Question: Since clouds contain tiny water droplets, why are rainbows not permanently present?

David Pyle answered on 20 Mar 2011:

Rainbows are part optical illusion. They form by refraction of (sun)light inside water droplets (rain drops), when the sun is behind the observer, and when there is rain falling in front of the observer. The angle between the light entering the droplet, and the refracted light coming out is about 40 degrees. When these conditions are not met, we won’t see rainbows. So we tend only to see rainbows when we have direct sunlight, and the sun is not too high in the sky (the rainbow would be too close to the ground). You also won’t get rainbows when the water droplets are frozen, as the optical properties of ice and snow are different! You may sometimes see related optical effects due to light scattering by water droplets in clouds when you are in an aircraft or on a mountaintop above the clouds. These are called ‘glory’ (in general), or Brocken spectres (if your giant shadow is also in the image), and look like haloes that appear to form round the aircraft shadow, or around the shadow of your head.