An amorphous, dark grey, rainy cloud layer reaching almost to the ground; 300-600 m vertical velocities of 0.05-0.2 m s-1.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2180 (https://doi.org/10.1351/pac199062112167)