Ask Ellen: Why are clouds white?

Ask Ellen

GRAND RAPIDS, Mich. (WOOD) — Today’s question comes from Addy! She wants to know, “why are clouds white?” This is an excellent question and once again has to do with how light travels and interacts with objects once it hits the atmosphere.

Let’s start at the beginning. Light traveling to us from the sun is known as white light. Even though it appears white to us, it actually contains all the colors of the rainbow.

Make a rainbow experiment!

You can do this at home! White light is all around us. You can see white light from the sunshine beaming into your house. Or you can use a flashlight to create your own beam of white light. Every single beam of white light contains all the colors of the rainbow.

Using a prism, a hose, or a cup of water with a mirror, transform a beam of white light into a splash of rainbow color.

Why is the sky blue?

Now that we know white light contains all the colors of the rainbow, it’s much easier to explain why the sky appears blue!

When white light hits the atmosphere the particles of air enter into the atmosphere and hit particles of air, the air does something special. The air scatters the higher wavelengths of light like red, orange, yellow and green, much less than color like Blue.

Because the air scatters back out blue light so much more, we see that scattered blue light as blue! This is why the sky looks blue.

A similar thing happens with objects all over the world. Grass looks green, for example, because grass will absorb all but green light. Grass will scatter green light back out, and so it appears green to us.

Why do clouds look white?

Water droplets are special. They are one of the substances that scatter light perfectly. Instead of absorbing one color and scattering another, cloud droplets scatter all colors of the light equally.

Sometimes clouds will appear gray when the water droplets stack up and begin to build shadows inside the cloud.

Ice crystals high in the sky also scatter white light perfectly. Often the brightest white clouds are the ones that are highest in the sky, like cirrus clouds.

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