In meteorology, the dimensionless ratio of the mass of a substance (such as water
vapour) in an air parcel to the mass of the remaining substances in the air parcel.
For trace substances, this is approximated by the ratio of the mass of the substance
to the mass of air. However, in the case of water vapour the mass of dry air is used.
In atmospheric chemistry, mixing ratios (molecular, molar, by volume, as well as by
weight) are used to describe relative concentrations of atmospheric trace gases and impurities.
Source:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2202