Monday, January 19, 2015

Red Bow

While taking the photos of the boat in the last post at low tide, we were treated to a rainbow. But not your usual multicolor bow. This bow was monochromatic.

It was a red bow (albeit a partial bow).

A red bow forms when the sun is very low on the horizon. The Earth's atmosphere scatters out the other colors, starting with long wavelengths at the blue end of the spectrum (thus the reason for the blue sky). As the sun sets the light travels through more of the atmosphere to reach us and increasing longer wavelengths are scattered out. We usually see this as reddish-pink clouds at sunrise and sunset. But sometimes you get lucky. And you get a red bow.

Another clue to the lowness of the sun is that the bow is almost vertical at the base. A rainbow is a ring centered on the anti-solar point. From our usual vantage point on the Earth's surface, part of that ring is below the horizon ( a full ring can be seen from a vantage point high enough above the surface). The closer the sun is to the horizon the higher the anti-solar point and thus more of the bow is above the horizon.

The last clue to the altitude of the sun is the blurriness of the trees and water. It was getting dark when I took these shots, necessitating long exposures, in which time the water flowed and the branches moved with breeze.

This is the only red bow I've seen. Which is not surprising as first you need a rainbow, then you need a low sun with a clear horizon. A not all that common combination.


Dennis Waters said...

Steve, is this a sort of sun pillar?

MevetS said...

Good morning Dennis, and thanks for stopping by.

No this is not a sun pillar, although because it is only a partial bow I could see why one might think so. What isn't obvious form the pictures in the post is that the first two images are the two sides of the bow, with the top arcing part missing (the third image is the same part of the bow as in the first shot).

Sun pillars are in the direction of the sun, appearing above and occasionally below the sun, while this was in the portion of the sky opposite the sun. The other difference is that sun pillars are reflections off of ice crystals, while rainbows are reflection and refraction in water drops, and there had been showers across the bay from where we were.

My favorite site for info on these type of phenomena is Les Cowley's Atmospheric Optics

Take care. said...

I know the truth - it is a UFO taking off after you had a conference with them, I knew Mevets was an Alien Name! Busted Dude!