An extensive cloud patch, sheet or layer, the greater part of which is sufficiently opaque to mask completely the Sun or Moon.
This term applies to Altocumulus, Altostratus, Stratocumulus and Stratus.
In Altocumulus, often, the base is even and its apparent subdivision into merged elements results from the irregularity of its upper surface, sometimes the base is uneven and the elements then stand out in true relief, particularly with a low Sun. The variety opacus occurs fairly often in the species stratiformis.
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