This link here is of an “atmospheric gravity wave.” What exactly is that?
I’ve never heard of an “atmospheric” gravity wave before, but, long before physicists were looking for quadrupole radiation from accelerating or decelerating masive objects, “gravity wave” was a term used by oceanographers to describe the waves you see on the ocean. The restorative force, pulling the waves back down isn’t elasticity, but the force of gravity. Hence the name. Confused the heck out of me when I first cracked open a book on the theory of ocean waves.
I assume that atmospheric gravity waves are oscillating disturbances in the atmosphere in which gravity pulls the atmosphere back into place. This plays havoc with my sense of order. The surface of the ocean has a pretty well-defined top, where you can observe and measure the displacement. But the atmosphere tails off exponentially (or thereabouts) into space. I suppose you could measure the 1/e point or something, though.
In other words, my guess is that these are disturbances of the top of the atmosphere, analogous to ocean waves.
Sorry, even after a supplementary Googling, I still don’t quite grok. The main thing is, what’s waving? I’ll toss out what conjectures I’ve been able to put together, and hopefully someone can correct my horrible, horrible errors.
It’s not a height wave like on the surface of water, but a pressure wave, sort of like a giant sound wave. Gravity tries to squeeze the atmosphere, heating tries to expand it. If everything were absolutely even they’d just balance at some point. This being the real world, something breaks the symmetry and the atmosphere starts vibrating in waves.
And for that reason, the quadrupole radiation from binary star systems and the like is properly called “gravitational waves” to avoid ambiguity.
I found this on the first try. Google is your friend.
Cool picture, though.
An atmospheric gravity wave is just a pressure wave that varies with height, horizontal displacement, as well as time. It’s like a ripple in a pond, except with the atmosphere instead of a pond. These disturbances are usually caused by some sort of displacement (i.e. a meteor creating a shockwave or wind flowing over a mountain) and continue propagating as a traditional wave.
In rare cases (mostly on the leeward side of a large mountain range) you can see clouds forming every time the wave hits a crest. Let me try to Google this phenomenon …
I suppose the problem is in overspecifying. That “atmospheric” modifier kills a lot.
Glider pilots refer to these as mountain waves or lee waves (because they appear in the lee - downwind - of a mountain ridge). They often produce useable lift that extends far above the height of the mountain - I have flown to over 25,000’ in a wave over New Hampshire’s Mt. Washington (6300’).
The lift is uncannily smooth. Your engineless plane may be climbing at over 15 kts (1500 feet per minute) but it is absolutely smooth - just as smooth as sitting on your sofa at home.
The clouds associated with wave lift are known as lenticular. The word means “lentil-shaped” but it’s often said to mean “lens-shaped” and indeed that is the cross-sectional shape of these clouds. They are unusual in that they are constantly forming at their upwind edge and dissipating at the downwind edge. They thus stay fixed with respect to the ground, even when the wind that forms them is blowing at 60+ kts.
It’s almost uncanny to fly along near such a cloud and watch the wind tearing through it at 70 or 80 mph, as determine by your ground speed, and the cloud just stands there. Flight service stations used to call them “standing waves” when giving the weather for mountainous areas. Maybe they still do.
I’ve heard the clouds called ACSL (alto-cumulus standing lenticular).