DYNAMIC METEOROLOGY. 



399 



remark that in the third term the integral is here present only as a cor- 

 rection whose value is comprised between very small limits. By com- 

 paring the adiabatic and the pseudo-adiabatic that start from the same 

 initial point we find that this latier curve is the one that most rapidly 

 approaches the axis of v. 



(D.) 'iHE SNOW STAGE AND THE HAIL STAGE. — Thesc are treated by 

 Bezold in a manner entirely similar to the preceding. He gives the 

 following exami)les of application of his methods. 



{A) Foehn. — In the Foehn, moist air expanding by ascending the 

 sides of a mountain is then compressed when it descends upon the other 

 slope, all without any addition or diminution of its heat. The curves 

 representing these changes of condition are therefore adiabatics. 



In Fig. 5, let a be the position 

 of the indicator for the condition ^ ** ^^ 

 corresponding to the initial con- 

 dition ; ISaSa the corresponding 

 line of saturation ; as the air ex- 

 pands the indicator moves cor- 

 respondingly along the adiabatic 

 ab down to the point b, where this 

 curve reaches the line of satura- 

 tion Sa. The arc ab is lo(;ated 

 really in a plane parallel to the 

 plane pov of the diagram, and at a 

 distance from this plane equal to 

 the mass x^ of the vapor of water 

 that accompanies 1 kilogram of the dry air. 



From a to b the adiabatic crosses a series of isotherms, of which the 

 first one only, namely, the dotted line Ta Ta is traced in the diagram, and 

 we see that the temperature has fallen continuously. At the moment 

 when the indicator cuts the line of saturation at b the dry stage ceases 

 and the rain stage begins. The adiabatic ab is now continued by the 

 pseudo-adiabatic be, which represents the rain stage. 



The air does not now cease to expand, but the temperature falls more 

 slowly ; this is why the curve be is less steeply inclined to the axis ov and 

 intersects continually the lines of saturation for quantities of aqueous 

 vapor that are steadily diminishing. 



However, the air arrives at the summit of the mountain and crosses 

 it and the compression begins ; at this moment the indicator of the con- 

 dition of the air is at c. Then we have the following alternatives : 



(1) All the water that is formed remains in suspension ; in this case 

 be is a true adiabatic, and here the change of condition of the air is 

 completely reversible. The indicator returns from c to fc in the rain 

 stage, and then from 6 to a in the dry stage. We find the same con- 

 ditions as to temperature, volume, and i)ressure, on either side of the 

 mountain. This case of complete reversibility always occurs when the 



Fig. 5.— Adiabatics. 



