Wet-Bulb Temperature Explained: The Number That Limits Human Cooling

Two afternoons with identical air temperatures can demand opposite survival strategies. A 38 °C desert afternoon is dry enough that sweat works; a 33 °C tropical evening with high humidity can be deadly, because the moisture in the air refuses to take your sweat. The number that separates the two is wet-bulb temperature — reading it requires nothing more exotic than a thermometer, a wet wick, and patience.

What a wet wick knows

Wrap a thermometer bulb in a damp cloth and waft air past it. Water evaporates, evaporation carries latent heat away, and the bulb cools below the air temperature — until it reaches equilibrium, where the cooling from evaporation exactly balances heat flowing back in from the air. That equilibrium reading is the wet-bulb temperature. In dry air, evaporation is fast and the wick cools a lot; in saturated air, evaporation stops entirely and the wet bulb climbs to meet the dry bulb. The gap between the two thermometers is called the wet-bulb depression, and it is a direct measurement of how much evaporative cooling the atmosphere still permits.

The reason this number matters to humans is uncomfortable: we are wet bulbs. When air temperature exceeds skin temperature (about 35 °C), the body cannot shed heat by convection — the environment is warmer than the radiator. The only remaining cooling path is evaporation, and sweat evaporating into air with a wet bulb approaching 35 °C can no longer carry away the body's metabolic heat. Core temperature then rises without limit.

The Stull formula

A real sling psychrometer — two thermometers, a wick, a swivel handle — is clumsy, so meteorologists compute the wet bulb from ordinary weather data. Roland Stull's 2011 paper fitted a closed-form expression to psychrometric data that needs only air temperature (°C) and relative humidity (%):

Tw = T·atan(0.151977·√(RH + 8.313659))
     + atan(T + RH) − atan(RH − 1.676331)
     + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035

The four trigonometric terms look arcane, but they are just a compact curve fit. Stull reported accuracy within about ±0.3 °C for relative humidity between 5 and 99% and air temperatures from −20 to +50 °C — tighter than the calibration error of most field humidity sensors, which is why the formula appears everywhere from weather models to sports-hydration guidance. Outside that envelope the fit degrades, which is why calculators built on it refuse inputs beyond the range rather than returning confident nonsense.

The survivability limit

In 2010, Sherwood and Huber asked a stark question: how hot and humid can the climate get before parts of Earth become uninhabitable? Their answer hinged on wet-bulb temperature. A resting human in the shade cannot survive a sustained wet bulb above about 35 °C, because evaporative cooling stops netting out metabolic heat. Actual danger arrives far earlier — hard exertion becomes hazardous above roughly 28 °C wet bulb, and industrial-hygiene guidance treats 31 °C as extreme — because sun exposure, clothing and workload all add heat the wick does not feel. The highest weather-station wet bulbs on record cluster in the low 30s along the Persian Gulf and South Asia, which is why those coastlines anchor every serious heat-mortality projection.

Reading the band

Wet-bulb bands translate directly into protocol. Below 20 °C, evaporative cooling works freely and heat stress is low. From 20 to 25 °C, intense exercise needs enforced hydration and rest cycles. From 25 to 28 °C, organise activity into short work-rest rotations. Above 28 °C wet bulb, hard outdoor work becomes dangerous without active cooling; above 31 °C it is extreme; approaching 35 °C is the theoretical wall. Notice what this ordering implies: a 38 °C desert afternoon with a 15 °C dew point yields a wet bulb in the low twenties — demanding but survivable — while a 33 °C humid evening can push past 30. The thermometer lies; the wick does not.

Try it yourself

Open the Wet-Bulb Temperature Calculator and run three comparisons. First, the desert-versus-tropics pair: 38 °C at 15% humidity against 33 °C at 85% — the first wet bulb lands near 20 °C, the second near 31 °C, and the bands tell opposite stories. Second, the saturation limit: at any temperature, push humidity toward 99% and watch the wet bulb close in on the air temperature, because evaporation is almost stopped. Third, your own conditions: take today's temperature and humidity, read the band, and ask whether the afternoon's actual stress matched it. For the moisture side of the same physics, pair it with the Dew Point Calculator; for the sun-angle piece of outdoor heat planning, the Sunrise & Sunset Calculator tells you when the radiant load begins and ends.