About the Heat Index & Exertion Safety Planner
When a school cancels practice or a crew goes to a mandatory work-rest cycle, the number on the sheet is almost never the heat index. It is Wet Bulb Globe Temperature — the metric in the athletic-training position statements, the military flag system and the OSHA technical manual — because it is the only common measure that accounts for radiant heat from the sun and the cooling effect of wind, not just temperature and humidity.
WBGT is not a published forecast variable anywhere, which is why almost every free tool shows you the heat index instead and calls it close enough. It is not close enough: on a bright, still afternoon the two can disagree by an entire risk category. So this tool computes it, hour by hour, from Open-Meteo's temperature, dew point, pressure, wind and direct and diffuse solar radiation, using the Dimiceli black-globe model developed at the National Weather Service office in Tulsa.
That implementation was validated before shipping against every worked example in the source paper: the intermediate quantities and the resulting globe temperature reproduce to better than 0.02 °C, and against the paper's own measured black-globe readings it lands within 0.67 °C. The output is then translated into the thing you actually need — how long you can practise, how many break minutes per hour, and what equipment comes off. Nothing you enter is stored, and this is planning guidance, not medical advice.
Peak effective WBGT per day with the coolest usable hour beneath it. Plan the hard sessions on the low bars.
How to Use the Heat & Exertion Safety Planner
Set the location to the actual field or job site, then pick the activity — this matters more than it looks, because gear changes the effective heat load by a whole category. The hero gives the effective WBGT: the computed Wet Bulb Globe Temperature plus the clothing adjustment for what people are wearing. Read the flag category next to it, follow the rules in the guidance panel, and if the current hour is unusable, look at the curve for one that is. Most “cancelled” sessions do not need cancelling; they need moving three hours earlier.
Why WBGT and Not the Heat Index
The heat index answers a narrow question: how hot does still air feel in the shade at a given temperature and humidity. That is the wrong question for anyone exercising or working outdoors, because it ignores the two factors that dominate an open field — radiant load from the sun and wind. WBGT weights three separate measurements:
- Natural wet bulb (70%) — a thermometer with a wet wick in moving air. It captures how much evaporative cooling is still available to you, which is the body’s main heat-shedding route. Humidity attacks this term directly.
- Black globe (20%) — a thermometer inside a matte black copper sphere, measuring radiant heat. On a clear afternoon a black globe can read 15–20 °C above air temperature.
- Dry bulb (10%) — ordinary air temperature, the smallest term.
The weighting is the insight: air temperature, the number everyone quotes, counts for one tenth. A 30 °C day at 30% humidity with a breeze is a routine training day. The same 30 °C at 80% humidity with no wind is genuinely dangerous, and only WBGT tells them apart.
How This Tool Gets a WBGT Without a Meter
No weather service forecasts WBGT as a standard variable, because two of its three terms require physical instruments. The workaround, developed at the National Weather Service office in Tulsa by Dimiceli, Piltz and Amburn, derives the black-globe temperature from an energy balance on the sphere — incoming direct and diffuse solar radiation and atmospheric longwave radiation in, radiative and convective losses out — using only routinely forecast quantities. The wet-bulb term comes directly from Open-Meteo. Our implementation was checked against every worked example in the source paper and reproduces the published globe temperatures to better than 0.02 °C, and the authors’ own measured readings to within 0.67 °C.
Two honest limitations. The globe model is very sensitive to wind speed, and its coefficients were fitted over a moderate range; we floor the wind at a light breeze rather than extrapolating into calm conditions where the model produces globe temperatures no instrument has ever recorded. And the calculation assumes an open, unshaded, grass-like site. It cannot know that your field is artificial turf, which routinely runs several degrees hotter, or that your stadium bowl blocks the wind. Where policy requires a WBGT reading, use an on-site meter.
