9th September 2005
Cirrus Kelvin-Helmholtz
Source: Excerpt from The Book " Weather "
Distribution: Worldwide. Height: Above 16,500 feet. Cause: Saturation of an air mass at high levels, combined with wind shear. Associated Weather: None Hazard Warning: Moderate to severe levels of turbulence at cloud level. Appearing as a slender, horizontal spiral of cloud, cirrus Kelvin-Helmholtz is one of the most distinctive cloud formations. However, it tends to dissipate only a minute or two after forming and, as a result, is rarely observed.
The shape of this kind of cirrus is the result of a particular type of wind shear. In general, wind shear occurs when one layer of air slides across another layer moving at a different speed or in a different direction (or both). This gives rise to vertical eddies that produce a regular pattern of air waves.
In most cases, wind shear creates a series of gently undulating cloud formations along the tops of the waves. In the case of the Kelvin-Helmholtz formation, however, the eddies are more powerful, and carry the cloud over the peak and down the other side, so that the waves "break" in the manner of ocean waves breaking as they approach the shore. As these waves complete a circulation, they create a distinctive corkscrew pattern.
This form of instability also occurs in fluids and in the Earth's outer atmosphere. It was first described in the late nineteenth century by Baron Kelvin (1824-1907), a Scottish physicist, and Hermann von Helmholtz (1821-94), a German physicist -- hence the name of the cloud.
Kelvin-Helmholtz waves are probably quite common in the upper troposphere, but generally there is insufficient moisture present to generate cloud and render the pattern visible.
The presence of this cloud indicates a degree of wind shear that is likely to produce moderate to severe turbulence at cloud level. In the absence of cloud, the same process can be a major source of clear air turbulence at high levels. As this turbulence is invisible and does not show up on radar, aircraft may encounter it unexpectedly.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
