DYNAMIC METEOROLOGY. 411 



vatiou, that is to say, the appearance of the sky at siiuset is recorded 

 as yellow, red, or jjreeu. These sky colors vary with the changing 

 amonnts of moisture through which the sun's rays pass. From these 

 appearances an observer can form an estimate of the general hygro- 

 nietric condition of large tracts of the atmosphere. 



(2) Condensation of vapor. — When a mass of air, whether dry or 

 moist, is, on account of its relative lightness, lifted up by the pressure 

 of the surrounding denser atmosphere, or is drawn up by any abnor- 

 mal diminution of the pressure above, or is pushed up the incline of a 

 mountain or plateau, it is raised into a region where the barometric 

 pressure is less than in its initial position. Consequently the rising 

 mass must expand to an extent proportional to the diminution of i^ress- 

 ure. In this expansion a great amount of both internal and external 

 work is done, corresponding, respectively, with the increased separation 

 of the gaseous molecules and the pushing aside of surrounding air; 

 this work is done at the expense of the internal heat of the rising air, 

 consequently the whole expanding mass of mixed air and vapor grows 

 cooler as it rises. 



The cooling process is known as a dynamic cooling, and takes place 

 uniformly and simultaneously throughout the whole rising mass of air. 

 It is a very different j^rocess from the much slower processes of cooling 

 by radiation or by convection. For dry air the rate of dynamic cooling 

 is almost the same for all pressures and temperatures, and is approxi- 

 mately 1° C. for every 100 meters of ascent or 1° Fahr. for 180.5 feet; 

 for moist air the rate of diminution of temperature is a little slower, 

 but the above value may also be used for it so long as no moisture is 

 condensed, that is to say, so long as. the temperature does not fall be- 

 low the dew-point. 



From the preceding it follows that the elevation above the ground 

 at which cloud or haze begins to be formed depends primarily upon the 

 depression of the dew-point of the rising air below its temperature at 

 the time when it starts from the ground. 



The third column of Table xxii gives an idea of this relation be- 

 tween altitude and temperature. 



