What is the dew point?
The dew point is the temperature to which air must be cooled, at constant pressure and constant water content, for it to become saturated with water vapour. Below that temperature the excess vapour condenses as dew, fog or condensation on surfaces. Any surface colder than the dew point of the surrounding air — a window pane, a cold pipe, an uninsulated wall — will collect condensation.
Examples computed by this page:
| Air temperature | Relative humidity | Dew point | Absolute humidity |
|---|---|---|---|
| 20 °C | 40 % | 6.0 °C | 6.9 g/m³ |
| 20 °C | 50 % | 9.3 °C | 8.6 g/m³ |
| 20 °C | 70 % | 14.4 °C | 12.1 g/m³ |
| 25 °C | 60 % | 16.7 °C | 13.8 g/m³ |
| 30 °C | 80 % | 26.2 °C | 24.3 g/m³ |
| 20 °C | 10 % | -11.2 °C (frost point) | 1.7 g/m³ |
At 100 % relative humidity the dew point equals the air temperature. The drier the air, the larger the gap between the two.
Frost point
When the air is dry enough for the result to fall below 0 °C, water vapour deposits directly as ice instead of condensing as liquid. The page then reports the frost point, computed from the vapour pressure over ice. On the chart this is the shaded area under the 0 °C curve.
How it is calculated
- The saturation vapour pressure at the air temperature comes from the IAPWS-IF97 saturation equation, the same one used by the water and steam calculator.
- The actual vapour pressure is the relative humidity times that saturation pressure.
- The dew point is the temperature at which that vapour pressure is the saturation pressure, obtained by inverting the same equation — no approximate (Magnus-type) formula is used. Below 0 °C the frost point is obtained from the IAPWS sublimation-pressure equation for ice.
Absolute humidity is the mass of water vapour per cubic metre of air; the mixing ratio is the mass of water vapour per kilogram of dry air. Total pressure only affects the mixing ratio.
Limits. Air temperature from 0 °C to 100 °C (the chart shows 0–60 °C). Humid air is treated as an ideal mixture: the small correction for real-gas effects (enhancement factor) is neglected. Relative humidity is taken with respect to liquid water.