266 



GENERAL AIR CIRCULATION. 



The solution of the problem will be greatly facilitated by the use of a diagram similar 

 to that first prepared by Hertz to show the cliauges in the physical state of air as it rises 

 and falls in the atmosphere. 



Figure 83 has been drawn by the method described by Hertz in 1884,* and a description 

 of which is found in most text-books of meteorology. The abscissa represent temperatures 



-60- 



— 50' 



—40° 



— 30° —20° 



Tetnperature °C 



-10° 



-+•10" 



20 



Fio. 83. Hertz diagram. 



and the ordinates when read on the logarithmic scale shown on the left of the diagram 

 represent pressure, and if read on the linear scale shown on the right of the diagram re- 

 present approximate heights m the atmosphere. 



The thin inchned hnes, e.g., D C, are the lines of adiabatic change. Thus if a mass 

 of air is taken at a temperature of -60°C. and 350 mm. of pressure, represented in the 

 diaiTam by the point A, it will be nearly 5 kilometres high in the atmosphere. At any 

 other height in the atmosphere its temperature and pressure will be determined by the line 

 A B, so long as no heat is allowed to enter or leave it. Thus if it is depressed to one 

 kilometre above the ground its pressure wiU be 650 mm. and its temperature -10°C. approxi- 

 mately. 



The broken Unes are ' lines of saturation.' 



If air having the temperature and pressure represented by any point on one of these 

 lines is saturated with moisture, its temperature and pressure may be altered at will, but if 

 no water is added or subtracted, whenever its temperature and pressure are those given by 

 another point on the same dotted line it will be saturated. For example if air, saturated 

 under the conditions represented by the point A, has its pressure and temperature altered 

 by any method rmtil they are those represented by the point F it wnll be saturated. One 

 way to do this will be to depress it rapidly in the atmosphere, imtil its pressure is the 

 same as that of F. This would raise its temperature from that represented by A to that 

 represented by M, for M is on the adiabatic line A B through A and also on the horizontal 

 pressure line through F. If now the air radiates its heat and so cools without changing its 

 height in the atmosphere it will cool down to the temperature at F. It will then be found 

 to be saturated. 



* Met. Zeit., 1884, page 421. See also Neuhoff Abhdlg d. Preusa. Met. Inst. I, No. 6, Berlin, 1900. 



