Iridescence is the property of certain surfaces that appear to change colour as the angle of view or the angle of illumination changes. Iridescence in clouds occurs when light from the sun or moon passes through an optically thin formation. This is a result of scattering in the cloud that produces colourful displays.
Scattering happens when different particles within a formation are different sizes, such as small particles like oxygen or nitrogen that reflect mostly blue or violet light as they are better at scattering shorter wavelengths. However, droplets of 20 micrometers are large enough to scatter all visible wave lengths, this is the reason that clouds appear white. However, when a cloud becomes to dense the incoming light cannot get through to the bottom, causing greyish tones. This type of scattering is referred to as Mie Scattering.
Coronas (like seen in the Cirrostratus) and iridescence are two closely related optical phenomenon that are closely linked. A corona exhibits colourful rings around the light source often referred to as a ‘halo’. This ring usually goes from a blue or green in the centre to red on the outside. Iridescence however, is much more random and the colours appear as swirling patches. Generally the colours are of a pastel tone due to the high content of white light and diffracted colours.
Iridescence happens when the average number of particle sizes differentiate within the cloud region or when this mean particle size is particularly small. The droplets that form lenticular clouds are usually made up of water droplets of narrow particle size, these particles are usually of a temperature as low as -36°C. However, if the temperature drops just slightly below this value it will freeze creating circular ice particles. Lenticular clouds can produce extremely clear coronas and iridescence because of these particle sizes.