Airplane Skywatching
Published on Nov 23, 2006 at 12:14 am.
3 Comments.
Filed under skywatching.
This is the Thanksgiving holiday here in the United States. It is one of the busiest travel times. A lot of people are going to be flying from here to there now and at Christmas. If you get to have a window seat, they you can look out and see the world pass by. If you pay attention, you can see some interesting sky atmospheric phenomina.
First of all, if you are flying high, notice that the sky is typically clear above you, and normally a deeper violet color that you don’t see from the ground. Commercial airliners fly at very high altitudes because the air is thinner (less drag), but at such high altitudes there is less turbulance.  Convection, the source of the atmosphere’s turbulance, occurs in the troposphere, the lowest level of the atmosphere. In the troposphere, the air is cooler with altitude. However, at sufficient altitude, a few miles up, ultraviolet llight from the sun breaks up oxygen atoms, and they recombine to form ozone. This ozone in turn absorbs UV, warming the air. The air gets warmer with altitude. We call this layer of the atmosphere the stratosphere. Convection can not be sustained if the air above is warmer than the air below. So, there is less turbulance.  There also aren’t many clouds, since there is no convection to lift water vapor past the troposphere. Less water vapor means the skies are less hazy, and a deeper color (the more water vapor, the whiter the sky looks). But, there are also fewer air molecules to scatter light, so the sky looks more violet than blue. If you fly even higher, as the Concorde did, the sky would even begin to look black.
Then, look down. If you are flying over clouds, then look for the airplane’s shadow. This will be directly opposite the Sun. You will sometimes see a colored halo surrounding the aircraft. This is caused by multiple reflections in water droplets, much like a rainbow. Now, the interesting part. 
The halo is centered directly opposite the Sun for you. That means that is is centered on your seat on the airplane! If you were standing on a cliff looking down at fog, you’d see the same thing, with the halo around your head. 
If you had a friend standing next to you, you’d see the halo only around your own head. This halo centered on your own shadow is called a glory. If the shadow if of a person, the shadow and glory together are called the Brocken Specter (or Specter of Brocken).
Another, similar thing, is if you look down on grass or trees (pine forests, not trees that have lost their leaves for the winter). Then, you will see simply a brightness surrounding your shadow. This is the heiligenschein. 
You
can see this in these two photos taken looking at the ground (it works better with wetter grass). In one, the ground around my head is brighter. In the other, with the camera held out to the side, it is brighter around the camera’s shadow. So, it is all perspective.
On the ground, or at lower altitudes, you can often see a halo around the Sun. This is caused by ice crystals in the sky. Different shaped ice crystals make the halo much brighter to the left and right of the Sun. These bright spots, generally only seen when the Sun is at fairly low elevations, are called parhelia, or sundogs.Â
So, just because you are flying somewhere, and you don’t have a telescope doesn’t mean that you can’t be observing the sky!
-Astroprof
You can also read a bit more about glories on Quasar9’s site.
Image Credits:Â
Glory by Franz Kirschbaum, Brocken Specter by Louis Greene, Heiligenschein by Astroprof, Halo and Sundogs by Philip Appleton
Quasar9 on November 24, 2006 at 3:13 pm: 1
Hi Astroprof,
Great post on glories & sun halos
Thanks for the mention
Hope you enkoyed Thanksgiving!
JANE PICKUP on April 6, 2008 at 2:25 am: 2
Could I use this image of the Broken Spectre on You Tube, please?
Astroprof on April 6, 2008 at 8:59 am: 3
The links to where I found the images are at the end of the posting. Be sure to credit the sources.