96 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



In fig. 20 are given side by side two charts of mutual topography obtained by 

 extrapolation from 219 stations at the earth's surface, and two as obtained from the 

 results of ascents in the air from 5 stations. If ascents had been made at a sufficient 

 number of places the curves of the latter charts would probably have had mainly 

 the same course as those of the extrapolated charts, but with slightly changed situa- 

 tions of the different curves, and it would have involved no difficulty to correct the 

 extrapolated charts by the fundamental values obtained by the ascents. 



As to the charts for higher levels, those of mutual topography or of pressure dif- 

 ferences are drawn directly from the results of the ascents. Afterwards we use the 

 following method for drawing the charts of absolute topography not obtained by 

 extrapolations from below : The chart of mutual topography of two surfaces is 

 placed upon that of the absolute topography of the lower one. Then the absolute 

 topography of the upper one is obtained by graphic addition. The chart thus 

 obtained is then corrected in accordance with the absolute heights found from the 

 ascents and from the observations on mountains of a sufficient height. For the 

 present, however, the latter observations must be used with caution because of our 

 ignorance of temperature gradients above mountains (section 58). This chart being 

 drawn, we place upon it the next chart of mutual topography, proceed in the same 

 manner, and so on. 



The charts of absolute pressure in the higher standard levels are found by a 

 completely analogous procedure. 



In drawing charts in this way, one after the other by graphic addition, there is 

 this advantage that the characteristic feature of the distribution of pressure as 

 known from the numerous observations from the earth's surface does not disappear 

 as we proceed upward, as would have been the case if each chart had been drawn 

 independently of the others by means of the small number of calculated values. 



69. Remarks on the Rapid Work Essential for Daily Weather Service. 



In the preceding articles we have shown in detail how to find and represent as 

 completely as possible the distribution of pressure and mass in the atmosphere. 

 Nothing would prevent the use of these methods in the daily meteorological service 

 for the forecasts of the weather. But then it becomes a question of vital impor- 

 tance how to be able to draw the whole system of charts with as short a delay in 

 time as possible. 



We have, then, first to make a choice between the two methods, developed side 

 by side that of representing the absolute and the relative topography of isobaric 

 surfaces, or that of representing the absolute and relative pressure in level surtaces. 

 There is no doubt as to what choice to make; the charts of absolute and relative 

 topography can be found by a smaller number of operations, and therefore be ready 

 within a shorter time. It may be possible that the method of constructing the iso- 

 baric charts in level surfaces might be developed to a greater degree of simplicity 

 than is done here. But it is not probable that the simplicity of the other method 

 could be reached. The preference in favor of the first method is due to the greater 

 theoretical simplicity of the problem of determining the height corresponding to a 

 given pressure compared with that of determining the pressure at a given height. 



