[Phys-L] Re: Hurricanes

[John Denker <jsd@AV8N.COM>]

Ludwik Kowalski wrote:
> A mass of air gains kinetic energy at the expense of thermal energy
> over the ocean. After hitting the land the kinetic energy is rapidly
> dissipated due to "friction." That is a very simplified description of
> the hurricane phenomenon. But is it oversimplified to the point of
> being wrong?

It has a kernel of truth, but it is oversimplified to the point of being
very, very much less than the whole story.

Sure, friction is part of the story.

Sure, warm water is part of the story.  But just saying "warm water" is
incomplete, and risks being very misleading, because if you had warm
moist air in thermodynamic equilibrium with the warm water, there
would be no energy transfer, no dynamics, no hurricanes, no nuthin.

It is not hard to imagine an atmosphere in equilibrium.  Indeed you
don't need any imagination whatsover;  you can just look at the
stratosphere, which has a uniform temperature (independent of
latitude, longitude, and altitude) to a good approximation.  There
is no weather in the stratosphere.

All weather is a consequence of the _uneven_ heating of the earth.
The heating is a strong function of latitude, obviously, and a
strong function of the time of year, especially near the poles.
Also, sunlight is much more effective at heating the surface of
the earth than heating the air itself, so the air is strongly
heated from below (by contact with the surface), which makes it
unstable against convection.  Rayleigh-Bénard and all that;  easily
demonstrated on a tabletop.
    http://www.google.com/search?q=rayleigh+convection+demonstration

As a consequence of convection, the troposphere is more nearly
adiabatic than isothermal.

The latent heat of water vapor can make things even more convective
than you might have guessed just looking at the temperature.  See
Feynman volume II chapter 9.

Dry air over warm water is the best way to make the air very, very
unstable against convection.

In the case of a hurricane, first it goes unstable vertically.  Then
the vertical forces cause horizontal motions, due to Coriolis effects,
because all this is happening relative to a rotating reference frame.

   I realize that most introductory physics courses don't say much
   about Coriolis effects, centrifugal fields, or non-inertail reference
   frames in general, and what they do say is often wrong ... but
   students have no chance of understanding weather unless/until they
   can cope with a rotating reference frame.

   For more on rotating reference frames, see Feynman volume 1 chapter 19, or
     http://www.av8n.com/how/htm/motion.html#sec-coriolis-effect

   Be warned that more than half of the web pages I've found purporting
   to explain the Coriolis effect are grossly wrong.

This is the essential nuts-and-bolts first-principles explanation of
all weather:  Uneven heating + rotating reference frame.  A hurricane
is just a special case.

For the next level of detail, I recommend the Encyclopedia Britannica
article on Climate and Weather.  It's a long, informative article,
correct AFAICT (which is a rarity among weather discussions), and
reasonably accessible to non-experts (but alas too advanced for typical
intro-level physics students in September).
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