KnowHOW team explains: Rain clouds appear black in colour because of an optical phenomenon, called diffraction, as well as a change of state from gas state to liquid.

Any cloud is a collection of a large number of water droplets or ice crystals in the air. The droplets move very slowly and accumulate or grow large enough to eventually descend as rain, hailstorm or snow.

The moisture responsible for the formation of clouds and rain evaporates from water surfaces over the earth. After rising up, the moist air expands and cools down. Finally, when the relative humidity becomes more than 100 per cent, the moisture condenses on various aerosol particles called condensation nuclei which are present in the cloud. At the cloud point temperature, a homogeneous liquid becomes cloudy due to separation into two phases when cooled under specified conditions. This is precisely what happens when there is a chance of rainfall.

The colour of the clouds, or the rings around the sun or moon which we see after a spell of rain are produced by diffraction of light through small spherical droplets. The colours are intensified by the uniformity of drop size, while the drop size determines the colours of the clouds. The heavy density of the clouds also adds to the dark colour of the cloud.

The question was sent by Pravin Sinha from Bokaro Steel City