How the Run Score works

The model (v4) is anchored on estimated WBGT (the heat-strain metric race directors use to flag risk) rather than air temperature or a "feels like" number. WBGT combines temperature, humidity, sun exposure, and wind into one measure of how hard your body must work to shed heat. From the full forecast, RunWeather derives a pace-adjustment estimate (how much conditions should slow you at even effort), a safety evaluation, and a comfort/experience read. Performance and experience contribute 77% and 23% to the baseline; safety supplies limits and warnings. Personal feedback adjusts the score for similar conditions within those limits, apart from a small heat relief on past runs (described below). These weights are product choices. The model's research basis includes ACSM heat-safety guidance (Mantzios et al., 2022) and NOAA/NWS WBGT and wind-chill references.

Dew point, not relative humidity, decides how well sweat evaporates. RunWeather scores it in five bands; above 70°F (21°C) the air is oppressive for running, and 81°F (27°C) is a hard safety gate regardless of fitness.

Peak performance sits at 50°F (10°C) to 54°F (12°C) air temperature. Above 59°F (15°C) the model reads warmth as working against you, above 77°F (25°C) it reads as hot, and above 86°F (30°C) heat becomes the dominant limiter. Cold slows you less than heat: performance holds well below 41°F (5°C), and the danger in winter is wind chill, not the thermometer.

A light 1 mph-4 mph (2-7 km/h) breeze actually helps by aiding evaporative cooling, so the scoring plateau sits there rather than at dead calm. Wind resistance becomes noticeable around 7 mph-9 mph, strong at 12 mph, and gusty above 16 mph. The wind factor, the experience read and the strong-wind cap use mainly the sustained wind and count only part of each gust, less in warm air, where moving air mostly cools you; the full gust-aware blend returns only when gusts near gale strength. Felt temperature, WBGT and wind chill keep the full blend, and in the cold, wind drives the wind-chill governor.

Safety gates cap the score no matter how good everything else looks. The limits are the same for every runner: neither the standard nor the hard limits relax for heat-acclimated runners. The one bounded exception to the cap is feedback-earned heat relief on your own past runs, described under personalization below.

WBGT heat-stress tiers used across the app and the live heat-risk board: moderate from 64°F (18°C), high from 73°F (23°C), extreme from 82°F (28°C). Three conditions gate the score to a near-zero floor — confirmed thunderstorms (storm code with strong precipitation backing), hazardous air quality (AQI 200+), and freezing rain or drizzle (WMO codes 56/57/66/67) with meaningful precipitation. There is no safe mitigation for lightning, toxic air, or black ice; the pace guide is suppressed for all three. Freezing drizzle under 0.5 mm — a glaze that may not yet be a continuous sheet — applies a soft ceiling instead (scores roughly 6–26), so hours within the band still rank; at or above 0.5 mm the hour is hard-unsafe. Mid-confidence storm codes (convective activity without strong precipitation backing, or a storm code the model only half-backs at 30–59% probability) also apply a soft ceiling, capping the score around 34–40 without treating the hour as a hard hazard: move or shorten, but it is not a “never go”. Dense fog (visibility below 400 m) raises a caution flag and applies a progressive ceiling — a foggy morning can score in the 30–50 range with lights and reflective gear, unlike lightning, hazardous air, or black ice where no mitigation helps.

Yes. The published thresholds are the defaults, and the safety limits and verdicts are the same for every runner. In the app you can say how much heat, humidity, cold, wind, rain and sun bother you, and your “felt harder” or “felt easier” feedback on past runs adjusts the score for similar conditions. Heat acclimatization relaxes neither the safety limits nor the pace guide: the app counts your recent hot runs, and on its own that count changes nothing. It only adds to relief your feedback has earned: if you said heat or humidity felt easier, your score for a past run on a caution or unsafe hot day, short of extreme heat, can rise by up to 6 points, even above its safety cap, and recent hot runs count toward that lift. The run's safety verdict does not change.

In the RunWeather app for iOS and Android (scored hour by hour with the best window to run), in the free Run Score calculator for any location, and on the hourly Heat Risk Today board, and the heat and humidity pace calculator turns the app's pace model into a target pace for your conditions. The calculator and the app run the exact same scoring engine, so the numbers always match. For how specific conditions feel in practice, see the guides on heat, humidity, cold, and wind.

For pollutant advice and unavailable AQI data, read running in poor air quality.