Why do clouds darken to a very deep grey just before it rains or prior to a thunderstorm?

Question

Why do clouds darken to a very deep grey just before it rains or prior to a thunderstorm?

Answer 1

Because they become so saturated with moisture that the sun's rays can no longer penetrate them enough to make them look light.

Answer 2

Because they are getting heavy with the rain that is about to fall.

Answer 3

I am afraid that the two previous answers aren't entirely correct. The clouds don't become 'heavier' or 'more saturated.' The simple reason that rainy clouds are darker is because they are bigger in size and therefore hide more light from the sun.

But why does a clound become to rain? It works like this: as moist air cools down, its water content has to condensate. As it continues to cool down (this assume convection, i.e. rising air) the air can sustain even less water and water droplets have to condense into larger droplets until gravity pulls them down in form of rain or snow.

But since water droplets need to give away heat in order to condensate and that they have a too small mass to do it, they depend on just anything like dust, smoke particles, etc. to condense on. That's the reason in some places aircraft 'seed' clouds with dust (silver iodide or dry ice) in order to get them to come as rain.