A principle used in forecasting the thickness of a layer between two given constant-pressure surfaces; more commonly used prior to the advent of numerical forecasting techniques. In practical use of the principle, specific thicknesses are considered to be associated with corresponding points on the lower of the two constant-pressure surfaces (the upper one can also be used). A prognostic chart is prepared for the lower surface and the assumption is made that the thickness of the layer at the corresponding points is unchanged at the same corresponding points on the prognostic map. The prognostic thickness chart is then constructed from thickness values at the corresponding points.