Stickleback Iridescent Cloud

January 14, 2012

Stickleback 2

Photographer: Martin Richard
Summary Authors: Martin Richard; Jim Foster

The photo above features an iridescent cloud showing Kelvin-Helmholtz instability as observed over Helena, Montana on November 6, 2011. Recently I spotted the Sun emerging from behind a small lenticular cloud but was particularly interested in this small, stickleback or cigar-shaped cloud just beneath it. I drove across several parking lots to get a shot unbroken by power lines. Fortunately, I happened to have my best camera with me at the time.

Kelvin-Helmholtz clouds form when wind shear at the cloud level is quite pronounced. Wind shear has lifted the crest portion of the undulated cloud surface from a relatively stable layer of air into the more unstable layer above it. Kelvin-Helmholtz clouds seem to be more often observed when altocumulus lenticularis clouds are present. Since lenticular clouds and other wave clouds are composed of water droplets rather than ice crystals, iridescence in sometimes detected along their edges.

Photo Details: Camera: NIKON D300S; Lens: 75.0-300.0 mm f/4.2-5.6; Focal Length: 240mm (35mm equivalent: 360mm); Focus Distance: 0.01m; Aperture: f/5.3; Exposure Time: 0.0006 s (1/1600); ISO equiv: 200; Software: Adobe Photoshop CS5 Macintosh. Adobe Camera Raw used to tweak the raw-format image, adjusting the exposure and white balance to bring out the color.