Eighteen degrees on the thermometer, an easy hour on the schedule. By halfway your heart rate sits ten beats above what the pace deserves. The run felt like a grind and the forecast said mild. The thermometer wasn’t lying. It was measuring the wrong thing.
Why dew point beats temperature
Dew point is the temperature at which air is saturated: the higher it sits, the fuller the air already is, and the less sweat it will take off your skin. That matters because running cools you by evaporation: sweat sitting on skin does nothing; sweat that evaporates carries heat away. Temperature says how hot the air is. Dew point says whether your cooling works.
That is why dew point drives heat stress more than temperature. At 28 °C in desert-dry air, sweat leaves as fast as you make it and the pace survives. At 21 °C with a dew point of 20 °C the air is nearly full: sweat sits, core temperature climbs, and your body answers the only way it can: more blood sent to the skin, a higher heart rate at the same pace. The mild-looking morning costs more than the hot dry one.
What heat shows up as in your numbers
A humid run leaves fingerprints. Heart rate climbs through the run at a held pace. That is the drift the sport calls aerobic decoupling. Efficiency falls with it: you cover less ground per heartbeat than the same effort earns on a cool day. Read without the weather, both look like lost fitness. They aren’t.
This is why debrief reads the conditions with the run. Temperature and humidity become a heat-stress index (a simplified wet-bulb globe temperature, for the textbook name), computed second by second across the session and reported beside the dew point. A slow split in a heatwave isn’t a slow split. But you can only say that with the weather in hand.
A rough guide for muggy days
Bands like these circulate among coaches with small variations. Treat them as starting points, not physiology. Runners differ in heat more than any table admits.
| Dew point | On the run |
|---|---|
| under 10 °C | Not a factor. Run the day as planned. |
| 10–15 °C | Comfortable. Noticeable, at most, late in a long run. |
| 16–18 °C | Easy days feel honest; hard days need patience. |
| 19–21 °C | Slow the easy runs. Judge hard days by effort, not pace. |
| over 21 °C | Survival running. Forget the watch; stay conversational. |
The easy run that sits at 5:40/km on an April morning is honest work at 6:10/km when the dew point reaches 20 °C. When in doubt, hold the effort and let pace land where it lands. The fitness is unchanged; the cooling bill is higher.
Your heat response is measurable from your own runs
A table can’t know you. Two runners on the same muggy morning pay different prices, and the same runner pays less in September than in June. The honest way to find your price is to measure it.
debrief fits it from your history: a robust regression of drift and efficiency against dew point across a year of your running. A time term in the model absorbs fitness change, so a spring of good training doesn’t get misread as heat tolerance. And the model stays silent until it has enough to stand on: twenty-five eligible sessions spanning at least 8 °C of dew point. No number is better than a fake number.
Once it speaks, the muggy long run gets read fairly: the drift the heat explains is set aside, and whatever remains is the part worth coaching.
Stop guessing on muggy days
Check the dew point before you check your pace, hold effort when the air is full, and stop reading humid splits as bad form. This is one piece of how debrief reads a run; the same eye goes over your training load and your long-run durability. For the whole picture, see what an AI running coach covers.