WEATHER 153 



animals to fly and balloons to rise from the surface of the 

 earth ; (8) it distributes moisture ; (9) it produces 

 waves and ocean currents ; (10) it is an important factor 

 in weathering and has many other functions. 



The atmosphere as we consider it in studying weather 

 includes much more than simply the mixture of the gases, 

 nitrogen and oxygen. It includes all the various sub- 

 stances floating in it, such as dust particles and minute 

 organisms in the form of bacteria, microbes, and decayed 

 plant and animal tissue. The dust of the air is thought 

 to be important in the precipitation of moisture. The 

 minute dust particle, by virtue of the law of gravitation, 

 becomes the nucleus of condensation. This little particle 

 of moisture increases in size until it forms a raindrop or 

 snowflake and then falls to the ground. 



Colors of the Atmosphere. On clear days the sky 

 has a beautiful blue color. This color is due to the break- 

 ing up of the sunlight by the countless number of small 

 particles floating in the air. If the particles are quite 

 large, other colors are produced. The smoke from the 

 lighted end of a cigar is blue because the particles in the 

 smoke are very minute. The smoke particles from the 

 other end of the cigar, however, are increased in size by 

 the moisture from the smoker's mouth, and a dull grayish 

 color is the result. Red and yellow colors predominate 

 in the sky near sunrise and sunset, because the sunlight 

 passes obliquely through a great thickness of atmosphere 

 near the earth's surface and the other colors are sifted out 

 by the large particles of dust which float near the ground. 



The twilight arch, a rosy arch of lighted air, may be 

 seen in the east after a clear sunset as the sun sinks below 

 the horizon. As the sun sinks lower and lower this arch 

 rises until it vanishes in the darkening shadow of night. 



