Wet Bulb Temperature Calculator

Calculate wet-bulb temperature and psychrometric depression from air temperature and relative humidity. Uses the Stull (2011) formula, valid for -20°C to 50°C and 5–99% RH.

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Calculation Examples

Calculation Case Result
Moderate summer day: 30°C at 50% RH Wet-bulb: ~22.6°C (moderate evaporative cooling potential)
Extreme humid heat: 35°C at 80% RH Wet-bulb: ~31.9°C (approaching danger threshold)
Arid climate: 35°C at 10% RH Wet-bulb: ~16.1°C (high cooling potential, evaporative cooling highly effective)

How to Use the Wet Bulb Temperature Calculator

Enter two inputs: the dry-bulb temperature (standard air temperature from a shaded thermometer) and the relative humidity as a percentage. Select your preferred temperature unit (Celsius or Fahrenheit — the calculator converts internally before applying the formula, which is defined in °C).

The calculator returns two values: the wet-bulb temperature (\(T_w\)) and the psychrometric depression (\(T_d - T_w\)). The depression is the gap between dry-bulb and wet-bulb temperature and serves as a direct indicator of atmospheric dryness and evaporative cooling potential. A depression of 10°C or more indicates very dry air with high cooling capacity; a depression near zero means the air is nearly saturated and evaporative cooling is ineffective. This is the condition that drives heat stress: at 100% RH, \(T_w = T_d\) and no further cooling through perspiration is possible.

The Wet-Bulb Temperature Formula

The most widely used computational method for wet-bulb temperature without a physical sling psychrometer is the Stull (2011) empirical formula, published by Professor Roland Stull of the University of British Columbia in the Journal of Applied Meteorology and Climatology: \[T_w = T \cdot \arctan(0.151977 \cdot (RH + 8.313659)^{0.5}) + \arctan(T + RH)\] \[- \arctan(RH - 1.676331) + 0.00391838 \cdot RH^{1.5} \cdot \arctan(0.023101 \cdot RH) - 4.686035\] where \(T\) is dry-bulb temperature in °C and \(RH\) is relative humidity in percent (not decimal).

The formula is valid for \(-20°C \leq T \leq 50°C\) and \(5\% \leq RH \leq 99\%\), covering virtually all inhabited terrestrial environments. Within this range, Stull reported a mean absolute error of approximately 0.28°C against direct psychrometric measurements — sufficient precision for heat stress assessment, HVAC design, and meteorological monitoring. Outside this range (for example, at very high altitudes where atmospheric pressure is significantly reduced), the formula loses accuracy because evaporative cooling rates depend on barometric pressure, which the Stull formula does not explicitly include. For those conditions, a full psychrometric calculation incorporating station pressure is recommended.

Wet-bulb temperature diagram showing the relationship between dry-bulb temperature, relative humidity, psychrometric depression, and the 35°C survivability threshold

Useful Tips 💡

  • When measuring dry-bulb temperature for input, shield the thermometer from direct sunlight and ensure adequate airflow around the sensor. Unshielded sensors can read 5–10°C above actual air temperature in direct sun.
  • If \(RH = 100\%\), the result will equal the dry-bulb temperature exactly (\(T_w = T_d\)). This is the physical saturation point where evaporative cooling stops entirely.
  • The Stull formula is calibrated for sea-level to moderate-altitude conditions. At elevations above 2,500 m, reduced barometric pressure increases evaporation rates, making the actual wet-bulb temperature slightly lower than the formula predicts.

📋Steps to Calculate

  1. Enter the dry-bulb temperature (standard shaded air temperature) and select Celsius or Fahrenheit.

  2. Enter the relative humidity as a whole number percentage (e.g., 65 for 65%). Do not enter a decimal.

  3. Click Calculate to get the wet-bulb temperature and the psychrometric depression (\(T_d - T_w\)).

Mistakes to Avoid ⚠️

  1. Entering relative humidity as a decimal (0.65) instead of a percentage (65). The Stull formula requires RH in percent; entering 0.65 produces a result that is not a wet-bulb temperature.
  2. Confusing wet-bulb temperature with dew point: both are lower than dry-bulb temperature, but dew point is the temperature at which air becomes saturated (RH reaches 100%), while wet-bulb temperature reflects the evaporative cooling limit, which is always equal to or above the dew point.
  3. Using the calculator outside its validated range: inputs below -20°C or above 50°C, or RH below 5% or above 99%, produce results with increasing error. For desert conditions below 5% RH, direct psychrometric calculation is more reliable.
  4. Treating wet-bulb temperature as equivalent to WBGT (Wet-Bulb Globe Temperature): WBGT also incorporates radiant heat (globe temperature) and is used for occupational safety standards, while wet-bulb temperature alone is a purely atmospheric measurement.

Practical Applications of Wet-Bulb Monitoring📊

  1. Occupational health and safety: Assess heat stress risk for outdoor workers, military personnel, and athletes using wet-bulb temperature as the primary input for WBGT calculations (ISO 7933, OSHA heat illness guidelines).

