Humidity and moist air
Dew point, relative humidity, and vapor pressure from each other, with the mixing ratio, virtual temperature, and how much lighter the moist air is than dry air.
Planning and education aid. Not for primary navigation. Full disclaimer
Experimental: not yet fully verified. How results are checked
Moist air here weighs 1.1362 kg/m³, against 1.1492 kg/m³ if it were dry; relative humidity is 70.3%.
- Dry air density
- Dew point
- Relative humidity
- Vapor pressure
- Saturation vapor pressure
- Mixing ratio
Provenance
- Computed by
- aviation.atmosphere.humidity 1.0.1, core 0.1.0
- Model
- e_s(T) = 6.1094 hPa × exp(17.625·T ÷ (T + 243.04)) (Alduchov and Eskridge 1996, over water); e = e_s(dew point) or RH × e_s(T); dew point by inverting the same form; mixing ratio = 622 × e ÷ (p − e) g/kg; virtual temperature T ÷ (1 − 0.378·e ÷ p); densities p ÷ (287.05287 × T)
- Accuracy
- The saturation pressure is within 0.4% from -40 °C to 50 °C, over liquid water; below freezing over ice it reads high. The densities follow from the ideal-gas law
- Notes
- 1 shown with the answer
- Cites
- Alduchov, O. A., and R. E. Eskridge, Journal of Applied Meteorology, Improved Magnus Form Approximation of Saturation Vapor Pressure; International Civil Aviation Organization, Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet))
Something look off?
How we got thisFormula, worked example, sources, and proof
Model: e_s(T) = 6.1094 hPa × exp(17.625·T ÷ (T + 243.04)) (Alduchov and Eskridge 1996, over water); e = e_s(dew point) or RH × e_s(T); dew point by inverting the same form; mixing ratio = 622 × e ÷ (p − e) g/kg; virtual temperature T ÷ (1 − 0.378·e ÷ p); densities p ÷ (287.05287 × T)
Show your work
Virtual temperature
Tv = T ÷ (1 − 0.378 × e ÷ p)303.15 K ÷ (1 − 0.378 × 29.78 ÷ 1,000)= 306.6 KMoist air density
ρ = p ÷ (287.05287 × Tv)100,000 Pa ÷ (287.05287 × 306.6 K)= 1.1362 kg/m³
The same steps an agent gets from the MCP server with explain: true.
Accuracy: The saturation pressure is within 0.4% from -40 °C to 50 °C, over liquid water; below freezing over ice it reads high. The densities follow from the ideal-gas law
Worked example: 30 °C, dew point 24 °C, 1,000 hPa. Source: add-aviation-suite humid-density scenario: moist air is lighter than dry air at the same temperature and pressure, and both are reported. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.
You enter
- Dew point
- 24 °C
- Air pressure
- 1000 hPa
- Temperature
- 30 °C
You get
- Moist air density
- 1.1362 kg/m³
- Dry air density
- 1.1492 kg/m³
- Dew point
- 24 °C
- Relative humidity
- 70.3
- Vapor pressure
- 29.78 hPa
- Saturation vapor pressure
- 42.37 hPa
- Mixing ratio
- 19.09 g/kg
- Virtual temperature
- 33.45 °C
Review: Not yet independently reviewed by a flight instructor (CFI).
Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.
Status: version 1.0.1, core 0.1.0. See this tool in the verification report.
Changes
- 2026-09-23, fixed: Plain-number results in a fixed unit now show that unit: the battery current reads 9.7 A where it read 9.7, and decibels, gigabytes, pulses per square meter, grams per kilogram, nautical miles per gallon, hours, nanoteslas, and minutes now carry their units too. The numbers themselves do not change. Each tool's patch version went up. Changelog
Checked against: 10 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.
Sources
- Improved Magnus Form Approximation of Saturation Vapor Pressure, Alduchov, O. A., and R. E. Eskridge, Journal of Applied Meteorology, Vol. 35, No. 4. pp. 601-609, equation 21 (AERK coefficients 6.1094 hPa, 17.625, 243.04 °C; within 0.4% from -40 to 50 °C).
- Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), International Civil Aviation Organization, 3rd edition, Doc 7488/3. Section 2 (defining constants and equations) and Table 1. Paid only.
Experimental means this tool has not yet met the stable bar: at least 20 golden vectors, differential tests, and an independent worked example.