From AMS Glossary
Synoptic analysis of change charts or of vertical differential charts (such as thickness charts) obtained by the graphical or numerical subtraction of the patterns of some meteorological variable at two times or two levels.
Spatial differential analysis from one surface to another in the vertical is useful because of its relation to hydrostatics: Differential analysis of pressure at low levels gives the mean isopycnic lines for the layer; differential analysis of temperature and potential temperature at two levels approximates the mean stability for the layer; and differential analysis of the topographies of two different isobaric surfaces gives the pattern of thickness or mean virtual isotherms for the layer.
Saucier, W. J. 1955. Principles of Meteorological Analysis. 136–137.