September 2, 2011

The Great Storm Irene
When Hurricane Irene was creating havoc on August 27, 2011 on the coast of North
Carolina and Norfolk, Virginia about 380 miles away, we could clearly see the outer upper edge of the hurricane from the clear and serene areas of Western North Carolina.  The horizontal band of clouds in the center of the photo is the sharply defined edge of the high altitude outflow from the hurricane.  Measurements on the photograph and satellite photos show that the high altitude cloud is about 60 miles to the east of Warren Wilson College, approximately over Hickory, NC.  None of the storm's massive fury of flooding up the East Coast and into New England affected parched Western North Carolina.

The image below shows a time-lapse of the scene.  One frame was taken every 10 sec and played back at 33 frames/sec compressing about 5 minutes into one second.  The motion of the cloud edge is clearly moving towards the left (north) in the photo.  Intrestingly, the low level clouds close by are moving in the opposite direction - towards the south - due to the passage of a cold front earlier in the day.  The high level cloudbank is circulating clockwise around the center of the hurricane - opposite the counter-clockwise rotation of the damaging winds at earth's surface.  The structure of a hurricane is shown in the NOAA diagram at right.  A hurricane is formed by the coalescence of several large thunderstorms in the tropics.  The strong vertical updrafts in the center of the storm draw surface winds toward the storm center.  The rotation of the Earth causes these converging winds to rotate counter clockwise in the northern hemisphere - this is called the Coriolis effect.  The rising warm air near the eye-wall rises to great heights - the tropopause - above which it loses its buoyancy and spreads out laterally.  This is the "anvil" top characteristic of thunderstorms, and consists of ice particles because the temperature at these heights is less than -40 deg C.  The center of the storm discharges the updraft to spread laterally, but the outward flow is again affected by the Coriolis effect from the Earth's rotation.  However, for an outward flow, the Coriolis effect induces a clockwise rotation.  Thus the surface winds in a hurricane in the northern hemisphere rotate counterclockwise while the upper-level outflow winds rotate clockwise. 

The overall rotation of the hurricane is clearly shown in the animated satellite photo (courtesy of NOAA) at left.  Near the storm center the storm clouds clearly show a counterclockwise rotation of quite strong winds.   The outer band - the sharply delineated band through western North Carolina - clearly rotates clockwise.  This band is indicated by an arrow in the first frame.

Additional calculations from the photo of the clouds, indicate that the altitude of the hurricane outflow is about 46,000 feet.  This is higher than the usual tropause in the temperate latitudes, but hurricanes involve such a large buoyancy due to the high ocean temperatures and the high humidity, that hurricanes often push the tropopause from about 30,000 ft to between 40,000 and 50,000 feet.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.


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