  2. HVAC and evaporative cooling: Determine the maximum cooling achievable by evaporative (swamp) coolers and cooling towers — both are physically limited by the ambient wet-bulb temperature.

  3. Climate risk assessment: Monitor proximity to the 35°C wet-bulb survivability threshold in heat-vulnerable regions, a metric highlighted in IPCC AR6 (2021) as a critical climate tipping point.

  4. Agriculture and horticulture: Predict frost risk for overnight dew point conditions, optimize greenhouse ventilation, and assess snow-making viability at ski resorts.

Questions and Answers

What is wet-bulb temperature and why does it matter?

Wet-bulb temperature (\(T_w\)) is the lowest temperature achievable by evaporating water into the air at constant pressure — it represents the physical cooling limit for any evaporation-based process, including human perspiration. At low relative humidity, \(T_w\) is well below air temperature and the body can shed heat effectively. As humidity rises toward 100%, \(T_w\) approaches air temperature and evaporative cooling becomes impossible. This is why a 35°C day at 90% RH is far more dangerous than a 40°C day at 10% RH: the wet-bulb temperatures are approximately 34°C and 18°C respectively.

How does wet-bulb temperature differ from dry-bulb temperature?

Dry-bulb temperature is standard air temperature measured by a thermometer shielded from moisture and radiation. Wet-bulb temperature is measured by wrapping the thermometer bulb in a water-soaked wick and passing air over it; evaporation from the wick absorbs latent heat and lowers the reading. The difference between the two — the psychrometric depression (\(T_d - T_w\)) — indicates atmospheric dryness: a large depression means dry air and effective evaporation; a depression near zero means saturated air where evaporation is negligible.

Why is wet-bulb temperature critical for heat stress assessment?

Wet-bulb temperature determines how effectively the human body can cool itself through sweat evaporation, which is the primary thermoregulation mechanism above approximately 35°C air temperature. Research published in Nature Climate Change (Sherwood and Huber, 2010) and confirmed by field studies identifies a sustained \(T_w\) of 35°C as the theoretical upper survivability limit for healthy adults at rest in the shade: at this point the body cannot lose heat fast enough to prevent hyperthermia even with unlimited water and ventilation. The IPCC AR6 (2021) identified expanding geographic exposure to wet-bulb temperatures above 30–35°C as a major climate risk for tropical and subtropical populations.

Can this calculator be used for indoor environments?

Yes. For indoor use, enter the indoor air temperature and the indoor relative humidity from a hygrometer. The result shows the wet-bulb temperature of the indoor environment, which determines the maximum cooling achievable by evaporative coolers in that space and helps HVAC engineers assess whether ventilation is sufficient to prevent heat stress. In industrial facilities, greenhouses, and data centers, indoor wet-bulb temperature is also used to calculate the Vapor Pressure Deficit (VPD) — a key variable for plant transpiration management and server cooling efficiency.

What is the relationship between wet-bulb temperature and WBGT?

Wet-bulb temperature (\(T_w\)) is one component of the Wet-Bulb Globe Temperature (WBGT) index, but the two are not interchangeable. WBGT is a composite heat stress index used by ISO 7933 and OSHA that weights three measurements: the natural wet-bulb temperature (70% weight), the globe temperature — a black-globe thermometer measuring radiant heat — (20% weight), and the dry-bulb temperature (10% weight). The 70% weighting reflects that humidity and evaporative potential are the dominant factors in heat stress. This calculator computes \(T_w\) (the psychrometric wet-bulb temperature); WBGT additionally requires a globe thermometer reading.

How accurate is the Stull formula used in this calculator?

The Stull (2011) formula achieves a mean absolute error of approximately 0.28°C across the validated range of -20°C to 50°C and 5% to 99% RH, based on comparison with direct psychrometric measurements. This is sufficient for heat stress monitoring, HVAC design, and meteorological assessment. For regulatory compliance (OSHA heat illness determinations, ISO 7933 occupational exposure limits) or laboratory calibration, a calibrated sling psychrometer or an Assmann aspirated psychrometer remains the primary reference instrument, as electronic calculations do not account for local air pressure variations. At elevations above 2,500 m, expect the formula to slightly overestimate \(T_w\) by 0.5–1.5°C.

What formula does this wet-bulb calculator use?

The calculator applies the Stull (2011) empirical formula published in the Journal of Applied Meteorology and Climatology (Vol. 50, pp. 2267–2269): \[T_w = T \cdot \arctan(0.151977 \cdot (RH + 8.313659)^{0.5}) + \arctan(T + RH) - \arctan(RH - 1.676331) + 0.00391838 \cdot RH^{1.5} \cdot \arctan(0.023101 \cdot RH) - 4.686035\] where \(T\) is dry-bulb temperature in °C and \(RH\) is relative humidity in percent. The formula uses arctangent functions to model the non-linear latent heat of vaporization relationship. It is valid for \(-20°C \leq T \leq 50°C\) and \(5\% \leq RH \leq 99\%\).

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Disclaimer: This calculator is designed to provide helpful estimates for informational purposes. While we strive for accuracy, financial (or medical) results can vary based on local laws and individual circumstances. We recommend consulting with a professional advisor for critical decisions.