Any idea how these clouds formed above San Francisco Bay (see photo)? They look like a corkscrew.
• The clouds are a manifestation of waves. Despite their appearance, they are not breaking waves like those you see on the seashore.
Kelvin-Helmholtz waves are produced when there is wind shear between layers of air, such as when two layers, one above the other, are moving at different velocities. The waves cause humid air to rise, cooling it below the dew point and allowing cloud droplets to form.
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are extremely short-lived, usually dissipating within a few minutes – and so often go unseen. I have seen them twice. On one of those occasions, just the wave crests were visible for less than a minute.
Kelvin-Helmholtz waves may also form when there is a difference in velocity across the boundary between two different fluids. They are observed in the ocean, and frequently occur at the boundaries between atmospheric belts on Jupiter and Saturn.
Storm Dunlop, Chichester, West Sussex, UK