The Cirrus Principal Cloud Type is made of various cirriform cloud elements which are white, delicate filaments, patches, or narrow bands. They appear as fibers and/or have a silky sheen. The ice crystal particles in Cirrus are often large enough to have a significant speed of fall giving the cloud elements considerable vertical extent. Wind shear (variations in wind direction and/or speed) cause these fibrous trails to be slanted or curved. Cirrus elements are too narrowto cause a complete halo.
Cirrus often develop from virga of cirrocumulus or altocumulus or from the upper part of the cumulonimbus. Cirrus is often hard to distinguish from Cirrostratus, especially near the horizon. However, Cirrostratus has a much more continuous structure and any banding is wider than Cirrus.