Tuesday, November 4, 2008

What causes this?

Yesterday I posted one of the sunset shots I took in Iowa a few days ago. It was a nice, normal and beautiful image of the sun setting behind the clouds. Fine.

But after the sun actually set there was this shaft of sunlight that rose straight up from the horizon. It lasted about 15 minutes. The colors and clouds kept changing but the shaft stayed constant. You can see some of the color changes just in these three shots.
Does anyone know what causes this? The clouds were changing to rapidly to think that it was a break in the clouds somewhere over the horizon. There are no mountains in Iowa (duh), so why this shaft of light? It stayed in the same place, as I said, for 15 minutes.


sh said...


I'm not sure what's causing it - it could be a volcano, additional air pollution caused by a weather inversion pattern, higher winds than usual kicking up additional dust, or some other cause. But the reddening and the polarization are definitely related.

The reddened sunset is caused by small particles and clumps of molecules scattering the sunlight. This process, called Rayleigh scattering, is wavelength-dependent, so the blue light is scattered more than the red light. At sunset, the light passes through a lot of these small particles, so the blue light is largely scattered out (spread over the entire sky), while the light reaching us on a more direct path is white light minus the blue (or redder). So the more particles, the redder it appears.

But why is it also polarized? If the particles (which are not generally spherical, but usually oblong) would be randomly distributed (all orientations equally likely), then the light wouldn't be polarized. But these particles are subject to earth's gravity, and so are more likely to be oriented vertically than horizontally (they present less air resistance as they fall under gravity). Then, the light which is "vibrating transversely" or along the axis of the particle "sees" a larger particle, and so is more likely to be scattered - think of the particle as being a larger target for the photon. This results in plane polarization!

Becky Meyers said...

that second shot is gorgeous!!

Anonymous said...

Actually, Iowa is America's Holy Land, I hope that helps figure this one out.

Wayne Larson said...

That's just God talking with Senator Tom Harkin.

Nathan Seldomridge said...

This is caused by ice crystals in the atmosphere, some of which are shaped like hexagonal flat plates, and have a preferred orientation (flat, like a parachute). The crystals above the the sun, from your perspective, reflect that light to your eye.

This is what I'm talking about:

I got to study this stuff a bit in grad school.

Jeff, this is my first comment, but I've followed your blog, and enjoyed "The Lord's Service".

Jeff Meyers said...

Nathan: thanks for the comment. That make sense. Cool.