Weather & air
Humidity & dew point
Translate relative humidity and dew point with an over-water model.
A clear answer. Here’s how.
Uses the NWS over-liquid-water approximation es(T)=6.11×10^(7.5T/(237.3+T)) hPa. Actual vapour pressure is RH×es/100 or es(dewpoint). Inverts the same equation for dew point and estimates absolute humidity with the ideal water-vapour gas constant 461.5 J/(kg·K). Frost and extreme cold/dry conditions need different models; dew point cannot exceed air temperature.
See a worked example
Worked example
A fixed demonstration, separate from the values you enter above.
- Air temperature (°C)
- 20
- Known humidity input
- Relative humidity
- Relative humidity (%)
- 50
Example result
- Relative humidity
- 50%
- Dew point (°C)
- 9.2692643463
- Actual vapour pressure (hPa)
- 11.6946784215
- Saturation vapour pressure (hPa)
- 23.3893568431
- Approximate absolute humidity (g/m³)
- 8.64423729439
Over-liquid-water approximation; frost and extreme cold/dry conditions are outside its useful accuracy.