How hot is too hot to run? And how do you cope with extreme heat on runs?

Any temperature above absolute zero.

What this joker's not telling you is how insanely rare this is. For example at 100F, you need RH over 80%.

People from Florida, Texas, whatever will swear to you this happens all the time, but can't back it up with an actual weather report. Virtually nowhere does RH even stay over 50% when it's that hot.

The amount of people I see wearing tights and long sleeve shirts on hot days… no wonder you’re gonna overheat if you dress like that in the heat.

You do hear people throwing around ridiculous #s but S.W. Fl. was a bit oppressive this summer with dewpoints averaging in the high 70s!

I think it depends how adapted you are and how dry vs humid it is. I often run when it's 100+ degreesF, but live in Arizona so my body is very used to it. slowly get used to it, wear sunblock, hydrate and just listen to the body. also, it's often said that heat is the poor man's altitude, so adjusting your pace accordingly is helpful and you'll still benefit from the effort

same here i live in vegas and anything up to 105 is fine to get ~5 miles in, longer runs or hotter temps i'll run at night with a headlamp (pretty much only run trails)

Pre-hydrating is important as well, even the night before. I like to run smaller loops so I can tap out if needed.

In the summer, the forest is filled with actual flies that won't leave you alone, even for an instant. It may be preferable to run on the roads. Shade is good though.

Usually it's the humidity/dew point that catches up to me. If it isn't humid, running in 90 degrees usually isn't too bad. Run early. Pay attention to heart rate. Start with ice in your hat. Etc.

What do you guys wear when it’s 90F/30C+?

Just shorts, get the rig out mate

Suns out guns out then aye?

Running in extreme heat can become dangerous once temperatures and humidity push your body past its ability to cool itself — typically above 27–30 °C (80–85 °F) for many people. To cope, run early or late in the day, stay hydrated, slow your pace, and wear light, breathable clothing.

Real Feel of 106 degrees when I ran today.

Run under arbors. If it's deep woods run shirtless.