APOD 3.4

For this week's APOD, I chose the picture called Iridescent Clouds from the Top of the World Highway.  It is a picture of a cloud that has many different colors to it.  This particular picture of an iridescent cloud was taken last year from the Top of the World Highway, which is right outside Dawson City, in the Yukon Territory in Northern Canada.  This highway was given its name because of its length, as it is 79 miles long.  It also skirts the crests of hills, giving a great sight on the valleys below.


These types of clouds are either known to display unusual colors vividly, or to show a whole spectrum of colors at the same time.  They are made of small water droplets which all have about the same size.  What happens is, when the Sun is in the right position and mostly hidden by big clouds, these thinner iridescent clouds can diffract sunlight, with different colors being deflected by different amounts. So, observers will see the colors in different directions.  A lot of these types of clouds start with having uniform regions of iridescence, but they rapidly become too thick and/or mixed up, or they are too far from the Sun to give off striking colors.  The colors are essentially in corona fragments.  Another name for iridescence is irisation, which is derived from the Greek language as well.  The bands and colors are usually seen at the clouds' edges, or in coronal rings in the clouds.  As the clouds evolve, the colors and bands come and go.  Iridescence occurs mostly in altocumus clouds, but can also occur in cirrocumulus and lenticular clouds.  Although some iridescence is seen far away from the Sun, it is more often seen close to the Sun.  One can also find iridescence in most rocket exhaust trails.