Cloud Iridescence
Photo: © Dieter Klatt

Sometimes you can see that certain parts of clouds show iridescent colours. In most cases this iridescence appears in clouds which form rapidly (e.g. altocumulus lenticularis). Especially the rims of these clouds have purple red, blue and green colours. This phenomenon is closely related to the coronae. Here the colours are also caused by the diffraction of light. The water droplets that cause the iridescence are very small. Small droplets generate very big coronae with wide rings of the same colour. This is the reason why great parts of the cloud have the same colour. The other colours in the iridescent cloud are less due to the changing distance from the sun, but to different sizes of the droplets. Different droplet sizes generate different coronae, what makes the colour differ despite the equal distance from the sun.

As the results of continuous observations of atmospheric phenomena show, about 12% of the cloud iridescence observed were visible in cirrocumulus clouds. The greatest part of these clouds consists of ice crystals while freezing water droplets make only a sma1l part of them. Even in cirrocumulus clouds iridescence is often observed more than 30� away from the sun, a fact that almost excludes the diffraction of light as a reason for the formation of the iridescence. So the latest theories assume that the colours are caused by interfering rays of light being reflected from the front or rear side of very thin plateshaped ice crystals or by interfering rays a part of which directly passes the cloud layer while the other rays are reflected once or several times inside the cloud layer.