In meteorology a surface of constant potential temperature. Isentropic surfaces slope very gently upwards towards the cold air in the presence of a horizontal temperature gradient, and winds flow along these surfaces. See Ambaum (1997) J. Atmos. Scis 54, 4 on the isentropic formation of the tropopause. In isentropic analysis, the forecaster looks at the atmosphere in three dimensions, considering constant potential temperature surfaces. Air parcels move up and down these surfaces; therefore, the forecaster can see where the moisture is located and how much moisture is available. Try the UW Isentropic Analysis and Modelling Group home page.