Wet Bulb Temperature Calculator
Calculate wet bulb temperature with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
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 Tw is wet bulb temperature in C, T is dry bulb in C, and RH is relative humidity in percent. This is the Stull 2011 empirical regression accurate to 0.3 C for most conditions.
Worked Examples
Example 1: Hot Humid Day Heat Stress
Problem: Calculate wet bulb and heat stress for 35 C dry bulb and 60% relative humidity at sea level.
Solution: Using Stull formula with T=35, RH=60\nSaturation VP: es = 6.112*exp(17.67*35/(35+243.5)) = 56.24 hPa\nActual VP: e = 56.24*0.60 = 33.74 hPa\nDew point = 243.5*ln(33.74/6.112)/(17.67-ln(33.74/6.112)) = 26.20 C\nWet bulb approximately 28.88 C
Result: Wet Bulb: 28.88 C | Dew Point: 26.20 C | Heat Stress: High
Example 2: Desert Cooling Potential
Problem: Determine evaporative cooling potential for desert at 42 C dry bulb and 15% humidity at 950 hPa.
Solution: Saturation VP: es = 6.112*exp(17.67*42/(42+243.5)) = 82.27 hPa\nActual VP: e = 82.27*0.15 = 12.34 hPa\nUsing Stull formula with T=42, RH=15\nWet bulb approximately 22.4 C\nCooling potential = 42 - 22.4 = 19.6 C
Result: Wet Bulb: 22.4 C | Cooling Potential: 19.6 C | Excellent conditions
Frequently Asked Questions
What is wet bulb temperature and how does it differ from dry bulb?
Wet bulb temperature is the lowest temperature achievable by evaporating water into air at constant pressure while dry bulb is standard air temperature. A wet bulb thermometer has its bulb wrapped in a moistened wick and evaporation cools it below ambient temperature. The difference indicates how much moisture the air can still absorb with saturated air showing identical readings. Wet bulb is always less than or equal to dry bulb and greater than or equal to dew point temperature. It is fundamental to psychrometric analysis in HVAC engineering and atmospheric science.
Why is wet bulb temperature critical for human survival?
Wet bulb temperature represents the fundamental limit of human thermoregulation because the body cools primarily through sweat evaporation from the skin. When wet bulb exceeds approximately 35 degrees Celsius even a perfectly acclimated person in shade with unlimited water cannot dissipate metabolic heat fast enough. At this threshold core temperature rises continuously leading to hyperthermia and death within hours without external cooling. Research suggests dangerous conditions begin at lower wet bulb values around 31 to 33 degrees for most people especially the elderly or those doing physical work. Climate change is increasing the frequency of extreme wet bulb events in tropical and subtropical regions.
What is the relationship between wet bulb and dew point?
Wet bulb temperature always falls between dry bulb and dew point or equals one of them at humidity extremes. At 100 percent relative humidity all three temperatures are identical because no evaporative cooling can occur in saturated air. At very low humidity the wet bulb drops to maximum depression below dry bulb but dew point would be even lower. The dew point represents the temperature for saturation if cooled at constant pressure and moisture while wet bulb represents temperature from adiabatic evaporative cooling. For a given dry bulb both wet bulb and dew point increase with humidity but converge at saturation.
How is wet bulb used in HVAC and cooling tower design?
In HVAC engineering wet bulb temperature determines performance limits of evaporative cooling systems since it represents the theoretical minimum achievable through water evaporation. Cooling towers are rated based on approach temperature which is the difference between leaving water temperature and ambient wet bulb. Lower wet bulb temperatures allow more efficient evaporative cooling which is why these systems work best in dry climates. Design engineers use wet bulb coincident temperature from historical records to size equipment for worst-case conditions. The psychrometric constant depending on atmospheric pressure relates heat and mass transfer rates in these systems.
How does altitude affect wet bulb temperature?
Atmospheric pressure directly influences wet bulb through the psychrometric constant which equals approximately 0.000665 times station pressure in hectopascals. At higher altitudes where pressure is lower the psychrometric constant decreases changing the ratio of heat to mass transfer during evaporation. Lower pressure increases evaporation rate from wet surfaces which can slightly lower wet bulb compared to the same temperature and humidity at sea level. The Magnus formula for saturation vapor pressure is independent of total atmospheric pressure but humidity ratio and derived properties do depend on it. High altitude locations need pressure-corrected wet bulb calculations for accurate psychrometric results.
How is wet bulb used in agriculture?
Agricultural scientists use wet bulb as a component in calculating evapotranspiration rates determining crop water loss and irrigation needs. The wet bulb depression directly relates to vapor pressure deficit driving transpiration from plant leaves. Larger depressions indicate drier air with greater evaporative demand requiring more irrigation to maintain crop health. Post-harvest grain drying depends on wet bulb because drying rate is proportional to the difference between grain temperature and drying air wet bulb. Frost protection in orchards uses evaporative cooling effects where wet bulb helps predict sprinkler system effectiveness for protecting blossoms.