Wet Bulb Temperature Calculator
What this calculator does
This wet-bulb temperature calculator estimates the temperature air can reach through evaporative cooling at a given temperature and humidity. Wet-bulb temperature is important in meteorology, cooling processes, and heat-stress interpretation because it links temperature with the evaporative capacity of the air.
When the air is dry, evaporation is efficient and wet-bulb temperature sits much lower than air temperature. When the air is already moisture-laden, evaporation becomes less effective and wet-bulb temperature climbs closer to the dry-bulb value. That relationship helps explain both cooling performance and some forms of environmental heat stress.
Inputs explained
- Air temperature: Enter the dry-bulb air temperature used as the starting condition.
- Relative humidity: Enter the moisture state of the air as a percentage.
- Internal unit handling: The Stull approximation is evaluated in Celsius inside the engine and the page reports both C and F outputs.
How it works / method
The calculator uses Roland Stull's compact empirical wet-bulb approximation. It is a practical shortcut for standard near-sea-level atmospheric pressure and is widely used for quick estimation when a full psychrometric or iterative solution is unnecessary. The method is especially useful when you need an online estimate from only temperature and relative humidity.
Formula used
Tw = T atan(0.151977 (RH + 8.313659)^0.5) + atan(T + RH) - atan(RH - 1.676331) + 0.00391838 RH^1.5 atan(0.023101 RH) - 4.686035
This is the Stull 2011 approximation for standard atmospheric pressure. It is a fit, not a full thermodynamic solver, so it is best treated as a practical estimate rather than an exact psychrometric reference value.
Practical note: Wet-bulb calculations depend on the chosen model, pressure assumptions, and input quality. In high-altitude or precision applications, a pressure-aware method or direct instrument reading is preferable.
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Step-by-step example
Suppose the air temperature is 32 C and the relative humidity is 50 percent. The wet-bulb value helps show how much evaporative cooling is still available.
- Enter 32 for air temperature.
- Enter 50 for relative humidity.
- The page estimates a wet-bulb temperature near the mid 20s C.
- That gap between dry-bulb and wet-bulb temperature reflects the cooling that evaporation can still produce.
- If humidity rises further, the wet-bulb result climbs and the available evaporative relief shrinks.
Use cases
- Checking whether evaporative cooling methods are likely to be effective in a given weather pattern.
- Comparing humid heat events where dry air temperature alone understates environmental strain.
- Supporting psychrometric interpretation in classrooms, field notes, and quick engineering estimates.
- Providing an input or comparison point for broader heat-stress discussions and related tools such as WBGT.
Assumptions and limitations
- This page uses an approximation that assumes standard atmospheric pressure and is not a universal replacement for full psychrometric methods.
- Results become less reliable outside the validated range described in the Stull paper and in unusual cold-dry combinations.
- The calculator does not directly account for site elevation, barometric pressure variation, radiation, or clothing.
- For critical occupational or medical safety decisions, measured wet-bulb or WBGT instruments are stronger evidence than an estimate.
Use this tool for interpretation and screening. If the decision is about exposure limits in sun or work-rest guidance, compare the result with the WBGT calculator and local protocols.
Frequently Asked Questions
Stull's empirical approximation is the everyday tool: Tw = T × atan(0.151977 × (RH + 8.313659)0.5) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 × RH1.5 × atan(0.023101 × RH) − 4.686035, with T in °C and RH as a percent. Valid for normal atmospheric pressure and moderate conditions. For lab-grade work use a psychrometric chart or iterative solution of the wet-bulb equation. At 30°C and 50% RH, Tw ≈ 22.0°C. Stull's error is below 0.65°C across the meteorological range.
Two routes. Empirical: Stull's approximation gives Tw directly from T and RH at sea-level pressure. Physical: solve the psychrometric equation iteratively using the wet bulb's vapour pressure plus a thermodynamic balance between sensible cooling and latent heat. Both need air temperature and humidity. The empirical form is faster and what most online calculators run. The physical form is what HVAC engineers use when pressure differs significantly from 101 kPa, such as in altitude or pressurised systems.
Either feed the two values into Stull's empirical formula, or use a psychrometric chart — locate the dry-bulb temperature on the bottom axis, follow the line up to the RH curve, then trace the constant wet-bulb line back to the saturation curve to read Tw. The chart is the visual version of the formula and is much easier to interpret. At 35°C and 30% RH, Tw lands near 22°C; at the same temperature with 80% RH, Tw climbs to 31°C.
The widely cited theoretical human survivability limit is 35°C wet bulb — at that level, your skin cannot shed heat by evaporation no matter how much you sweat. Recent physiological studies suggest the real limit is closer to 31°C wet bulb for healthy young adults, and lower for the elderly, infants, and the ill. Above 31°C wet bulb, sustained outdoor exposure becomes life-threatening. A few hours at 35°C wet bulb is fatal even at rest in shade with unlimited water.
Theoretical: 35°C, the temperature at which sweat evaporation cannot cool skin below core. Empirical: 31°C, based on actual heat-tolerance experiments. Below 31°C wet bulb, healthy people can lose enough heat to survive indefinitely with shade and water. Above it, core temperature climbs and organ failure follows in hours. Climate research now tracks how often these thresholds are breached in places like the Persian Gulf, South Asia, and equatorial Africa — frequencies that were rare in the 20th century and are rising.
It tells you how cool the air can become through pure evaporative cooling — basically, the lowest temperature your skin can reach by sweating in that air. If wet bulb is well below body temperature (37°C), evaporation is effective and you stay cool. If wet bulb approaches body temperature, evaporation cannot keep up. So wet bulb is the single most predictive variable for heat-stress risk in humid conditions, more useful than air temperature alone or RH alone for medical decision-making.
Wet bulb is a temperature, in °C or °F. Relative humidity is a ratio, in percent. They describe related properties of moist air but are different quantities. RH says how saturated the air is at its current temperature; wet bulb says how cool that air can become through evaporative cooling. You can compute either from the other if you know the dry-bulb temperature. For human physiology and HVAC engineering, wet bulb usually communicates the practical impact better than RH.
No, they are different metrics for different situations. Wet bulb is a thermodynamic temperature reflecting evaporative cooling potential — purely physics. Heat index is an apparent-temperature estimate based on regression against human comfort surveys — physiology folded into a number. WBGT is yet a third thing, combining wet bulb, globe, and dry bulb readings for occupational heat stress. Each has its place: wet bulb for survivability research, heat index for public weather forecasts, WBGT for sport and labour safety.
Sources & references
- NOAA National Weather Service - Dew point glossary
- NOAA National Weather Service - Relative humidity glossary
- NOAA National Weather Service - Dry bulb, wet bulb, and dew point definitions
- NOAA National Weather Service - Vapor pressure glossary
- American Meteorological Society - Stull wet-bulb approximation
- NIST Chemistry WebBook - Water data
- Schema.org - FAQPage and WebApplication vocabulary