Cloud PointField Guide

Supplementary feature (fluctus)

Kelvin-Helmholtz

Breaking ocean waves, frozen in the sky

Written by Laurence WayneEntry No. 19Rare

Kelvin-Helmholtz: Breaking ocean waves, frozen in the sky
a. Crest curls forwardb. Shear boundaryc. Even spacingd. Gap between rolls
Plate XIXPhotograph by N O E L | F E A N S, CC BY 2.0.

Kelvin-Helmholtz clouds — officially named fluctus — are a rare and fleeting sight: a neat row of curling crests that look exactly like ocean waves about to break. They form at the boundary between two layers of air sliding past each other at different speeds, where the faster upper layer rolls the cloud tops over into breaking waves. They usually last only a minute or two before smearing away.

How to tell it apart

Kelvin-Helmholtz is regularly confused with the entries below. Compare the plates before you commit to a name.

Not thisUndulatus — Ripples revealing waves in the air itselfUndulatusXLIIIWind shear between air layers; little direct weather significance
Not thisAsperitas — The newest, most ominous wavy undersideAsperitasXUnsettled, turbulent air, but usually disperses without a storm

Worth knowing

  • The same Kelvin-Helmholtz instability that curls these clouds also ripples the boundary between ocean and air, stirs the solar wind, and helps shape the banded clouds of Jupiter and Saturn.
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 do Kelvin-Helmholtz clouds look like?

  1. aA row of curling crests, like ocean waves about to break
  2. bA smooth, featureless grey sheet
  3. cA bright ring around the sun
  4. dA single towering thunderhead
Show answer

AnswerA row of curling crests, like ocean waves about to break

They appear as an evenly spaced row of breaking-wave shapes, all curling the same way, like surf frozen in the sky.

What sets off the breaking-wave shape of a Kelvin-Helmholtz cloud?

  1. aHeavy rain dragging the cloud down
  2. bWind shear — a faster layer of air sliding over a slower one below — rolling the boundary into waves
  3. cLightning inside the cloud
  4. dThe cloud freezing and cracking apart
Show answer

AnswerWind shear — a faster layer of air sliding over a slower one below — rolling the boundary into waves

Where a faster, upper layer of air slides over a slower layer beneath, the velocity difference (shear) rolls the boundary up into a train of curling vortices that scoop the cloud tops into breaking waves.

Beyond clouds, where else does the same Kelvin-Helmholtz instability appear?

  1. aOnly ever in clouds
  2. bAt the ocean-air boundary, in the solar wind, and in the cloud bands of Jupiter and Saturn
  3. cOnly deep inside volcanoes
  4. dOnly in laboratory experiments
Show answer

AnswerAt the ocean-air boundary, in the solar wind, and in the cloud bands of Jupiter and Saturn

Kelvin-Helmholtz instability arises wherever two fluids shear past each other — it ripples water under wind, stirs the solar wind, and helps shape the banded clouds of the giant planets.

Nearby in the guide