86 



DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



This recorded curve is the curve of true temperature. Using the virtual-tem- 

 perature divisions, we draw the curve to the right, the curve of virtual temperature 

 for saturated air. No humidity having been registered, we suppose the relative 

 humidity to have had the average value of 70 per cent and draw the curve of virtual 



yivenu?* temperature at a dis- 



WrfuaZ'' tance from the curve 



'XtempemHwe f true temperature 



Tressune of standard , . 



m- z bars ' sheets'^ equal to 70 per cent 



~t of the distance be- 

 tween the two curves. 

 Then the circle arcs 

 are drawn, which, in 

 this system of coordi- 

 nates, represent the 

 average virtual tem- 

 peratures of the 

 standard sheets, as 

 well as the arc (invis- 

 ible in the figure on 

 account of its short- 

 ness) representing the 

 average virtual tem- 

 perature (+12) of the 

 air between the lowest standard surface and the earth. Using this temperature and 

 the pressure (1005.2) at the station, we find by tables iim and 12M the height (42 

 dynamic meters) of the lowest standard surface above the station, and adding the 

 height (39) of the station, we find the height (81 dynamic meters) of this surface 

 above sea-level. The figures to be telegraphed 



60 so 



W 30 20 10 , O 10 



Temperature C. 

 Fig. 12. Meteorogram, Berlin, July 4, 1901. 



SO 



(*) 



81, 13, 9, 3, -2, -10, -21, 



36, -54 



are thus found. The time from the moment the meteorologist has obtained the 

 meteorogram until he has found the figures {b) ought not to exceed five minutes. 



58. Extrapolation of the Virtual-Temperature Diagrams. The virtual- 

 temperature diagram obtained from the observations of a meteorological ascent may 

 be prolonged some distance upward, so as, for instance, to attain the next standard 

 isobaric surface. If the prolongation be not too long, a slight deviation from the 

 course which real observations would have given will have no great influence upon 

 the calculated heights or pressures. Short extrapolations of this kind have been 

 used occasionally in the examples given above. 



A special kind of extrapolation will be of great importance, namely, those from 

 the earth's surface. Complete observations along a vertical in the atmosphere will 

 always remain rare, while we may get abundant observations from stations at the 

 earth's surface. The observation of pressure, temperature, and humidity gives one 



