FIELDS OF PRESSURE AND MASS IN THE ATMOSPHERE. 97 



The superiority of the charts of absolute and relative topography being admitted, 

 the meteorologists at the central bureau have to work out such charts from two 

 sets of telegrams, giving (1) the observations from the common meteorological 

 stations, (2) the height of the lowest standard surface and the virtual temperature 

 of the standard sheets above the aeronautical stations from which ascents have been 

 made (section 57). 



From the first set of observations the charts of absolute and relative topography 

 are drawn as described above for the lower levels. They can be drawn independently 

 of each other, and accordingly simultaneously by different workers. As the drawing 

 of each chart is of precisely the same nature as the drawing of an isobaric chart for 

 sea-level, nothing prevents the whole set from being ready within an interval of 

 time not exceeding that required for drawing the single isobaric chart for sea-level. 



From the telegraphed values of the virtual temperatures of the standard isobaric 

 sheets the higher-level charts of relative topography are drawn. In doing this it is 

 not necessary first to change by table 9 m the telegraphed virtual temperatures into 

 heights. The curves for constant thickness of a sheet are curves for certain con- 

 stant values of the virtual temperature. We may therefore note these tempera- 

 tures on the chart and draw the curves for constant thickness of the sheet directly 

 from them, table 9 m showing which virtual temperature corresponds to a required 

 value of the vertical distance. 



The charts of relative topography being drawn, the corresponding charts of 

 absolute topography are found by the method of graphic addition. Finally, if 

 required, the charts are corrected according to the absolute heights, which to save 

 time may have been calculated by another computer. But as long as observations 

 from- the open air are rare, one man will probably be able to perform all the work 

 for the higher-level charts during the time required by the other workers to draw 

 the lower-level charts. Thus, by good organization there is nothing to prevent the 

 whole system of charts giving the distribution of pressure and mass in the atmos- 

 phere for all heights reached by extrapolations from below and by direct ascents in 

 the air, being ready within an interval of time not greatly exceeding that required 

 for drawing such charts as are now used for sea-level. 



70. Example i. Atmospheric Conditions over North America, September 

 23, 1898. The first simultaneous meteorological kite ascents were organized by 

 the U. S. Weather Bureau during the summer of 1898.* September 23 seven 

 ascents succeeded, five of which were fairly simultaneous, between 7 and 1 1 o'clock 

 in the morning, and thus simultaneous also with the common meteorological obser- 

 vations at 8 o'clock, time of the seventy-fifth meridian. Two of the ascents, from 

 North Platte and from Dodge City, came between 2 and 5 in the afternoon. Dur- 

 ing the days September 21 to 24, kite ascents were made also from the Blue Hill 

 Meteorological Observatory near Boston.f None of them were simultaneous with 



*See H. C. Frankenfield : Vertical Gradients of Temperature, Humidity, and Wind Direction. A preliminary 

 report on the kite observations of 1898. Weather Bureau Bulletin F. Washington, 1899. The original results 

 of the kite ascents have not been published. Those used below have been kindly communicated by the Weather 

 Bureau. 



tH. Helm Clayton: Studies of Cyclonic and Anticyclonic Phenomena with Kites. Bulletin No. 1, 1899, 

 of Blue Hill Meteorological Observatory. 



