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Geopotential height
In ERA5 (and in the IFS), and often in meteorology, heights (the height of the land and sea surface, or specific heights in the atmosphere) are not represented as geometric height, or altitude (in metres above the spheroid), but as geopotential height (in geopotential metres above the geoid). Note, that ECMWF usually archives the geopotential (in m2/s2), not the geopotential height.
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Geometric height is not represented in ERA5 (nor in the IFS). However, the geometric height or altitude (alt) can be approximated by the following formula (neglecting horizontal variations in the Earth's gravitational acceleration):
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