Monday Dec. 3, 2007

A crossectional view of a mature hurricane and a picture like you might see on a satellite photograph. 

Sinking air in the very center of a hurricane produces the clear skies of the eye, a hurricane's most distinctive feature.  The eye is typically a few 10s of miles across, though it may only be a few miles across in the strongest hurricanes. 

A ring of strong thunderstorms, the eye wall, surrounds the eye.  This is where the hurricane's strongest winds found. 

Additional concentric rings of thunderstorms are found as you move outward from the center of the hurricane.  These are called rain bands.  These usually aren't visible until you get to the outer edge of the hurricane because they are covered by high altitude clouds.



Out at sea, the converging surface winds create surface currents in the ocean that transport water toward the center of the hurricane.  The rise in ocean level is probably only a few feet, though the waves are much larger.  A return flow develops underwater that carries the water back to where it came from.

As the hurricane approaches shore, the ocean becomes shallower.  The return flow must pass through a more restricted space.  A rise in ocean level will increase the underwater pressure and the return flow will speed up.  More pressure and an even faster return flow is needed as the hurricane gets near the coast.

 Here is a link to the storm surge website (from the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Labororatory).  It has an interesting animation showing output from the SLOSH model used to predict hurricane storm surges.


Hurricanes can, of course, be very destructive.  Out at sea the main hazards are the strong winds and the large waves.  The Perfect Storm by Sebastian Junger describes the sinking of the Andrea Gail in a strong hurricane like storm in October 1991.  The exact fate of the fishing ship is not known but it may have been turned end over end by a large wave (pitch poled).  Large waves can also flood a ship and begin to fill it with water.

Along a coast the greatest threat is from the hurricane winds and the storm surge.   Large waves are superimposed on the storm surge.

The hurricanes winds  slow quickly as it moves onshore, though tornadoes may form.  The biggest threat is from flooding.  Hurricanes can easily drop a foot or more of rain on an area as they pass through.