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Re: [Phys-L] conserved momentum thru gas...

Thank you both, John Sohl, John Denker.

Useful information and insights. My original question (actual mechanism of
transferring momentum through, say, 100 kms of gas molecules) is still
something I see as having unanswered aspects, but this information from all
the responses is helpful and appreciated.


On Thu., Dec. 22, 2022, 8:52 p.m. John Sohl via Phys-l, <> wrote:

A few quick comments:

OA mentions "counter-intuitive" winds such as a SSW-ish wind. Keep in mind
the scale of things here. At the equator the linear rotational (tangential)
velocity is on the order of 1,000 MPH. Even tornado level winds are much
smaller than that and certainly a "typical" SSW breeze is likely in the
range of 5 to 20 knots. (Close enough to 5 to 20 MPH.)

Point is, these are minor flows within the rotating atmosphere.

The second comment is one of scale and complexity. The atmosphere is
wickedly complex. This is why there are multiple models, all run on
supercomputers, and they do not agree. Just taking two of these models, the
US "GFS" model and the "European Model" more than a few hours out, they do
not agree. Even on large 16 km cubes they don't agree and they certainly
don't agree on small "point forecast" scales. A common question I get is
"which one is better: the GFS or the European model?" The answer is
"neither is better, that's why we use both." Sometimes one of them gets it
right and sometimes the other one gets it right.

(When I'm running a personal forecast for myself, I usually use the HRRR =
High Resolution Rapid Refresh model. But that only works for a day or two
out at best.)

Why is it complex? Because of many things, including: uneven heating (both
from above (Sun) and below (land vs. water)), uneven frictional forces with
the surface (mountains, plains, oceans, lakes, etc.), Earth's axial tilt
(seasons), volcanos, city heat islands, pollution, sea surface
temperatures, clouds and moisture content (there is more moisture moving in
the atmosphere than in all the rivers and creeks on Earth combined, what we
call "the atmospheric river"), etc. Lots of curious things happen, for
example, the northern hemisphere and southern hemisphere atmospheres don't
actually interact very much and it gets really wild when you look at the
area above the "thermal equator" called the Intertropical Convergence Zone.
Adding to the complexity, the ICZ happens in both the air and oceans.

Relative to the original question though, even massive hurricanes are
small-scale compared to the entire rotating atmosphere.

Let me leave you with a simple demo. Get in your car and drive down the
highway. Have a passenger pull out a cup of hot cocoa and stir it. If you
are traveling at 120 kph, so is the cocoa. Yet, there are currents in the
cocoa too, and those currents go in all different directions. There is also
steam rising from the cup resulting in air currents inside the car. None of
those currents approach 120 kph and even if they did, it wouldn't matter,
it is just motion being measured inside a particular FoR, your car.

- - - -
John E. Sohl, Ph.D.
WSU Brady Presidential Distinguished Professor
Department of Physics and Astronomy
Department of Environmental Science
Weber State University
1415 Edvalson St., Dept 2508
Ogden, UT 84408-2508

Office: TY 326
Office phone: (801) 626-7907
cell: (801) 476-0589 (Text me, I don't answer the phone if you are not in
my contacts.)
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