1) One who sees a rainbow and did not make the blessing immediately, may make the blessing afterwards so long as the rainbow can still be seen. (Latter Authorities)
2) Even though the rainbow is a natural phenomenon caused by the splitting up of the light rays by the raindrops, there is still a sign of a covenant, as before the flood the rainclouds were always very thick and heavy and covered all of the heavens to the extent that it was impossible for any sunbeams to break through them. However, after the flood the clouds were made to be much thinner and weaker and they therefore spread out very easily while the rain is falling so that there is a possibility that the sunbeams will break through them and the rainbow will appear in the clouds. (Malbim Bereishis 9)
