See the plan HeatWise would have given a coach on the day Houston hit 109°F, built from that day's archived weather reports. A historical replay, not a forecast.
3. Your heat plan
This is a SYNTHETIC sample day (not a real forecast), so you can see how HeatWise works.
HISTORICAL REPLAY, not a forecast: Houston Intercontinental Airport (IAH) on Thursday, August 24, 2023, when it reached 109°F (tying Houston's all-time record) during an NWS Excessive Heat Warning. Built from that day's archived hourly weather reports.
Safest window
WBGT (wet-bulb globe temperature) is the heat-stress number that Texas UIL flags and OSHA/NIOSH limits use. It counts sun, wind and humidity, so it reads higher than the air temperature on a sunny day.
Planning only: once the WBGT reaches the yellow flag (82.0 °F in Class 3, 79.7 °F in Class 2), UIL requires schools to check it with an on-site meter or an approved app within 15 minutes before practice and every 30 minutes during it.
Next 48 hours
Show hour-by-hour table
* simple estimate at night/cloud: likely reads high (safer side)
Water plan
Acclimatization planner
New to the heat this season? NIOSH says build up time in the heat over 7–14 days.
This is NIOSH's worker schedule; athletes should also follow their school's acclimatization rules.
Warning signs & first aid
- Heat stroke: confusion, slurred speech, passing out, seizures, hot dry skin or heavy sweating, very high body temperature (above 103 °F). CALL 911 and cool them with ice or cold water until help arrives. Don't give them anything to drink.
- Heat exhaustion: dizziness, nausea or vomiting, heavy sweating, weakness, thirst. Move them to shade or A/C, loosen their clothes, cool them with cold wet cloths, and give sips of water. Stay with them. Get medical help right away if they vomit, get worse, or aren't better within 1 hour.
- Heat cramps: muscle spasms or pain, often in the legs, arms or stomach. Stop, rest somewhere cool and sip water. Get medical help if cramps last more than 1 hour.
- Fastest cooling: put the person in cold water or an ice bath (OSHA calls this the best method).
When in doubt, cool them and call 911. Never leave someone with heat illness alone.
One-page coach plan
Print it for the clipboard or send it to parents and staff. Pick the day and the language; “Both” prints an English page and a Spanish page.
How the math works
- Forecast: for U.S. places, hourly air temperature, humidity and wind come from the National Weather Service gridded forecast (api.weather.gov, the same grid behind the NWS heat index and WBGT forecasts); sunshine (solar radiation) and pressure come from Open-Meteo, because the NWS grid has none. Outside the U.S., or if the NWS does not answer, every input comes from Open-Meteo. Both are free with no key, and the plan says which source it used.
- WBGT in the sun (wet-bulb globe temperature, the heat-stress index used by OSHA, NIOSH, the military and Texas UIL) is estimated with the Liljegren et al. (2008) physics model: it solves the heat balance of a black globe and a wet wick using the sun's angle, sunshine, wind, humidity and temperature, then WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Ta. HeatWise's JavaScript port is tested against the open-source reference implementation (pywbgt) to within 0.05 °C. If sun or wind data are missing it falls back to the simpler Bureau of Meteorology formula
WBGT ≈ 0.567·T + 0.393·e + 3.94. - Texas UIL mode: the WBGT is compared with UIL's WBGT activity flags (Class 3: green < 82.0 °F, yellow 82.0–86.9, orange 87.0–90.0, red 90.1–92.0, black ≥ 92.1; Class 2 thresholds are 79.7 / 84.7 / 87.7 / 89.8 °F), and HeatWise shows UIL's practice limits for each flag.
- Work mode: effective WBGT = WBGT + clothing factor (OSHA/NIOSH: 0 to +11 °C), compared with the OSHA/NIOSH limit for the workload: light 28/30 °C, moderate 25/28, heavy 23/26, very heavy 21/25 (unacclimatized/acclimatized). HeatWise's ±2 °C bands around that limit are its own design choice.
- Safest window: among every run of consecutive allowed hours, pick the one whose worst hour is coolest (a minimax problem), solved in O(n) with a sliding-window maximum.
- Heat index (shade, shown for familiarity) uses the NWS Rothfusz regression.
- Pronóstico: en lugares de EE. UU., la temperatura del aire, la humedad y el viento por hora vienen del pronóstico en cuadrícula del Servicio Meteorológico Nacional (api.weather.gov, la misma cuadrícula de los pronósticos de índice de calor y WBGT del NWS); el sol (radiación solar) y la presión vienen de Open-Meteo, porque la cuadrícula del NWS no los trae. Fuera de EE. UU., o si el NWS no responde, todo viene de Open-Meteo. Ambos son gratis y sin clave, y el plan dice qué fuente usó.
- WBGT al sol (temperatura de globo y bulbo húmedo, el índice de estrés por calor que usan OSHA, NIOSH, el ejército y la UIL de Texas) se estima con el modelo físico de Liljegren et al. (2008): resuelve el balance de calor de un globo negro y una mecha mojada con el ángulo del sol, la radiación, el viento, la humedad y la temperatura; luego WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Ta. La versión en JavaScript de HeatWise se compara en pruebas con la implementación de referencia de código abierto (pywbgt), con una diferencia menor a 0.05 °C. Si faltan datos de sol o viento, usa la fórmula simple de la Oficina de Meteorología de Australia
WBGT ≈ 0.567·T + 0.393·e + 3.94. - Modo UIL de Texas: el WBGT se compara con las banderas de actividad WBGT de la UIL (Clase 3: verde < 82.0 °F, amarilla 82.0–86.9, naranja 87.0–90.0, roja 90.1–92.0, negra ≥ 92.1; en Clase 2 los límites son 79.7 / 84.7 / 87.7 / 89.8 °F), y HeatWise muestra los límites de práctica de la UIL para cada bandera.
- Modo trabajo: WBGT efectivo = WBGT + factor de ropa (OSHA/NIOSH: de 0 a +11 °C), comparado con el límite OSHA/NIOSH para la carga de trabajo: ligera 28/30 °C, moderada 25/28, pesada 23/26, muy pesada 21/25 (no aclimatado/aclimatado). Las bandas de ±2 °C alrededor del límite son una decisión de diseño de HeatWise.
- Horario más seguro: entre todos los bloques de horas seguidas permitidas, elige el que tiene la peor hora más fresca (un problema minimax), resuelto en O(n) con un máximo de ventana deslizante.
- Índice de calor (a la sombra, como referencia) usa la regresión de Rothfusz del NWS.
Sources: UIL Heat Stress & Athletic Participation Plan · OSHA Heat Hazard Recognition · NIOSH Workplace Recommendations · OSHA First Aid · NWS Heat Cramps, Exhaustion, Stroke · Liljegren et al. 2008 · BoM Thermal Stress · NWS Heat Index · NWS API (api.weather.gov) · Weather data by Open-Meteo.com (CC BY 4.0)
HeatWise is a planning and screening tool, not medical advice. Forecasts are not on-site measurements; OSHA recommends a WBGT meter at the site for the most accurate assessment.