Definition
The lowest altitude of the visible portion of a cloud, measured above ground level. The cloud base forms at the altitude where rising air cools to its dew point and water vapor begins condensing into visible droplets.
Plain English
The bottom of a cloud — the height where you start to see it when looking up.
Context Anchor
Seen in weather reports, preflight planning, and discussions of the temperature and dew point relationship.
Derivation
‘Base’ comes from the Greek ‘basis,’ meaning the bottom or foundation of something. The cloud base is literally the floor of the cloud — the surface you’d meet first if you flew up into it.
Why Pilots Care
It determines ceiling height, affects whether visual flight rules can be maintained, and influences route, altitude, and alternate airport decisions.
Grounding Statement
If you stand on the ground and look up at a cloud, the cloud base is the lowest part you can see — the altitude where the cloud begins.
Intuition Check
Cloud base does not mean the cloud starts from the ground or from sea level. It means the bottom of the visible cloud layer, usually described by its height above the ground at that location.
Example Sentence 1
The cloud base was reported at 3,500 feet, giving us plenty of room to remain clear of clouds during the cross-country.
Example Sentence 2
With the temperature-dew point spread narrowing, the lowering cloud base forced an earlier descent into the pattern.