lower temperature they maintain through radiation to space, just as, and for the same reason that it collects on blades of grass and other exposed good radiators. But in order that the marked radiation, essential to the formation of the water droplets, may take place, it is necessary that the atmosphere above them be dry, for water vapor does not allow radiation freely to pass through it. Hence a gray morning sky implies a dry atmosphere above the dew droplets, and, therefore, justifies the expectation of a fair day, or even a clear one, for the droplets themselves to which the gray is due are soon evaporated by the rising sun, and convection, in this case, because it mixes the moist lower with a dry upper air, seldom causes precipitation.
A red morning sky commonly implies that the lower and heavier dust particles have been protected from excessive night radiation by a blanket of overlying moisture, else it would be gray; and at the same time it also implies the presence, in the lower atmosphere, of sufficient moisture to enlarge the dust particles through incipient condensation, else the sky would have some shorter wave-length color, such as yellow to green. Hence when the morning sky is red the whole atmosphere, to considerable elevations, is moist, and rain, therefore, probable.
Convection in the main, as we have seen, prepares the way for the phenomena of the evening sky, and radiation for those of the morning sky. Hence the amount and distribution of moisture most favorable to any given sky color, such as a gray or a red, are radically different in the two cases. There is, therefore, a real physical basis for, and much truth in, the proverbs that declare one result to follow the red of morning and quite another that of evening. There is also justification for some proverbs, two of which have already been given, that refer to or include other colors.
Additional good examples of the latter are as follows:
"Evening gray and morning red
Make the shepherd hang his head."
"An evening gray and a morning red
Will send the shepherd wet to bed."
"Evening red and morning gray
Two sure signs of one fine day."
"Evening red and morning gray
Help the traveler on his way;
Evening gray and morning red
Bring down rain upon his head."
Corons and Haloes
"For I fear a hurricane;
Last night the moon had a golden ring,
And to-night no moon we see."
—Longfellow.
