Guide

What is wet bulb temperature?

Wet bulb temperature is what a thermometer reads when its bulb is wrapped in wet cloth and ventilated — the lowest temperature evaporation can produce in that air. It matters because human cooling is evaporative: at a wet bulb of about 35°C, sweating cannot shed heat at all, whatever the shade or the water supply.

The measurement

Take two thermometers. Leave one alone; wrap the other's bulb in wet muslin and blow air across it. The wet one reads lower, because water evaporating from the cloth carries heat away. Where it settles is the wet bulb temperature, and the gap between the two is a direct measure of how much evaporation the air will accept.

In a desert the gap is enormous — 40°C air at 10% humidity gives a wet bulb near 19°C. In a tropical downpour there is no gap at all.

This is the original instrument behind every humidity measurement: a sling psychrometer is just those two thermometers on a handle you whirl around your head.

Why it is the number that decides survivability

The human body runs at roughly 37°C and skin sits near 35°C when heat is flowing out. Heat only moves from warmer to cooler. So if the wet bulb temperature of the surrounding air reaches 35°C, there is no longer anywhere cooler for the heat of metabolism to go, even with unlimited sweat.

That is a different kind of statement from a comfort index. The heat index says conditions are dangerous. The wet bulb limit says the physics has run out.

Two important refinements:

35°C is a theoretical ceiling, derived rather than measured. Sherwood and Huber's 2010 paper set it as an upper bound on adaptability, not a threshold observed in people.
Experiments put the practical limit lower. Work at Penn State exposing young, healthy subjects to controlled conditions found critical environmental limits — the point at which core temperature begins an uncompensated rise — nearer 31°C wet bulb in humid conditions, and lower still for older adults or anyone doing physical work.

So a forecast wet bulb of 31°C is not comfortably inside the safe zone. It is already at the edge of what controlled trials describe as survivable for a resting young adult.

Worked example: two 40°C days

Phoenix: 40°C, 15% humidity → wet bulb 21°C
Persian Gulf coast: 40°C, 75% humidity → wet bulb 35°C

14°C of difference in what the body can do about it, with identical thermometer readings. One is a hot afternoon; the other is at the theoretical limit of human tolerance.

Where the world gets closest

Published analyses of station data find the highest wet bulb temperatures clustering in a few specific places: the coast of the Persian Gulf around Dhahran and Bandar Mahshahr, the northern Indus valley in Pakistan, the southern Red Sea, and parts of the Gulf of California. The common feature is not simply heat — it is hot air over very warm shallow water, so the air arrives both hot and nearly saturated.

Notably, the hottest air temperatures on Earth do not produce the highest wet bulbs. Death Valley is dry.

Wet bulb, natural wet bulb and WBGT

Three related things, easy to conflate:

The distinction that matters when reading standards.
Quantity	Conditions	Used for
Psychrometric wet bulb	Shaded and ventilated	The survivability discussion; what "wet bulb temperature" means in conversation and in climate science
Natural wet bulb	Left out in the open, sun and ambient wind	An input to WBGT, weighted 0.7
WBGT	Weighted average of natural wet bulb, black globe and dry bulb	Occupational and sports heat-stress standards

The number this site's wet bulb calculator gives is the first one. If you need the third, the WBGT estimator is the right page, and WBGT explained covers how the three fit together.

Reading a wet bulb figure sensibly

Compare it to 31°C, not 35°C, if the question is about actual risk to people.
Remember it assumes shade. In sun, the relevant quantity is the natural wet bulb, which reads higher.
Check the altitude. Wet bulb depends on pressure; calculators, including this one, generally assume sea level.
Do not use it as a comfort scale. It is a limit. For everyday decisions the heat index and its four published bands are the more useful instrument.

Common questions

What is a dangerous wet bulb temperature?

There is no official band table, but the literature gives a shape. Below about 25°C, heat is a matter of exertion and duration. From about 28°C, sustained heavy work becomes hard to do safely. From about 31°C, prolonged exposure is dangerous even at rest — this is where recent experimental work puts the practical limit. 35°C is the theoretical ceiling.

Has 35°C wet bulb ever been recorded?

Yes, briefly and locally. Analyses of weather station records have found short episodes at or above 35°C on the Persian Gulf coast and in parts of the Indus valley, generally lasting hours rather than days, and almost always in coastal locations where hot air meets very warm sea water.

Why is wet bulb lower than air temperature?

Because evaporation cools. Water leaving the wet cloth takes latent heat with it, so the bulb settles below the air temperature. The drier the air, the more water can leave, and the larger the gap. At 100% humidity nothing can evaporate and the two temperatures are equal.

Is wet bulb temperature the same as dew point?

No, though both describe moisture. Dew point is the temperature at which air would become saturated if cooled without adding water. Wet bulb is the temperature reached by evaporating water into the air. The wet bulb always sits between the dew point and the air temperature.

Can a fan help above 35°C wet bulb?

Not by cooling. A fan works by accelerating evaporation from skin, and above the limit that evaporation cannot remove enough heat regardless of airflow. Above roughly 35°C air temperature a fan can actively add heat by blowing hotter air across the skin. What helps is cooler or drier air, water immersion, or air conditioning.

Sources

Sherwood, S. & Huber, M. (2010). An adaptability limit to climate change due to heat stress — PNAS — the origin of the 35°C figure
Vecellio, D. J. et al. (2022). Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects — Journal of Applied Physiology — experimental limits nearer 31°C
Raymond, C., Matthews, T. & Horton, R. M. (2020). The emergence of heat and humidity too severe for human tolerance — Science Advances — observed exceedances
Stull, R. (2011). Wet-Bulb Temperature from Relative Humidity and Air Temperature

Published 30 July 2026