Mixing line

From Glossary of Meteorology

mixing line

A method of thermodynamic analysis where the conserved variables for two different states of air (i.e., air parcels) are plotted on a thermodynamic diagram, and the ending state of a mixture of the two parcels is found on the straight line connecting the two initial states, with relative distance along the line proportional to the relative amounts of each parcel in the mixture.

Mixing line analysis can help determine the origin of air within clouds.
See conserved variable diagram.