Article — Wet Bulb Temperature Calculator
Wet Bulb Temperature Calculator: The Real Heat-Stress Number
Wet bulb temperature combines heat and humidity into one number. The Stull 2011 approximation computes it from dry-bulb temperature and relative humidity with ±0.38 °C accuracy. Sustained wet bulb above 35 °C exceeds the human body's ability to cool itself.
If you have ever worked outside on a humid day and felt as though no breeze could cool you down, you have experienced high wet bulb temperature. It is not the temperature on the thermometer that matters most for heat stress — it is the temperature your body can actually reach by sweating. The wet bulb reading tells you that.
What wet bulb temperature actually means
The wet bulb temperature is the lowest temperature air can reach by evaporative cooling alone. Wrap a thermometer's bulb in a piece of damp muslin, swing it through the air, and the water evaporates, cooling the bulb until equilibrium. The reading you get is the wet bulb temperature.
This number captures both temperature and humidity in a single value. In dry air, evaporation is fast and the wet bulb reading sits well below the dry bulb. In saturated air, no evaporation happens and the two thermometers agree exactly. Wet bulb is therefore always at or below the air temperature, never above.
The Stull wet bulb formula
The exact thermodynamic wet bulb temperature requires iteratively solving a coupled energy and mass balance. For everyday use, Roland Stull's 2011 empirical fit is good enough: it predicts the iterative answer to within ±0.38 °C across the range –20 to +50 °C and 5 to 99 percent relative humidity. The formula is a sum of four arctangent terms plus a constant, and the calculator above evaluates it for you in milliseconds.
30 °C, 60 % RH Tw ≈ 23.9 °C35 °C, 50 % RH Tw ≈ 26.0 °C35 °C, 80 % RH Tw ≈ 31.6 °C40 °C, 90 % RH Tw ≈ 38.4 °CWet bulb vs heat index vs WBGT
Three closely related numbers describe heat stress. Heat index is the apparent temperature in the shade, estimated by NOAA from air temperature and humidity. Wet bulb temperature is a physical measurement of cooling potential. WBGT (Wet Bulb Globe Temperature) combines wet bulb with black-globe temperature (which captures solar radiation) and air temperature.
For occupational safety, OSHA, ACGIH, and the US military use WBGT, not heat index. WBGT is the standard for outdoor sport, military training, and industrial work because it accounts for the sun and wind, which the heat index ignores. Wet bulb temperature is the largest single component of WBGT.
The wet bulb thermometer dates to the 1820s, when August invented the psychrometer at the University of Berlin. The same principle is still used today: a wet thermometer and a dry thermometer, side by side, read out humidity directly from a psychrometric chart.
The 35 °C wet bulb survival limit
A 2010 paper by Sherwood and Huber in PNAS identified 35 °C wet bulb as the theoretical limit of human survival. The reasoning is simple. Normal skin temperature is about 35 °C, and the body cools itself by losing heat from skin to air. When the air's wet bulb temperature reaches 35 °C, the temperature gradient driving evaporative cooling vanishes, even at rest in the shade with unlimited water. Hyperthermia follows in a matter of hours.
The threshold is theoretical and assumes a healthy young adult. In practice, dangerous heat stress can occur at wet bulb temperatures of 28 to 32 °C, especially for the elderly, the very young, and people on certain medications. Recent research at Pennsylvania State University showed lab subjects became unable to maintain core temperature at wet bulb readings as low as 30.6 °C.
Wet bulb categories for safety
Occupational guidelines convert wet bulb temperature into action thresholds. Below 18 °C, normal activity is fine. From 18 to 24 °C, monitor outdoor workers. From 24 to 28 °C, schedule water breaks every 30 minutes. From 28 to 31 °C, limit heavy work and shorten breaks. Above 31 °C, suspend strenuous outdoor work; above 35 °C, evacuate if possible.
- Phoenix, July 2023 peaks at Tw ≈ 25 °C — dangerous but survivable for acclimatised people
- Persian Gulf, August 2015 recorded Tw ≈ 34.6 °C at Bandar Mahshahr, Iran — within 0.4 °C of the survival limit
- Jacobabad, Pakistan, July 2010 hit Tw ≈ 33 °C during a long heat wave
- South Asian heat dome, 2022 caused dozens of brief Tw ≥ 32 °C events
- Climate projections suggest Tw > 35 °C events could become common in the Gulf and Indus by 2050
Wet bulb temperature and a hotter planet
Climate change is shifting the wet bulb distribution upward. A 2020 Science Advances paper by Raymond, Matthews, and Horton found that the global frequency of extreme wet bulb events has doubled since 1979. The Persian Gulf, the Indus Valley, and parts of the US Gulf Coast already see brief excursions above 31 °C. Without aggressive mitigation, regions where outdoor work becomes physiologically impossible for parts of the year are projected to expand significantly.
How to measure wet bulb in practice
Three methods are common. A sling psychrometer spins a wet and dry thermometer through the air; a few seconds of rotation gives a reliable wet bulb reading. Aspirated psychrometers pull air past the wet thermometer with a fan, eliminating the need to swing the instrument. Electronic sensors (capacitive humidity probes plus thermistors) compute wet bulb from measured RH and T, exactly as this calculator does, with response times under a second.
When checking weather forecasts for outdoor events, ask for the wet bulb or WBGT, not just the temperature or heat index. Many national weather services now publish wet bulb forecasts during summer heat events.
Common wet bulb mistakes
Wet bulb temperature and dew point both describe humidity, but they answer different questions. Dew point is the temperature at which condensation starts. Wet bulb is the temperature evaporation can drive an exposed surface to. Wet bulb is always above dew point, often by several degrees.
The other recurring error is assuming dry heat is always safer than humid heat at the same air temperature. It usually is, but not always. Phoenix at 45 °C and 10 percent RH has a wet bulb of about 24 °C, well within safe limits for fit adults. Houston at 35 °C and 85 percent RH has a wet bulb of about 32 °C, in the extreme-stress category. Pay attention to the wet bulb, not the headline temperature.