Cloud Irisation

Cloud irisation (also called iridescence) is caused by sunlight diffracting around cloud droplets Diffraction is the bending of light as it passes between small gaps or past edges. For this to occur, the cloud droplets must be of a similar size to the wavelength of light. Note that this is different to the refraction that causes rainbows – refraction is the bending of light when it passes from one substance into another, e.g. from the air into a raindrop. Diffraction also causes a beam of light to ‘spread out’. The amount that the light spreads out depends on the size of the gap and the ‘wavelength’ (colour) of the light. This process is depicted in the diagram below.

Since different colours of light diffract by different amounts, this causes the colours to separate. The image below shows the pattern you see if you diffract a beam of white light through a single slit on a piece of card. You can see here that there is a bright white line in the middle, followed by many paler coloured lines on either side.

So how does diffraction cause cloud irisation? As light passes between the raindrops, diffraction causes the different colours of the spectrum to fan out, meaning we see many different colours. However, due to the irregular shape of the cloud, the colours are not neatly organised like the image above, and are often mingled together.  
The photo below shows a good example of cloud irisation, with rainbow colours appearing above the sun in the picture.

Photographed over Christchurch by Glenn, @grf_my_perspective on Instagram, in November 2016.