Q: Why do clouds usually appear white but look grey when they’re going to rain?
Vineeth E.K.
A: Scattering by water droplets in the clouds is to blame.
When the Sun shines over clouds, water droplets in the latter act like prisms, splitting white sunlight into its component colours. They send these rays of light of different colours (frequencies) flying in different directions, at haphazard angles. These rays often manage to recombine because there are several droplets in clouds all scattering sunlight, creating white light. This is why clouds are white.
But just before clouds are going to rain, the water droplets are swollen. They coalesce to form larger droplets, of a few millimetres or more each. These droplets absorb more light and transmit less to the base of the clouds. As a result, these clouds have a greyish appearance; only their base scatters white light (as above) to observers on the ground. However, because enough sunlight illuminates the clouds’ upper decks, they continue scattering white light to anyone viewing them from space or from aboard an aeroplane.
Such scattering phenomena manifest in other ways too. Dust scatters white light the same way clouds do for the same reason, in a process called Mie scattering. It takes place only when the size of the scattering particle is comparable to the wavelength of the light.
- Karthik Vinod, intern with The Hindu
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