Iridescent Pileus imaged by Mike Kellstrand (Photography) near Boston, MA July 9, '08. Image ©Bourbon Street Photography, shown with permission.

"I had just left the house to go for a walk, heard some thunder, and looked up.  There was an ominous storm cloud moving through the area and I spotted a small colorful bright band at the top of the cloud."

The warm air of thunder clouds boils upwards and pushes layers of moist air above them to higher altitudes. The air expands and cools, its moisture condenses out to form a pileus or 'cap' cloud.

Pileus shines with colour because its droplets, formed at the same time, are similar sized and the cloud is thin. Iridescence is the result of diffraction by individual droplets. Clouds do not usually reveal the colours because the diffraction patterns of different sized drops average together to white and because in thick clouds light is scattered by many drops before emerging.