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 overcast with a uniform gray, then we know that numerous water droplets are present, and that the dust particles, in spite of the heat they absorbed from sunshine, have become loaded with much moisture. Obviously, then, to produce this effect, the atmosphere, at considerable elevations, 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 particles in the air—dew that has collected on them because of the slightly
