HVAC and refrigeration
Cooling tower performance and evaporative cooler capacity are set by wet bulb, not dry bulb. It is the design input.
Wet bulb temperature is the lowest temperature air can be cooled to by evaporating water into it. Wrap a thermometer bulb in a wet wick, move air across it, and the reading drops as the water evaporates. Where it settles is the wet bulb temperature.
That number matters because it is the same physics your body uses. Sweating is evaporative cooling, and the wet bulb temperature is the floor that cooling can reach. When the air is dry, the floor is far below body temperature and sweating works well. When the air is saturated, the floor rises toward air temperature and sweating stops working at all.
Wet bulb temperature calculator
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—Wet bulb depression
—Dew point
—Heat index
Method. Psychrometric wet bulb temperature from the Stull (2011) approximation, accurate to within a few tenths of a degree Celsius for relative humidity between 5 and 99 percent at sea level. This is the thermodynamic wet bulb, not the natural wet bulb used inside WBGT, and not the same thing as WBGT itself.
Four temperatures people confuse
These describe the same air and answer different questions.
Dry bulb temperature
Ordinary air temperature, measured by a shaded thermometer. What a forecast reports.
Wet bulb temperature
The lowest temperature evaporation can reach. Always at or below dry bulb; equal only at 100% humidity.
Dew point
The temperature at which air becomes saturated and condensation begins. Always at or below wet bulb.
Wet bulb globe temperature
A composite index for heat stress, weighting natural wet bulb, black globe and dry bulb. Not the same thing as wet bulb temperature.
How much cooling the air will allow
The gap between dry bulb and wet bulb is the wet bulb depression, and it is the clearest single measure of how much help evaporation can give you.
| Relative humidity | Wet bulb temperature | Depression | What it means |
|---|---|---|---|
| 20% | 66.7°F | 28.3°F | Large margin. Sweat evaporates freely and cools effectively. |
| 40% | 76.1°F | 18.9°F | Comfortable margin for most exertion with fluids. |
| 60% | 83.3°F | 11.7°F | Narrowing. Sustained work needs structured breaks. |
| 80% | 89.5°F | 5.5°F | Little margin left. Cooling is badly impaired. |
| 100% | 95°F | 0°F | No evaporative cooling is possible at all. |
This is why a 95°F day in Arizona and a 95°F day in Florida are not the same event. The air temperature is identical and the physics available to your body is not.
How to measure it
The classic instrument is a sling psychrometer: two thermometers on a frame, one with a wetted wick, whirled through the air.
- Wet the wick with distilled water. Tap water leaves mineral deposits that change the reading over time.
- Ventilate it. Whirl the sling, or use an aspirated psychrometer with a fan. Still air gives a falsely high reading.
- Keep whirling until the wet bulb reading stops falling, then read both thermometers quickly.
- Subtract to get the wet bulb depression, and read the relative humidity from a psychrometric chart if you need it.
- Keep the instrument out of direct sun and away from your own body heat while reading.
Note that this gives the psychrometric wet bulb temperature. The natural wet bulb used inside WBGT is deliberately measured unaspirated and unshielded, so it reflects the actual wind and sun at the site. The two are not interchangeable.
The 35°C limit
Human core temperature sits around 37°C. To shed metabolic heat, the body needs somewhere cooler to send it. A wet bulb temperature of 35°C, about 95°F, is the theoretical point at which that stops being possible: even a healthy, resting, fully shaded, fully hydrated person cannot cool down, because evaporation cannot take their skin below the surrounding wet bulb temperature.
At that point core temperature rises regardless of behaviour, and hyperthermia follows within hours. It is a physical limit, not a comfort threshold, and no amount of fitness or acclimatization moves it.
Readings that high remain rare, and have been recorded only briefly in a handful of places, mostly around the Persian Gulf and parts of South Asia. More recent work suggests the practical limit for real people is lower than 35°C, closer to 31°C for young healthy adults, because the idealised assumptions behind the number do not hold in practice.
For planning actual work or training, none of this is the operative number. Danger begins far below any survivability limit, which is what the heat stress indices exist to describe.
Wet bulb temperature compared with WBGT
The names are similar and the uses are not.
| Wet bulb temperature | WBGT | |
|---|---|---|
| What it is | A physical temperature | A weighted composite index |
| Inputs | Temperature and humidity | Natural wet bulb, black globe and dry bulb |
| Accounts for sun | No | Yes |
| Accounts for wind | Only through ventilation of the instrument | Yes |
| Used for | Psychrometrics, HVAC, cooling towers, climate limits | Occupational and athletic exposure limits |
| Appears in standards | As an input | As the measure itself, in ISO 7243 and ACGIH |
Wet bulb temperature is one of the ingredients in WBGT, weighted 0.7 outdoors. If you are deciding whether practice runs or a crew works, you want WBGT, not wet bulb temperature alone.
Who uses wet bulb temperature directly
Climate science
The 35°C threshold is used to describe where and how often parts of the planet become physiologically uninhabitable.
Meteorology
Wet bulb temperature predicts whether falling precipitation arrives as rain or snow, and drives evaporative cooling in downbursts.
Industrial hygiene
As the heavily weighted input to WBGT, and in its own right when evaluating evaporative cooling options.
Live wet bulb and WBGT for your location
The app shows wet bulb temperature alongside WBGT, humidity, wind and solar radiation, so you can see both the physical temperature and the heat stress index that depends on it.
- Live wet bulb temperature for anywhere
- WBGT computed with the Liljegren model
- Hourly and 14-day forecasts
- Fahrenheit or Celsius
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Sources
- Liljegren JC, Carhart RA, Lawday P, Tschopp S, Sharp R. Modeling the wet bulb globe temperature using standard meteorological measurements. J Occup Environ Hyg. 2008;5(10):645–655. doi:10.1080/15459620802310770
- International Organization for Standardization. ISO 7243:2017, Ergonomics of the thermal environment — Assessment of heat stress using the WBGT (wet bulb globe temperature) index.
Common questions
What is wet bulb temperature?
The lowest temperature that air can be cooled to by evaporating water into it. It is measured with a thermometer whose bulb is wrapped in a wet wick and ventilated, and it reflects how much cooling evaporation, including sweating, can actually achieve.
How do you calculate wet bulb temperature?
From air temperature and relative humidity. The calculator above uses the Stull (2011) approximation, which is accurate to a few tenths of a degree Celsius between 5 and 99 percent humidity at sea level. Traditionally it is measured directly with a sling psychrometer.
What is the difference between wet bulb and dry bulb temperature?
Dry bulb is ordinary air temperature from a shaded thermometer. Wet bulb is what a wetted, ventilated thermometer reads, and it is always lower except at 100 percent humidity. The gap between them is the wet bulb depression and it shows how much evaporative cooling the air will permit.
What wet bulb temperature is dangerous?
Risk rises well before any absolute limit. A wet bulb temperature of 35°C, about 95°F, is the theoretical point at which a healthy resting person can no longer cool by sweating, though recent research suggests the practical limit is nearer 31°C. For work and training decisions, use WBGT thresholds rather than wet bulb temperature.
Is wet bulb temperature the same as WBGT?
No. Wet bulb temperature is a physical temperature from two inputs. WBGT is a composite index that weights a natural wet bulb reading at 0.7, a black globe reading at 0.2 and dry bulb at 0.1 outdoors, so it also accounts for sun and wind.
Is wet bulb temperature the same as dew point?
No, though they are related and both equal the air temperature at 100 percent humidity. Dew point is the temperature at which air becomes saturated. Wet bulb temperature sits between the dew point and the dry bulb temperature.
Related WBGT guides
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