Why 87 °F can be more dangerous than 105 °F
A dry 105 °F afternoon in Phoenix and a humid 87 °F afternoon in Miami sound like they aren't close. On a thermometer they aren't. On your body they can be — and the humid one can be the harder of the two.
This isn't a quirk. It's the single most important thing air temperature fails to tell you.
Your body's only real cooling system
You shed heat mainly by sweating, and sweat only cools you when it evaporates. The liquid leaving your skin carries energy away with it. Sweat that beads up and drips off does almost nothing — it costs you water and buys you no cooling.
Whether evaporation happens depends on how much moisture the surrounding air can still absorb. Dry air is thirsty and takes it readily. Air already near saturation cannot take much more, so evaporation slows, and your cooling system throttles down exactly when you need it most.
At 105 °F and 15% humidity, sweat evaporates fast and you can shed a lot of heat. At 87 °F and 85% humidity, it can't, so heat accumulates even though the number on the thermometer is 18 degrees lower.
What the wet bulb is measuring
This is why the "wet bulb" measurement exists. Wrap a wet cloth around a thermometer and the water evaporating off it cools the bulb — the drier the air, the more it cools, and the lower it reads. The wet-bulb temperature is therefore a direct measure of how much cooling evaporation can still give you.
In dry air, wet bulb sits far below air temperature. In saturated air, the two converge — and when they converge at a high value, evaporative cooling has essentially stopped working.
Where WBGT comes in
Wet bulb captures the humidity problem, but it isn't the whole picture either: standing in direct sun is harder than standing in shade, and wind helps by carrying moist air away from your skin.
Wet-bulb globe temperature (WBGT) combines all four — temperature, humidity, wind, and radiant sun — into one number. That's why occupational and athletic safety bodies set their limits on WBGT rather than on air temperature: it's the measure that doesn't get fooled by a dry heat or a humid one.
For a fuller treatment of how WBGT is built, see what is WBGT, and for how it differs from the more familiar heat index, see WBGT vs. heat index.
The practical version
- Don't compare cities by their highs. A humid 88 °F day and a dry 100 °F day are not ranked by the thermometer.
- Watch the dew point. It's the quickest humidity signal most forecasts give you. Above roughly 70 °F it's genuinely oppressive; above 75 °F, evaporation is badly impaired.
- Humidity punishes exertion hardest. At rest your heat load is small. Work or play hard, and the cooling you've lost is exactly what you needed.
This is the reason a heat forecast built on air temperature alone can read as reassuring on the days that deserve the most caution.