Why are clouds dark and grey when there's about to be a thunderstorm, while white and pure when it's sunny?

Question

Is it due to the water vapour concentration or what?

Answer 1

A cloud's colour chiefly depends upon the cloud's relationship to the sunlight.If all the wavelengths( red to violet) are not blocked,but scattered EQUALLY,the result is white light.So, if the sun is shining and the cloud is not covering the sun,the cloud is white in general.
On the other hand,if a cloud covers the whole visible area of the sky,the depth of the cloud determines the colour of the cloud; the deeper it is,the darker it is.
Cumulonimbus is the cloud which produces thunderstorm and rain and is usually black.This is because,light cannot penetrate through it as it is deep and densely packed with water droplets,ice and snow particles.

Answer 2

You answered your own question.

In thunderstorms, the cloud is thick and no sunlight penetrates, so the underside of the clouds is dark.

When there are few clouds around they tend to be high thin clouds (cirrus) or tall clouds (cumulus), with lots of gaps between. The sun can then shine through the clouds or on the sides of the clouds, which then appear white. Even an isolated thunder cloud will look white when the sun shines on it.

Answer 3

When there's a buildup in water vapor in the clouds, light from the sun connot penetrate through the clouds. That give the clouds the dark grey color. On sunny days, the clouds are less dense, and the light from the sun can shine through the clouds.

Answer 4

The clouds for thunderstorms hold precipitation thats why they look dark and grey.

Answer 5

Cloud brightness is actually from density of moisture contained in the cloud.

Dark clouds have lots of water droplets in them, deflecting and diffracting the light, so it can't shine through.

Bright white clouds have less (and smaller) water droplets, so the light diffracts and deflects a little bit. but most of shines through for you to see.

Answer 6

U had almost answered ur own question. Good attempt.