Why Are the Clouds So Dark?

A question from Bryan about the clouds:

"Can you help me out? My 3 yr old daughter asked me why the clouds were a little bit grey today, I told her they had only a little bit of rain in them."

When you're in an airplane, seeing the clouds from above, no matter how tall they are, they're white.  That's because you're seeing them from the top where the sunlight is hitting them directly.

A good example of clouds shadowing themselves:
The clouds are bright white at the top where the sunshine
is hitting them directly.  But they turn dark near the
bottom where not as much light gets through.

From the ground, we usually see the bottom, shadowed side of the clouds.  The taller the cloud, the less sunlight gets through it and the darker it looks from below.

It's just like holding your hand up to block the sun from your eyes or standing under a leafy tree; it's darker in the shadow.  The top part of the cloud starts blocking the sunlight and it gets darker and darker as more layers of the cloud block and scatter the light even more.

In many cases, taller clouds are more likely to include rain.  So, while there seems to be a connection between the rain and the darkness of the clouds, that's really just a misconception.  It's all about the shadows.

Big, puffy cumulus clouds may block a lot of the sunlight but not have any rain to drop, as in the photo above.  Thick stratus clouds may not be as tall and may not block as much of the sunlight (so they look lighter) but still have lots of rain with them (in which case, they become nimbostratus).