Fulgurites sometimes
form when lightning strikes soil. The heat from the lightning
channel fuses the sand into a hard tube like or root like formation
that remains after the lightning is over. A couple of small
pieces of fulgurites were passed around in class.
We will
spend the next two classes on Hurricanes.
A good place to begin is to compare hurricanes (tropical cyclones) with
middle latitude storms (extratropical cyclones).
Hurricanes and middle latitude
storms have a couple of characteristics in common:
a. they both form around surface centers of
low pressure (that is why the term cyclone appears in the names of both
types of storms). Upper level divergence can
lower the surface pressure
which then can cause both types of storms to intensify.
1,2. Middle latitude storms (MLS) are form at middle latitude (between
30 and 60 degrees latitude) and are generally bigger than
hurricanes. A large middle latitude storm might cover half of the
United States. Hurricanes form in the tropics, a big hurricane
might fill the Gulf of Mexico.
3. MLS form in the prevailing westerly wind belt and move from west to
east. Hurricanes form in the trade winds and move from east to
west.
4,5. MLS can form over land or water. Fronts separate warm, cold,
and cool air masses. Hurricanes only form over very warm ocean
water and are made entirely of warm moist air.
6. The strongest MLS form in the winter and
early
spring. Peak hurricane activity occurs in the late summer into
the fall.
7. MLS can produce a variety of different 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
tropical cyclone
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
the
world's most active hurricane zone. Hurricanes are
very rare off
the east
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
don't
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
level
convergence
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
they are
past it . The convergence will cause air to rise and
thunderstorms
to begin to develop.
In an average year, in the N.
Atlantic, there will be 10 named
storms
(tropical storms or hurricanes) that develop during hurricane
season. 2005 was, if you remember, a very unusual
year. There
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
traffic
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
sometimes
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.
That was
about all the new material we had time to cover in class because a
20
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
1979.
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.
The Saffir simpson
scale is used to rate hurricane intensity or damage potential.
We'll talk more about hurricane damage in class on Friday.
