Wet-Bulb Temperature (Stull 2011)

Wet-bulb temperature is the lowest temperature evaporation alone can reach. Wrap a thermometer bulb in a wet wick, blow air over it, and the water evaporates; evaporation takes latent heat out of the wick, the bulb cools, and it keeps cooling until the heat arriving from the air by conduction exactly balances the heat leaving as vapour. Where that balance sits depends on how dry the air is. In saturated air nothing evaporates and the wet bulb equals the dry bulb; in genuinely dry air the depression can exceed 15 K.

Its central property is that it is a limit, not a preference. No evaporative process can go below it. A cooling tower's cold basin water approaches the ambient wet bulb and can never reach it, which is why tower performance is quoted as an approach in kelvin rather than as a temperature. An evaporative cooler's supply air is bounded by it. Human survivability at high heat is bounded by it, because sweat is evaporative cooling and a wet bulb near body temperature means sweating stops working regardless of how much water you drink. And a snowmaking droplet, atomised into cold air, cools itself by evaporating and arrives at the wet-bulb temperature rather than the air temperature — which is why snow guns run in above-freezing air.

Getting the number has historically been awkward. The exact relation is implicit: the wet bulb appears inside the saturation vapour pressure of the wet bulb, so you iterate, or you read a psychrometric chart, or you swing a sling psychrometer and read it directly. Roland Stull's 2011 paper closed the loop with a single expression — a curve fit, in Journal of Applied Meteorology and Climatology 50:2267, built by regressing an exact psychrometric solution over the whole useful range and then finding a closed form that tracked it. It is four arctangents and a power, it needs no iteration, and it is now in a great deal of code that used to iterate.

Respect its stated band, which the paper is unusually clear about. Relative humidity 5 to 99%, air temperature −20 to +50 °C, at sea-level pressure. Accuracy is roughly 0.3 K RMS with a worst case near −1 K, and the errors are largest in the corner where the air is both cold and dry at once — where the fit runs warm. That corner matters here, because it is exactly where marginal snowmaking decisions live, and an error on the warm side is the flattering direction. If a decision turns on a fraction of a degree, use a chart or a psychrometer.

The pressure caveat is the one that bites hardest in this shard and the equation cannot signal it. This is a sea-level fit. At altitude, air at the same relative humidity and temperature holds proportionally more vapour and the true wet-bulb depression is larger, so the equation runs warm again — and a ski hill at 2,000 m is precisely where the wet bulb is being read. Treat mountain answers as conservative rather than accurate.

Two last notes. Wet bulb is not dew point: dew point is where condensation begins on cooling at constant humidity, wet bulb is where evaporation stops, and the wet bulb always lies between the two. And below about 0 °C a real wet-bulb thermometer becomes an ice bulb, with a different latent heat and a different saturation curve, which is a genuine discontinuity that this smooth fit papers over.