434 THE POPULAR SCIENCE MONTHLY 



and number, as they do near the surface of the earth, or in size only, 

 even at the expense of numbers, as happens in a moist atmosphere, be- 

 cause of their hygroscopic property, light of the shorter wave-lengths 

 becomes more completely absorbed and the sky assumes some longer 

 "wave-length color. Finally, when the particles are large enough to 

 reflect as mirrors the sky becomes whitish. Hence both the morning 

 and the evening twilight sky often shows a series of colors ranging from 

 red, near the horizon, through orange and yellow to a green or even 

 blue-green with increase of elevation and consequent decrease in the 

 number and size of dust particles along the path of light from the sun 

 to that part of the sky in question and thence to the observer. 



When the air is filled with fog, or other particles of similar size, the 

 whole sky becomes uniformly gray. This is because the water droplets 

 that together make fog and cloud, though usually so small that it would 

 take from 2,000 to 3,000 of them to make a row an inch long, neverthe- 

 less are large enough to reflect, as would little mirrors, and to refract, 

 or transmit in a new direction, light of every color. 



It remains now, in preparing the way to an understanding of the 

 weather significance of morning and evening colors, briefly to outline 

 the essential conditions and processes of cloud formation and rain. 



Probably that one of these conditions with which the general public 

 is least familiar is the presence, in large numbers, of some sort of 

 nuclei about which water vapor can condense. We can safely assume, 

 too, that in the open atmosphere these nuclei consist only of dust par- 

 ticles, though it is possible in the laboratory, under conditions that 

 rarely, if ever, exist naturally, to obtain condensation without the aid 

 of dust of any kind. 



Besides the presence of dust particles, a certain relation between 

 temperature and water content of the atmosphere is also essential to 

 condensation. The warmer the air, so long as the temperature is below 

 the boiling point, the greater, and, for ordinary temperatures, at a 

 rapidly increasing rate, the amount of water vapor it can contain in 

 the form of a transparent gas. 



In reality the relation above discussed is between the temperature 

 and amount of moisture per unit volume, a quantity which does not 

 appreciably change with the presence or absence of other gases. But it 

 is allowable, because of this constancy, to use the popular, though un- 

 scientific, expression, " water content of the atmosphere," provided one 

 thinks of the atmosphere as a mixture of gases (chiefly nitrogen and 

 oxygen) coexisting with the undisturbed water vapor, and not as a sort 

 of sponge that mechanically holds it in suspension. 



If, then, air, which always has dust particles in it, containing all or 

 nearly all the water vapor it can hold, is cooled to a distinctly lower 

 temperature, a corresponding amount of condensation will take place 



