4D chart

From Glossary of Meteorology

A chart showing the field of D-values (deviations of the actual altitudes along a constant- pressure surface from the standard atmosphere altitude of that surface) in terms of the three dimensions of space and one of time.
It is a form of a four-dimensional display of pressure altitude. The space dimensions are represented by D-value contours, and the time dimension is provided by tau-value lines. 4D charts are used primarily for relatively long-distance preflight planning over ocean areas.
See also S-values.

The quantity D describing the altitude of a point on a constant-pressure surface by its departure from "standard" altitude:

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