# Mixing ratio

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.

## Mole ratio

In atmospheric chemistry, mixing ratio usually refers to the mole ratio $r_{i}$ , which is defined as the amount of a constituent $n_{i}$ divided by the total amount of all other constituents in a mixture:

$r_{i}={\frac {n_{i}}{n_{tot}-n_{i}}}$ The mole ratio is also called amount ratio.

If $n_{i}$ is much smaller than $n_{tot}$ (which is the case for atmospheric trace constituents), the mole ratio is almost identical to the mole fraction.

## Mass ratio

In meteorology, mixing ratio usually refers to the mass ratio $\zeta _{i}$ , which is defined as the mass of a constituent $m_{i}$ divided by the total mass of all other constituents in a mixture:

$\zeta _{i}={\frac {m_{i}}{m_{tot}-m_{i}}}$ The mass ratio of water vapor in air can be used to describe humidity.