Absolute Humidity Calculator

Calculate the absolute humidity from temperature and relative humidity.

Absolute humidity13.787 g/m³mass of vapour per cubic metre

Relative humidity60%vapour pressure 18.97 hPa against a saturation value of 31.617 hPa

Dew point16.698 °Ccool the air to this and it saturates — a genuinely absolute measure of moisture, unlike RH

Saturation vapour pressure31.6174 hParoughly doubles every 10 °C, which is why warm air holds so much more water

Actual vapour pressure18.9704 hPa

Mixing ratio11.8671 g/kggrams of vapour per kilogram of DRY air — conserved as air rises and cools, which is why meteorologists prefer it

Specific humidity11.7279 g/kgper kilogram of MOIST air — very close to the mixing ratio, and often confused with it

Virtual temperature27.151 °Cthe temperature dry air would need for the same density — moist air is LIGHTER than dry air at the same temperature, because water is lighter than nitrogen

Wet-bulb temperature19.503 °CStull's approximation, good to about 0.3 K — the lowest temperature evaporation alone can reach, and the limit for any evaporative cooler

Wet-bulb depression5.497 Kthe cooling an ideal evaporative cooler could deliver here

Heat indexnot applicable herethe regression is only defined above about 27 °C and 40% humidity — below that the apparent temperature is essentially the air temperature

Convective cloud base1,038 mabout 125 m per degree of dew-point spread — rising air cools at 3 K/km faster than its dew point falls, so they converge at the condensation level

At 10 °C warmer, same vapour33.77% RHthe same air warmed 10 K — the humidity roughly halves without a molecule of water changing

The formula

RH = e ÷ e_s(T); w = 0.622e ÷ (p − e)

Relative humidity is a ratio, not an amount

Saturation vapour pressure roughly doubles every 10 °C, so relative humidity compares the moisture present against a target that moves steeply with temperature. Air at 50% RH and 30 °C holds about twice the water of air at 50% RH and 20 °C. Warming air without adding water halves its RH, which is exactly why heated indoor air in winter feels so dry.

Dew point and mixing ratio are the absolute measures. Dew point is the temperature at which the air would saturate, so it tracks actual moisture content directly. Mixing ratio — grams of vapour per kilogram of dry air — is conserved as air rises and cools, which is why meteorologists work in it rather than in RH.

The wet-bulb temperature is the floor for evaporative cooling, and it matters for survivability: above about 35 °C wet bulb, sweating cannot shed metabolic heat at all, regardless of shade or wind.

A counter-intuitive result worth stating: moist air is lighter than dry air at the same temperature and pressure. Water has a molar mass of 18 against nitrogen's 28, so replacing nitrogen with vapour reduces density. That is what the virtual temperature captures.