Tim; One of my Second year Physics Lecturers; who taught the Sound and accoustics part of Physics; had a waves on water fetish.

It didn’t matter what subject he got started on; he could eventually bring it around to waves on water; whcih he loved.

Speaking of diffraction gratings; have you ever considered the idea, that a wave pattern on the surface of the water, is itself an accoustic diffraction grating.

It came to me once, while standing out on the Florida Keys Tarpon flats (on a boat) fly rod in hand, and waiting for a tide change that was calculated to bring the thundering herd of critters right past our stake out spot.
There was an almost imperceptible faint breeze blowing that created a slight wave pattern on the water, which was good for the tarpon to not be so skittish; but not so much wave as to slap against the hull, creating a noise which would blow them off our location.

Yet it wasn’t quiet; my guide commented about the interminable; but faint rushing sound in our ears; with no apparent source. So during a pregnant pause in the action; he asked me what the hell the noise was all about. That gave me an idea that set me bobbing up and down to get my ear as close to the water as possible (flats boats are very low freeboard).

Sure enough; as I changed my ear height, the sound changed its character.

Then I realized that the winds from far away were in fact generating a lot of noise all around us, but it was only audible for the sounds coming from a region close enough to us (but all around us) to be audible.

The somewhat chaotic surface wave pattern was acting as a surface diffraction grating of somewhat undefined but not totally random spacing, that was diffracting the essentially white noise into a frequency spectrum that was distributed roughly in verticaql angle from that apparent audible source radius.

So my up and down bobbing was simply scanning the sound spectrum, and listening to the frequency shift.

Needless to say; my guide (The Scientific American Recipient) was bloody impressed; and of course he made full use of his lecture episode to bamboozle his fellow guides at the bar apres fish. Darn sure we won ourselves a few free drinks at some of those bar meetings.

Yes waves are fun visual things for students; if you can turn just one on with them, it is worth the effort. You can show them interference and diffraction all in a shallow tank with simple apparatus; and hopefully get some surf bums to pay attention more often.

George

PS if you have some apparatus you can use with soap films to demonstrate the surface tension, and its independence of displacement, unlike a balloon skin. Like a couple of parallel wires with one movable end, so you can demonstrate that the work done is simply T x the change in area of the film.
Remember soap films are double sided so it is 2T x delta (a).

My grey matter is a little rusty so I don’t have a mental figure for the order of magnitude of T for modern soap films.

Thanks a lot for the review of water waves. I knew that they had some real deviations from simple harmonic motion, but I hadn’t heard the reason explained so clearly before. The equations for water wave speed as a function of depth is rather complex. I’ve been trying to assign a student project to design a diffraction grating that would work with water waves, as well as a “lens” that would use a variable water depth to focus the wave. But it’s hard to get a usable effect in the ripple tank without going extremely shallow (~< 1/16"). Next time I try it, I'll have a good explanation of why it works at all, though.

Digging a little deeper around the web, it would appear that this cloud form is already defined as stratus or altostratus undulatus, undulatus referring to the wavy bottom, and that the cited examples of “asperatus” at CAS are just good examples of the form. You might make a case for some examples being “mammatus undulatus”, depending on their genesis.

At any rate, another word doesn’t seem necessary or desirable, and they are certainly not “new storm clouds”. “”"

Paul, I suspect that the only difference between these aparatus clouds, and lenticular clouds, is one of persistence.

With lenticulars, they seem to form right over the generating peak, as a result of being forced up in altitude; till condensation occurs; but they re-evaporate, once the waveform dips lower, thus cutting off the lenticle.

George

The undulation pattern in the clouds might be from “gravity waves” “”"

Tim, ordinary deep ocean waves are “gravity” waves; typically wind powered, and as you state, the restoring force on a displaced particle is gravitational, and the wave propagation is non dispersive, so the waves are simple harmonic motion, and all frequency components have the same wave velocity.

In water, you also have another restoring force, that is surface tension; which acts to try and minimise the surface area. But whereas the restoring force in gravity waves is proportional to displacement; (which is the differential equation definition of simple harmonic motion); the surface tension restoring force is absolutely independent of particle displacement, and depends only on the surface tension of water (Newtons per metre).
So surface tension waves are anything but SHM, and the are highly dispersive; so the short wavelength higher frequencies travel faster than the longer wavelenght lower frequencies.

Out in deep ocean waters; the surface tension is totally swamped by the gravity , so deep ocean waves travel non disp[ersively at the same velocity independent of wavelength.

But as the water shallows near shores, and beaches, the power in the gravity waves is reduced while the surface tension remains constant, so the surface tension waves start to dominate, and the higher frequencies run ahead of the lower frequencies, so the wave shape changes from sinusoidal for the SHM gravity waves to a sawtooth waveform, with the water poiling up at the front edge of the wave. Well eventually the high frequencies go right over the cliff, and at a given water depth, depending on the height of the waves, the wave leading edge collapses in the mess that beach bums and surfers travel the world to find.

The atmosphere doesn’t really have anything comparable to surface tension, so we never see those aparatus waves breaking on an upside down beach.

You do need air flow over the lip of the organ pipe (hills) to get the waves started; which is why you don’t see these over the flat midwestern cornfields; and notice that the waveforms are pretty nice symmetrical sinusoids, althoguht they are travelling in several directions and interfering with each other. The interference pattern is going to depend on the ridge line of the hills, and how many indivdual sources it creates; via passes, and peaks.

George

http://www.giurfa.com/venice.jpg

“There would probably need to be quite a lot of heat around to produce the energy needed to generate such dramatic cloud formations.

Well the new name suggests itself then:

Nimbyus Nostradamus

http://www.khou.com/topstories/stories/khou090601_tnt_global-warming-hurricanes.3ab467cf.html

Take a valium before you read it. It’s going to raise your blood pressure.

[snip]

Reply: While that is not a [snip] site per se, and in fact is an anti [snip] site, in order to prevent discussions of [snip], I’m snipping the anti [snip] so as not to promote a reaction from the [snip] gallery. ~ charles the [snip] moderator.

http://news.bbc.co.uk/today/hi/today/newsid_8076000/8076805.stm

So this particular and unique cloud travels around the world where it is spotted by different cloudspotters at different times? Or is the explanation that the writer has little grasp of the English language?

We get these wave clouds all the time over silicon valley; they are so ho-hum.

Partly it is because the south Bay is hemmed in by some hills up to 4000 feet high, and then you get the daily vacuum cleaner winds coming through the golden gate; which play the valley like an organ pipe.

More importantly though, the bottom of those wave clouds are exactly like the bottom of the Arctic sea ice looks. (yeah I know it is crummy grammar).

There’s a picture on the table of contents page of the May 2009 issue of OPN; which is the Optics & Photonic News monthly bulletin. (Page 2)

It’s a USGS/NASA Landsat image looking donw on some mountains somewhere buried in cloud cover, where the whole area is an interfering wave pattern just like that bottom view above, and there is one sharp peak sticking up through the clouds, that is creating a turbulent diffraction pattern around the mountain that stretches down wind for about 15 times the diameter of the peak protruding through the cloud layer, with a whorl pattern that is cellular with cells about the size of the protruding peak.

Much more interesting than those apparatus clouds above.

George