Definition
A stationary, dome- or cap-shaped cloud that forms over the summit of a mountain when moist air is forced upward by the terrain, cools, and condenses at the peak. The cloud appears to sit on the mountain like a hat and remains in place even though the air is moving through it.
Plain English
A cloud that looks like a cap sitting on top of a mountain. It forms because air is pushed up the mountain, cools as it rises, and turns into cloud right at the summit.
Context Anchor
Seen in mountain flying, weather briefings, and visual checks of ridges, peaks, and passes.
Derivation
From the everyday word "cap," meaning a covering worn on top of the head. The name comes directly from how the cloud looks — a cap placed on the mountain peak.
Why Pilots Care
Cap clouds indicate strong updrafts, possible icing, and turbulence that can affect climb performance and safety near mountains.
Analogy
Like a hat resting on the mountain's head, formed when wind forces air upward and it condenses at the peak.
Grounding Statement
Picture damp wind climbing a mountain slope, cooling as it rises, and turning into a cloud right over the summit.
Intuition Check
Do not read “cap” as a general cloud layer or ceiling. Here it means a cloud covering the top of terrain, usually a mountain or ridge.
Example Sentence 1
On the preflight weather check, the pilot noted a cap cloud sitting on the ridge and decided to delay the crossing until winds aloft eased.
Example Sentence 2
A cap cloud forming on the peak signaled rising air that could produce turbulence on the lee side.