http://www.theairlinepilots.com/met/cirrusuncinus.htm wrote:
Cirrus Uncinus
Distribution: Worldwide.
Height: Above 16,500 feet.
Cause: Saturation of air mass at upper levels, combined with strong wind immediately below cloud level.
Associated Weather:
May indicate an approaching frontal system.
<<This often spectacular variation of cirrus cloud is also known as hooked cirrus (uncinus is the Latin for hook), or cirrus mares' tails, a reference to the cloud's resemblance to a horse's tail.
Cirrus uncinus forms in much the same way as other cirrus formations. However, its distinctive pattern of filaments is the result of a high-speed wind below the level at which the ice crystals form. As the crystals descend under the influence of gravity, this wind rapidly smears them across the sky, forming the distinctive, elongated, hooked shapes.
Like other cirrus clouds, uncinus is a result of high-level moisture, and is therefore often associated with the approach of a frontal system. Since it is also evidence of a high-speed, high-level wind, it may indicate the presence of a jet stream.
Normally, cirrus uncinus produces no significant weather on the ground, although snow showers may be visible immediately below cloud level. These usually evaporate well before reaching the ground and are therefore classified as virga.
As cirrus uncinus generally indicates the presence of high- speed winds, pilots often associate this cloud with turbulence. In most cases, however, the turbulence would cause little discomfort to pilots or passengers.
Excerpt from The Book " Weather "
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.>>