when to run

Wet-bulb globe temperature folds temperature, humidity, sun load, and wind into one heat-stress number — the standard for outdoor athletics. It omits your fitness, acclimatization, and pace. The words follow the National Weather Service bands: low, elevated, moderate, high, extreme — a veto at 90°F and above.

These figures are estimated from the gridpoint forecast, not measured. A black-globe sensor on the course reads the real thing and can sit a few degrees higher in full sun on asphalt — treat a forecast 88° as “maybe 90° on the asphalt.”

The absolute moisture in the air, which sets how well sweat can evaporate. Below 55°F is comfortable; at 70°F and above evaporation stalls and heart rate drifts up at any pace. It omits temperature itself; that is WBGT’s job.

The same moisture as the dew point, read against the temperature — so it climbs toward dawn even while the absolute moisture holds steady. A high-RH dawn is not wetter air than a moderate-RH afternoon; it is cooler air holding the same water. Use dew point to judge sweat; use RH to expect fog and soaked grass at first light.

The EPA scale for sunburn speed on exposed skin — at UV 9, damage begins in about 15 minutes. It says nothing about heat: a dawn window can read UV 0–2 while the same day’s noon hits 10. Shade, cover, and sunscreen change your exposure, not the index.

The EPA Air Quality Index, taken from the worst nearby monitor and its dominant pollutant — a protective reading, not an average. Moderate air is fine for most runners; unusually sensitive people may shorten hard sessions. 201 and above vetoes the hour.

Sustained speed plus gusts, with the vane in the ledger showing where it blows from. In heat, wind helps — it moves saturated air off your skin. In cold it cuts; the wind-chill veto sits at −10°F. It omits direction changes along a loop; see the route note.

Advisories are cautions, not stop orders: an advisory banners the page but strikes nothing out. Vetoes are different — lightning at 30% thunder probability or more, WBGT at 90°F and above, or AQI past 200 remove the hour from play with the reason named in the row, never averaged away. Rain alone rarely cancels a run.