Clothing and Equipment: The Adjustment Most People Skip
Evaporation is how the body sheds most of its heat once the air is warm, and evaporation requires sweat reaching air. Anything that covers skin blocks it. The standard occupational approach is a clothing adjustment value added to the measured WBGT before comparing against a threshold: ordinary work clothes are the zero baseline, double-layer woven clothing adds about 3 °C, and a vapour-barrier suit adds around 11 °C. A full football uniform with helmet and pads behaves much like the double-layer case. That is why a measured WBGT of 86 °F — comfortably in the “use discretion” band for a cross-country runner in a singlet — is an effective 91 °F for a lineman in full pads, which is a one-hour, no-equipment, no-conditioning day.
Acclimatisation: Why the First Two Weeks Are the Dangerous Ones
Heat acclimatisation is a real, measurable physiological change: you begin sweating earlier and more freely, sweat becomes more dilute so you lose less sodium, plasma volume expands, and heart rate at a given workload drops. Most of that develops over 10 to 14 days of progressively longer exposure, and it decays within a few weeks of stopping. The practical consequence is stark — exertional heat illness clusters heavily in the opening days of preseason, when athletes arrive unadapted from an air-conditioned summer and immediately do the hardest conditioning of the year. Phase in duration first, then intensity, then equipment, and treat the published WBGT thresholds as too generous for anyone in their first week.
Recognising Trouble
Heat exhaustion presents as heavy sweating, weakness, dizziness, nausea, headache and cool clammy skin, with a rising but not extreme core temperature. Move the person to shade, remove equipment, cool them and give fluids. Exertional heat stroke is a medical emergency and the distinguishing sign is central nervous system dysfunction — confusion, aggression, staggering, collapse or loss of consciousness — alongside a core temperature above roughly 40 °C. Sweating may or may not have stopped, and waiting for dry skin is a mistake. The evidence strongly supports cold-water immersion begun immediately, before transport, because outcome tracks how long the body stays above the critical threshold. Call emergency services and cool first.
This tool is planning guidance, not medical advice, and it does not replace an on-site emergency action plan or a qualified athletic trainer. For the other outdoor hazards, pair it with the Air Quality & Exercise Planner and the UV Exposure & Vitamin D Planner. More in Health & Fitness tools.
Frequently Asked Questions
What is the difference between WBGT and the heat index?
The heat index combines only air temperature and humidity, and it is defined for shade. WBGT also accounts for direct sun and wind, which is why it is the standard for outdoor exertion. On a bright, still afternoon WBGT can indicate dangerous conditions while the heat index still looks moderate, and the sun is the reason. Neither replaces medical judgement.
What WBGT is too hot for sports practice?
The widely adopted athletic thresholds are in degrees Fahrenheit: below 82.0 is normal activity, 82.0 to 86.9 calls for discretion and extra breaks, 87.0 to 89.9 limits practice to two hours with reduced equipment, 90.0 to 92.0 limits it to one hour with no protective equipment and no conditioning, and above 92.1 outdoor workouts should be cancelled or moved. Youth, unacclimatised and previously heat-ill athletes need stricter limits than the table.
Why does football gear matter so much?
Helmets and pads block evaporative cooling over a large fraction of the skin, and evaporation is how the body sheds most of its heat in hot conditions. Standard clothing adjustment practice treats a full pads-and-helmet uniform as adding roughly 3 degrees Celsius to the effective WBGT, which is often a whole flag category. This tool applies that adjustment and shows both the measured and effective values.
What is heat acclimatisation and how long does it take?
It is a set of physiological adaptations, including earlier sweating, more dilute sweat and increased plasma volume, that substantially improve heat tolerance. Most of the benefit develops over ten to fourteen days of progressively longer exposure, which is why preseason guidelines phase in practice duration and equipment. An unacclimatised athlete is at far higher risk at the same WBGT, and the first few days of a season are when exertional heat illness most often occurs.
How accurate is a calculated WBGT compared with a real meter?
A calculated value is a good planning tool but it is not a field measurement. The black-globe model used here reproduced its source paper's measured readings to within 0.67 degrees Celsius, but it assumes an open, unshaded site and cannot know about your specific surface. Artificial turf, a windless stadium bowl or a dark track can all run several degrees hotter than an open grass field. Where policy requires a WBGT reading, use an on-site meter.
