236 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 9 



ffrrar^ment of s/af/on cfafa 

 Specific Hum/c/ifi/ 

 Pressure » Concfensaf/or) Pressure 

 ffe/afiVe /fumiV/fy 

 ^ _ _ /soiiars of acfuo/ Pressure 

 — — • " " com/snsaffon Pressure 

 S/}acfecf area - Pey/on of Safuraf/on 



Feb. J, /938 

 Q = 29S 



Figure 9. — Au isentropie chart : A synoptic map of the surface in the upper air over 

 which the potential temperature at the time was everywhere 295°. The places at which 

 the condensation pressure becomes equal to the current pressure outline a cloud-covered 

 region in most of which precipitation occurred at the surface of the earth. 



the surface itself, is intimately related to the occurrence of cloudiness 

 and precipitation, and other meteorological processes. Isentropie 

 charts have shown considerable promise both for purposes of hydro- 

 dynamical investigations of atmospheric motions and for practical 

 forecasting. 



It must be emphasized that the physical analysis of maps and 

 cross sections is quite distinct from the making of a forecast; the 

 analysis must be followed by a determination of the expected dis- 

 placements, transformations, and developments of the fronts, air 

 masses and pressure formations, and an estimate of the weather 

 conditions that will accompany them. Important progress has been 

 made in developing methods for this purpose also on a quantitative 

 physical basis; but the unusual demands of practical meteorological 

 work, particularly the exceedingly limited time within which fore- 

 casts must be completed and issued, together with the extraordinary 

 complexity of the phenomena involved, the great volume of data 

 necessary and the wide areas from which they must be gathered, 

 make it peculiarly difficult to adapt the results of theoretical investi- 

 gations to meet the needs of applied meteorology. The majority of 

 attempts to achieve anything that approaches an exact mathematical 

 calculation of future weather on the basis of dynamical and physical 



