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Billow clouds are created from instability associated with flows
having marked vertical shear and weak thermal stratification.
The common name for this instability is Kelvin-Helmholtz instability.
One may visualize these instabilities as a row of horizontal eddies
aligned within this layer of vertical shear.
[Image: example of kelvin helmholtz waves (61K)]
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.