altostratus cloud in atmospheric chemistry

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 $2000 m$ - $6000 m$ ; vertical velocities of $0.05 m s −1$ - $0.1 m s −1$ .

Source:

PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2180