Free Wet Bulb Calculator

As global temperatures continue to break records—Canada’s 49.6 °C is a vivid example—relying solely on air temperature can underestimate the true heat stress that people face. The combined effect of heat and humidity is captured by the wet‑bulb temperature, a measure that tells how effectively the environment can cool the human body through evaporation. The Wet Bulb Temperature Calculator is a free online tool that uses the Stull formula to compute wet‑bulb temperature from just two inputs: air temperature and relative humidity. It also serves as a Humidity Calculator and provides the Wet Bulb Globe Temperature (WBGT), a more comprehensive heat‑stress index. This article explains what these metrics mean, how they are calculated, and why they are essential for safety and decision‑making in fields ranging from construction to outdoor sports.

What Is Wet‑Bulb Temperature?

Wet‑bulb temperature is the reading of a thermometer whose bulb is covered with a water‑soaked cloth and then ventilated. It represents the lowest temperature a given air mass can reach solely through the process of evaporative cooling. When the air is fully saturated (100 % relative humidity), the wet‑bulb temperature equals the ordinary dry‑bulb temperature. As humidity drops, the wet‑bulb temperature falls below the dry‑bulb value.

This concept is directly linked to how the human body regulates heat. Sweating cools the body by evaporating moisture from the skin; in humid air, evaporation proceeds more slowly, reducing the cooling effect and causing core temperature to rise. Research has shown that a wet‑bulb temperature sustained above 35 °C (95 °F) greatly increases the risk of hyperthermia and heat stroke. Therefore, the wet‑bulb reading provides a more accurate indicator of thermal danger than air temperature alone.

How to Calculate Wet‑Bulb Temperature (Stull Formula)

Many empirical equations have been proposed for wet‑bulb temperature, but the Wet Bulb Temperature Calculator relies on the Stull formula because of its good accuracy over a wide range of common conditions. It performs well for relative humidities between 5 % and 99 % and for air temperatures from –20 °C to 50 °C. Under marginal conditions—very low humidity and low temperature—the error stays within about –1 °C to +0.65 °C.

The Stull formula is:

$$\begin{aligned} T_w &= T \cdot \arctan\!\Bigl(0.151977\,\sqrt{RH+8.313659}\Bigr) + \arctan(T+RH) \\ &\quad - \arctan(RH-1.676331) + 0.00391838\,RH^{3/2}\cdot\arctan(0.023101\,RH) - 4.686035 \end{aligned}$$

where:

• $T$ – dry‑bulb (ambient) temperature in °C
• $RH$ – relative humidity as a percentage (e.g., 40 for 40 %)
• $T_w$ – wet‑bulb temperature in °C

Because the formula is designed for the Celsius scale, any temperature supplied in Fahrenheit must first be converted to °C. The calculator can handle this conversion automatically if you enter a value in °F.

Practical Applications of Wet‑Bulb Temperature

Although less familiar than the temperature read on a standard thermometer, the wet‑bulb value plays a key role in several practical areas:

• Construction and building design – Materials behave differently under varying moisture levels; wet‑bulb data helps engineers choose appropriate materials and design effective HVAC systems for different climates.
• Snowmaking – Producing artificial snow requires a precise balance of low temperature and low humidity. The wet‑bulb temperature indicates whether the air is dry enough for efficient snow production.
• Meteorology – Forecasters use the wet‑bulb temperature profile of the lower atmosphere to distinguish between rain, snow, and freezing rain, improving precipitation‑type predictions.

Wet Bulb Globe Temperature (WBGT)

The Wet Bulb Globe Temperature is an apparent temperature index that incorporates temperature, humidity, wind speed, and solar radiation. It was created to estimate the heat stress experienced by a person in direct sunlight and is widely adopted by the military, industrial hygienists, and athletic trainers to set safe limits for physical activity and fluid intake.

WBGT is determined by two common formulas:

• Outdoors – with direct solar radiation: $$\text{WBGT} = 0.7\,T_w + 0.2\,T_g + 0.1\,T$$
• Indoors or shaded – no direct sunlight: $$\text{WBGT} = 0.7\,T_w + 0.3\,T_g$$

In these equations, $T_g$ is the globe thermometer temperature, measured by a thermometer placed inside a blackened copper globe. The globe reading accounts for the heating effect of solar radiation. By combining the wet‑bulb temperature (evaporative cooling), globe temperature (radiant heat), and dry‑bulb temperature (ambient air), WBGT provides a realistic measure of the total heat load on the body. On hot days, following WBGT‑based guidelines—often recommending a threshold around 28–30 °C for heavy work—is critical for preventing heat‑related illness.