Kelvin-Helmholtz Clouds Observed Over Helena Valley, Montana

May 06, 2019


May 2019 Viewer's ChoicePhotographer: Hannah Martin
Summary Author: Rodney Benson

This photo shows a Kelvin-Helmholtz cloud as observed from the Helena Valley in Montana, looking west, on January 26, 2019. Mount Helena can be seen on the left, and the distant horizon marks the Continental Divide. Also known as fluctus or billow clouds, these distinctive clouds were named after Lord Kelvin (1824-1907) and Hermann Von Helmholtz (1821-1894) who identified the type of instability responsible for these unique waves. Such clouds are relatively rare and may only last for a few minutes.

The waves form at the boundary between layers of air that have different densities and wind speeds (wind shear). Air in the layer above the cloud is moving faster than air in the layer containing the cloud. Development of waves on the cloudy layer is similar to what happens when waves form on the ocean as wind blows across the water. In the photo, the wavy air layer is denser than the clear air flowing above it -- just as water is denser than the air blowing over its surface.

The type of motion that causes the wave pattern is not that uncommon in the atmosphere, although we usually don't see it. For us to view it, clouds must be present in the lower layer, as they were when the photo was taken. One of the nice things about clouds is they provide clues about the type of motion currently happening in the atmosphere.

Photo Details: Camera: Apple iPhone 7; Exposure Time: 0.0002s (1/5988); Aperture: ƒ/1.8; ISO equivalent: 25; Focal Length (35mm): 37.