A dappled layer of patch or cloud composed of flattened globules that may be arranged in groups, lines or waves collectively known as billows; 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)