Friam Digest, Vol 27, Issue 31

[Mike Oliker]

Nick,
 
(1) The atmosphere is in a high energy state (cool DENSE air on the top,
warm THIN air on the bottom, if they can switch places they shift to a much
lower energy state, but where to break symmetry?  Buoyancy drives the
system.  Humidity comes in to it by keeping the warm air warm much longer
than sensible heat alone can do.
 
(3) If two fluid streams collide, they can either deflect around one
another, or undergo mixing.  Mixing will only occur if there are strong
shears with sharp velocity gradients to mix the streams turbulently.  By
deflecting when possible, the streams keep the shear forces down, the
gradients small, and easily preserve their angular and linear momentum.  It
is the path of least resistance.  So, in the free air, they will be
deflected mostly, and act as though their boundaries were obstacles.

-MikeO

-----Original Message-----
Message: 1
Date: Wed, 28 Sep 2005 20:26:57 -0600
From: "Nicholas Thompson" <[hidden email]>
Subject: [FRIAM] Hurricanes
To: Friam at redfish.com
Message-ID: <410-22005942922657409 at earthlink.net>
Content-Type: text/plain; charset=US-ASCII

thank you, Eric, Robert, and Mike for these wonderful posts.  I want to
consider each in detail, but in the meantime, a few random thoughts.

(1) I understand the movement of heat from low latituds to high lattitudes
as a "function" of hurricanes but am less certain about the vertical
assymetry in temperature.  A stable atmosphere is three to five degrees F
-- average --- cooler for every thousand feet of altitude up to the
tropopause ... somewhere around 35kfeet or so.  .  So, for instance, in an
atmosphere that is, say  80 degrees at the surface is symmetrical if the
temperature is, say,  zero F at 20 kfeet.  (Dont hold me to the exact
numbers.)  So if temperature cannot be the name of the property of an
atmosphere that is UNstable.  Are we talking about relative temperature,
the lifted index..... what IS that property.   Is latent heat the sort of
thing there can be an assymetry in?

(2)  Having read hundreds of hurricane discussions in the last year
assiduously -- some would say obsessively --, I dont get the impression that
hurricanes are "trying" to go north.  They just go wherever the wind blows
them.  The currents that move them along are miniscule compared to the
currents within the storm, which is why I asked if a hurricane is more like
a top, a whirlpool, or a dervish.  Hurricanes start where it is warm and are
dirrected by the currents that move along the bottom of a subtropical high.
When they hit the west end of the high they turn north.
The reason they appear to be a north-going thing is that they start in the
south and if they get north they die.  but they can as easiy be carried
south if they get into a south moving current. over water that is warm
enough to sustain them.  Every rare once in a while,  when the bermuda high
pressed low enough a hurrican (or tropical storm) will come up the coast of
the US, turn eastward over the atlantic, travel back toward africa, and
then head south around the EASTERN end of the Bermuda High.   The Bermuda
High is itself a mixer but so far as the high is concerned, if a hurrican
wants to walk around the top of it to africa, it is happy to comply.

(3) Finally, I wonder if anybody could help with the intuition block that
makes it difficult for me to think of a whirling column of air as an
obstacle.  I can see how it might suck me in but I cannot see how it might
bounce me off.

Nick

Nicholas S. Thompson
Professor of Psychology and Ethology
Clark University
nickthompson at earthlink.net  <http://home.earthlink.net/~nickthompson/>
http://home.earthlink.net/~nickthompson/
 nthompson at clarku.edu


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