Vezelwolken

Vezelwolken, in meteorology often translated as fibrous clouds, is a Dutch term used for a subset of high-level ice clouds characterized by a thread-like, filamentary appearance. They belong to the cirrus cloud family and are typically observed at altitudes from about 5,000 to 13,000 meters, though measurements vary with geographic location. Vezelwolken are composed primarily of ice crystals and form where winds in the upper troposphere stretch and shear moisture into slender filaments. They may occur alone or in thin bands, and are usually white and translucent, catching sunlight to appear bright.

In classification, the fibrous appearance corresponds mainly to cirrus fibratus, and to some extent to cirrus uncinus when the strands have hooked ends. Vezelwolken do not produce precipitation at the surface, but their presence often signals an approaching weather front or changes in the upper-air flow within the next 24 to 48 hours, especially when they thicken into cirrostratus or altostratus clouds.

Observers typically describe vezelwolken as delicate, hair-like clouds that give the sky a feathery or net-like look. They may be accompanied by halos due to the diffraction of light by ice crystals.

See also: Cirrus clouds, Cirrus fibratus, Cirrus uncinus, Weather forecasting.

References: International Cloud Atlas (World Meteorological Organization) describes cirrus species and varieties, including fibrous appearances.