Optical Effects
Rainbow I
Rainbow II
Rainbow III
Polarisation Rainbow
Halo I
Halo II
Sun Pillar
Sun through clouds
Crepuscular Rays



This nacre-like optical phenomenon which is called iridescence arises when sunlight is refracted at under-cooled water droplets or ice crystals. It is especially distinctive in zones of the clouds where cloud particles have the same or similar sizes. The noticeable colours depend on the position of the sun and the size of the particles. Therefore the distribution of the colours sometimes changes when clouds are vaporizing. Close to the sun the iridescence shows circular patterns, further away the individual colours tend to follow the contours of similar droplet size. Mostly iridescence appears within 25 degrees to the sun, sometimes up to 40 degrees. This phenomenon is also especially known in connection with polar stratospheric clouds (see Polar Stratospheric clouds, PSC5.jpg).

Typically and relatively often this phenomenon can be noticed in a subconscious form as in the Cirrus clouds shown in Iridescence.jpg. Air flow and droplet growth conditions are rarely so that spectacular views as demonstrated in the mountain Leewave clouds (Cirrus clouds) of Iridescence.1-5.jpg occur. However, the upper troposphere had to be characterized by strong, low-turbulent flows coming predominantly from Western Europe on 4th of January 2005, because similar as in the images before, very strongly iridescent Cirrus clouds came up in the late forenoon also in Rhinehesse. Iridescence6.jpg and Iridescence7.jpg present the optical effect at Cirrus clouds.

They can be noticed in the upper part of the image, whereby the direct radiation of the sun is blocked by Cumulus clouds located in front of them. Without this masking the sun would be so bright that the iridescence could not be noticed.

Iridescence.jpg: S. Borrmann, Ingelheim, Germany 30 June 2004, 7:06 p.m.

Iridescence1.jpg to Iridescence5.jpg: Elisabeth Arnold, Ramsau near Dachstein, Germany, 4 January 2005, 10:59 a.m. to 11:01 a.m.

Iridescence6+7.jpg: M. Krämer, Aracatuba, Brazil, TROCCINOX Field Campaign, February 2005