It takes both the temperature and the humidity
The wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. Dry air evaporates a lot and cools far below the thermometer reading; saturated air cannot cool at all. That is why sweat works in a dry heat and fails in a humid one, and why you need humidity as well as temperature to get the number.
The calculator uses the Stull equation (accurate to about ±0.4 C); the line above is the quick mental version. At 100% humidity the wet-bulb equals the air temperature — there is nowhere for evaporation to go.
Same air temperature, very different danger
A dry 45 C afternoon can be worked through with water and shade. A humid 35 C day can be lethal, because the wet-bulb temperature — the real measure of heat stress — is far higher.
The cooler-looking humid day sits ~9 C higher in wet-bulb terms — close to the limit of human tolerance, while the scorching dry day is merely hard work.
What each wet-bulb band means
| Wet-bulb | Heat-stress level | What it means for activity |
|---|---|---|
| Below 18 C | None | Normal activity, no special measures |
| 18–24 C | Low | Watch for symptoms, keep drinking |
| 24–28 C | Moderate | Rest breaks, steady hydration |
| 28–31 C | High | Frequent breaks, cut heavy work |
| 31–35 C | Extreme | Suspend strenuous work |
| 35 C and above | Critical | Survivability limit even for a healthy person at rest |
Vulnerable people — older adults, anyone with heart or kidney disease, and those doing hard labor — reach danger well before these thresholds. Work-safety limits are lower again for heavy tasks and for people not acclimatized to heat.
The number the forecast hides
- Not the temperature you see. Wet-bulb is always below the air temperature (except at 100% humidity), so a "38 C" forecast can still carry a dangerous wet-bulb load once humidity is high.
- Not the dew point. Dew point is the saturation temperature; wet-bulb is the cooling you get from evaporation. They are close in very humid air but not the same.
- Humidity moves it slowly, heat moves it fast. Every +10% humidity adds roughly 1–2 C; every +5 C of air temperature adds about 4–5 C at fixed humidity.
Common questions
Why do I need humidity, not just the air temperature?
The wet-bulb temperature is how cool the air can get by evaporating water into it, and dry air evaporates far more than humid air. With temperature alone you cannot tell whether sweat will actually cool you, so both the dry-bulb temperature and the relative humidity are required.
Is a wet-bulb temperature of 35 C really fatal?
At 35 C wet-bulb the body can no longer shed heat by sweating, so a healthy person at rest overheats within hours. In practice people die well below that, around 24 to 28 C wet-bulb, when they are old, ill, working hard, or without shade and water.
How is this different from the heat index?
The heat index tells you how hot it feels in the shade. Wet-bulb temperature measures how much cooling your body can actually get. For work-safety and survivability the wet-bulb value (or WBGT) is the one that matters.
Can I estimate the wet-bulb temperature without the formula?
A rough rule is wet-bulb is about air temperature minus (100 minus humidity) divided by 5. At 25 C and 60% humidity that is 25 minus 8, so about 17 C. It is within a degree or two, enough for a quick read.
