Why are rainclouds black?

Dr Bolouswki · 13-07-2004 11:41pm #1

I was wondering the other day - why are rain clouds black? I mean, I get regular clouds, its evaporated water, looks like steam, its white... grand, but the black ones? Is it just cos they are menacing and it fits their purpose better?

14-07-2004 9:57am

{warning-non technical assumption}

I'd always assumed that was an optical illusion caused by the fact that the thicker and higher the cloud and consequently the more precipitation in it, the more light it blocks off and consequently the darker it's appearance.

Really heavy cloud (even fog) being capable of turning the athmosphere to Dusk.

Dappergent I hand this one over to you for the scientific answer

Zulu · 14-07-2004 10:14am

It is indeed caused by the light being defracted as it passes through the cloud.
With the heavier dark rain clouds, the water vapour has formed little droplets, and the droplets are joining up which will continue until the droplets get too heavy and fall to earth. These heavier droplets defract the light more than light white clouds of vapor.
The bigger/denser the cloud the darker it is underneath, as less light gets through.

Actually, that is pretty much the same reason the sky is blue (replace rain droplets with athmospheric dust)....

redspider · 14-07-2004 3:49pm

Yes, its to do with the size of the droplets and the number of them or density. Also the amount of sunshine and light where you are viewing from. ie: underneath or above. For example, if you are above a heavy thunderstorm cloud (say viewing from an airplane) the top of it will be white. If you are on the ground, that same cloud may look very dark grey indeed.

Also note that there are water droplets in the air at all times. Its all around us even in clear air. When there is more water vapour (tiny droplets) in the air this is what gives us the effect of haze. We can measure it with humidity.

But the blue sky is not caused by dust. It is due to an effect known as scattering - this is where the constituent parts of white light are refracted by differing amounts (depending on their wavelngth) when coming down through the layers of atmosphere (which is made up of Nitrogen and Oxygen mainly).

So at noon, the red part is refracted "away" so the sky looks blue. At sunset we can receive more of this refraction and hence red sunsets. In fact, dust in the atmosphere such as after volcanoes can enhance sunsets.

I'm sure the people on the astronomy forum (mike65?) can provide a more in-depth explanation.

Dr Bolouswki · 14-07-2004 9:43pm

I bow to enlightenment - thanks folks

DapperGent · 19-07-2004 11:41am

Originally posted by Earthman
Dappergent I hand this one over to you for the scientific answer

It would appear Zulu got there first.

(And I knew this one too!)

Why are rainclouds black?

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