# How is a hurricane’s eye formed/maintained?

What’s (physically/thermodynamically) going-on with a hurricane eye? I can’t get a satisfying answer from the web. Most all the sites I viewed claimed that even the experts aren’t sure how these things work.

The theory I like best is that some of the flow is coming back down the eye, though I don’t buy it. I’m told that the point of lowest pressure is found at the eye. If the lowest pressure is in the eye, then there ain’t no air being pushed down there.

So, what gives?

This article seems like it’d help answer some of your questions. And if it doesn’t, the articles it cites might do so. The Formation of Moist Vortices and Tropical Cyclones in Idealized Simulations,Journal of the Atmospheric Sciences, September 2015. https://journals.ametsoc.org/doi/10.1175/JAS-D-15-0027.1

I’m not sure about describing formation, but I think it’s quite intuitive to understand how a vortex is maintained, and why the center is at low pressure. Assume that a vortex has already formed, and consider the movement of molecules at an instant in time. Considered instantaneously, all the rotating molecules are moving tangentially, i.e. in straight lines parallel to the circumference of the eye. With no other force applied, a molecule will tend to keep moving in a straight line. But a continued straight line would takes these tangentially moving molecules away from the center. Fewer molecules at the center means reduced pressure at the center, creating a pressure gradient that will tend to bend the molecules around. So there’s a dynamic equilibrium condition where molecules are all moving tangentially, and there’s enough of a pressure gradient (low pressure in the middle) to bend the molecules around into a perfectly circular motion.

Given that there are three dimensions with ground below, it’s also easy to see intuitively that there’s a tendency for this low pressure in the center that’s created by the circulation to pull air down from above. So the downward movement of air is an effect, not a cause.

ETA: oh, and of course other things being equal, with downward movement of air clouds tends to evaporate.

But all the above is just an intuitive idea of what’s going on in general terms, I’m not claiming that any of the above is sufficient to explained the detailed features of a hurricane, like why there is a well-defined eye with a wall.

There is sinking air, esp. sort of towards the eyewall. There just isn’t nearly enough sinking air to counterbalance the low pressure produced by the vortex. And sinking air “dries out” in a way (pressure increases a bit), so it’s likelier to be clear.

Think about this. The vortex is really trying hard to send a lot of air up (and when it gets high enough it spreads out away from the eye). This drags air around the eyewall up. This air is on the “wrong” side of the vortex so air from the other side can’t move in to replace it. It has to come from somewhere. So it comes from above.

Keep in mind that the normal ranges of pressure of rising/sinking air don’t apply to hurricanes. So a small rise in pressure doesn’t compare to what else is going on.

Just tossing this out there:

Does thinking of a tornado help? It too is a vortex and it too has a (relatively) calm center.

Isn’t it just basic centrifugal force? I mean, you spin something fast enough, the stuff in the middle will move towards the sides,leaving an empty space.

I hadn’t thought of the molecular - or even particle- perspective, so I enjoyed Riemann’s explanation. And I can understand low pressure at the center and the vortex. However, everywhere else in the storm the rising air is violently condensing, but not in the center. I’d expect there to be more convection and condensation in the eye of the storm, given its low pressure.

The falling air is understandable, and I would expect it. But not if the lowest pressure is measured under that column of air. The armchair meteorologist in me would expect the lowest pressure in the eye-wall, where the updrafts are greatest.

Exactly. The molecules are moving in circles, but there is nothing to “tether” them, so they will have a tendency to fly outward with centrifugal force. But to the extent they do that, there are fewer molecules in the the center, lowering the pressure in the center. So there’s an equilibrium condition where the centrifugal force that tends to make the molecules fly outward is exactly balanced by the pressure gradient that’s pulling them back.

Yes, I think the links from there will explain it. They’re not quick reads, but it’s good stuff. The abstracts are almost funny as I struggle to make sense of them, but the articles themselves seem to be great resources.