Definition
A stationary, plume-shaped cloud that forms on the downwind (lee) side of an isolated mountain peak when moist air is forced up and over the summit. Air flowing past the peak experiences a drop in pressure on the lee side, causing it to expand, cool, and condense into a cloud that streams away from the peak like a flag.
Plain English
A cloud that looks like a flag streaming off the back of a sharp mountain peak. It stays attached to the peak even though the wind is blowing through it.
Context Anchor
Seen in mountain weather discussions, route planning near high terrain, and visual flying near peaks or ridges.
Derivation
‘Banner’ comes from the Old French ‘baniere’, meaning a flag or standard carried into battle. The cloud is named for its appearance — it streams off the peak the way a flag streams from a flagpole.
Why Pilots Care
Signals strong winds over mountains and warns of possible turbulence or rotor activity downwind.
Analogy
It looks much like a flag attached to a mountain, except the “flag” is made of cloud and is shaped by the wind.
Grounding Statement
Picture the Matterhorn on a windy day with a long, ragged cloud trailing off its summit — that trailing cloud is the banner.
Intuition Check
A banner cloud is not just any long, thin cloud. It is connected with wind flowing around high terrain, usually a mountain peak or ridge.
Example Sentence 1
On the climb out, the banner cloud streaming from the ridge told us the winds aloft were stronger than forecast.
Example Sentence 2
A banner cloud forming on the lee side confirmed the forecast for strong winds aloft.