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In ERA5, 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 metres above the geoid, which is represented by the mean sea level in ERA5). Note, that ECMWF usually archive the geopotential (in m2/s2), not the geopotential height.
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where Re is the radius of the Earth. This Note, this geometric height is relative to mean sea level and it is assumed that the Earth is a perfect sphere - for more information see ERA5: data documentation - spatial reference systems.
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