Why are rain clouds dark?

Asked by Jono80

The main reason why rain clouds are dark is due to their particulate density. As clouds are formed from minuscule water droplets and/or ice crystals, the denser they are packed, the more light radiating from the Sun is scattered and dispersed by them, lending a darker appearance to their lower sections.

This is best visualised by a trip on an aeroplane, with clouds on an overcast day seeming grey/black in colour when viewed on the runway, yet a bright white when viewed from above during flight. Essentially, the deeper and denser a cloud formation is, the less light will pass through it.