Wet-Bulb Temperature Calculator

What is the wet-bulb temperature?

The wet-bulb temperature is the lowest temperature that air can reach by evaporating water into it. Physically, it is the temperature you would read on a thermometer whose bulb is wrapped in a wet cloth and ventilated: as water evaporates, it draws heat from the bulb until evaporation and cooling reach equilibrium.

Because our bodies cool themselves the same way — by sweating — the wet-bulb temperature is a direct measure of how well heat stress can be relieved. When the air is dry, the wet-bulb temperature sits well below the air temperature and sweat evaporates quickly. When the air is humid, the two temperatures move closer together and cooling becomes harder.

This calculator uses air temperature and relative humidity to estimate the wet-bulb temperature. Enter the temperature in °C or °F and the relative humidity as a percentage; the result can be viewed in either temperature unit.

How does the calculator work?

Computing the exact wet-bulb temperature normally requires an iterative psychrometric solution. In 2011, Roland Stull published a single-equation empirical fit that reproduces the exact value to within a few tenths of a degree over the range of conditions found at sea-level pressure, roughly relative humidities of 5–99% and temperatures of −20 °C to 50 °C.

The calculator plugs your air temperature $T$ (in degrees Celsius) and relative humidity $RH$ (as a percentage, so 50% is entered as 50) into that formula and returns the wet-bulb temperature $T_w$ in degrees Celsius, converting to Fahrenheit if you switch the output unit.

Formula

$T_w = T \cdot \arctan\!\left[0.151977\,\sqrt{RH + 8.313659}\right] + \arctan(T + RH) - \arctan(RH - 1.676331) + 0.00391838\,RH^{3/2}\,\arctan(0.023101\,RH) - 4.686035$

Here $T$ is the air temperature in °C, $RH$ is the relative humidity in percent, and every $\arctan$ is evaluated in radians. The result $T_w$ is in °C.

Worked examples

Example 1 — a warm, moderately humid day. With $T = 30\,^{\circ}\mathrm{C}$ and $RH = 50\%$:

$T_w \approx 22.3\,^{\circ}\mathrm{C}$

So the air can be cooled by evaporation only down to about 22.3 °C — comfortable, but noticeably muggy.

Example 2 — cooler and more humid. With $T = 25\,^{\circ}\mathrm{C}$ and $RH = 60\%$:

$T_w \approx 19.5\,^{\circ}\mathrm{C}$

Reference values (wet-bulb temperature in °C)

Notes and practical uses

• A sustained wet-bulb temperature of about 35 °C is widely regarded as the theoretical limit of human survivability, because at that point the body can no longer shed metabolic heat even at rest in the shade.
• Because it folds temperature and humidity into one number, the wet-bulb temperature is used in heat-stress indices, HVAC and cooling-tower design, snow-making, and agriculture.
• The Stull fit assumes standard sea-level pressure. At high altitude the true wet-bulb temperature differs slightly.
• For a related measure of how the weather “feels”, see the air density calculator and the Beaufort scale calculator.

FAQ

Is the wet-bulb temperature always lower than the air temperature? Yes, unless the air is fully saturated (100% relative humidity), in which case the wet-bulb temperature equals the air (dry-bulb) temperature.

What is the difference between wet-bulb temperature and the “feels like” heat index? The heat index estimates perceived warmth on a scale calibrated to comfort, while the wet-bulb temperature is a physical quantity — the actual lowest temperature reachable by evaporative cooling.

How accurate is this estimate? Stull’s formula typically agrees with the exact psychrometric wet-bulb temperature to within about 0.3 °C over the range of normal weather conditions.