Two Hurricane Questions

In light of Isabel…

a) This will vary, but at what rpms does a hurricane turn, on avg?
b) Why doesn’t a hurricane fall apart under sheer? Similarly, why does parts (esp. outer parts) go flying off tangent to the circular path? - Jinx

(a) ??? Rpms? Please explain what you are talking about. A cane turns depends upon the steering currents in the upper atmosphere.

(b) Circulation around a cane in this hemisphere is counter-clockwise, as it is in all low pressure areas, around the center. When Isabel was Cat 5, it was perfectly symmetrical and there were no “parts” going off in a tangent. When you get away from the immediate center, there are bands of thundershowers spiraling around the center in a counter-clockwise direction.

Sheer certainly helps to weaken a cane, and that is one reason Isabel dropped to Cat 2. Another reason was an influx of dry air. The sheer will cause some of the rain bands to go off in a tangent and breaks up the energy that a cane needs. But a cane is a very powerful force, and some sheer is not going to destroy it. Sheer does destroy lesser storms, such as tropical waves, tropical depressions, and weak tropical storms.

You have to get a basic understanding on how canes form. They get their energy from the water, which must be at least 80 degrees F. Since the sea surface temperaturs at the projected landfall are below that, that will also weaken Isabel. Isabel is expected to strengthen since the sheer has weakened and the dry air has been hung up on the east coast, but IMHO it will weaken again before landfall due to the low SSTs.

There is subsidence in the center and therefore there must be a weak high pressure area aloft for a hurricane to form.

barbitu8: This is the very first time in my life I’ve ever seen a hurrican referred to as a cane.

Or a hurrican. Gaudere’s Law Strikes Again!

And it is SHEAR!!!

I think I understand what Jinx was asking. How come hurricanes don’t fly apart?

The centripetal force of atmospheric pressure differential is constantly pushing them inwards towards that gigantic low.

As for the shear force, I’d bet that accounts for tornaditos and wind shear in the nasty bits…

The “rpms” in my question refers to the rotational speed of the hurricane. As the time-lapsed radar pictures always show, these storms have translational and rotational motion. These storms rotate, as Barbitu8 even admits. Surely, it has an avg rotational speed…as well as a translational speed…the latter of more importance.

Overall, these storms are not point masses, and it is hard to imagine how such a mass of moisture maintains its integrity (overall) for such great distances. If you cut the hurricane in half altitude-wise, why should the bottom half move along with the top half? What really keeps the whole storm moving along as one entity? Even without help from other currents, shouldn’t even the smallest “clump” of moisture start “spreading out”…just from translation alone! I mean, do clouds have some kind of special property making water vapor suddenly highly viscous?

In general, weather is a puzzling science…how can science work without boundary conditions? For example, how do you build pressure without boundaries? (And, how can a clear sky, with presumably fewer molecules…equate to HIGH pressure, and vice versa? It’s counter-intuitive. More molecules, more collisions, greater pressure.) - Jinx

Jinx, atmospheric physics/meteorology are extraordinarily complex sciences. Weather modeling programs make Cray supercomputers whimper and beg and ask for their mommies. A hurricane, indeed, is not an object, it is a complex system.

IIRC, the reason there is not much concern about “rpm” (or rather, rph) on a hurricane is precisely because it is not a solid object AND it is hundreds of miles wide. Different “bands” at different distances and altitudes have different rates of motion depending on distance to center, pressure differential, surface temperature effects, sheAr effects, coriolis force, etc. Unlike a solid spinning disc, the innner part of the storm is moving faster than the outer (e.g. if the eye wall is 10 miles across and max wind is 100mph, the air mass right outside the eye wall makes 1 “turn” every 3 hours. But at the outer border of the system, the air mass may be making 1 “turn” every 2 days). The whole system of all these motions make up the hurricane. And “rpm” number is not very significant.

As to what keeps all those clouds together: the very thermal energy that generates the storm makes the air mass have a very, very high capacity for carrying water vapor. Those clouds ARE continuously raining down, but the system is at the same time sucking up water vapor from the surface, the falling rain, and any peripheral air mass influxes, feeding further clouds. So what you are seeing is not a specific fixed cloud, going around and around, but an ever-renewing formation of clouds along the lines of wind flow.
Once the system moves over cooler waters, the input of energy drops, and the storm starts losing power precisely from shear and the dissipation of energy caused by the rain.

“Highs” and “lows” in the atmosphere have to do with temperature, circulatory eddies, surface irregularities, earth’s rotation, etc. locally causing a variation in the air mass. Air, being highly compressible, has a certain “elasticity”: a large enough irregularity in pressure won’t instantaneously equalize. “High” pressure is associated with clear skies (but it is not necessarily synonimous) because weather disturbances will be carried away by the air as it moves between the H and the L, IOW if you’re sitting under the H, the weather IS happening, but it’s moving away from you.

(And yes, all the above was stupidly oversimplified!)

All the pictures I’ve seen of Isabel make it look exactly like a spiral galaxy, with two arms flying away from the core. So, yes, it does “fly apart” in a sense. And the core will break down too once it reaches land.

Isabel from orbit. Yesterday’s Astronomy Picture of the Day.

A real time picture of Isabel.

Well the translational speed is an order of magnitude less than most of the rotational speeds. Storms loiter along at 10s of mph while the winds circulate at 100 mph +.

Think of storms as a big dimple in the atmosphere. There is a great deal of air in the vicinity trying desperately to restore equilibrium. The more air runs in towards the center, the more it’s raised, the more clouds are formed, the more people run…

Don’t think of those clouds you see in the time-lapse shots as being the ‘same’ clouds over the course of the shot. They are constantly being renewed from fresh inflow.

As for why a dimple should appear in the first place I have to defer to a real atmospheric geek.

I’m not an atmospheric geek so I cannot answer the entirely question completely. However, as I said before, the “dimple” is a result of a weak high pressure area aloft, which provides the subsidence at the eye. Most of the hurricanes that hit the USA this time of the year originate from storms that flow across Africa. When these storms hit the ocean, they acquire the latent energy of the warm waters which fuel tropical storms. The main things about tropical storms in comparison with non-tropicall storms, are (1) they are not associated with a front and (2) the center of the low pressure area has the lowest pressure and is concentrated at one geographical location. If you look at the Lows (and the associated barometric readings) that transverse the States, you will see that the center is spread out, not circular, and does not have the steep gradients that tropical storms have (and, of course, are associated with a front). Wind is caused by changes of barometric pressure, and the steeper the change, the greater the pressure.

For the storm to intensify and become a hurricane, with the associated eye (which forms at 75 mph), you must have the subsidence in the center.