Dew Point & Absolute Humidity

The engineering

The Magnus (Magnus–Tetens) approximation with the widely used Alduchov–Eskridge constants a = 17.625, b = 243.04 °C — accurate to a few hundredths of a degree over ordinary conditions (−40 to 60 °C, over water). Any surface colder than the dew point row will sweat: that is the condensation check for cold pipes, windows, compressed-air lines, and electronics brought in from a cold van.

Absolute humidity comes from the vapor pressure via the gas law for water vapor and answers the drying and ventilation question RH cannot: 25 °C at 60% RH carries ~13.8 g of water per cubic metre. RH is a ratio, so 'humid' winter air at 90% RH can hold a third of the water of 'dry' summer air at 40% — the g/m³ row is the honest number.

Where this math comes from

John Dalton established around 1802 that water vapor exerts its own partial pressure with a definite saturation curve; Heinrich Gustav Magnus published his careful exponential fit of that curve in 1844, and Regnault's dew-point hygrometer measurements of the same decade gave it data worth fitting. The formula has been re-fitted ever since — the constants here are Alduchov and Eskridge's 1996 revision — but it is still Magnus's equation.

Willis Carrier turned the whole subject into an industry: his 1911 'Rational Psychrometric Formulae' paper gave engineers the chart that runs every air-conditioning calculation, born from the very practical problem of a Brooklyn printing plant whose paper swelled with the weather.

1. 1802John DaltonPartial pressures and the saturation curve of water vapor.
2. 1844Heinrich Gustav MagnusThe Magnus saturation-vapor-pressure formula.
3. 1911Willis CarrierRational psychrometric formulae — air conditioning gets its chart.
4. 1996Alduchov & EskridgeModern refit of the Magnus constants used here.

See the full timeline of the math behind every calculator →

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