Australian Weather Calendar: November 2015

November photgraph by Giselle Goloy

Kelvin–Helmholtz wave cloud over Jervis Bay

The wave-shaped clouds in Giselle Goloy's photograph are a result of Kelvin–Helmholtz instability. Lighter, warmer air lying over denser, colder air forms two separate layers, and different wind speeds in each layer cause rippling at the boundary. The effect is similar to a flag flapping, caused by differences in wind speed on either side of the cloth.

Giselle's picture is such a good example of this phenomenon that it has been used in lectures at the University of the Philippines Marine Science Institute (by Dr Laura David, a friend of Giselle's mother) and in the Cloud Appreciation Society's Cloud Collector's Handbook. Kelvin–Helmholtz instability isn't restricted to our skies; it is also responsible for Jupiter's red spot and the curling shapes in the bands of Saturn's atmosphere.

Giselle was using a Canon EOS 400D camera, at a focal length of 28 mm.

Diagram