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Geopotential height is the altitude at which the air is at a given pressure at a given moment. When meteorologists mention "upper-level low pressure systems", they are referring to low geopotential heights, usually at the 500mb level. Low geopotential heights are often associated with storm systems and cold air. Geopotential height anomaly is the departure of the geopotential height from average at a given level. On our weather maps, geopotential height and geopotential height anomaly are given in decameters.