Psychrometric Calculator: Humid Air Properties
Humidity ratio, dew point, wet-bulb temperature, enthalpy and specific volume of moist air.
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How this calculator works
Air always holds some water vapour. Relative humidity says how close it is to saturation at its temperature. The saturation pressure of water vapour rises quickly with temperature, so warm air can hold much more moisture. From the dry-bulb temperature, the relative humidity and the pressure, the calculator finds the actual vapour pressure and the humidity ratio, which is the mass of water per kilogram of dry air.
The dew point is the temperature at which the air would be saturated if cooled at constant moisture content: below it, water condenses. The wet-bulb temperature is what a wetted thermometer reads in moving air. The equations are the ASHRAE ones for moist air treated as an ideal-gas mixture.
Formulas used
- p_w = \phi\,p_{ws}(T)
- W = 0.622\,\frac{p_w}{p - p_w}
- h = 1.006\,T + W(2501 + 1.86\,T)
Worked example
With the default numbers in the calculator above:
- Saturation pressure of water vapour at the dry-bulb temperature: 4.246 kPa
- Actual vapour pressure: 2.548 kPa
- Humidity ratio: 0.016041 kg/kg
- Degree of saturation: 58.97 %
- Specific enthalpy: 71.19 kJ/kg
- Specific volume: 0.8809 m³/kg
- Dew-point temperature: 21.39 °C
- Wet-bulb temperature: 23.81 °C
Change the numbers in the calculator to see the full working, every step, for your own case.
Common questions
What is the dew point?
The temperature at which moisture starts to condense out of the air when it is cooled.
Why does the humidity ratio matter more than the relative humidity?
Because it is a real amount of water. Relative humidity changes just by heating or cooling the air, while the humidity ratio does not.
What is the wet-bulb temperature used for?
To find the humidity from two thermometer readings, and in sizing cooling towers and evaporative coolers.
Related calculators
Further reading: ASHRAE Handbook: Fundamentals, chapter on psychrometrics; Cengel and Boles, Thermodynamics: An Engineering Approach; Moran and Shapiro, Fundamentals of Engineering Thermodynamics.
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