The first part deals with methods of isentropic analysis on the scaled values of data transmitted for the purpose by the United States Weather Bureau. The practical application of shear-stability ratio vectors is demonstrated. A logical order of analysis is presented, designed to make the fullest possible use of all data available. The preferred isopleths for direct representation are those of temperature, acceleration potential and specific humidity; a system of isopleth spacing convenient in conjunction with surface weather maps is employed.

In the second part the properties of various surfaces for upper-air analysis are systematically studied. It is found that severe restrictions exist in the use of surfaces of constant entropy or potential temperature. All the properties discussed are however adequately satisfied by a surface of constant potential virtual temperature, a quantity newly introduced and defined as the potential temperature of matter of standard composition, in this case dry air, at the same pressure and density as the sample. The thermodynamic configuration of this surface is described by isotherms of virtual temperature.

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*Revision of paper read at Inst, of Aeron. Sci. meeting, Jan. 25, 1941; the full paper was published in the Journ. of Aeron. Sci. for May, 1941.