Why are Clouds Grey?

imageWe’ve found out about clouds and why clouds are white in earlier posts.  I anticipated what the next question would be – mixing paint does not make white – ha, wrong or at least not the first question asked.

The question is: Grey clouds aren’t white, why are they grey.

Why Clouds are White – recap

We’ve learnt that clouds are white because the water droplets inside them are big enough to scatter / refract light from the visible spectrum.  Light from the seven wavelengths in the visible spectrum combines to produce white light.

Why do Clouds Appear Grey

imageClouds look grey when shaded by other clouds or if the top of a cloud casts a shadow on its base.

Also, the cloud’s darkness depends on the background sky.  A cloud looks darker when surrounded by a bright sky and lighter when in front of darker clouds.  A dark cloud does not always mean rain.

Why are Clouds Dark Grey

imageA cloud becomes thicker and more dense as it collects more and more water droplets and ice crystals.

The thicker a cloud gets, the more light it scatters.  So, less light actually makes it through to the bottom of the cloud.

The bottom of a rain cloud has little light left to scatter, so the bottom looks grey when we look up at it.

Summary

Clouds reflect and pass light from the sun, but they do not absorb any one of the colors from the visible spectrum more than any other. So we see

  • white when the clouds are thin and well lit,
  • grey when they are thick and
  • near black clouds when they are very thick and dense, which block most of the sun’s rays.

When light from the Sun reaches the top of a cloud it scatters in all directions. Some of it scatters farther into the depth of the cloud. Then, this light scatters as well, now there’s less light in the depth of the cloud yet it still scatters farther into the cloud. The deeper into the cloud you go the less light it gets from scattering above.  The deeper and denser a cloud is, the less light will pass through it.

A Note to You and Me

Think about when we are on an aeroplane.  At take off clouds maybe grey. As the aeroplane climbs we fly through the clouds.  As soon as we have climbed above the clouds we see they are lovely and white.

Thank you for reading our post.  We have written other posts about clouds.  Here are their links if you would like to have a look:
Why are Clouds White?
What are Clouds?
Clouds – (a poem.)

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