Definition
The lowest altitude at which a cloud or cloud layer begins, measured from the ground up to the bottom of the visible cloud.
Plain English
The bottom edge of a cloud — the height where the cloud starts when you look up at it.
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 clouds, so knowing the base altitude lets pilots anticipate where icing may begin and plan accordingly.
Grounding Statement
As you climb from clear air into cloud, the cloud base is the height where the aircraft first reaches the bottom of that cloud.
Intuition Check
Do not assume cloud bases are the same everywhere or fixed like a solid ceiling. They can vary by location, weather system, and time, and the height reference must be understood from the report or context.
Example Sentence 1
The pilot reported cloud bases at 2,500 feet, which gave plenty of room to fly underneath the layer in visual conditions.
Example Sentence 2
With cloud bases forecast at 3,000 feet, the instructor briefed the student on possible icing once entering the clouds.