Billow Clouds
produced by Kelvin-Helmholtz instability

Billow clouds are created from instability associated with air flows having marked vertical shear and weak thermal stratification. The common name for this instability is Kelvin-Helmholtz instability. These instabilities are often visualized as a row of horizontal eddies aligned within this layer of vertical shear.

Photograph by: Rilling

Billow clouds are created in the upward branch of each of the eddies if the air within this branch has a high enough relative humidity that, upon lifting, the air parcel reaches saturation. Individual billow clouds generally have life times of a few minutes. The presence of billow clouds provides a visible signal to aviation interests of potentially dangerous turbulence.

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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.