Hurricane Isaac--What's the "reason" for hurricanes?

Cover for my book
Due out February 18, 2013
Norton Press

A dear friend of mine who is facing her first big hurricane in New Orleans said that she heard a meteorologist on the Weather Channel comment today that there was a "reason" for hurricanes:

I found it interesting, though, that [the meteorologist] said a “reason” as if either 1) God had  a plan for earth and hurricanes are a part of it or 2) hurricanes were living beings with an innate reason for doing something, like fish swimming upstream or something.

Is there a “reason” for hurricanes?  Do scientists consider “reason” the appropriate word?  Or is the better word “consequence” or “result”? 

Hurricane Isaac is eerily following the same path as Hurricane Katrina and the same time-line--seven years later. Due to make landfall in Louisana later today, Isaac is likely to drop 15-18" of rain just off the eastern toe of Louisanna and southern toe of Mississippi. 

Predicted rainfall over 5 days for Isaac. Prediction is for 5-day period
beginning 8:00 a.m. EDT August 28 through 8:00 a.m. EDT Sept. 2.
NOAA/NWS/NCEP/HPC
The maximum rainfall is in the small lime-green circle just
off the eastern coastline of Louisana and southern coastline of
Mississippi.

What is the "reason" for hurricanes? Leaving aside the tendency of meteorologists (and all of us) to  anthropomorphize hurricanes (after all, why give them names when numbers would do just as well or better?), there is a scientific "reason" for hurricanes--it is part of the process that results in the most efficient way that the earth has of redistributing heat away from the equator toward the poles.

Energy pours into the earth from our sun, but it does not pour in equally over the surface of the earth. Much more energy accumulates at the equator than at the poles. The atmosphere and oceans work as a coupled system to transport the energy from the hotter regions toward the colder regions. In a (very) simple world, there would be a single convection cell doing this transport: Warm air would rise at the equator and flow toward the poles, cooling along the way. Near the poles it would sink and flow back along the surface of the earth toward the equator, warming along its path. This model, first proposed by George Hadley, in the 18th century, consists of a single giant convection cell. Unfortunately, it doesn't explain the way that the atmosphere works.

A hundred years after Hadley proposed this model, Gaspard-Gustave de Coriolis and William Ferrel came up with models that explained the major wind directions on the surface of the earth. Coriolis pointed out the effect of the rotation of the earth on wind direction, and Ferrel proposed, correctly, that there were three, not one, convection cells, each spanning roughly 30 degrees of latitude. The three cells are called, in order of distance away from the equator, the Hadley, Ferrel, and polar cells. 

The temperature gradients across the Hadley and polar cells are fairly small: a traveler could traverse a Hadley cell in the northern hemisphere from south to north and the weather would change only from tropical to subtropical conditions. Across a polar cell, it would change only from polar to subpolar climates. But, a tourist traveling through the northern hemisphere Ferrel cell would go from the humid warmth of New Orleans almost up to the freezing conditions at the Arctic Circle.

The very large temperature gradients in the Ferrel cells, combined with the complicated role of the continental land masses, causes enormous turbulence in the atmosphere. Turbulence is, loosely, a very complicated flow regime in which individual parcels of fluid mix on many scales, from the tiniest molecular scale to the largest scale that has the dimensions typical of the whole atmosphere. The individual "parcels" of fluid are "eddies," and the largest eddies carry most of the kinetic energy of the motion. 

You can envision this process by observing a pot of water come to a boil on your stove. Initially, when you turn on the heat, the water is at a nearly uniform temperature--whatever the temperature was when it came out of the faucet. As heat pours into the bottom of the water, the temperature gradient across it increases. Initially, the heat is transported by conduction--molecule-by-molecule collisions of water. Then, convection (transport by physical motion) starts. Initially the convection is "gentle" (the technical word is "laminar"), but when the temperature gradient becomes too large, the convection becomes "turbulent." The water is roils in the pot as hot and cold parcels mix and merge to transport the heat as efficiently as possible.

So, too, in the atmosphere, turbulent mixing is the process that makes heat transport around the planet as efficient as possible. The fluid parcels that mix range in scale from huge (the dimensions of the atmosphere, tens, hundreds, even thousands of kilometers) down to "tiny" (dust devils or tornadoes). Hence, the "reason" for hurricanes is they play a major role in transporting heat around the planet. The words "consequence" and "result" are, as my friend said, much better than "reason": hurricanes are the inevitable consequence of the uneven distribution of solar heating of the earth, and the laws of thermodynamics that describe how this unevenness is smoothed out by heat transport.

These concepts are explained in more detail in my book "The Dynamic of Disasters," due out on February 18, 2013, by Norton Press. Here's a description on Amazon!