The Alan Sondheim Mail Archive

Kelvin-Helmholtz clouds

Kelvin-Helmholtz instability

(Of all the cloud phenomena I've seen, this is the most beautiful. It's also 
remarkable - the vortices were astonishing and more defined than in other 
examples. In addition there was an unique event perhaps previously unrecorded - 
a 180 degree rotation between left and right wave-trains.)

Video at (series of stills 1 fps).
Separate images up for a short time - kh jpgs, 1600 x 1200 resolution.
Photographed in northern New Mexico off Interstate 25.

From Wikipedia, the free encyclopedia

A KHI on the planet Saturn, formed at the interaction of two bands of the
planet's atmosphere [see article at Wikipedia]

A KH instability rendered visible by clouds over Mount Duval in Australia
Kelvin–Helmholtz instability can occur when velocity shear is present
within a continuous fluid or, when there is sufficient velocity difference
across the interface between two fluids. One example is a wind blowing
over a water surface, where the wind causes the relative motion between
the stratified layers (i.e. water and air.) The instability will manifest
itself in the form of waves being generated on the water surface. The
theory can be used to predict the onset of instability and transition to
turbulent flow in fluids of different densities moving at various speeds.
Hermann von Helmholtz studied the dynamics of two fluids of different
densities when a small disturbance such as a wave is introduced at the
boundary connecting the fluids.

For some short enough wavelengths, if surface tension can be ignored, two
fluids in parallel motion with different velocities and densities will
yield an interface that is unstable for all speeds. The existence of
surface tension stabilises the short wavelength instability however, and
theory then predicts stability until a velocity threshold is reached. The
theory with surface tension included broadly predicts the onset of wave
formation in the important case of wind over water.

For a continuously-varying distribution of density and velocity, (with the
lighter layers uppermost, so the fluid is RT-stable), the onset of the KH
instability is given by a suitably-defined Richardson number, Ri.

Typically the layer is unstable for Ri<0.25. These effects are quite
common in cloud layers. Also the study of this instability becomes
applicable to inertial confinement fusion and the plasma-beryllium

From the Cloud Appreciation Society

The classic 1964 surfing documentary, The Endless Summer, followed the
adventures of three surfers traveling from Malibu to Ghana, via Nigeria,
Australia, New Zealand, Tahiti, and Hawaii in search of the perfect wave.
Cloudspotters can experience their own perfect wave too, without ever
having to leave home - all they need is a lot of patience. The only down
side to the ultimate aerial breaker, is that it has the particularly
un-hip name of the "Kelvin-Helmholtz cloud."

The breaking waveforms of "Kelv-Helmz" (as they aren't known) are the
result of shearing winds up at cloud level. A particular type of
turbulence can develop in a layer of Cirrus cloud, which happens to form
below an inversion* between air currents of differing speeds and/or
directions. Sea waves break as their bases are slowed down upon reaching
shallow water and their crests surge ahead. Cloud waves break in the same
way: when their crests are pushed ahead of their troughs by the difference
in air currents.

Though spotter-dudes wonÂ’t have to cross the world to see this formation,
those in search of the perfect Kelvin-Helmholtz will have to wait a while.
This most beautiful and transient of formations may appear over most
regions of the world but it only ever does so on the rarest of occasions.

*An inversion is a region where air temperatures change with altitude in
such a way as to act as an invisible "ceiling" that stops clouds from
rising through it.

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