436 THE POPULAR SCIENCE MONTHLY 



If the evening sky, not far up, but near the western horizon, is 

 yellow, greenish, or some other short wave-length color, then all the 

 greater is the chance for clear weather, for these colors indicate even 

 less condensation (smaller particles) and therefore a dryer air than 

 does red. Hence we can accept the following lines from Shakespeare as 

 the expression of a general truth : 



"The weary sun hath made a golden set, 

 And by the bright track of his fiery car 

 Gives token of a goodly day to-morrow." 



If, however, the evening sky has none of these colors, but is over- 

 cast with a uniform gray, then we know that numerous water droplets 

 are present, and that the dust particles, in spite of the heat they ab- 

 sorbed from sunshine, have become loaded with much moisture. Obvi- 

 ously, then, to produce this effect, the atmosphere, at considerable eleva- 

 tions, must be practically saturated, a condition that favors rain and 

 justifies the familiar proverbs : 



"If the sun set in gray 

 The next will be a rainy day." 



"If the sun goes pale to bed 



'Twill rain to-morrow, it is said." 



The above discussion of color phenomena applies to the evening sky 

 only. It remains to explain the origin of similar morning effects and 

 to point out the differences in the processes by which they are brought 

 about. 



A gray morning sky means, just as does a gray evening one, that 

 the atmosphere is filled with water globules which are large enough, 

 and even the smallest of them are, to refract and specularly reflect light 

 of every color. The difference, then, must be in the processes that lead 

 to the formation of the evening and the morning droplets. And these 

 processes are not the same, for the dust of the day sky is heated by 

 sunshine, as are also, to a greater or less extent, both the air and the 

 earth beneath, while the dust in the night sky, as does everything else 

 that is freely exposed, loses of the heat it possesses and cools through 

 radiation to space. Besides, the atmosphere during the day time, and 

 especially in the afternoon, is cooled by convection, which, as already 

 explained, leads to more or less condensation of moisture on the dust 

 that is present; while at night there is no strong upward movement, 

 there being no surface heating, and consequently but little dynamic 

 cooling of the air. The slight condensation here considered is due by 

 day chiefly to convectional cooling, by night mainly to loss of heat 

 through radiation. 



Evidently, then, the gray of the morning sky may often be caused 

 by water droplets that have gathered as so much dew on the dust par- 

 ticles in the air — dew that has collected on them because of the slightly 



