Opacity of clouds.

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Main Question or Discussion Point

Clouds form in the atmosphere when moist air cools.
This is due to some of the water vapour as a gas condensing and instead forming into minute droplets of liquid water.
Why does this make the cloud opaque though? - both pure water vapour and pure liquid water are transparent.
 

Answers and Replies

  • #2
ProfuselyQuarky
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Unlike the water in a glass or swimming pool, clouds are not contiguous bodies of water. Rather, they are more like millions a water droplets scattered throughout with air in between. Whenever light passes through each of the individual droplets (which, of course, it does), the light is scattered and refracted--but only for a short distance because the refracted light will eventually hit another droplet.

So I guess what I'm trying to say is that photons that find themselves in clouds are constantly changing directions and the wavelengths of the light waves become so scattered that a cloud is seen, rather than just being a clear mass.
 
  • #3
ProfuselyQuarky
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I should add that one of the reasons why rain clouds are dark (not white) because they are thicker and denser. If you fly in a plane above a rain cloud, it will be white like any cloud on a sunny day because the sun is still beating down on it, giving the top of the cloud light to scatter. The droplets in the bottom of cloud (the part we see from the ground), however, don't receive enough light to scatter, thus, it appears grey and dark.
 

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