Q: How come rainbows do not always develop when there is sunshine right after a rainfall?
A: A rainbow occurs when there are raindrops in front of you with the sun at your back. The sunlight is refracted (or bent) as it enters the raindrop, is reflected by the back of the raindrop, and then is refracted more as the light comes back out into the air toward you.
Sunlight is a combination of all colors of the visible spectrum. The refraction of each color occurs at slightly different angles, which creates the colorful display.
Because the sun is single point of light and the refraction is a constant set of angles, it creates an arc of colors.
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Technically, there would be a complete rainbow circle, but the bottom half of the circle is blocked by the land on the horizon. You sometimes can see the complete circle when spraying a garden hose with your back to the sun.
Given that many, but not all, of our storms have some sort of a westerly component, rainbows are more common during the evening hours in the east with the departing shower while the sun is to our west.
During the morning with the rising sun to the east, you sometimes can see a rainbow to the west before an approaching shower. These are a little less common, as shower and thunderstorm activity is increased during the afternoon and evening hours, when the ground is the warmest and the air is typically the most unstable.
How can there be no rainbow despite sunshine returning? It could be that the sun is too high in the sky to allow you to see the refracted circle light. Also, there can be times that the sun is on the same side of the sky as the departing storm.
Q: What causes what I call rainbow clouds?
A: What you are describing is likely a circumhorizontal arc. While this event has a nickname of “fire rainbow,” it is not technically the same thing as a rainbow. We learned in the previous question that rainbows are formed with the sun at your back and rain in front of you.
In the case of these arcs, the sun is on the same side of the sky as thin, high clouds. The light of the sun is refracted by the ice crystals in these high, thin clouds. Sometimes this refraction will cause a complete circle of light around the sun or the moon.
Most of the times the refracted light that makes it through the clouds is not intense to see separate colors, but occasionally it can be colorful with the sun.
That is the case in what you are seeing.
And sometimes you can only see part of the arc which really does make it look like a mini-rainbow in the sky. In order for this to all come together right, there must be certain flat ice crystals in the clouds, and the sun must be quite high in the sky, greater than 58 degrees high in the sky.