Plain English Meaning

The bottom edge of a cloud — the height where the cloud starts when you look up at it.

Full Definition

The lowest altitude at which a cloud or cloud layer begins, measured from the ground up to the bottom of the visible cloud.

Context Anchor

Seen in weather briefings and instrument weather planning, especially when deciding when an aircraft may enter cloud and possible icing conditions.

Derivation

Cloud comes from an old word for a mass or hill, later used for the masses seen in the sky. Base comes from words meaning a foundation or bottom support. In aviation, cloud base means the bottom boundary of the cloud layer.

Why Pilots Care

Structural icing occurs inside , so knowing the base altitude lets pilots anticipate where may begin and plan accordingly.