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20 July 2009 @ 09:36 pm
A Rainbow in the Clouds  

Today at lunch I was trying to decide whether or not to go out to Hurdman to search for butterflies, and so I looked out the window to see if the sun was shining. It hadn't been very sunny the last few days, with mostly cloudy skies and a few rain showers on the weekend. As butterflies are easier to find on sunny days, I wanted to make sure that the sky wasn't too cloudy. When I looked out the 26th floor window of our office building downtown, however, I was puzzled to see what appeared to be a rainbow within a single cloud! The rainbow was contained within a high, thin, wispy cloud which itself was surrounded by thick, fluffy cumulus clouds. Having never seen anything like it before, I grabbed my camera and started taking pictures!

Circumhorizontal Arc

Unfortunately (or not, depending on how you look at it!) the windows in our building don't open so I was taking pictures through the glass. Any UFOs that might appear in the sky in these pictures are most likely smudges of dirt on the window!

Rainbow in a cloud

This phenonmenon is actually called a circumhorizontal arc, or circumhorizon arc. This beautiful, large, rainbow-coloured halo is rarer than other haloes formed by atmospheric ice crystals, such as sundogs or the common 22° halo . For it to appear, the sun has to be very high in the sky, with an angle of approximately 58° or higher. In northern latitudes, the sun only reaches such an elevation for a few days of the year (around mid-summer) during the hours around noon.

As such, one has a better chance of seeing this atmospheric phenomenon in the south when the sun is higher above the horizon longer. The circumhorizontal arc cannot be seen in countries that lie more than 55° in latitude from the equator because the sun there is always lower than 57.8°.

Circumhorizontal Arc

The arc is found beneath the sun, parallel to the horizon, approximately twice as far from the sun (two hand spans) as the 22º halo. It is formed by sunlight passing through a cluster of perfectly aligned (i.e. nearly perpendicular to the sunlight) ice crystals, such as those found in cirrus clouds. The sunlight passing through these crystals is refracted, resulting in a pure spectrum of colour and a vivid light display. As such, usually only fragments of the arc are visible where there happen to be cirrus clouds.

Here are a few more photos of the circumhorizontal arc taken from the 26th floor:

The colours were all very vibrant, but at one point the hues of green surrounding a cumulus cloud became very noticeable. They reminded me of the Northern Lights I used to see in Alberta when I lived there as a teenager.

Cumulus Cloud in front of Circumhorizontal Arc

Seeing this beautiful phenomenon really brightened my day. It is always a precious, special moment to see something like this, especially in the summer when it is difficult to look for sundogs and sun pillars because the sun rises so early in the morning. Now that I'm aware of this atmospheric phenomenon, I'm going to have to pay more attention to the sky in the summer around lunchtime!