Message-ID: <Pine.NEB.4.64.0812011105070.12645@panix3.panix.com>
From: Alan Sondheim <sondheim@panix.com>
To: Cyb <cybermind@listserv.aol.com>, Wryting-L <WRYTING-L@listserv.wvu.edu>
Subject: Kelvin-Helmholtz Clouds and a unique event
Date: Mon, 1 Dec 2008 11:05:42 -0500 (EST)
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 http://www.alansondheim.org/kh.mov (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 KelvinHelmholtz 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 interface. 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 wont 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.