DYNAMIC METEOROLOGY. 



413 



This evolution of latent heat without wholly checking the rate at 

 which the ascending air is cooling, diminishes its rate nearly in propor- 

 tion to the amount of vapor condensed ; therefore the interior of aclo:id 

 is warmer and lighter than the interior of a corresponding njass of clear 

 dry air that has attained the same level. The rate at which the tem- 

 perature diminishes within a mass of ascending, expanding and con- 

 densing, saturated air, such as that of which clouds are composed, de- 

 pends almost wholly on thermodynamic adiabatic conditions, and is 

 shown in the following table (xxiii), by Hann, for various initial 

 pressures and temperatures : 



Table XXIII, — Diminution of iemperature in a rising mass of saturated air, expressed 

 in degrees Fahrenheit, for each 1,000 feet of ascent. 



Note.— The approximate altitudes in feet are computed on the assumption of 50° Fahr. as the 

 average temperature of the air column. 



Thus we see for instance, that with a dew-point of 32° Fahr. the foggy 

 air, which near sea-level cools 3o.5 for 1,000 feet of ascent, will, when it 

 becomes a cloud at an elevation of about 15,500 feet or when the press- 

 ure is about 17 inches, be cooling at the rate of 2o.8 per 1,000 feet, and, 

 in general, the higher the saturated air ascends the slower it cools. So 

 far, then, as concerns this important cause the ascent of moist currents 

 would continue indefinitely, but the continued loss of water by its fall 

 as rain, and the more rapid loss of heat by surface radiation at the 

 higher altitudes, or the absorption of solar heat by the clouds probably 

 determines the limiting form and the height of the outer or upper sur- 

 face of the clouds. 



The average altitudes at which these clouds are respectively formed 

 may be approximated from the following table (xxiv), for Berlin, by 



