gram per kilogram
Humidity ratioexact by definition
The gram per kilogram is a thousandth of the SI form and is the usual working unit on metric psychrometric charts, since it puts ordinary indoor values in the range of 5 to 12 rather than 0.005 to 0.012. The factor of 0.001 is exact, being a decimal prefix. Air at 22 °C and 50 percent relative humidity carries about 8.2 g of vapour per kilogram of dry air.
Watch out: Humidity ratio and relative humidity measure different things. Relative humidity is the ratio of the vapour pressure to the saturation vapour pressure at that temperature, so it changes when air is heated even though no moisture has moved. Humidity ratio does not change on heating or cooling until condensation occurs, which is why it is the honest measure of how much water the air is actually carrying.
| 1 g/kg | 1 g/kg |