I'm a newbie in meteorology so sorry if this is a dumb question.

I get the general idea of how cyclones form, but one thing I can't wrap my head around is why must the air diverge laterally when forming the spin, instead of following the original drift by Coriolis force?

For example, the formation of a cyclone in the northern hemisphere, enter image description here

why isn't it the case on the left? What really happens when the cyclone starts to form that gives its direction, before gaining the rotational inertia that sucks more air in that direction?


1 Answer 1


It may help to focus on how pressure centers aren't initially about rotation.

Picture you are air... you start moving towards a new cyclone (because of a lowered pressure). Straight inwards...

enter image description here

Then you start to turn to the right because of Coriolis (in the NH).

enter image description here

That results in only the right diagram (try adding the arrows in from other directions)

The thing is, even if you managed to get a clockwise circulation like your left diagram... its basic forces don't suggest it can continue to work. Because the two forces of note typically directing the air (horizontally) in pressure systems... both point towards the low pressure if the low is rotating clockwise; coriolis (turning to the right, NH) and the pressure gradient both pull the wind directly inward towards the center. The circulation would cease.

Circulation is all about balance.

It is actually indeed possible to get a clockwise low pressure circulation. But, given those pair of forces it would need to counteract, you need something relatively hefty to make it happen. Probably the most viable method to start up a clockwise low capable of persisting is by the tilting of vertical rotation (i.e. wind shear), which will bring a counterclockwise area of rotation north (NH) of an updraft in a thunderstorm. (See Figure 3 from Dynamics Of Tornadic Thunderstorms, Klemp, 1987) So this means there are clockwise rotating mesocyclones.

From there, you can have enough rotation speed that centrifugal force can begin to be a bigger factor, which does point outwards [that's the balance that keeps satellites rotating around the planet... gravity pulls them in... centrifugal force pushes them out... result is they can maintain their height]. And that can allow them to persist, and even tighten up... leading to anticyclonic tornadoes, which are indeed low pressures that are rotating clockwise.

There are other options for forced clockwise circulation, like a downdraft/rear inflow jet surging, leading to a diagram like this one from SPC's bow echo education:

enter image description here

(Note the anticyclonic on the bottom)
However, these tend to weaken more/aren't in a favorable spot for continuation.

But it's only that such clockwise rotation can't develop "on its own" without those additional forces/motions circumstances helping, and then needs just the right balance to continue (counterclockwise rotation does need just the right balance too, but those two opposing forces mentioned [PGF+Coriolis] tend to really help the balance be much more stable in the face of disturbance).

For a little more discussion of the balancing tendency in a cyclonic (counterclockwise in NH) low pressure, see also this question here.

  • $\begingroup$ No dumb questions at all, either, when you're trying to learn! Do note that meteorologists may find your phrase "laterally diverging" a bit confusing/incorrect technically... see this diagram for how we use the terms... low pressures at low levels tend to be convergent, whether CW or CCW, due to friction (lost balance or ageostrophic forces [meaning basically outside "extra" forces] can help either convergence or divergence, so you can still see a mesocyclone for example tend to drift apart, but lows by in large are convergent) $\endgroup$ Commented Jul 29, 2023 at 19:42
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    $\begingroup$ got it! thanks for the detailed answer! $\endgroup$
    – user29373
    Commented Jul 29, 2023 at 21:02
  • $\begingroup$ I realize now you may be saying that it's diverging because of the arrows pointing away from the cyclone. But those aren't the path it takes, it's the inner circular arrows. We sometimes draw the "away" arrows to show that coriolis is turning it right (but there's also pressure pulling it in), which can also be drawn $\endgroup$ Commented Jul 30, 2023 at 23:56

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