Michigan is drowning right now in some of the worst rains on record. Every day for the last two weeks, from Midland to Grand Rapids to Traverse City, my feed has been full of images of people losing everything to flooding. PLEASE send rainbows.
Just so this post isn’t a totally depressing send-off for your weekend, let me call in one of my favorite websites, Atmospheric Optics. Regarding secondary rainbows or “double rainbows” they say that the secondary is nearly always fainter than the primary, with colors reversed and more widely separated:
Light can be reflected more than once inside a raindrop. Rays escaping after two reflections make a secondary bow.
The secondary has a radius of 51º and lies some 9º outside the primary bow. It is broader, 1.8X the width of the primary, and its colours are reversed so that the reds of the two bows always face one another. The secondary has 43% of the total brightness of the primary but its surface brightness is lower than that because its light is spread over its greater angular extent. The primary and secondary are are concentric, sharing the antisolar point for a center.