24.81
°C
76.65
°F
5.19
°C
24.81
°C
76.65
°F
5.19
°C
The wet bulb temperature is the lowest temperature that air can reach through evaporative cooling alone. It is measured by wrapping a standard thermometer in a wet cloth and allowing air to pass over it — as water evaporates, it draws heat from the thermometer, lowering the reading until an equilibrium is reached between heat gain from the air and heat loss from evaporation.
This measurement is critically important in many fields. In meteorology, it helps predict precipitation type (rain vs. snow). In industrial hygiene, the wet-bulb globe temperature (WBGT) is the standard metric for assessing heat stress in workplaces. In climate science, a wet bulb temperature exceeding 35 °C is considered the upper survivability limit for humans, because the body can no longer cool itself through sweating. In HVAC engineering, wet bulb temperature determines the performance of cooling towers and evaporative coolers.
Our Wet Bulb Calculator uses Stull's empirical formula (2011), a closed-form approximation that provides accurate results without iterative psychrometric calculations. Enter the air temperature (°C) and relative humidity, and the tool returns the wet bulb temperature in both Celsius and Fahrenheit, along with the evaporative cooling potential — the temperature drop achievable through evaporation.
The calculator uses Stull's empirical formula (2011), which provides a direct, non-iterative approximation of the wet bulb temperature:
$$T_w = T \cdot \arctan\left[0.151977\sqrt{RH + 8.313659}\right] + \arctan(T + RH) - \arctan(RH - 1.676331) + 0.00391838 \cdot RH^{1.5} \cdot \arctan(0.023101 \cdot RH) - 4.686035$$
where T is the air (dry bulb) temperature in °C and RH is relative humidity in percent. The formula uses multiple arctangent terms to approximate the complex psychrometric relationship between temperature, humidity, and wet bulb temperature.
Stull's formula is accurate to within ±0.3 °C for relative humidity between 5% and 99% and temperatures between -20 °C and 50 °C, making it suitable for virtually all terrestrial conditions.
The evaporative cooling potential is $$T - T_w$$, representing the maximum temperature reduction achievable through evaporation. This is the working principle behind swamp coolers, cooling towers, and the human sweating mechanism.
A wet bulb temperature below 20 °C indicates good evaporative cooling potential and comfortable conditions. Between 20–27 °C, evaporative cooling is still effective but heat stress risk increases with physical exertion. Between 27–32 °C, heat stress becomes a serious concern for outdoor workers. Above 32 °C, the body's cooling ability is severely compromised. At 35 °C, sustained human survival is impossible even in the shade with unlimited water, as the body can no longer shed metabolic heat through sweating.
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At 35 °C with 75% humidity, the wet bulb is ~31.9 °C — extremely dangerous. Only 3.1 °C of evaporative cooling is available, severely limiting the body's ability to cool itself.
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At 40 °C with only 15% humidity, the wet bulb drops to ~21 °C. Nearly 19 °C of evaporative cooling is available, making swamp coolers and sweating highly effective despite the extreme heat.
Wet bulb temperature is the temperature a parcel of air would reach if cooled solely by evaporating water into it at constant pressure until saturation. Physically, it is measured with a thermometer wrapped in a wet wick exposed to air flow. It always falls between the dew point (minimum) and the dry bulb temperature (maximum), equaling both only when the air is fully saturated (100% RH).
At a wet bulb temperature of 35 °C, the air is too warm and humid for sweat to evaporate, which is the body's primary cooling mechanism. Without evaporative cooling, core body temperature rises uncontrollably even at rest in the shade, leading to fatal hyperthermia within hours. This limit was identified by Sherwood and Huber (2010) and represents a hard physiological threshold independent of fitness, acclimatization, or hydration.
In HVAC, wet bulb temperature determines the performance of cooling towers and evaporative coolers, both of which rely on evaporating water to reject heat. The approach temperature (how close the cooled water gets to the wet bulb) is a key design parameter. Lower wet bulb temperatures mean more effective evaporative cooling, which is why swamp coolers work well in dry climates but poorly in humid ones.
Both relate to atmospheric moisture but measure different things. The dew point is the temperature at which condensation begins — it is purely a moisture content indicator. The wet bulb temperature is the lowest temperature achievable through evaporative cooling — it depends on both moisture content and the air's ability to accept more moisture. The wet bulb is always between the dew point and the dry bulb temperature.
Stull's 2011 empirical formula is accurate to within ±0.3 °C for relative humidity between 5–99% and temperatures between -20 °C and 50 °C. This covers virtually all terrestrial weather conditions. For extreme precision in laboratory or industrial settings, iterative psychrometric calculations using the full Clausius-Clapeyron equation may be preferred, but Stull's formula is more than sufficient for weather, safety, and engineering applications.
Yes. As global temperatures rise, wet bulb temperatures are also increasing, particularly in tropical and subtropical regions. Several locations in the Persian Gulf, South Asia, and the Amazon have already experienced wet bulb temperatures approaching 35 °C during extreme events. Climate projections suggest more frequent and widespread exceedances of dangerous wet bulb thresholds in coming decades, posing significant risks to outdoor laborers and vulnerable populations.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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