What do the heat stress levels mean?
Updated
RunFast grades heat stress by WBGT, one number for the air, humidity, sun and wind, the reading race medical teams set their flags by. Low is under 18.3 °C (65 °F), Moderate from 18.3 °C (65 °F), High from 22.8 °C (73 °F) and Extreme from 27.8 °C (82 °F).
The four levels
| Level | WBGT | What it means for your run |
|---|---|---|
| Low | Under 18.3 °C (65 °F) | Run your race |
| Moderate | 18.3 to 22.8 °C (65 to 73 °F) | Drink early, ease the climbs |
| High | 22.8 to 27.8 °C (73 to 82 °F) | Slow down, drink at every table |
| Extreme | 27.8 °C (82 °F) and over | Race medical teams’ highest flag |
These are the bands the big city marathons use for their heat flags.
Why WBGT, not the temperature
WBGT (wet bulb globe temperature) puts the air temperature, humidity, sunshine and wind into one number. A humid, sunny morning can reach High at an air temperature that looks mild on a weather app. It’s the heat of the place, the same number a medical tent’s meter would read at that spot and hour. Read more in WBGT for runners.
Where you’ll see it
- Map your run: “Heat stress Moderate” beside the rating, and the HEAT STRESS tile with the WBGT at its peak. Drag along the profile to see it for any stretch.
- The reason line, for example “Heat stress High from km 7”.
- Following someone live: HEAT HERE on their screen, and a coloured heat label on their card.
Dew point and feels like
- Dew point is how sticky the air is: Comfortable under 10 °C (50 °F), Noticeable 10 to 15 °C (50 to 59 °F), Sticky 15 to 18 °C (59 to 64 °F), Hard 18 to 21 °C (64 to 70 °F) and Oppressive above that. See the dew point guide.
- Feels like is worked out at running pace. Running, there’s little breeze to cool you, so a sunny climb feels hotter than a weather app says, and a cold, windy start feels colder.
Not medical advice
Heat stress is a guide to plan by. On race day, the race’s flags and its medical teams come first.