You ask, I answer (cloud color)

MDK asked about cloud color. Something along the lines of, is rain from white clouds cleaner than rain from grey clouds?

Imagine taking off in an airplane on a rainy day with the sky covered by gray clouds. As your airplane flies through the clouds, the sky becomes brighter and brighter and eventually you see bright sunlight and look down: the tops of those gray clouds you've flown through.

Now, the clouds are white.

It's a little more complicated than this, but basically, clouds look gray when they block sunlight. The thicker the cloud, the more light it blocks.

There's nothing blue in the sky, it's a matter of blue light reflecting from one molecule to another in the sky with some of these reflections from all parts of the sky reaching our eyes.

The tiny water drops or ice crystals that make up clouds don't scatter, or reflect, only one color or a few colors that make up sunlight. Instead, they scatter all of the colors, which together add up to white.

Cloud particles also absorb some light. In other words, as a "sun beam" goes into a cloud, some of it is reflected back to make the cloud look white to someone who's on the same side of the cloud as the sun.

But if you were in an airplane descending through the cloud — imagine it's a thick one — it would get darker and darker as more and more light is absorbed and scattered in all directions, including back toward the sun.

In general, when a cloud is around 3,000 or so feet thick, hardly any sunlight will make its way through the cloud.

You often see cumulus clouds that are white on top, and dark at the bottom.

Different things could be going on here. First, the bottom of the cloud could be in the shadow of other clouds, or the sun could be setting and is shining only on the tops of towering clouds — this is common since thunderstorms are more likely in the late afternoon than at other times.

Also, larger drops of water, such as raindrops near the bottom of a cloud, absorb more light than smaller drops.

I'm sure you've noticed that gray clouds don't always mean it's going to rain or snow.
To see why, let's take a brief look at why rain or snow falls from some clouds, but not from most of them.

Clouds stay in the air, even though their water drops and ice crystals are heavier than air, because slowly rising air holds up the water drops and ice.

When the water drops or ice crystals grow too large for the rising air to hold them up, they begin falling as rain or snow. Often when we see a cloud as gray the water drops or ice crystals aren't large enough to fall.

To learn more about the color of clouds you really have to look into books. I've done a good amount of looking on the Web, and found some good information. But I've yet to find a Web site that puts the basic science together in a way that's easy to understand.

I found this information on USA Today.