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.
Plain English
The blanket of clouds beneath you when you are flying above them.
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 IFR descent or alternate routing.
Analogy
It is like standing on a hill above a thick blanket of fog: the air above you may be clear, but the ground below is hidden.
Grounding Statement
From the cockpit, the sky above may look clear while the earth below is blocked by cloud.
Intuition Check
Undercast does not mean the weather is above you. It means the cloud cover is underneath you.
Example Sentence 1
Cruising at 9,500 feet, the pilot reported a solid undercast extending from the departure airport all the way to the coast.
Example Sentence 2
With an undercast below the pattern altitude, the instructor elected to fly the approach using instruments.