alto-stratus is a grey or bluish veil through which the sun and moon are faintly visible, occasionally giving rise to coronæ.
Through openings in these clouds an upper layer of cirro-stratus or alto-stratus may almost invariably be seen.
Plural altostrati (āl'tō-strāt'ī')
A mid-altitude cloud that extends in flat, smooth sheets or layers of varying thickness. Altostratus clouds generally form between 2,000 and 6,100 m (6,560 and 20,000 ft) and often produce long, steady rain showers. See illustration at cloud.