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Sunday, February 20, 2011
Great atmospheric phenomenon this morning! Not the best one I've ever seen, but they are rare enough that I'm tickled pink anytime I can see one, and also photograph it.
This is called a sun pillar. Actually, they can form in moonlight, or even from streetlights. I've never seen a moon pillar. Now that would be COOL!
These are seen when the air contains flat, hexagonal ice crystals. When the plates are larger than 20 micrometres across they tend to fall with their large flat side parallel to the ground, as if you dropped a dinner plate. (When they are smaller, they just tumble randomly). Because of the hexagon shape, they also tend to gather in formations, sort of like a flock of birds. Light bounces off their edges and because of the non-random crystals, we see the light as a column. The light source must be very low in the sky for us to see the phenomenon.
There is a good explanation at Weather Doctor
They tend to be short, but somewhere, from the past, I have a picture of a really tall one I saw once. If I run across it, maybe I'll use it for a future post.