isohypse

A line joining points of equal geopotential height on a given surface. In Meteorology, isohypse maps are produced (colloquially known as upper-air maps) for certain standard pressure levels. The most common are - from bottom to top - those at 850, 700, 500, 300, and 200 hPa. Isohypses are, therefore, the contour lines - expressed in geopotential meters - of a pressure surface in the atmosphere.