Alan Edwards, of Belleknowes, asks :-

Why are different sorts of clouds different colours, from white to very dark grey?

Esther Haines, a physicist at the University of Otago, responded.

We see clouds because the water droplets or ice crystals making up the cloud reflect light from the sun into our eyes.

The sunlit side of a cloud is always appears white because the light reflected by the cloud has been scattered many times by the small droplets of water that make up the cloud. If we are looking at clouds from the other side they appear white, grey or black depending on how thick the cloud is and how many droplets there are in each cubic metre of the cloud.

A thick cloud with many droplets appears black because relatively little light makes it all the way through the cloud. A thin cloud with few droplets can appear bright white because most of the light makes it through the cloud. At sunrise and sunset we sometimes see spectacular red, orange, yellow or pink clouds. This happens because molecules in the atmosphere scatter blue light more effectively than the other colours making up white light from the sun. At sunrise and sunset the length of the path of light through the atmosphere is longest so more of the blue light has been removed. Hence the light reflected by the clouds is predominantly red.

Clouds are responsible for other spectacular sights. Rainbows form when light from the sun enters a water droplet, reflects off the opposite surface and exits from the water droplet travelling at an angle relative to the original direction of the light. There is a special angle, which is about 42°, where the light is especially bright. The angle is slightly different for different colours of light, for example, it is 42.4° for red light and 41.5° for blue light, which is why we see circular bands of colour in a rainbow.

The thin, wispy cirrus clouds that form high in the atmosphere are made of ice crystals. Ice crystals are responsible for the haloes that we sometimes see around the sun and the moon. Haloes are formed by light that changes direction as it enters an ice crystal and then again when it leaves the ice crystal. The light cannot change direction by more than about 22° because of the hexagonal shape of the ice crystal. This leads to a bright ring round the moon or the sun. Unlike rainbows, to observe a halo you need to be looking at the light source. Do not look directly at the sun.

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