How this instrument works
Wet-bulb temperature is the reading a ventilated thermometer gives once its bulb is covered in a wet wick and water evaporates freely from it. Evaporation pulls latent heat from the wick, cooling the bulb below the surrounding air's own dry-bulb reading, until the cooling from evaporation exactly balances the sensible heat the moving air keeps supplying back to it. That balance point sits between the dew point and the air temperature, and the two converge only when the air is already saturated — at 100% relative humidity there is no spare capacity left to evaporate anything, so wet-bulb and dry-bulb read the same.
The catch is that the true wet-bulb temperature comes from an implicit energy-balance equation with no algebraic solution — the classic method is an iterative search or a printed psychrometric chart, both slow for a single lookup. Roland Stull published a way around that in 2011: an explicit formula built from arctangent terms, fit by regression directly to output generated by solving the implicit equation across a wide grid of temperature and humidity pairs. None of its six constants, 0.151977 through 4.686035, represents a physical quantity on its own; together they reproduce the iterative answer to within roughly 0.3°C on average, without a single loop.
That accuracy holds from about 5% to 99% relative humidity at sea-level pressure, the window this instrument checks its inputs against. HVAC engineers lean on a fast wet-bulb figure to size cooling towers and evaporative condensers, industrial-hygiene teams feed it into the natural-wet-bulb term of a heat-stress index, and meteorologists use it to flag when falling snow will partially melt on the way down. Push the humidity toward the very dry end, or well below freezing, and the fit's error grows past that average, which is why demanding engineering work still keeps the iterative solution on hand as a check.
- Enter the still-air reading in Air (dry-bulb) temperature — the ordinary thermometer value, in °C or °F.
- Enter Relative humidity, % from a hygrometer or sling psychrometer; keep it above about 5% for the fit to hold.
- Read Wet-bulb temperature: it always lands between the dew point and the dry-bulb value you entered.
- Subtract the result from the dry-bulb reading to see the wet-bulb depression — a wide gap means air with plenty of evaporative cooling capacity left.
Worked example — 20°C air at 50% relative humidity
A sling psychrometer read outdoors on a mild spring afternoon shows Air (dry-bulb) temperature at 20°C and Relative humidity, % at 50. Feeding both into Stull's formula — the arctangent of the humidity-scaled square root, plus the two arctangent terms in T and RH, plus the RH^1.5 correction, minus the fixed offset — returns Wet-bulb temperature of 13.699341969°C, which the field rounds to 13.70°C.
The gap between the two readings, 20 minus 13.70, is a wet-bulb depression of about 6.3°C — a moderate spread that a portable evaporative cooler run at 100% efficiency could theoretically pull the supply air down toward, though real units fall well short of that ideal. A meteorologist reading the same pair off a psychrometric chart, or solving the implicit energy-balance equation by iteration, would land on essentially the same 13.7°C, which is the entire point of using Stull's fit: matching the chart without the chart.
Questions
What is wet-bulb temperature measuring, physically?
The equilibrium temperature of a thermometer bulb kept wet and ventilated: evaporation drains heat from the wick until that cooling exactly balances the sensible heat the passing air keeps delivering back. It always sits between the dew point and the dry-bulb (air) temperature, equal to dry-bulb only when the air is already saturated at 100% relative humidity, since saturated air has no capacity left to evaporate any more water.
Why use a curve fit instead of solving the real psychrometric equation?
Because the real equation is implicit — wet-bulb temperature appears on both sides through the saturation vapor pressure at that same unknown temperature — so it has no closed-form solution and traditionally needed iteration or a printed chart. Stull's 2011 regression was fit directly to a table generated by solving that equation, reproducing it to within about 0.3°C on average from one explicit formula, no loop required.
Is this the same figure as wet-bulb globe temperature (WBGT) used for heat-stress warnings?
No. WBGT is a separate, weighted index — commonly about 0.7 times a natural (unventilated, sun-exposed) wet-bulb reading plus 0.2 times a black-globe temperature plus 0.1 times dry-bulb temperature — used by OSHA and similar bodies to set work-rest limits. This instrument returns the plain, ventilated psychrometric wet-bulb temperature, one plausible input among several to a WBGT calculation, not WBGT itself.
Why does wet-bulb temperature equal the air temperature at 100% humidity?
Because evaporative cooling needs somewhere for the evaporated water to go, and saturated air already holds all the vapor it can at that temperature. With no evaporation possible, the wick sheds no extra heat, so the wet-bulb reading stops falling below dry-bulb and the two converge exactly. Watch the result field approach the entered air temperature as relative humidity climbs toward 100%.
What happens outside the roughly 5%–99% relative humidity range?
The instrument flags relative humidity below about 5% as outside Stull's calibrated window, where the fit's error grows beyond its typical 0.3°C average and the arctangent terms were not validated against the underlying psychrometric table. For humidity that low, or for non-standard pressure such as high altitude, the iterative energy-balance solution or a pressure-corrected psychrometric chart remains the more trustworthy source.
Why do evaporative coolers get mentioned alongside this number?
Because wet-bulb temperature is the theoretical floor a direct evaporative (swamp) cooler's supply air approaches as its saturation efficiency nears 100%. A real cooler at 80% efficiency only closes 80% of the gap between dry-bulb and wet-bulb temperature, which is why the wet-bulb depression calculated here — dry-bulb minus wet-bulb — is the number evaporative-cooling equipment is sized against, not the dry-bulb reading alone.