What the dew point tells you
Air holds water vapour up to a limit that rises steeply with temperature. The dew point is the temperature to which the air must be cooled, at constant pressure and moisture content, for that limit to be reached: below it, water condenses on surfaces. Unlike relative humidity, the dew point does not change when the air is heated or cooled, so it is the quantity to watch in compressed air, cold rooms, electrical cabinets and drying processes.
How the calculation works
The calculator uses the saturation vapour pressure formula recommended by the World Meteorological Organization (WMO-No. 8, after Sonntag 1990):
ew(t) = 6.112 × exp(17.62 t / (243.12 + t)) hPa
The actual vapour pressure is e = ew(t) × RH / 100. The dew point is the temperature at which ew equals e, found by inverting the same formula. Below 0 °C the calculator also gives the frost point, using the ice formula 6.112 × exp(22.46 t / (272.62 + t)). Enter either the relative humidity or the dew point; the other is calculated.
From the vapour pressure follow:
- Absolute humidity, grams of water per cubic metre of air: e / (Rv T) with Rv = 461.5 J/(kg·K)
- Mixing ratio, grams of water per kilogram of dry air: 622 × e / (p − e); it depends on the air pressure
- Specific enthalpy, the heat content per kilogram of dry air used in HVAC and drying calculations
Worked example
Air at 20 °C and 50 % relative humidity, sea-level pressure:
- Vapour pressure 11.66 hPa (saturation 23.33 hPa)
- Dew point 9.3 °C: a cold-water pipe at 8 °C will sweat
- Absolute humidity 8.62 g/m³, mixing ratio 7.24 g/kg
- Enthalpy 38.5 kJ/kg
At 25 °C and 60 %, the dew point rises to 16.7 °C.
Relative humidity below 0 °C
By convention relative humidity is referred to liquid water, also below freezing; weather services and most humidity sensors report it that way. Over ice the saturation pressure is lower, so air that shows less than 100 % against water can already be saturated against ice: frost forms at the frost point, which lies slightly above the dew point.
Accuracy
The formula agrees with the IAPWS reference values to within about 0.3 % from −45 to 60 °C, far better than any field humidity sensor. In practice the uncertainty comes from the measurement: a sensor with ±2 % RH at 20 °C and 50 % gives a dew point uncertainty of about ±0.6 °C. At high pressure the enhancement factor adds about 0.4 % to the vapour pressure; for compressed air at line pressure, use a pressure dew point sensor rather than converting an atmospheric reading.
Common mistakes
Comparing relative humidity across temperatures. 50 % at 30 °C contains more than twice the water of 50 % at 15 °C. Compare dew points or mixing ratios.
Ignoring local cold spots. Condensation forms on the coldest surface, such as a cabinet wall in winter or an uninsulated pipe, not at the air temperature measured in the room.
Mixing up dew point and frost point. Below 0 °C, specifications for instrument air and freezers sometimes mean the frost point; check which one a data sheet uses.
Frequently asked questions
Does it work in °F?
Yes. Use the °C/°F switch at the top of the page; all temperatures follow it.
Which pressure should I enter?
The total air pressure. 1013.25 hPa is standard sea level; it only affects the mixing ratio and enthalpy, not the dew point or relative humidity.
Can I convert a humidity transmitter’s current output?
Scale the 4–20 mA signal to % RH or °C dew point with the 4–20 mA converter, then enter the value here.