Plain English Meaning

The blanket of clouds beneath you when you are flying above them.

Full Definition

A layer of clouds observed from above, lying below the altitude of the aircraft. The term is used by pilots flying on top of a cloud layer to describe that layer when reporting conditions or planning a descent.

Context Anchor

Seen in weather reports, pilot reports, and in flight when an aircraft is above a cloud layer.

Derivation

Built from 'under' (below) and 'cast' (in the weather sense of an overlay or covering, as in 'overcast'). An overcast is a cloud layer seen from below; an undercast is the same kind of layer seen from above.

Why Pilots Care

Prevents visual reference to the ground, forcing reliance on instruments and potentially requiring an descent or routing.