The isentropic chart is a plot of meteorological data which uses potential temperature as the vertical coordinate. These charts are of most use to operational meteorologists and synoptic/dynamic meteorologists who are interested in three-dimensional perspectives of atmospheric flow. Potential temperature is a variable which depends on both temperature and pressure. We plot temperature (oC) on the upper left, mixing ratio (g/kg) on the lower left [mixing ratio is a measure of atmospheric water vapor content], pressure (mb or hPa) on the upper right, and isentropic separation (mb or hPa) on the lower right. The latter quantity, usually negative because potential temperature usually increases with decreasing pressures, is a measure of stability of the atmosphere; the larger (negative) the value of pressure separation, a measure of pressure change between two adjacent isentropes, the more stable the atmosphere.

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