My question is, why are the clouds white? At midday, when the sun is high in the sky or the clouds, light scattered off the water droplets in clouds is composed of all the colors of the rainbow. When you mix every color of visible light — all the reds and greens and yellows and blues — you see white.
On the other hand, at sunset, the clouds look reddish. That’s because sunlight from a low sun has to travel through more atmosphere before it reaches your eyes. Air molecules scatter blue light, and they scatter it best around sunset, when there’s more air between you and the sun. Meanwhile, the red and yellow components of white sunlight travel a straighter path to our eyes even at sunset — so the sunset clouds look those colors.
Meanwhile, stormclouds look very different from above than from below. Satellites flying above stormclouds see them as brilliant white. There are typically lots of ice crystals at the tops of these clouds — these crystals scatter light so efficiently that the light never makes it to the bottoms of the clouds. So the bottoms of storm clouds, as we see them on the ground, look a sinister gray.
Science Made Simple: Why is the Sky Blue?
Q: Why does the sunlight have to travel through more clouds atmosphere at sunset?
Here’s how I think about it. Imagine you have a large grapefruit and a flashlight and are sitting in a dark closet. Point the sun at the grapefruit. Only half of the fruit lights up. If you were an ant living on the grapefruit, the sun would be directly overhead if you were sitting in the middle of the illuminated area. That’s midday on the grapefruit.
Sunset on the grapefruit would be the edge of the illuminated circle.
Now imagine the thick skin of the grapefruit is actually the atmosphere, and the ant lives down below. If you were to stretch a thread between an ant in the midday position and the sun, that thread would pass through less grapefruit rind than it would if it were stretched to the ant sitting at the sunset position.
Studying the Clouds: NASA’s CloudSat Mission