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Isohypse (aka height contour)
A line of equal geopotential height. Geopotential assumes the earth is perfectly flat and a perfect sphere. The geopotential height is the distance above the Earth’s surface if it was a perfect and flat sphere. Isohypse are shown on a constant pressure surface. For example, when looking at a 850 mb chart, all isohypses no matter their value are located at 850 mb. Regions of low isohypse values are correlated with low pressure (trough) while high isohypse values are correlated with high pressure (ridge). This 850 chart has height contours. The lines are the isohypses. Above 850 mb, the wind flows close to parallel to the height contours. A curving down of the height contours represents a trough while a curving up a ridge.