Heat and humidity are the number one thing that wrecks my training. Not hills, not shoes, not sleep. When the air gets thick, everything falls apart: pace slows, heart rate climbs, and runs that should feel easy start feeling like survival.
For a long time I blamed "heat" in a general way. Hot day, bad run, move on. But once I started actually logging dew point next to pace and heart rate, a much clearer pattern showed up. Temperature barely predicted how a run would feel. Dew point predicted it almost every time.
What dew point actually tells you
Temperature measures how hot the air is. Dew point measures how much moisture is in it. You can have a 68 degree morning that feels fine, and a 68 degree morning that feels brutal, and the difference is dew point. Once it climbs, your sweat stops evaporating well, your body can't cool itself, and your heart has to work harder to do the same job.
That's the part that matters for training. It's not just uncomfortable. It's your heart doing extra work to hit the same pace.
The threshold showed up more than once
I didn't go looking for a number. It found me, over and over, across an entire Chicago summer training block.
April 2: dew point 65, air temp 72, humidity 81 percent. Average heart rate 155 at only a 9:23 pace. On paper it felt like a legs problem. It wasn't. It was a heart rate problem wearing a legs costume.
April 4, two days later: dew point 66, humidity 93 percent, air temp only 68. Cooler thermometer reading, worse run. If I'd only checked temperature, I would have expected an easier morning than April 2. Dew point told the real story.
That's the threshold showing up early in the year. Then peak week hit, and it showed up three more times in the space of two weeks.
Aug 16: the wall you can't pace your way around
This was an 18-miler at a 72 degree dew point, and it's the clearest example I have of a run where pacing strategy stops mattering entirely. I was forced to stop at mile 11, mile 13, and mile 15. Marathon-pace work wasn't slow, it was impossible. My heart rate sat pinned in easy-effort range no matter what pace I tried to hold.
This is the case to recognize in your own training: when heart rate won't come down regardless of how much you slow down, the run is done. You're not negotiating with the heat at that point. There's no smarter pacing plan that fixes it. The only move is to stop, or cut the workout down to something your body can actually do.
Aug 21: same runner, five days later, totally different run
Five days after the Aug 16 wall, I ran 19 miles: 11 easy plus 8 at marathon pace, averaging around 8:11 per mile. Heart rate peaked at 168, no forced stops, best run of the entire block.
Same fitness. Same training block. Five days apart. The only thing that changed was the air. That's the pairing I want people to sit with: it's tempting to read a bad heat run as a fitness problem or a bad day, but Aug 16 and Aug 21 prove it's neither. It's conditions, full stop.
Aug 30: the late start that cost me the back half
This one isolates the variable better than any other run in the block. The plan was a 6am start. A storm pushed it to 8:48am instead, a nearly three-hour delay on a 73 degree, humid, rainy day.
The first 7 miles held right on target, 8:26 to 8:43 pace. Miles 8 through 13 faded hard: 8:40, then 8:52, then 8:53. Average heart rate was 153, max 167, and it climbed steadily through the run instead of settling the way it does on a clean easy day.
Same runner, same plan, same effort level going in. The only thing that changed was the start time. That's why 6am starts aren't a preference for me during peak weeks, they're a rule. A three-hour delay in July or August isn't a scheduling inconvenience, it's a different workout.
Sep 1: when the cost shows up after the run, not during it
This was supposed to be a tempo run. 77 degree air, 71 degree dew point, 90 percent humidity. I wasn't pushing pace at all, just holding an easy 8:43 per mile. Heart rate still climbed steadily from 125 to an average of 147, peaking at 166, on effort that should have stayed flat. I cut it at 4 miles because I got nauseous.
The part worth flagging: recovery afterward took roughly double my normal time. That's the piece people skip when they think about heat. It's not just what happens during the run, it's what it costs you for the next day or two. I started watching resting heart rate and HRV for 48 hours after runs like this instead of just shrugging off a bad workout and moving on.
Between Aug 16, the faded back half of Aug 30, and this one, that's three separate heat-wall events in a single training block. That's not bad luck. That's a measurable, recurring pattern across a Chicago summer.
What it costs you in heart rate and pace
Here's where it gets concrete. Small jumps in dew point cost real seconds per mile, even when the temperature barely moves.
| Comparison | Conditions | Heart Rate | Pace |
|---|---|---|---|
| March 31 | 68°F | 142 bpm | 9:11/mi |
| April 1 | 72°F | 143 bpm | 9:39/mi |
Same heart rate, basically. A 4 degree jump in temperature cost 28 seconds a mile.
| Comparison | Conditions | Heart Rate | Pace |
|---|---|---|---|
| April 3 | Lower humidity | ~165 bpm | 9:23/mi |
| April 4 | Higher humidity, 86% | ~154 bpm | 10:07/mi |
This is the one that convinced me. Ten beats per minute lower, and still 44 seconds per mile slower. Lower effort by the heart rate number, worse outcome. Humidity was the only thing that changed.
Aug 19: what training is supposed to feel like
In the middle of that same rough stretch, one morning came in cool: 59°F air, dew point 51. That run was 8:28 per mile at an average heart rate of 143, peaking at just 156. That's a full minute per mile faster than my typical heat pace, at a noticeably lower heart rate.
I'm the same runner in that data point as I am in the Aug 16 wall and the Sep 1 nausea run. Nothing about my fitness changed between those days. The air did. If you want a single before-and-after to hold in your head, this is it: same training block, same person, and the dew point is the only variable that explains the gap.
Let heart rate make the call, not pace
Once I trusted the pattern, I stopped trying to hit pace targets on bad dew point days and started running by heart rate ceiling instead.
On that same April 4 run, I hit a heart rate spike of 175 at mile 7. Original plan was 8 miles. I stopped at 7. That's the habit worth building: when your heart rate crosses your ceiling, the run is over, not "let's see how the next mile feels."
The payoff of running that way showed up in the splits. Six laps that day landed within 13 seconds of each other, in 86 percent humidity. That's not from chasing a pace number. That's from holding effort steady and letting the watch decide, not my ego.
The rule of thumb I actually use
- Dew point under 60: normal effort, normal pace expectations.
- Dew point 60 to 65: pace will slow some, don't fight it.
- Dew point 65 and up: heart rate decouples from pace. Run by effort, not by watch pace.
- Dew point 70 and up: there's no pace that makes this safe. Shorten it, slow it way down, or move it.
Check dew point the night before a key run, not just temperature. It's a five second look at your weather app and it'll tell you more about how the run is going to go than anything else on the screen. Our Chicago Marathon weather history breakdown shows why October race days tend to be kinder than August long runs, and the marathon training block guide has more on building peak weeks that survive Chicago summers.