Decide If It Is Safe to Run Outside Today
Three checks in under a minute. If you train regularly, this is the routine that prevents heat illness without forcing you indoors on every warm day.
- 1
Check the heat index, not the temperature
Open the calculator on this page and enter the actual air temperature and humidity (most weather apps show both). If the heat index is below 90 F, you are in normal training territory. 90 to 100 F is reduce-intensity zone. Above 100 F is run-early-or-skip zone.
- 2
Adjust for sun and acclimatization
Heat index assumes shade. Add 15 F if your route is in direct sun. Subtract effective reduction if you have been training in heat for 2+ weeks (your acclimatized body handles 5 to 10 F higher than the chart suggests). If you have been indoors for two weeks (sick, traveling, in AC), treat the chart numbers as conservative.
- 3
Pick the right time window
Air temperature peaks 2 to 4 hours after solar noon. If solar noon in your city is 1 PM, peak heat is 3 to 5 PM. Run before 9 AM or after 7 PM in summer. The 6 to 8 AM window is consistently 15 to 20 F cooler than peak. Avoid 10 AM to 4 PM for anything strenuous above HI 85 F.
- 4
Set a fluid plan, then commit
Pre-hydrate 17 to 20 oz two hours before the run. During the run, 7 to 10 oz every 15 to 20 minutes if it is more than 45 minutes. Carry it with you or run a loop near water. If heat index is above 90 F, switch to a sports drink with 100 to 200 mg sodium per 8 oz, plain water at that volume risks hyponatremia.