Amount of Moisture content in air

Humidity Ratio/Specific Humidity(HR):
The mass of water vapour carried by 1 lb of dry air.
= mass of water vapour / unit mass of dry air
= lb / lb of Dry air

Absolute humidity:
The mass of water vapour carried by 1 ft3 of dry air.
= mass of water vapour / unit volume of air
= lb / ft3 of Dry air
The same can be expressed in grains.
= grains of water vapour / unit volume of air
= grains / ft3 of Dry air
1 lb of water = 7000 grains

Degree of saturation:
The mass of water vapour carried by lb of dry air compared to the mass of water vapour carried by 1lb of saturated air.
= mass of water vapour / unit mass of dry air ÷ mass of water vapour / unit mass of saturated air @ same temperature.
= lb / lb of Dry air ÷ lb / lb of Saturated air

Relative Humidity (RH):
The mass of water vapour carried by 1ft3 of dry air compared to the mass of water vapour carried by 1ft3 of saturated air.
= mass of water vapour / unit volume of dry air ÷ mass of water vapour / unit volume of saturated air @ same temperature.
= lb / 1ft3 of Dry air ÷ lb / 1ft3 of Saturated air