Cloud PointField Guide

Special cloud — orographic

Banner cloud

The flag streaming off a sharp peak

Written by Laurence WayneEntry No. 18Rare

Banner cloud: The flag streaming off a sharp peak
Plate XVIIIPhotograph by Kuhnmi, CC BY-SA 4.0.

A banner cloud is a plume that streams horizontally from the downwind side of a steep, isolated peak — like a flag flying from the summit — while the surrounding sky stays clear. The classic example flies from the Matterhorn. Strong wind hitting the sharp peak creates a swirling, rising eddy in its lee, where the air cools and condenses into the trailing banner.

Worth knowing

  • Counter-intuitively, a banner cloud forms on a peak's sheltered downwind side, not the side facing the wind — and it stays anchored to the summit even as air rushes through it, condensing as it enters and evaporating as it leaves.
Note

Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.

Test yourselfThree levels

Choose a difficulty

What does a banner cloud look like?

  1. aA cloud streaming like a flag from a single sharp peak while the sky around stays clear
  2. bA coloured ring around the sun
  3. cA flat grey sheet over the whole sky
  4. dTiny rippled grains high overhead
Show answer

AnswerA cloud streaming like a flag from a single sharp peak while the sky around stays clear

A banner cloud trails horizontally from the downwind side of an isolated, pointed summit, like a flag flying from the peak.

What creates a banner cloud on the downwind side of a sharp peak?

  1. aSnow blowing off the summit
  2. bA rising lee-side eddy where the air cools and condenses behind the peak
  3. cSunlight refracting through ice
  4. dSmoke from climbers' fires
Show answer

AnswerA rising lee-side eddy where the air cools and condenses behind the peak

Strong wind striking the steep peak sets up a recirculating eddy with strong upward motion in its lee; air drawn upward there cools and condenses into the trailing banner.

On which side of the peak does a banner cloud form, and what is surprising about it?

  1. aThe windward side, where air is forced up the slope
  2. bThe sheltered, downwind side — and it stays fixed to the peak even as wind blows through it
  3. cDirectly on the summit, where it freezes solid
  4. dIt drifts freely away from the peak like an ordinary cloud
Show answer

AnswerThe sheltered, downwind side — and it stays fixed to the peak even as wind blows through it

Unusually, the banner forms on the leeward (downwind) side, where a rising eddy condenses the air. The cloud stays anchored to the peak, continually forming as air enters and evaporating as it leaves.

Nearby in the guide