A grey, uniform, striated or fibrous sheet but without halo phenomena, and through which the sun is seen only as a diffuse, bright patch or not at all; usually at elevations 2000-6000 m; vertical velocities of 0.05-0.1 m s-1.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2180 (https://doi.org/10.1351/pac199062112167)