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(~300K) and the upper troposphere (~ 200K).
Air spirals into the surface low, rises adiabatically
in the eye wall cloud to the upper troposphere,
and then descends outside the storm, completing
a Carnot energy cycle (the most efficient cycle
possible for converting a given amount of thermal
energy into work) with an efficiency of about
33 percent. Recent work suggests that Saharan
dust events may tend to influence hurricane
development due to the role of the dust in
suppressing cloud formation, and by the
associated very dry Saharan air that is advected
over the eastern tropical North Atlantic. These
processes are believed to have operated during
the less active 2006 Atlantic hurricane season.
In the eye, or innermost region of the
storm (see Figure 11.9 ), adiabatic warming of
descending air accentuates the high tempera-
tures, although since high temperatures are also
observed in the eye-wall cloud masses, subsiding
air can only be one contributory factor. Without
this sinking air in the eye, the central pressure
could not fall below about 1000mb. The eye has a
diameter of some 30-50km, within which the air
is virtually calm and the cloud cover may be
broken. The mechanics of the eye's inception are
still largely unknown. If the rotating air conserved
absolute angular momentum, wind speeds would
become infinite at the center and clearly this is not
the case. The strong winds surrounding the eye are
more or less in cyclostrophic balance, with the
small radial distance providing a large centripetal
acceleration (see p. 149 ). The air rises when the
pressure gradient can no longer force it further
inward. It is possible that the cumulonimbus
anvils play a vital role in the complex link between
the horizontal and vertical circulations around
the eye by redistributing angular momentum in
such a way as to set up a concentration of rotation
near the center.
The supply of heat and moisture combined
with low frictional drag at the sea surface, the
release of latent heat through condensation and
the removal of the air aloft are essential conditions
for the maintenance of cyclone intensity. As soon
as one of these ingredients diminishes, the storm
decays. This can occur quite rapidly if the track
(determined by the general upper tropospheric
flow) takes the vortex over a cool sea surface
or over land. In the latter case, the increased
friction causes greater cross-isobar air motion,
temporarily increasing the convergence and
ascent. At this stage, increased vertical wind shear
in thunderstorm cells may generate tornadoes,
especially in the northeast quadrant of the storm
(in the Northern Hemisphere). However, the
most important effect of a land track is that cutting
off of the moisture supply removes one of the
major sources of heat. Rapid decay also occurs
when cold air is drawn into the circulation or
when the upper-level divergence pattern moves
away from the storm.
Hurricanes usually move at 16-24km hr -1 ,
controlled primarily by the rate of movement of
the upper warm core. Commonly, they recurve
poleward around the western margins of the
subtropical high pressure cells, entering the
circulation of the westerlies, where they die out or
regenerate into extra-tropical disturbances.
Some of these systems retain an intense
circulation and the high winds and waves can
still wreak havoc. This is not uncommon along
the Atlantic coast of the United States and
occasionally eastern Canada. Similarly, in the
western North Pacific, recurved typhoons are a
major element in the climate of Japan (see D, this
chapter) and may occur in any month. There is an
average frequency of 12 typhoons per year over
southern Japan and neighboring sea areas.
To sum up: a tropical cyclone develops from
an initial disturbance, which, under favorable
environmental conditions, grows first into a
tropical depression and then into a tropical storm.
The tropical storm stage may persist for four to
five days, whereas the cyclone stage usually lasts
for only two to three days (four to five days in
the western Pacific). The main energy source is
latent heat derived from condensed water vapor,
and for this reason hurricanes are generated and
continue to gather strength only within the
 
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