October 22, 2026 · Climate & Energy · 5 min read

Wet Bulb Temperature: The Heat Number That Matters Most

Weather reports give you temperature and humidity separately, but your body experiences their combination. Wet bulb temperature is the number that fuses them — and it's the one that decides whether humans can safely work, exercise or even survive outdoors.

What it measures

Wrap a thermometer bulb in a wet cloth and the evaporating water cools it. How much it cools depends on humidity: in dry air, evaporation is strong and the reading drops far below air temperature; in humid air, evaporation stalls and the reading barely falls. Wet bulb temperature is that cooled reading — the lowest temperature achievable by evaporation, and a direct proxy for how well your own sweating can cool you.

The Stull formula

Meteorologist Roland Stull found a simple, accurate approximation that works for most everyday conditions (air temperatures −20 to 50 °C, humidity 5–99%):

with T and Tw in °C and RH in percent. For a 35 °C day at 60% humidity it gives about 29 °C — already dangerous for sustained exertion, even though 35 °C sounds "manageable."

The 35 °C line

A wet bulb temperature of 35 °C is roughly the theoretical survival limit for a healthy human at rest: above it, evaporative cooling can no longer shed metabolic heat, and body temperature climbs regardless of how much you sweat. Fortunately that's rare on Earth — but wet bulbs of 28–31 °C, which are far more common, already force serious cutbacks for laborers and athletes.

Practical thresholds

• Below 18 °C wet bulb: heavy work fine for most people.
• 18–23 °C: schedule heavy exertion early, enforce hydration breaks.
• 23–28 °C: reduce intensity and rest frequently; heat illness risk rises fast.
• Above 28 °C: avoid sustained exertion; above 32 °C even light activity is hazardous.

The same physics explains why a fan helps in dry heat but not in humid heat — and why the air conditioner you sized with cooling load basics matters more in muggy climates than the thermostat number suggests.

Then size the machine that fights it: Heat Pump Sizing covers both the heating and cooling sides.