Heat
Sweat cools you only when it evaporates. Humidity determines whether it does, which makes it a more consequential number than temperature across much of the summer.
A 100°F day in Phoenix and a 90°F day in Houston are not comparable exposures, and the hotter one is frequently the safer one. The reason is that evaporative cooling — which accounts for the overwhelming majority of heat dissipation during exercise in warm conditions — depends on the vapour pressure gradient between wet skin and surrounding air.
Radiation transfers heat to cooler surroundings as infrared. It reverses when the environment is hotter than skin, which happens routinely on sunlit pavement.
Conduction transfers heat to contacting surfaces. Minor in air, substantial in water.
Convection transfers heat to moving air. It improves with wind and declines as air temperature approaches skin temperature, ceasing entirely above about 35°C.
Evaporation removes roughly 580 calories per gram of water vaporised. In warm conditions during exercise it does most of the work — and unlike the other three, it can still function when air temperature exceeds skin temperature.
That last point is the crux. Above roughly 35°C ambient, evaporation is essentially the only cooling route remaining. If humidity disables it, there is nothing left.
Relative humidity is a ratio to the air's capacity at the current temperature, and that capacity changes with temperature. Sixty percent relative humidity at 60°F and sixty percent at 90°F describe very different quantities of water in the air.
Dew point is the temperature at which air becomes saturated, and it is an absolute measure of moisture content. It is what actually determines the vapour pressure gradient.
| Dew point | Perception during exertion |
|---|---|
| Below 55°F | Comfortable; evaporation efficient |
| 55–60°F | Noticeable but manageable |
| 60–65°F | Sticky; measurable performance cost |
| 65–70°F | Uncomfortable; significant impairment |
| 70–75°F | Oppressive; substantial risk with hard effort |
| Above 75°F | Dangerous for sustained exertion |
Endurance athletes tend to track dew point rather than temperature for exactly this reason, and it explains why a 75°F morning with a 72°F dew point feels far worse than an 85°F afternoon at a 50°F dew point.
Core temperature rises faster at any given workload. Heart rate climbs for the same pace as cardiac output is redirected toward skin perfusion — a phenomenon called cardiovascular drift, which is markedly amplified in humid conditions. Perceived exertion rises disproportionately. Sweat rate increases while cooling per unit of sweat falls, accelerating fluid loss without corresponding benefit.
The result is that pace-based training targets become misleading. Effort-based or heart-rate-based pacing is considerably more appropriate in humid heat.
Move the session earlier. Dew point is relatively stable across a day while temperature is not, so early morning generally offers the lowest combined load. Note that relative humidity is typically highest at dawn, which misleads people who track the wrong metric — the absolute moisture is the same, and the temperature is lower.
Expect and accept slower paces. Reductions of 20 to 30 seconds per mile or more are normal at high dew points, and attempting to hold cool-weather pace is the common route to heat illness.
Maximise exposed skin and airflow. Loose, light, minimal clothing. Anything that traps a saturated boundary layer against the skin removes what evaporative capacity remains.
Use external cooling. Pouring water over the head and neck provides evaporative and conductive cooling that does not depend on your own sweat production. Ice in a hat or bandana works well.
Seek moving air. Wind replaces the saturated boundary layer at the skin surface and restores part of the vapour pressure gradient. An exposed ridge in a breeze can be more tolerable than a sheltered shaded valley with still air.
Raise sodium intake. Sweat losses are higher and the sessions tend to run longer in duration relative to the work accomplished.
In dry heat, shade solves most of the problem. In humid heat, shade helps with radiation and does nothing for the variable that is actually limiting you.
As wet bulb temperature approaches core temperature, evaporative cooling capacity approaches zero and core temperature rises regardless of behaviour. This threshold sits near 35°C wet bulb — achievable at 35°C air temperature with 100 percent humidity, or 40°C at around 75 percent, or 45°C at around 50 percent.
These conditions are rare but no longer hypothetical. In them, no hydration strategy, acclimatisation state or fitness level makes sustained outdoor exertion survivable. Shade and air conditioning are the only answers.
The heat index and hydration calculator combines temperature and relative humidity into the National Weather Service apparent temperature and scales the sweat estimate accordingly, so the humidity term is carried through the whole calculation rather than noted and forgotten.