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I often hear the vague explanation that the Coriolis Effect is why air rotates into pressure systems. That doesn't explain it for me though.

Is pressure gradient directly toward the center of the low? And does the air turn only because the pressure system itself is turning? And is that not just due to friction with the surface where the surface is moving faster at a point at the Equator than toward the poles?

  1. Where does Coriolis come into that?

  2. I'm assuming that there is some kind of friction or force/acceleration of some kind (gravity of the sun on the atmosphere?) slowing the atmosphere down slightly compared to the earth's surface, so that the latitudinal difference in friction with the surface explains rotation of pressure systems. Is this correct?

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    $\begingroup$ Why wouldn't it be due to friction with the surface (or friction with lower layers of air, which in turn experience friction from even lower layers of air, and so on, until the bottom layer is driven by friction from the surface)? $\endgroup$ – probably_someone Jul 17 at 10:25
  • $\begingroup$ Indeed.. that is what I mean.. but that assumes some form of friction opposing the transfered ground friction.. otherwise the upper layers, if nothing else was effecting them, would have long since accelerated to the same speed as the lower layers/ground $\endgroup$ – T.Donaldson Jul 17 at 11:16
  • $\begingroup$ I think you might be conflating a few things here. The air moving with the surface of the Earth is different than the Coriolis effect. The question in the title and the body of your question seem completely different. $\endgroup$ – JMac Jul 17 at 11:30
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The pressure gradient is toward the center of the low. That's pretty much what a low pressure area means.

On a flat and non-rotating Earth, that pressure gradient would drive air to move straight toward the center of the low. But Earth is both spherical and rotating...

Imagine a bit of air directly to the north of the center of the low in the northern hemisphere. (The air is in Chicago, and the air is somewhat south of that) Because Earth is rotating, that air is already moving around Earth's axis eastward at the velocity that'll take it around that axis every 24 hours. Now the pressure moves it south. But it keeps that same eastward velocity. As it moves south, because Earth is round, it's now farther from the axis: It's not moving east fast enough to get all the way around the axis in 24 hours. Put another way, the air to the south is moving east faster, and the air from Chicago falls behind: It moves west, or turns to the right.

enter image description here

Note that a bit of air that has to move north to get to the low has the opposite problem: It's moving closer to the axis, so it's going too fast. It'll pull out in front of the air already there and move east. But that's again a right turn, because it's moving north.

How about some air west of the low? To catch up to the center of the low, it has to go faster than the surrounding air. It's moving faster around the axis. The resulting centrifugal force is directly away from the axis, not away from the center of Earth because Earth is round. That means it has a component to the south, to the right, and pushes the air in that direction.

enter image description here

Friction has no part in causing any of this.

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  • $\begingroup$ If the atmosphere weren't rotating along with the Earth, this wouldn't happen. Isn't friction between the surface and the atmosphere what keeps the atmosphere rotating along with the Earth? Otherwise, turbulence would cause the large-scale rotation of the atmosphere to eventually dissipate into a bunch of smaller-scale motions, right? $\endgroup$ – probably_someone Jul 17 at 12:34
  • $\begingroup$ "If the atmosphere weren't rotating along with the Earth", nobody would care about this effect because 1000mph winds would scour the surface down to bed rock. But of course that's not the case, because the atmosphere does rotate with the earth on average. Winds are the (very small by comparison) variations from that. To put it another way, nothing is needed to "keep() the atmosphere rotating along with the Earth". It does that anyway. $\endgroup$ – Bob Jacobsen Jul 17 at 13:06
  • $\begingroup$ Those 1000mph winds would eventually stop once enough angular momentum is transferred from the Earth to the atmosphere. The atmosphere pushes on the Earth and the Earth pushes on the atmosphere. It's precisely this transfer mechanism that I'm saying shouldn't be neglected, as without it, the atmosphere would not be kept rotating along with the Earth. You can call it what you want; I call it "friction" because it's a partially-dissipative interaction that happens at the boundary between two objects when they are traveling at different local velocities. $\endgroup$ – probably_someone Jul 17 at 13:11
  • $\begingroup$ You may have a different word for this interaction, and that's fine, but unequivocally saying that "friction has no part in causing any of this" without defining specifically what you mean by "friction" may cause confusion. $\endgroup$ – probably_someone Jul 17 at 13:12
  • $\begingroup$ "To put it another way, nothing is needed to "keep() the atmosphere rotating along with the Earth". It does that anyway." - So you're saying that, once you stir a cup of tea, it will keep flowing round and round forever? $\endgroup$ – probably_someone Jul 17 at 13:13

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