Friday Nov. 21, 2008
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No music before class today. Instead a few minutes from the 1999
Le Tour de France. You saw the finish of the first really tough
mountain stage in the race that year (they finished in the town or
resort of Sestrieres). Lance Armstrong started the day leading
the race and wearing the Yellow Jersey, but many people thought he
would crack in the mountains. Well that didn't turn out to be the
case. He won the stage and would go on to win the race that year
for the first time. A bicycling video seemed appropriate because
the El Tour de Tucson is tomorrow.
spend the next two classes on Hurricanes.
A good place to begin is to compare hurricanes (tropical cyclones) with
middle latitude storms (extratropical cyclones). The following
figure was on a handout distributed in class.
1. Middle latitude storms (MLS) are generally bigger than
hurricanes. A large middle latitude storm might cover half of the
United States. A big hurricane might fill the Gulf of Mexico.
2,3. MLS can form over land or water. At middle latitudes they
are in the prevailing westerly wind belt and move from West toward
East. Hurricanes can only form over warm ocean water (80 F or
above). The trades winds cause hurricanes to move from east to
a. Both MLS and hurricanes from around surface centers of
low pressure (that is why the term cyclone appears in the names of both
types of storms).
b. Upper level divergence can lower the surface pressure
which then can cause both types of storms to intensify.
4. Warm and cold air masses collide along fronts in MLS.
You only find warm moist air in a hurricane.
5. MLS intensify with altitude. Hurricanes weaken with
altitude - the low pressure at the bottom center of the storm actually
becomes high pressure at the top center of the storm. The fact
that hurricanes weaken with altitude came up in the video tape shown at
the end of class. In the video a NOAA reconnaisance plane was
flying into the center of a very strong hurricane. Normally the
plane would fly in at an altitude of 5,000 feet. This particular
hurricane was so strong however that they decided to play it safe and
to fly in at 10,000 feet altitude.
6. The strongest MLS form in the winter and early
spring. The peak of hurricane season is already behind us.
7. MLS can produce a variety of types of precipitation.
Hurricanes mostly just produce very large amounts of rain.
8. Hurricanes receive names (when they reach tropical storm
strength). The names now alternate male and female. The
names of particularly strong or deadly hurricanes (such as Katrina) are
retired, otherwise the names repeat every 6 years.
The figure above shows the relative frequency of
development in different parts of the world.
The name hurricane, cyclone, and typhoon all refer to the same type of
storm (tropical cyclone is a general name that can be used
anywhere). In most years the ocean off the coast of SE Asia is
world's most active hurricane zone. Hurricanes are
very rare off
and west coasts of South America.
Hurricanes form between 5 and 20 degrees latitude, over warm ocean
water, north and south of the equator. The warm
layer of water
must be fairly deep to contain enough energy to fuel a hurricane and in
order that mixing doesn't bring cold water up to the ocean
surface. The atmosphere must be unstable so that thunderstorms
can develop. Hurricanes will only form when there is very little
or no vertical wind shear (changing wind direction or speed with
altitude). Hurricanes don't form at the equator because there is
no Coriolis force there (the Coriolis force is what gives hurricanes
their spin and it causes hurricanes to spin in opposite directions in
the northern and southern hemispheres.
Note that more tropical cyclones form off the
west coast of the US than
off the east coast. The west coast hurricanes don't generally get
much attention, because they move away from the coast and usually
present a threat to the US (except occasionally to the state of
Hawaii). The moisture from these storms will
sometimes be pulled up into the southwestern US where it can lead to
heavy rain and flooding.
Hurricane season in the Atlantic
officially runs from
June 1 through to November 30. The peak of hurricane season is in
September. In 2005, an unusually active hurricane season in the
Atlantic, hurricanes continued through December and even into January
2006. Hurricane season in the Pacific begins two weeks earlier on
May 15 and
runs through Nov. 30.
Some kind of meteorological process that produces low
is needed to initiate a hurricane. One possibility, and the one
that fuels most of the strong N. Atlantic hurricanes, is an "easterly
wave." This is just a "wiggle" in the wind flow pattern.
Easterly waves often form over Africa or just off the African coast and
then travel toward the west across the N. Atlantic. Winds
converge as they approach the wave and then diverge once
past it . The convergence will cause air to rise and
to begin to develop.
In an average year, in the N.
Atlantic, there will be 10 named
(tropical storms or hurricanes) that develop during hurricane
season. 2005 was, if you remember, a very unusual
were 28 named storms in the N. Atlantic in 2005. That beat the
previous record of 21 names storms that had been set in 1933. Of
the 28 named storms, 15 developed into hurricanes.
In some ways winds blowing through an easterly wave resembles
on a multi-lane highway. Traffic will back up as it approaches a
section of the highway with a closed lane. Once through the
"bottleneck" traffic will begin to flow more freely.
Another process that causes surface
winds to converge is a "lee side low."
Winds blowing over mountains on the
west coast of Mexico will
form a surface low on the downwind side of the mountains. Surface
winds will spiral inward toward the center of the low. Note
there are generally a few more tropical
storms and hurricanes in the E. Pacific than in the N. Atlantic.
They generally move away from the US coast, though the Hawaiian Islands
are sometimes affected.
about all the new material we had time to cover in class because a
minute segment from a NOVA program (PBS network) on hurricanes was
shown. A film crew was on board a NOAA
reconnaissance plane as it flew into the narrow eye of hurricane
GILBERT. Gilbert set the record low sea level pressure reading
for the Atlantic ocean (888 mb). That record stood until the 2005
hurricane season when WILMA set a new record of 882 mb. The world
record low sea level pressure, 870 mb, was set in a SE Asian typhoon in
Here are some of the comments written down during the video (these were
on the back of the handout distributed in class. We
will review the Saffir Simpson scale in class on Wednesday and look at
the 3-dimensional structure of hurricanes in more detail.
One of the most distinctive
features of a hurricane is the clear eye in the center. The eye
is produced by sinking air. Once in the eye, the people in the
NOAA plane where able to see blue sky when they looked and and saw the
ocean surface when they looked down. The eye of a hurricane is
something that very few people will ever see. The eye is
surrounded by the eye wall, a ring of strong thunderstorms.