What this calculator does
Wet bulb temperature is the lowest temperature air can reach through evaporative cooling alone, at constant pressure. It is what a thermometer reads when its bulb is wrapped in a wet wick and air is passed over it: water evaporating from the wick pulls heat away, so the reading sits below the ordinary "dry-bulb" air temperature unless the air is already saturated.
The gap between dry-bulb and wet-bulb temperature depends on relative humidity. In dry air, evaporation is fast and the wet bulb reads well below the air temperature. In saturated air, at 100% relative humidity, no more moisture can evaporate, so the wet bulb and dry bulb temperatures converge to the same value.
The formula
This calculator uses the Stull (2011) empirical approximation, a published formula that estimates wet bulb temperature directly from dry-bulb temperature and relative humidity, without needing atmospheric pressure as an input. It is accurate to within about 1 °C across the range it was fitted to and is widely used because it avoids the iterative psychrometric calculation the exact thermodynamic method requires.
| Term | Meaning |
|---|---|
| Dry-bulb temperature | The ordinary air temperature, as read by a standard thermometer. |
| Wet-bulb temperature | The temperature air would cool to through evaporation alone, at the current humidity. |
| Relative humidity | How much moisture the air is holding, as a percentage of what it could hold at that temperature before saturating. |
| Wet-bulb depression | The gap between dry-bulb and wet-bulb temperature: a rough gauge of how much evaporative cooling the air still has left in it. |
The inputs explained
| Field | What to enter |
|---|---|
| Air temperature (dry-bulb) (°C) | The current air (dry-bulb) temperature, in degrees Celsius. |
| Relative humidity (%) | The current relative humidity, as a percentage from 0 to 100. |
When to use it
Judging heat stress risk
Wet-bulb temperature matters for heat stress because it reflects how well sweat can still evaporate and cool the body. A high wet-bulb temperature means the air is already close to saturated, so evaporative cooling barely works, which is far more dangerous than the same dry-bulb reading at low humidity.
Evaporative cooling and swamp coolers
Evaporative ("swamp") coolers work by pushing air through a wet pad, cooling it toward the wet-bulb temperature. A low wet-bulb temperature means an evaporative cooler has plenty of room to work; a wet-bulb temperature close to the air temperature means it will do very little.
Aviation and industrial process checks
Wet-bulb temperature feeds into density-altitude and cooling-tower calculations, where the gap between it and dry-bulb temperature determines how effectively a process or engine can shed heat into the surrounding air.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How wet-bulb temperature changes with humidity at 30 °C
A fixed 30 °C air temperature, across a range of relative humidity levels.
| Relative humidity | Wet-bulb temperature | In Fahrenheit | Wet-bulb depression (dry-bulb minus wet-bulb) |
|---|---|---|---|
| 30% | 18.4 °C | 65.1 °F | 11.6 °C |
| 50% | 22.3 °C | 72.1 °F | 7.7 °C |
| 70% | 25.6 °C | 78.1 °F | 4.4 °C |
| 90% | 28.6 °C | 83.5 °F | 1.4 °C |
How wet-bulb temperature changes with air temperature at fixed humidity
A fixed 50% relative humidity, across a range of dry-bulb air temperatures.
| Air temperature | Wet-bulb temperature | Wet-bulb depression (dry-bulb minus wet-bulb) |
|---|---|---|
| 20 °C | 13.7 °C | 6.3 °C |
| 30 °C | 22.3 °C | 7.7 °C |
| 35 °C | 26.6 °C | 8.4 °C |
Questions
Is wet-bulb temperature the same as dew point?
No, though the two are related and both come from air temperature and humidity. Dew point is the temperature air would need to cool to (at constant moisture content) before condensation starts. Wet-bulb temperature is the temperature evaporative cooling alone would reach. Wet-bulb temperature always sits between dew point and dry-bulb temperature, and only equals dew point when the air is already saturated. The dew point calculator covers that related figure.
Why does high wet-bulb temperature matter for human safety?
The human body cools itself mainly by sweating, which relies on evaporation. Once wet-bulb temperature gets close to normal skin temperature, sweat can no longer evaporate effectively, and the body starts to overheat even at rest and in the shade. This is why very humid heat is more dangerous than dry heat at the same air temperature.
How accurate is the Stull approximation used here?
Stull (2011) fitted this formula to match the exact thermodynamic (psychrometric) calculation to within about 1 °C, across the humidity and temperature ranges it was tested on. It is a widely used shortcut precisely because it avoids an iterative calculation while staying close to the exact answer for ordinary weather conditions.
Can wet-bulb temperature ever be higher than air temperature?
No. Wet-bulb temperature can only equal dry-bulb (air) temperature, at 100% relative humidity, or sit below it. Evaporative cooling can only remove heat, never add it, so the wet bulb reading is always the lower of the two, or equal when the air is fully saturated.
For the related figure of when condensation starts, see the dew point calculator.