Friday, March 23, 2007

Rainbows in clouds

When the sun is behind you and you look at some rainstorms, you see a rainbow. Since the droplets in a cloud are (I assume) spherical, why don't we see rainbows in clouds? My hypothesis is that they are there, but they are superimposed on a bright cloud where secondary scattering dominates, so the rainbow is just washed out by contrast. Recently I was on a flight around noon and could see below me the shadow of my plane and when the clouds became thin enough that primary scattering probably dominates, there was a perfect circular rainbow with the dark terrain below it. So why no rainbows from thin clouds when the viewer is not on a plane? My guess is the sky behind it is bright. But I am unsure. Another possibility is that what I saw was not a rainbow. But the ring's angular radius looked about right...


Adrian McCarthy said...

I suspect it's just a matter of the bright sky behind the rainbow.

Is the halo you see surrounding a high sun or bright moon the same as a rainbow (due to water vapor) or some other sort of atmospheric effect? It's an circular area of brighter sky centered on the sun or moon and confined by what looks like a faint rainbow.

Peter Shirley said...

I think that that is a different effect caused by ice, but I may be wrong. That is what I surmised from this excellent page: atm. optics

Kevin said...

Here's a photograph similar to what you describe from that site:

Kevin said...

Sebastien said...
This comment has been removed by the author.
Sebastien said...

I saw a full-circle rainbow on a plane once too (in the morning though, so I didn't have to look down), and I'm pretty sure it was a regular rainbow. You can also make these rainbows by spraying water in a large enough "cloud" and positioning yourself correctly.

I'm thinking, if it were the brightness of the sky, we wouldn't see regular rainbows against a bright sky either, would we? This looks like a counterexample.

Perhaps it has to do with the packing of the droplets in clouds vs. rain?

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