Dry air (source code)
= 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>.<Psychrometrics in the 2017 ASHRAE Handbook Fundamentals>[Chapter1, Section1: composition of dry and moist air]{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 $0.010/1.010\approx0.00990$ kilograms per kilogram of moist air. These differ because their denominators describe different physical samples.<Psychrometrics in the 2017 ASHRAE Handbook Fundamentals>[Section5, Eqs7–9: humidity ratio and specific humidity]{ref}
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