Earth Science

Why are clouds gray?

By George Garza
Info Guru,

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Dark clouds
Dark clouds do not always indicate stormy weather
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Discover what makes clouds gray.

In a previous article, we discussed why the sky is blue. We also have looked at why clouds are white. So now we look at why clouds are gray, and why they can look colored. Why are clouds gray?
In the same way that skies are blue because of scattering, clouds are white because their water droplets or ice crystals are large enough to scatter the light of the six wavelengths (red, yellow, green, cyan, blue, and violet), which combine to produce white light. White means all colors are present.

Clouds are made up of tiny water droplets or ice crystals, usually a mixture of both. The water and ice scatter all light, making clouds appear white.


But not all clouds are pure white. For those that are, it is due to their altitude and the reflection of sunlight.
Different clouds reach different altitudes. The cirrus will appear above 18,000 feet; the alto clouds from 6,500 to 18,000 feet; and the stratus clouds up to 6,500 feet. Also, cumulus and cumulonimbus grow vertically. The combination of clouds many merging together or appearing on top of one another will affect the color. Why do clouds turn gray?

The Shadow Effect

So why are clouds gray? If the clouds get thick enough or high enough that all the light above does not make it through, the cloud will look gray or dark. Clouds will appear dark or gray when either they are in another cloud's shadow, or when the top of a cloud casts a shadow upon its own base. If there are lots of other clouds around, their shadow can add to the gray or multicolored gray appearance.

The darkness of a cloud also depends on the background sky. A cloud will look darker when it is surrounded by a bright sky and lighter when it is in front of darker sky. A dark cloud does not always forecast rain.
Some of the whitest, most pure light can be observed when dark clouds break apart and sunlight filters through. The reason we experience dark rainy days is because clouds are blocking sunlight.

Clouds with Different Colors

Now back to our discussion on cloud color, which is usually white. But some clouds can appear with different colors.
If the light hitting the clouds does not have all colors of light, then the clouds will scatter only the colors present in the light hitting the cloud. Remember that if all of the colors are present, you get white clouds; if the prevalent color wavelength is a combination of red and green, you will get a variety of yellow. Think of it like this: If you are shining a red light on a piece of paper, the paper will look red. This is because the light hitting the paper is red to begin with.

Why is that the case with clouds? When the sunlight is traveling in a horizontal direction from the sun near sunset or sunrise instead of vertically down during the middle of the day, the wavelength will interact with air molecules and scatter differently, showing different colors. Clouds can act like a canvas for the reddish sunset sky, like the piece of paper.

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