SEATTLE — Spectacular rainbows were seen in parts of the Puget Sound region on Thursday evening, with some folks getting treats of a “double rainbow” or even a very rare “triple” or “quadruple” rainbow!
Rainbows form when the sun is relatively low in the sky (below halfway to the horizon) and rain is present nearby. However, to see the rainbow, the sun needs to be generally at the viewer’s back — behind them — while rain is ahead.
When viewed from just the right angle, the countless raindrops act as prisms, “bending” the sunlight as an actual glass prism does, creating the visible ranges of light from red all the way to violet.
A “double rainbow” occurs when the light is reflected twice within raindrops. It’s not as common, but isn’t extremely rare.
What KIRO 7 viewer Pauly Kostora captured in Kirkland Thursday night, however, is very rare. This was at least a “quadruple rainbow!”
But the actual term is supernumerary rainbow!
This occurs when we have raindrops of very uniform, or similar, size and shape. The droplets also need to be pretty small to get this to occur. As the precipitation on the eastern side of Puget Sound was pretty light Thursday evening, that’s the ingredient Pauly had in place!
As the scattered light waves exit these almost perfectly similar drops, the light waves that emerge actually interfere with each other in a process that winds up “canceling out” some of the colors of the rainbow and introducing intervals of light and darkness until the subsequent “rainbows” fade into nothing.
What a catch!
Here’s more detailed information on the phenomenon from Atmospheric Optics: https://www.atoptics.co.uk/blog/supernumeracy-rainbows/