If you've trained through a summer, you've probably had the run I had last July. Same easy loop I'd been running all spring, same effort, and somehow my pace was 40 seconds slower and my heart rate was 15 beats higher by the second mile. Nothing had gone wrong with my fitness. It was 84 degrees at 7am, and that was pretty much the whole story.
What the heat is actually doing
When it's hot, your body has two jobs competing for the same blood supply: feeding the muscles doing the work and shedding heat through your skin. The hotter you get, the more blood gets routed to the surface to cool you down, which leaves a little less for the running. Your heart covers the gap by beating faster, so your heart rate climbs at a pace that felt comfortable back in April. Somewhere north of 80°F you can expect to run 20 to 30 seconds per mile slower at the same effort, and worse than that once the humidity is high enough to stop your sweat from evaporating.
Run by effort, not the watch
This is where a lot of us get into trouble. You see the goal pace on your plan, the one that was right in cooler weather, and you chase it anyway. In the heat that just digs a hole: a higher core temperature, worse recovery the next day, and a workout that cost more than it gave you. I've done it enough times to finally take the lesson. On a hot day, run the effort and let the pace be whatever it turns out to be. The same 9:30 that's genuinely easy in October is a moderate run in late July, and pretending otherwise doesn't make you any fitter.
Acclimation is faster than you'd think
The good news is your body adapts to heat pretty quickly, faster than it adapts to almost anything else in training. Give it around 10 to 14 days of consistent exposure and real changes show up: your blood plasma volume expands, you start sweating earlier and more, and your sweat turns more dilute as you hold onto salt. The first week is genuinely miserable, and I won't pretend it isn't. But somewhere in the second week a 75-degree morning that would have flattened you starts to feel almost manageable, and that's the adaptation doing its job.
Water isn't the whole story
Hydration matters, but drinking more water doesn't buy you as much as you'd think. If you want to know your actual sweat rate, weigh yourself before and after an hour-long run with no clothes on, dry both times; each pound you've dropped is roughly 16 ounces you didn't replace. Most people lose more than they'd guess. The part that tends to get skipped is sodium. You're losing salt along with the water, and if you drink a lot of plain water on a long hot run without replacing any of it, you can dilute your blood sodium enough to feel worse rather than better. For most runs an electrolyte tab or a pinch of salt in a bottle covers it. (This is one of those places where more isn't automatically better, so it's worth a little attention.)
Letting the plan flex
One last thing, and this is the part where the app I'm building comes in. Heat doesn't only slow your runs, it shows up the next morning as a higher resting heart rate and often shorter, worse sleep, both of which read as lower recovery. Wave already accounts for that, so a brutal hot week nudges your plan toward easier days instead of holding you to paces that were set when it was 20 degrees cooler. You'd adjust for the heat on your own anyway. It just helps to have something watching the trend so you don't have to talk yourself into it every morning.
None of this makes summer running feel good, exactly. But once you stop fighting the heat and start training with it, the fitness you quietly build in August has a way of showing up when the weather finally breaks. Worth the sweat, I think.