FIELDS OF PRESSURE AND MASS IN THE ATMOSPHERE. 119 



Table S contains the absolute heights of the lowest standard isobaric surfaces, 

 as well as the heights from surface to surface as obtained by the method of extra- 

 polation from the common meteorological stations.* Among those from which 

 observations have been available 219 have been chosen. Their situation is seen 

 from the last chart of fig. 19. The principle in choosing has been to get as many 

 stations as possible on different levels. The chart therefore contains a great number 

 of stations in mountainous regions, and relatively few in low land. 



By means of the figures contained in tables R and S the charts of absolute and 

 of mutual topography (figs. 19 and 20) of the standard isobaric surfaces have been 

 drawn in full accordance with the directions given in sections 66 and 68. The two 

 lowest charts of mutual topography obtained by extrapolation from the 219 com- 

 mon stations and the two corresponding obtained from the ascents from the five 

 aeronautical stations are given side by side in fig. 20, no attempt having been made 

 to mold the corresponding charts into one. (Compare section 68.) The charts 

 of absolute pressure in the four lowest levels (fig. 22) have been derived from the 

 corresponding topographic charts of fig. 19 by the graphic method described in 

 section 67. The charts of pressure differences (fig. 23) are drawn exclusively from 

 the pressure differences contained in table T. 



As in the preceding example, we have dotted all isobaric or level curves running 

 below the earth's surface, the lines of intersection of the isobaric or the level sur- 

 faces with the earth being marked as thick curves. Thus the half of the 1000 

 m-bar surface is below the earth, while the 900 m-bar surface passes below the earth 

 only in the mountainous parts of Scandinavia, southern Europe, and adjacent parts 

 of Asia. Above the 800 m-bar surface only the higher parts of the Alps and of the 

 Caucasus rise as small islands. 



Figs. 21 and 24 are vertical sections containing profile curves, the first of iso- 

 baric and isosteric, the second of equipotential and isopycnic surfaces. These sec- 

 tions are worked out from ascents from Strassburg, Berlin, and St. Petersburg. 



Figs. 19 to 24 thus described give the distribution of pressure and mass in a 

 cyclone having its center above Finland. Here the isobaric charts show a mini- 

 mum of pressure, and the topographic charts deep depressions of the isobaric 

 surfaces. A striking feature of the topograph}- of the isobaric surfaces is their 

 inclination as we proceed upwards. It is characteristic of the other method of 

 representation that the isobaric charts do not show noticeably greater difference of 

 pressure in one level than in another. Thus the topography of the isobaric surfaces 

 is in some sense a more sensitive indicator of the distribution of pressure in higher 

 levels than the isobaric charts for given levels. A most striking feature of the charts 

 of mutual topography is that they indicate decreasing specific volume towards the 

 cyclone center. In the same manner the charts of pressure-differences show an 

 increasing density of the air as we approach this center. Thus both indicate a 



*The observations are taken from the meteorological year books published by the different countries. Unfor- 

 tunately some of them (Italy, Spain) only contain different average values, not original observations, and are there- 

 fore of no use for investigations in atmospheric dynamics. As is well known, the simultaneity of the observations 

 from the different countries is not very satisfactory. This will be a great difficulty for dynamical investigations. 

 For our present purpose we may treat them as if they were true simultaneous observations. 



