Wet Bulb Temperature Calculator
Find wet-bulb temperature from air temperature and relative humidity using the Stull (2011) empirical formula, no psychrometric chart needed.
🌡️ What is Wet-Bulb Temperature?
Wet-bulb temperature is the lowest temperature air can be cooled to through evaporative cooling alone, at constant atmospheric pressure. It's physically measured with a thermometer whose bulb is wrapped in a water-soaked wick and ventilated, as water evaporates from the wick it absorbs latent heat from the surrounding air, lowering the reading below the ordinary (dry-bulb) air temperature.
Meteorologists and engineers rely on wet-bulb temperature constantly, to assess outdoor heat stress risk for workers and athletes (the basis of the Wet Bulb Globe Temperature index), to size and evaluate evaporative air conditioners and industrial cooling towers, to forecast the type of precipitation falling through a shallow warm layer (snow can survive briefly above freezing if wet-bulb temperature stays near 0 C), and to protect crops from frost using overhead sprinklers that exploit evaporative cooling limits.
A common point of confusion is assuming wet-bulb temperature is close to air temperature. In dry climates it can be dramatically lower, a 40 C day at 10 percent relative humidity might have a wet-bulb temperature near 15 C, which is exactly why evaporative coolers work so well in arid regions but do almost nothing in humid ones, where wet-bulb and air temperature converge.
This calculator computes wet-bulb temperature directly from air temperature and relative humidity using the Stull (2011) empirical formula, a fast, validated approximation that avoids the need for an iterative psychrometric chart lookup.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Mild Reference Case
T = 20°C, RH = 50%
Example 2 — Hot, Dry Afternoon
T = 35°C, RH = 30% (desert-like heat)
Example 3 — Cool, Humid Morning
T = 10°C, RH = 80% (damp, near-saturated air)
❓ Frequently Asked Questions
🔗 Related Calculators
What is wet-bulb temperature?
Wet-bulb temperature is the lowest temperature air can be cooled to by evaporating water into it at constant pressure, physically measured with a thermometer whose bulb is wrapped in a wet wick and ventilated. It's always at or below the actual (dry-bulb) air temperature, since evaporative cooling only lowers temperature.
How do you calculate wet-bulb temperature from temperature and humidity?
This calculator uses the Stull (2011) empirical formula: Tw = T*atan[0.151977*sqrt(RH+8.313659)] + atan(T+RH) - atan(RH-1.676331) + 0.00391838*RH^1.5*atan(0.023101*RH) - 4.686035, where T is air temperature in Celsius, RH is relative humidity as a percentage, and all arctangent terms are evaluated in radians.
Why is wet-bulb temperature always lower than air temperature?
Wet-bulb temperature reflects evaporative cooling, as water evaporates from the wet wick, it absorbs latent heat from the surrounding air, lowering the thermometer reading. The only time wet-bulb equals air (dry-bulb) temperature is at 100 percent relative humidity, when the air is already saturated and no further evaporation, and therefore no further cooling, can occur.
What is the Stull (2011) wet-bulb formula and how accurate is it?
Published by meteorologist Roland Stull in the Journal of Applied Meteorology and Climatology (2011), it's a direct empirical formula that estimates wet-bulb temperature from air temperature and relative humidity without needing an iterative psychrometric solver. It's accurate to roughly 0.3 C at standard sea-level pressure across temperatures from -20 to 50 C and relative humidity from 5 to 99 percent.
Why does wet-bulb temperature matter for heat safety?
Wet-bulb temperature captures both heat and humidity together, which determines how effectively the human body can cool itself through sweat evaporation. A wet-bulb temperature above roughly 35 C is considered a survivability threshold, since sweat can no longer evaporate fast enough to keep the body from overheating, even in shade with unlimited water.
What is wet-bulb depression?
Wet-bulb depression is the difference between air (dry-bulb) temperature and wet-bulb temperature. A large depression means dry air with strong evaporative cooling potential, useful for evaporative cooler sizing, while a small depression (close to zero) means humid air near saturation, where evaporative cooling barely helps.
How is wet-bulb temperature different from dew point?
Dew point is the temperature air must cool to at constant moisture content to reach saturation. Wet-bulb temperature is the temperature reached through active evaporative cooling, which also adds moisture to the air. Wet-bulb temperature is always between dew point and air temperature (dew point <= wet-bulb <= dry-bulb), except at 100 percent RH where all three are equal.
Does wet-bulb temperature depend on air pressure?
Yes, true psychrometric wet-bulb temperature depends slightly on atmospheric pressure because evaporation rates change with pressure. The Stull (2011) formula used in this calculator is fit for standard sea-level pressure (about 1013 hPa), so results at very high altitude will have slightly more error than the roughly 0.3 C typical accuracy at sea level.
Where is wet-bulb temperature used besides weather forecasting?
Wet-bulb temperature is used in industrial cooling tower design (it sets the theoretical minimum achievable water temperature), evaporative air conditioner ('swamp cooler') sizing, occupational heat stress standards like the Wet Bulb Globe Temperature (WBGT) index used by OSHA and sports organizations, and agricultural frost protection via overhead sprinkler irrigation.
What is Wet Bulb Globe Temperature (WBGT) and is it the same as this calculator's output?
WBGT is a composite heat stress index that combines natural wet-bulb temperature, black globe temperature (radiant heat), and dry-bulb air temperature, used in workplace and sports safety guidelines. It is not the same as the plain psychrometric wet-bulb temperature this calculator computes, WBGT additionally accounts for solar radiation and wind, which this simplified formula does not.
What input range does this calculator accept?
This calculator accepts air temperatures from -20 to 50 degrees Celsius and relative humidity from 5 to 99 percent, matching the validated range of the Stull (2011) approximation at standard sea-level pressure.