Rainbows

i) Why is the rainbow the sign of Noah's covenant? An obvious reason is that rainbows signal the end of a rainstorm. Some rainstorms produce flash flooding. So there's a natural symbolic association. 

ii) Another reason may be the universality of rainbows, comported to regional phenomena like the Northern lights. The universality of rainbows match the universality of Noah's covenant. A covenant with creation. A covenant that signifies ordinary providence. 

iii) Because rainbows are generated by sunlight and rain water, they evoke the Creator God of Gen 1–the Maker of the sun and rainclouds. That points to the divinity of the Son of Man in Ezk 1:25-28, whose nimbic aura resembles a rainbow. 

iv) In a dry climate like the Middle East, rainbows have a beneficent connotation. They signify life-giving rain. A sign of divine favor and blessing. An emblem of divine benevolence.