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Do you remember how rainbows form? There’s a ton of videos that explain them.
The physics is pretty easy. Light enters tiny water droplets, bends or refracts, bounces,
bends again, and different colors end up in your eye.
Easy, right?
But if you’ve ever looked up in the sky and seen a bright halo around the sun or the
moon, or a fiery explosion of colors … that’s not a rainbow. It’s way cooler.
In fact, you might even say… it’s ice cold.
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In an old video, we talked about the science of snowflakes, and how the shape of H2O gives
frozen water crystals a six-sided symmetry all the way down to the atomic level.
But way up in the atmosphere, where it’s cold and dry, instead of forming the snowflakes
you know from ugly Christmas sweaters, ice crystals end up more like this...
These crystals can act like tiny sun-bending prisms, and if they end up between you and
the sun, like inside a cirrus cloud, they can form those amazing halos. So how does
it happen?
Inside that high cloud, there’s prisms arranged in all different directions.
If we zoom in on a slice like this, crystals in different positions will be rotated in
just the right way, so that the light coming out the other side will be bent towards your
eye.
Different rotations refract light at different angles, but since those angles average about
22˚… the halo is brightest right there. And since no light is bent at less than 21.7˚
we see a dark hole in the center.
When the sun is lower in the sky, even cooler things can happen.
How cool? Ummm… I’m trying to think of a pun and I seem to be… freezing up.
Sorry.
Instead of prisms, some ice crystals end up more like six-sided frisbees. And just like
autumn leaves, they tend to fall with their flat sides down because of air resistance.
This means that close to the horizon, those crystals are oriented the same way, which
creates more concentrated refraction in very specific spots. It can look like there’s
three suns! Because these fake suns obediently follow
their sun master through the low sky, we call them sun dogs.
You know how in a rainbow, longer wavelengths are bent slightly less? Well that happens
here too, tinting the inside of a sundog slightly red, and the outside slightly blue
On rare occasions you can see this, a ring stretching around the entire sky.
Instead of refraction, those flat ice crystals are reflecting like mirrors.
This stuff can even happen around the moon!
These are just a few of the amazing things that can happen when light meets ice.
If the right conditions are just right, it can look like Bob Ross went nuts and dragged
his icy paintbrush all over the sky.
Between clouds, sunsets, rainbows and now all THIS icy awesomeness, it can be hard to
pick a favorite atmospheric phenomenon. Luckily, you don’t have to. Enjoy them all.
Keep looking up, just not directly at the sun.
Stay curious