Dry air

Dry air is the atmospheric gas mixture considered with its water vapor removed; reference descriptions also exclude contaminants such as suspended smoke. It is still a mixture, principally nitrogen and oxygen, with argon and smaller constituents. Dryness does not mean chemical purity or absence of oxygen.^[ref]

The distinction is useful for keeping a fixed basis while water vapor changes. If a sample contains $1.00\mathrm{kg}$ of dry air and $0.010\mathrm{kg}$ of water vapor, its moist total mass is $1.010\mathrm{kg}$. The humidity ratio is $0.010$ kilograms of vapor per kilogram of dry air, whereas the specific humidity is $0.010/1.010\approx 0.00990$ kilograms per kilogram of moist air. These differ because their denominators describe different physical samples.^[ref]