Physics Photo of the
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
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 firstname.lastname@example.org.
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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