Thursday, April 12, 2012

The sky: not as empty as it looks

Rainbows are so commonplace and are known so universally that they barely even elicit awe these days.  Sure, there was that guy on youtube who got pretty excited about one, but that was a double rainbow, for Pete’s sake.  Have we reached the point as a humanity where we need multiple rainbows just to pause Angry Birds and look out on a sunny day once in a while?  I’ll bet you’d look up if this was in the sky:

atmospherethroughthelens.weebly.com



Yes, as it turns out, the atmosphere is capable of producing grand displays of many and varying optical phenomena.  The image above depicts my personal favorite: ice halos.

I’ll get to their formation in a moment, but first, a lengthy refresher on what was surely one of many children’s first interesting, observational questions, before we became calloused to the simple joys of life: What is a rainbow?  A rainbow is band of light that has been separated into its constituent wavelengths by water droplets in the air.  Now, let me take you back one more step, before my plot line becomes clear.   

When you drive your car through a puddle, it slows you down, right?  If the puddle spans the entire lane, both of your front tires will hit it at the same moment, and the whole car will be slowed at the same time.  If the puddle is only on the side of the road, it will grab your outside tire while the inside one keeps on going, potentially causing you to veer off the road.  Now, imagine a photon: a single, discrete “packet” of light, moving through the air.  When the photon hits glass or water or the lens of your eye, it slows down just like the car in the puddle.  If the photon hits at an angle, the first part to make contact gets slowed before the rest, so the photon changes course. 

Now think of a photon from the sun zinging through Earth’s atmosphere and colliding with a water droplet.  When it hits the convex, outer face of the droplet, it changes course. 

physicsforums.com

The exact angle of refraction depends on the wavelength of light, so a red photon will bend less than orange, and so on all the way to violet.  When you look at an area of the sky with a lot of water droplets, all of them are refracting all the colors of the rainbow, but your particular position will happen to be right in the path of blue light coming from some droplets, green from others, and yellow from still others.   

Ice crystals in the air also bend light, but not in the same way droplets do.  They have no curving faces, only flat ones, so they typically cannot bend light enough to separate out its constituent wavelengths into the rainbow colors.  In great enough density, however, they can redirect large quantities of white light, producing ice halos like the ones above.  Of course, displays like that are rare enough that you may have to camp out at the South Pole for months to see them. 

But you certainly do not have to go anywhere special to see these amazing spectacles.  Ice crystals are present in the atmosphere no matter how hot it is on the ground.  The most common ice halo is the 22 degree halo, which is easy enough to form that it is a daily occurrence throughout most of the world.   

Here's one over a Cambodian ruin.

The 22 degree halo only requires standard hexagonal prism ice crystals, but there are tons of variations on that structure that can refract and reflect light in some really unusual ways.  There are thin, flat plates and long columns, as well as various non-equilateral hexagons.  Given the proper crystal structure, orientation, and density, there are more possible halos than can be seen in a single lifetime.  Some, like the elusive kern arc, are rare enough that they have only been reported a handful of times worldwide.

Other awesome atmospheric sights include irisation:

Photo: Paul Moss, NZ

 
Solar and lunar glories (in this case solar, and complete with a Brocken Spectre):

Wikipedia
And the elusive Fata Morgana mirage:

photobucket.com

So take a look around, eh?  There's some awesome optical stuff going on!  If you're interested, here's a link to the Optical Picture of the Day.  And this one is the Astronomy Picture of the Day.  Set one as your homepage!

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