Tornadoes, thunderstorms,
and hurricanes are macroscale dissipative processes also driven by
gradients. All winds are organized by gradients,
with steeper gradients leading to gustier winds. Cyclones, or hurricanes,
are relatively dispersed, often violent whirling wind systems in which
higher wind speeds correlate with greater pressure and temperature
gradients. National Weather Service data confirm for hurricanes a
direct relationship between barometric gradient and wind speeds .
The pressure-volume-temperature relationships discovered in the early
days of thermodynamics remain crucial to modern meteorology. Clouds
rise over a mountain range, for example, because air expands as it
is lifted. This in turn reduces pressure and cools the wafting masses
of moist air. The air then reaches its dew point, and water vapor
condenses. It rains.
Hurricanes range in size from fewer than one hundred
miles across to as much as a thousand. A storm must enjoy wind speeds
of greater than seventy-five miles an hour to gain "hurricane" status. Hurricanes
originate along small atmospheric and ocean fronts, and generally form
over warm water. As in a Taylor vortex or Bénard cell, small instabilities
are amplified into larger-scale coherent actions. Often a small low-pressure
system with ascending winds draws warm moist air upward. The gradient
that drives a hurricane is between the warm ocean, about 27°C and
above, and much cooler temperatures higher in the atmosphere. This temperature
gradient between warm ocean and cool air creates an updraft of warm air.
Air sucked up in the storm creates low pressure at the ocean's surface,
thereby rendering not only a temperature gradient within the storm but
also a pressure gradient between the inside and outside of the storm.
Producing the infamous eye of the hurricane—a relative vacuum in
the storm's center at the ocean's surface—the vertical temperature
gradient drives the horizontal pressure gradient; combined with the
effects of Earth's rotation, known as the Coriolis force, a hurricane
spins into existence.
Cooler air above and a divergence in flow at the top of the storm allows
the system to intensify. As in thunderstorms, moisture is wrung from
the ascending air by the cooler temperatures found thousands of feet
above. Some of this dry but still warm air falls back into the eye of
the hurricane. The reported clear cloudless center of the hurricane is
the result of such dry air flowing down inside the middle of the massive
storm. The flow of the wind around the hurricane is an exquisite balance
of forces and flows. Again, the play of gradients organizes the context
of the system, which comes into being with limited materials at hand,
cycling them until the organizing differences are gone and relative chaos
again reigns.
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Part
II: The Complex
8. Swirl World
9. Physics' Own "Organisms"
10. Whirlpools and Weather
Tornado in a Bottle is a simple connector that attaches two soda bottles neck
to neck. When the upper bottle is partially filled with water, set on end, and
given a small rotational twist, a water tornado forms from the gravity-induced
pressure gradient. This highly organized hydrodynamic structure, with billions
upon billions of water molecules working in concert, is a far cry from Boltzmann's
random distribution of molecules at equilibrium.
Satellite photograph of the 1998 Caribbean hurricane Mitch. Mitch
was responsible for over nine thousand deaths in Central America and was
one of the deadliest Atlantic tropical cyclones in history. The 905-millibar
minimum central pressure and estimated maximum sustained wind speed of
155 knots made it the strongest October hurricane since systematic record
collecting began in 1886. Note the distinct eye in the center of the storm,
the counter-rotation of the storm, and the entrainment of surrounding air
masses. (Photograph courtesy of the National Weather Service of the National
Oceanic and Atmospheric Administration.)
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