Because it denotes that there are various ærial currents of different temperatures and densities, producing atmospheric disturbance.
"And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud."—Genesis ix.
The refraction of the sun's rays by the falling rain.
487. Why does the rainbow exhibit various colours?
The colours belong to the elementary rays of light; and these rays having different degrees of refrangibility, some of them are bent more than others; they are therefore separated into distinct rays of different colours.