April 2022

Every Cloud has a Silver Lining

… And some have multi-coloured ones too. That’s when the edges of a cloud are embellished with delicate pastel colours known as cloud iridescence. One of the best formations for producing this subtle optical effect is the humble Cumulus cloud that forms on a sunny day. The iridescent tints can include pale blues and greens, but most typical are the delicate yellows, golds and pinks displayed by this Cumulus spotted by Sven Lorenz (Member 39,532) over Essen, Germany.

Iridescence at the edges of a Cumulus is best viewed when the main body of the cloud acts as a sunshade. The cloud middle is thick enough to stop much of the light from shining through. Sunlight is scattered in all directions by the cloud’s dense array of droplets and so little is able to penetrate and emerge from the other side, making the centre of the cloud look dark on the side away from the Sun. You must take care not to damage your eyes by looking towards a Sun that is too bright, so a dense cloud like a Cumulus can be helpful for sky gazers.

By contrast to the middle, the edges of the cloud look bright. This is where the droplets are far less plentiful. The sunlight can shine through more easily here, and the same light-scattering effect of the droplets now sends some of it towards you. That’s why a cloud has a silver lining. But why do the edges sometimes show colours?

The iridescence appears only when the droplets at the edges are extremely small and at a very consistent size. If they’re tiny enough, they scatter the different wavelengths of visible light by different degrees. We see different wavelengths of visible light as different hues, and these appear as distinct fringes of colour when the droplets are the same size and their scattering effects add up.

Droplets are only small enough and even enough at the edges when a Cumulus is beginning to dissipate away. That’s when its outer droplets are evaporating away, when its edges appear frayed: a form known as Cumulus fractus. So if you want to prove to yourself that not every cloud has a silver lining, just wait for a fair-weather Cumulus to drift in front of the Sun as it is growing old. Don’t worry, you won’t have to wait long. They only live to the ripe old age of 15 minutes.

Cumulus fractus displaying cloud iridescence, spotted over Essen, Germany by Sven Lorenz (Member 39,532).

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