© Keith Brown
It looked like a rainbow, but we didn't have any rain Monday. These really aren't rainbows. They are called circumhorizontal arcs.
They form by light passing through wispy, high-altitude cirrus clouds. They are rather rare because they are only seen when the sun is very high in the sky, more than 58 degrees above the horizon. Another important factor is that the hexagonal-shaped ice crystals within the high, thin cirrus clouds need to be thick plates with their faces parallel to the ground. So, as you can imagine, this is often a difficult task to get these events to occur at the same time.
When the sun's light enters through a vertical side of this ice crystal and leaves from the bottom face, it refracts, or bends in the same way that light passes through a prism. If the cirrus's crystals are aligned just right, the whole cloud lights up in a spectrum of colors.
© Keith Brown
Comment: A few days before, another not-so-rare circumhorizontal arc was seen in North Carolina. It appears to be happening more often due in part to atmospheric conditions that include increased particulates in the atmosphere from a rise in volcanic ash, dust and 'meteor dust' left by fireballs exploding in our atmosphere.