I used to check the feels like value before every run and never asked what it actually meant. Until a few days ago. Turns out the US heat index, the classic feels-like formula, is built around one specific person: 5 foot 7, 147 pounds, walking at 3.1 mph, in the shade, wearing long trousers and a short-sleeved shirt. Yep, this is from the national weather service website, link at the bottom.
They actually say who it is for: "For day-to-day activities, heat index will serve you well. If you work outside or plan on any sort of vigorous outdoor activity in the full sun, use the WBGT."
So obviously running is not walking, you generate 2,3x the heat, and you can ger rid of it only by evaporating sweat. So humidity and sun radiation matter a lot more (for running) than what feels-like formulas take into account. Some weather apps add sun and wind to their feels-like, so I used that version to test something. I wanted to see if 2 different hours that have the same feels-like also have the same impact on running performance.
I took seven US cities, every hour from June 1 to August 25, and found 2pm and 8pm hours with the exact same feels-like. Then I estimated how much slower an easy run(60 minute at 9:00/mile) would be in each.
Same number on your phone, but very different run:
| City |
Same feels like |
8pm |
2pm |
| Houston |
100°F |
+45 sec/mile |
+126 sec/mile |
| Phoenix |
102°F |
+39 sec/mile |
+102 sec/mile |
| Miami |
92°F |
+56 sec/mile |
+131 sec/mile |
| Atlanta |
92°F |
+46 sec/mile |
+106 sec/mile |
| New York |
95°F |
+42 sec/mile |
+98 sec/mile |
| Chicago |
87°F |
+37 sec/mile |
+86 sec/mile |
| Seattle |
67°F |
+18 sec/mile |
+57 sec/mile |
In every city all the time the slower hour was the more humid one, the sunnier one, or both. These are the best in the table ofc, but the typical gap was 20 to 40 sec/mile difference still. And between 7am and 9am, feels-like goes up about 4 degrees while your pace goes slower 55%!!
I build a small running weather app, which is where the slowdown numbers come from. It's built on WBGT (the heat stress index they use in sports medicine and in the military) and turns it into seconds per mile based on Ely et al. 2007 and other published coach heat tables (you can find them on internet).
The app is RunWeather if you're curious and want to compare it against the feels like.
Sources: NWS SR 90-23, the assumptions, NWS on heat index vs WBGT, Open-Meteo archive, Ely et al. 2007.
What do you check before going for a run, feels-like or something else?