MONTGOMERY, Ala. (WSFA) - If you saw what looks like a rainbow on clouds Tuesday afternoon, you were looking a rare optical phenomenon called a circumhorizontal arc. Yes, that’s a very real and scientific term!
You may have seen one of these before and referred to it by its other name -- a “fire rainbow.”
That name may be commonly used, but these circumhorizontal arcs actually have nothing to do with rainbows or fire. The only similarity between these and rainbows is the presence of the same 7 colors in order -- red, orange, yellow, green, blue, indigo, and violet (ROYGBIV).
Circumhorizontal arcs require a specific set of conditions to be met in order to form...
First, the sun has to be at least 58 degrees high in the sky -- usually during the late morning and early afternoon hours. Second, there have to be thin and wispy cirrus clouds present. Third, the flat, hexagonal ice crystals within those cirrus clouds must be aligned horizontally in order to refract the sun’s light in a collectively similar manner.
If these conditions are met, you can get what looks like a rainbow to form on those cirrus clouds that lies parallel to the horizon well below where the sun is in the sky. The reason you get the display of colors is due to the way the sun’s light enters and leaves the ice crystals. The ray of light must enter the crystals through their vertical side faces and exit through their lower horizontal basal faces.
Think of light shining through a prism!
A circumhorizontal arc may be complete with all 7 colors of a rainbow, or it may look fragmented. It all depends on the orientation of the cirrus clouds in question. These beautiful displays can last a few seconds or several minutes depending on the exact setup, so be sure to act quickly if you want to snap a photo!
But there’s another optical phenomenon that looks very similar to circumhorizontal arcs that can form in our skies in Alabama. The image above courtesy of the National Weather Service in Wilmington, North Carolina, shows what’s referred to as cloud iridescence.
Cloud iridescence is a display of colors on clouds found high up in the atmosphere. These are usually cirrus, cirrocumulus, altocumulus, or lenticular clouds.
The difference is this forms due to the sun’s light being diffracted by water droplets or small ice crystals suspended within the high-level clouds. This process results in the scattering of the sun’s light, resulting in a random assortment of color with no organization to it. Not every color is guaranteed to be present in iridescence either.
This phenomenon also occurs very near where the sun actually is in the sky, which is also different from circumhorizontal arcs.
Cloud iridescence is relatively rare. The cloud(s) must be thin and have lots of water droplets or ice crystals of about the same size. When that is the case, the sun’s rays encounter just a few of the droplets at a time, resulting in the random display of colors.
Semi-transparent clouds or clouds that are just forming are the ones you should look for when hoping to catch iridescence in the Alabama sky!