Stairstep Wave Clouds

March 27, 2009


Photographer: Phillip Lachman
Summary Authors: Phillip Lachman, Jim Foster

Just by chance, I looked out of our lounge room window at 6:00 a.m. and saw this spectacular stairstep sky. It had been a typical Sydney (Australia) summer night - a low of 72 degrees F (22 degrees C) and tropical-like humidity. These wave clouds were at the rear end of an overnight storm that was passing out to sea, to the east. Summer is the wet season in Sydney and most of our rain results from thunderstorms.

Wave clouds form when stable air is forced to rise over elevated terrain. The resulting waves may persist hundreds of miles from where they initially formed. As is the case here, mid-level altocumulus and altostratus clouds often show iridescence if they pass relatively near the Sun. This iridescence or irisation is caused by diffraction of sunlight around minute water droplets of various sizes. The intensity of the color that’s displayed is predominantly affected by the cloud droplet size. Photo taken in early January of 2009.

Photo details: FUJIFILM FinePix S7000 camera; f/8; ISO 400; 1/2000th second exposure. Software used: Corel Paint Shop Pro Photo X2.