Wet Bulb Temperature Calculator
Calculate wet bulb temperature with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Wet Bulb Temperature Calculator
Calculate wet bulb temperature from dry bulb and relative humidity using the Stull formula. Includes heat stress classification, WBGT estimate, and evaporative cooling potential.
Last updated: December 2025Reviewed by NovaCalculator Mathematics Team
Calculator
Adjust values & calculateFormula
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.
Last reviewed: December 2025
Worked Examples
Example 1: Hot Humid Day Heat Stress
Example 2: Desert Cooling Potential
Background & Theory
The Wet Bulb Temperature Calculator applies the following established principles and formulas. Earth science calculators draw on a wide range of measurement scales and physical principles that quantify natural phenomena across geological, atmospheric, and hydrological systems. Earthquake magnitude is most precisely described by the Moment Magnitude Scale (Mw), which replaced the original Richter scale for larger events. Mw is calculated as Mw = (2/3) log10(M0) − 10.7, where M0 is the seismic moment in dyne-centimeters. The Richter scale, while still referenced colloquially, is a local magnitude (ML) measurement derived from peak seismograph amplitude at a standard 100 km distance. Wind intensity is classified using the Beaufort Scale, a 13-point empirical scale (0–12) relating wind speed in knots to observable sea and land effects, with Beaufort 12 corresponding to hurricane-force winds above 64 knots. Tropical cyclone intensity is further categorized by the Saffir-Simpson Hurricane Wind Scale, which assigns Categories 1 through 5 based on sustained wind speed, correlating with expected structural damage. Mineral hardness is quantified on the Mohs scale (1–10), comparing scratch resistance relative to reference minerals from talc (1) to diamond (10). Soil composition analysis measures the proportions of sand, silt, and clay by particle size, alongside organic matter content, bulk density, and porosity, which together determine engineering and agricultural suitability. Seismic wave velocity in rock varies by material: P-waves travel at approximately 5–7 km/s in granite and 1.5 km/s in water, while S-waves travel at roughly 60% of P-wave speeds. Atmospheric pressure decreases with altitude according to the barometric formula: P = P0 × exp(−Mgh / RT), where M is molar mass of air, g is gravitational acceleration, h is altitude, R is the universal gas constant, and T is temperature in Kelvin. Standard sea-level pressure is 101,325 Pa. Tidal calculations use harmonic analysis of gravitational forcing by the Moon and Sun, with the principal lunar semidiurnal tidal constituent (M2) having a period of approximately 12.42 hours.
History
The history behind the Wet Bulb Temperature Calculator traces back through the following developments. The systematic study of Earth's structure and processes spans millennia, but the scientific foundations were laid in the seventeenth century. In 1669, Danish naturalist Nicolas Steno published his principles of stratigraphy, establishing the laws of superposition, original horizontality, and lateral continuity — foundational rules for reading rock layers that remain in use today. Scottish geologist James Hutton introduced the concept of uniformitarianism in 1788, proposing that geological processes observable in the present have operated throughout Earth's history at broadly consistent rates. This idea of deep time challenged prevailing biblical chronologies and set the stage for modern geology. Charles Lyell systematized these ideas in his landmark three-volume work Principles of Geology, published beginning in 1830, which directly influenced Charles Darwin's thinking on biological evolution during the voyage of the Beagle. The nineteenth century saw growing curiosity about continental shapes, but a coherent theory awaited Alfred Wegener, a German meteorologist who proposed continental drift in 1912, arguing that the continents had once formed a supercontinent he called Pangaea. His evidence included matching fossil records and geological formations across the Atlantic, but his mechanism was disputed for decades. The theory gained acceptance in the 1960s when seafloor spreading was confirmed through paleomagnetic studies, and plate tectonics emerged as the unifying framework of modern geoscience. The United States Geological Survey was established by Congress in 1879 to classify public lands and examine the geological structure, mineral resources, and products of the national domain. The twentieth century brought instrumental advances, including the global seismograph network deployed after World War II, initially to monitor nuclear tests, which dramatically improved earthquake detection and characterization. Satellite Earth observation began in earnest with the Landsat program launched in 1972, enabling continuous global monitoring of land use, glacier retreat, and vegetation patterns. Today, GPS networks, LIDAR scanning, and ocean-floor mapping provide centimeter-scale precision for tracking tectonic motion, sea level rise, and volcanic deformation in near real time.
Frequently Asked Questions
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.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy