Colourful Weather, Ep 4: Those rainbow colours on a cloud aren’t really what they seem

NATE BYRNE :

Ah, a bungee jumper. A flying manta ray. An oil slick? Those rainbow colours on the cloud are called iridescence. It might look a little bit like a rainbow on a cloud, but the way the colours are created for rainbows and for iridescence are completely different.

JONATHAN HOW :

Rainbows are created by a process called refraction, but iridescence is caused by defraction.

NATE BYRNE :

When light defract , it is gets get bent around very small object . The light is bends bend or defract slightly on the outside of thing like water , droplet and ice particle in a cloud . Defraction is scatters scatter light wave into a ring – like pattern of colour around the droplet . When you have a whole lot of light scatter around the outside of a cloud fill with tiny water droplet , that light is spreads spread out into the iridescent colour . The scattering is means mean that the colour do n’t usually appear in neat band like a rainbow . Sometimes the bands is be of colour will be wavelike , but usually the pattern is uneven and the colour are randomly spread out . Also , because the light wave are defracte , you is get ‘ll get to see amazing colour like hot pink and teal . The iridescence is sometimes compare to colour on a soap bubble , or a peacock feather , or a Christmas beetle . usually the colours is be wo n’t be as bright as that though . They is be ‘ll be a little bit faint or pastel . Defraction is happens happen when the droplet are tiny , like really , really tiny , less than a micron wide . How small is a micron is is ?

JONATHAN HOW :

Take a human hair. That’s about 100 microns wide, and a water droplet is just one micron wide.

NATE BYRNE :

And as well as being very small, all the droplets in the cloud need to be around about the same size. That’s more likely to happen when a cloud is brand new. Because the droplets all start out about the same size, they haven’t had enough time to grow or bump into each other and collect.

JONATHAN HOW :

Iridescence is also more vivid where there aren’t too many droplets in the cloud. A fewer droplets makes it more likely that the light wave will defract off only one droplet.

NATE BYRNE :

When there are few droplet in a cloud , it is ‘s ‘s usually a lot thin . That is ‘s ‘s why you ‘ll see iridescence at the edge of cloud , where there are less droplet , or in semi – transparent cloud . The kinds is are of cloud that iridescence occur most often in are one that are high up in the atmosphere like altocumulus serum , cirrocumulus and cirrus cloud . They ‘re often made of ice . Of course you is need need light as well . So the close the sun is to the cloud , the strong the iridescent colour . Now that bright light is means right in your eyeball mean that it can be hard to see iridescent . So a good tip is is is to shield the sunlight from your eye and then make sure you can still see the cloud . Ah , that is ‘s ‘s well . beautiful . Is n’t it is Is ?