action required, not just a number staff have to interpret themselves.
If a school runs practices at multiple off-site venues, like a tennis team practicing miles from the main campus, Perry Weather can triangulate from nearby towers, schools, and airport stations to give a more accurate reading for that specific venue, even without hardware installed there directly.
If you want to see what automated WBGT and air quality monitoring looks like on your campus, get a quote today.
Live Q&A From the Webinar
Are the category thresholds arbitrary?
No. They come from 14 years of hourly WBGT readings collected at 217 airport weather stations across the contiguous U.S., including both hot-dry and hot-humid climates. Cooler regions (Category 1) get lower thresholds because athletes there are acclimatized to milder conditions — not because the rules are stricter for the sake of it.
How do we find out which WBGT category our school is in?
Take a loop at the Perry Weather lookup tool.
You can enter your schools name in and it will return the region.
Does CIF require an on-site WBGT reading, or can we use an app?
CIF’s rule recommends to take measurments on-site and lists that NOAA public data sources can be used as a fallback when a WBGT device is not available. Best practice is a WBGT reading from your school’s area — ideally an on-site device — rather than an estimate pulled from a distant airport station.
Are there any talks about the desert region getting its own Category 4?
Not a fourth category exactly, but there has been proposed state legislation (Assembly Bill 2503) that would direct CIF to revise guidelines specifically for schools in extreme desert climates within Category 3, allowing outdoor activity up to a WBGT of 93.5°F if schools add safety measures like hydration breaks every 15 minutes and enhanced acclimatization protocols. Until anything like that is formally adopted, the panelists were clear that schools should follow the current guidelines rather than anticipating a change, since deviating now would be difficult to justify if an incident occurred.
Does Perry Weather account for conditions at field level versus where the station is mounted?
Yes. The hardware uses a vetted algorithm to calculate the ground-level wind speed and apply it to your WBGT reading. This method is validated through field research — including a recent three-week study at White Sands Missile Range comparing a 10-meter-mounted device against seven ground-level handhelds.
If we travel to a hotter zone for a game, whose guidelines apply?
Best practice is to follow the more conservative category — your athletes’ bodies are acclimatized to the conditions they train in, not the host site’s. In practice, California’s 7 p.m. varsity kickoffs rarely create conflict; the friction shows up at 3 and 5 p.m. lower-level contests.
An athlete started acclimatization late and double days have begun. What can they do?
Technically they should start from day one of the progression. The non-negotiable piece is rest: best practice is a minimum three-hour rest period in a cool environment between sessions. Walkthroughs and conditioning sessions are permitted within that framework.
What about athletes returning from illness or injury?
Acclimatization decays. An athlete home sick for a week or two — or one sidelined with an injury who’s been watching practice rather than participating — has lost adaptations. Passive heat exposure doesn’t produce the same physiological changes. Reintroduce them gradually rather than dropping them into a full-length practice.
Should WBGT be used for elementary and middle schools too?
The risk profile is different. Younger kids generally stop when they feel bad; high school athletes push through because of external pressure. Exertional heat stroke is rare in young populations. That said, if budget allows, monitoring conditions at competitive middle school is worth doing — and watch for hot playground equipment as a burn risk at recess.
Is there a way to see the active policy on the app home screen?
Yes, Perry Weather shows you the exact WBGT zone that you are in at each location in the mobile and web dashboard.
