Halos in the sky

I just got back from this evening's Cafe Scientifique — where were you guys? — and I got to see lots of pretty pictures of halos and sundogs and light pillars. One of the nice things about living in Morris is that we actually get a lot of that weird atmospheric phenomena here, because we have lots of the raw material for them here: ice crystals. Vast drifting clouds of hexagonal crystals, flat and columnar, of various proportions, floating in the sky at various orientations to both refract and reflect light into our eyes.

I won't go into all the details, since you weren't there. And since most of you live in a less blessed place than the cold crisp upper midwest in the wintertime, you won't get to see them, because your wicked heat melts all those sharp edged crystals into sludgy droopy droplets. Sorry. But I wanted to pass along one tip.

There's some free software called Halosim that lets you do simulations of ice crystal distributions in the atmosphere. You specify their sizes and proportions and shapes, and then it traces the paths of light rays and produces an idealized image of what you should be able to see.

It's very cool. You can tinker and see that to make dramatic sundogs, for instance, you need lots of flat hexagonal platelets floating in a mostly horizontal orientation, and presto, you'll get a pair of virtual suns 22° to either side of the real one.

Well, maybe you can do that. It's PC only, so I can't run any of the simulations on my home computers myself. I'll have to settle for looking at the real thing, darn it.