Caused by the reflection and refraction of the rays of the sun shining on falling rain. It was appointed as a witness of the divine faithfulness (Genesis 9:12-17). It existed indeed before, but it was then constituted as a sign of the covenant. Others, however (as Delitzsch, Commentary on Pentateuch), think that it "appeared then for the first time in the vault and clouds of heaven." It is argued by those holding this opinion that the atmosphere was differently constituted before the Flood. It is referred to three other times in Scripture (Ezek. 1:27, 28; Revelation 4:1-3; 10:1).

Noah Webster's New International Dictionary of the English Language

(n.) A bow or arch exhibiting, in concentric bands, the several colors of the spectrum, and formed in the part of the hemisphere opposite to the sun by the refraction and reflection of the sun's rays in drops of falling rain.