In chemistry and physics, the dimensionless mixing ratio is the abundance of one component of a mixture relative to that of all other components. The term can refer either to mole ratio or mass ratio.
The mole ratio is also called amount ratio.
If is much smaller than (which is the case for atmospheric trace constituents), the mole ratio is almost identical to the mole fraction.
In meteorology, mixing ratio usually refers to the mass ratio , which is defined as the mass of a constituent divided by the total mass of all other constituents in a mixture:
The mass ratio of water vapor in air can be used to describe humidity.