Wet Bulb Temperature Calculator

Wet bulb temperature is the equilibrium reached when air is cooled adiabatically to saturation by evaporating water into it, with the latent heat drawn from the air itself. Physically it is set by a balance between sensible heat flowing into a wetted surface and latent heat leaving it, which is why the governing psychrometric relation e = es(Tw) - gamma * (T - Tw) contains the psychrometric constant gamma, approximately 0.000665 times station pressure in hPa, or about 0.67 hPa per degree C at sea level. Solving that relation for Tw requires iteration because es appears as the Magnus exponential 6.112 * exp(17.67 * T / (T + 243.5)). Roland Stull's 2011 regression avoids the iteration, reproducing Tw from dry bulb and relative humidity alone with root-mean-square error near 0.3 degrees C over roughly -20 to 50 degrees C and 5 to 99 percent RH at sea-level pressure. The ordering Td is less than or equal to Tw is less than or equal to T always holds, collapsing to a single value at saturation. The gap T minus Tw, the wet bulb depression, is the evaporative cooling potential: 15 to 20 degrees C in desert air at 15 percent RH, where swamp coolers work well, but only 2 to 5 degrees C in humid tropical air, where they do not. Human thermoregulation depends on the same physics, which is why 35 degrees C wet bulb is treated as the theoretical survivability limit for an unclothed, resting, fully shaded person, though laboratory work at Pennsylvania State University published by Vecellio and colleagues in 2022 found critical limits nearer 30 to 31 degrees C in humid conditions for young healthy subjects. Two caveats matter: the Stull fit assumes about 1013 hPa and degrades at altitude, and the WBGT reported here is a shade estimate that omits the black globe radiant term, so it will understate outdoor heat stress in direct sun.

The wet bulb dates to the psychrometer, an instrument pairing a dry thermometer with one wrapped in a moistened wick. James Hutton described the evaporative cooling principle in the 1780s, and John Leslie in Scotland experimented with wet-bulb hygrometry around 1800, but the practical instrument is generally credited to Ernst Ferdinand August in Berlin, whose 1825 psychrometer gave the device its name and the psychrometric constant its early empirical values. Henri Victor Regnault refined the underlying vapor pressure measurements in the 1840s, and Sir Napier Shaw and others put the aspirated design on a firmer footing; Richard Assmann's aspirated psychrometer of 1887, which forced a steady ventilation rate past both bulbs, became the reference instrument and remained standard in weather services for a century. The engineering framework came later. Willis Carrier presented Rational Psychrometric Formulae to the American Society of Mechanical Engineers in 1911, converting the wet-bulb relation into the design mathematics of air conditioning and giving rise to the psychrometric chart that ASHRAE still publishes. The heat-stress application arrived through the military: the Wet Bulb Globe Temperature index was developed by David Minard and colleagues for the United States Marine Corps at Parris Island around 1956 after a series of recruit heat casualties, weighting natural wet bulb at 0.7, globe temperature at 0.2, and dry bulb at 0.1 outdoors. It was later codified as ISO 7243. The survivability threshold framing came from Steven Sherwood and Matthew Huber in a 2010 paper in the Proceedings of the National Academy of Sciences, and Colin Raymond and coauthors reported in Science Advances in 2020 that brief 35 degree C wet bulb readings had already occurred in the Persian Gulf and Indus Valley.