The azure tint of the cloudless sky is due to the decomposition and
refraction of light, as it passes through layers of air successively
increasing in density. The blue and violet, being more refrangible than
other colors of the solar spectrum, are diffused through the atmosphere;
and being reflected from its particles, they impart to it their own
color.
The clouds, floating in the atmosphere, absorb the more refrangible
rays, and reflect the less. At sunrise and sunset, when the light
traverses the greatest depth of atmosphere, all the colors are absorbed
except the red and the yellow; and these, being deflected from the
particles of vapor, produce the brilliant coloring of sunrise and
sunset.