May: Katherine displays a palette of delicate colours

Enthusiastic photographer Peter Ostry frequently visits friends in the Northern Territory and particularly enjoys photographing the territory’s famous thunderstorms. But one tranquil afternoon in December 2005 he was “doing the tourist river cruise” on the Katherine Gorge when the peace was broken by his niece Angela who cried: “Look at that!” while pointing to rainbow-like colours amid the clouds. Peter, a refinery production operator at Geelong in Victoria, reached for his camera and captured an unusually bright cloud iridescence. “It was a spectacular hour-long display of intense colour during the build-up to the Wet season,” he recalls. “I’d only seen fleeting examples before.”

Iridescence (or irisation) commonly occurs when sunlight passes through water droplets in recently formed clouds and is split into a spectrum of delicate colours as in a rainbow (see March). This often occurs in the thin parts of clouds. The pattern of colours changes as the cloud develops and can appear in random patches or in bands. Cloud iridescence commonly occurs in Altocumulus, Cirrocumulus and lenticular clouds and is brightest when the sun is hidden behind the clouds. The phenomenon is named after the Greek goddess Iris, the personification of the rainbow and a messenger of the gods.

Note: The photograph in the printed calendar is NOT watermarked with a copyright symbol and name.

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