Asked by: Anonymous
It’s odd because clouds contain air and water – both transparent substances that hardly absorb any visible light.
Crucially, however, some of this water exists as tiny, sparsely dispersed droplets. Light can travel many metres into cloud, but eventually it’s likely to hit a droplet. That can scatter the light, changing its direction, although often only slightly. In thick cloud, each particle of light may hit many droplets in turn. Follow each circuitous trail, and you’ll see light is eventually thrown back out of the cloud in a random direction, often near the side it entered (explaining the darker bit of cloud on the non-illuminated side).
So a cloud’s colour is basically a mix of all the light put into it. Daylight is usually white: light straight from the Sun mixed with a little blue skylight. But you see non-white clouds at sunset, or over lit cities at night. Multiple scattering of small particles also explains the whiteness of milk, sugar, beer foam, and whisked Marmite.