Whoa, look out for that gravity storm

Yes, yes, I know it might seem lazy to post a video linked to in the ‘Related Videos’ column from a previous post (the one directly below this one, even), but you’ve got to admit it’s kinda cool. This is a 36-minute time lapse of an atmospheric event I had previously only heard referenced by Jimmy Buffett: a gravity storm. Or, either since it’s not actually storming per se or because the person that posted the video says so, a gravity wave:

I consulted the wiki elves to help patch this hole in my understanding of how the world works:

In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two mediums (e.g. the atmosphere or ocean) which has the restoring force of gravity or buoyancy.

Which totally cleared it up for me.

Errr, ummm, wait. Maybe I didn’t catch that on the first pass.

Since the fluid is a continuous medium, a traveling disturbance will result. In the earth’s atmosphere, gravity waves are important for transferring momentum from the troposphere to the mesosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first waves propagate through the atmosphere without affecting its mean velocity. But as the waves reach more rarefied air at higher altitudes, their amplitude increases, and nonlinear effects cause the waves to break, transferring their momentum to the mean flow.

So, what they’re saying is that, with the right gravitivity and/or polaritude, these waveforms cause an inverse yet reciporocal… okay, sorry. Lost it again.

The phase speed c of a linear gravity wave with wavenumber k is given by the formula

c = √ g/k,

where g is the acceleration due to gravity. Since c = ω / k is the phase speed in terms of the frequency ω and the wavenumber, the gravity wave frequency can be expressed as


The group velocity of a wave (that is, the speed at which a wave packet travels) is given by

cg = dw/dk,
and thus for a gravity wave,

cg = ½ √g/k = ½c.

The group velocity is one half the phase velocity. A wave in which the group and phase velocities differ is called dispersive.


blink, blink.

Okay, then. Honestly, though, none of this helps make that Buffett song make any more sense.


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