In some ways winds blowing through
an easterly wave resembles
traffic
on a multi-lane highway. Traffic will slow down and start to
bunch up as it approaches an obstruction. This is like the
convergence that occurs when air flows into an easterly wave.
Once through the
"bottleneck" traffic will begin to flow more freely.
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.
Normal hurricane activity
in the Atlantic
|
10 tropical storms per year
6 reach hurricane strength
2 hit the US coastline
|
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.
Another process that causes surface
winds to converge is a "lee side low."
A small center of low pressure
sometimes forms on the downwind side of mountains along the west coast
of Mexico. Surface winds will start to converge into the low, air
starts to rise, and clouds form.
Normal hurricane activity
in the Pacific
|
16 tropical storms per year
8 reach hurricane strength
0 hit the US coastline
|
There is usually a little more
hurricane activity in the Pacific off the west coast of North America
than in the Atlantic. We don't usually hear as much about this
because the storms move from east to west and away from the coast
(though occasionally a hurricane will affect Hawaii)
This is an
important figure. It tries to explain how a
cluster of thunderstorms can organize and intensify into a hurricane.
1. Converging surface winds
pick
up heat and moisture from the ocean. These are the two mains
sources of energy for the hurricane.
2. Rising air expands, cools, and thunderstorm clouds
form. The
release of latent heat during condensation warms the atmosphere.
The core of a hurricane is warmer than the air around it.
3. Pressure decreases more slowly with increasing
altitude
in the warm core of the hurricane. The result is that pressure at
the top center of the hurricane is higher than the pressure at the top
edges of the hurricane (pressure at the top center is still lower than
the
pressure at the bottom center of the hurricane). Upper levels
winds diverge and spiral outward
from the top center of the hurricane (you can sometimes see this on
satellite photographs of hurricanes).
4. The upper level divergence will cause the surface
pressure at the center of the hurricane to decrease. The speed of
the converging surface winds increases
and the storm intensifies. The converging winds pick up
additional heat and moisture which warms the core of the hurricane even
more. The upper level high pressure and the upper level
divergence increase. The increased divergence lowers the surface
pressure even more.
Here's another view of
hurricane development and intensification
In the figure
at left the upper level
divergence is stronger than the
surface convergence. Divergence is removing more air than is
being added by surface convergence. The surface low pressure will
decrease. The decrease in surface pressure will cause the
converging surface winds to blow faster.
In the middle picture, the surface low pressure is lower, the
surface
convergence is stronger. The upper level divergence has also been
strengthened a little bit. The upper level divergence is still
stronger than the surface convergence so the surface so the surface low
pressure will
decrease even more.
In the right figure the surface low pressure has decreased enough
that
the surface convergence now balances the upper level divergence.
The storm won't strengthen any more.
Generally speaking the lower the surface pressure at the center of
a
hurricane the stronger the storm and the faster the surface winds will
blow.
This figure tries to show the
relationship between surface
pressure and surface wind speed. The world record low
sea level pressure reading, 870 mb, was set
by Typooon Tip off the SE Asia coast in 1979. Sustained winds in
that storm were 190
MPH. Three 2005 Atlantic hurricanes: Wilma, Rita, and Katrina had
pressures in the 880 mb to 900 mb range and winds ranging from 170 to
190 MPH.
The stages
of storm development that lead up to a hurricane are shown
at the bottom of p. 143a in the photocopied ClassNotes.
A tropical disturbance is just a
localized cluster of thunderstorms
that a meterologist might see on a satellite photograph. But this
would merit observation because of the potential for further
development. Signs of rotation would be evidence of organization
and the developing storm would be called a tropical depression.
In order to be called a tropical storm the storm must
strenthen a little
more, and winds must
increase to 35 knots. The storm receives a name at this
point. Finally when winds exceed 75 MPH (easier to remember than
65 knots or 74 MPH) the storm becomes a hurricane. You don't need
to remember all these names, just try to remember the information
highlighted above.
A crossectional view of a mature
hurricane (top) and a
picture
like you might
see on a satellite photograph (below).
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. Generally speaking the
smaller the eye, the stronger the storm.
A ring of strong thunderstorms, the eye wall, surrounds the
eye.
This is where the hurricane's strongest winds are 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 layer
clouds.
That was 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 comments shown during the video. We
will review the Saffir Simpson scale in class on Monday.
Here's a simplified version of the
Saffir-Simpson scale used to rate hurricane strength or intensity.
You should remember that the scale
runs from 1 to 5 and that winds need
to be 75 MPH or greater in order for a tropical storm to become a
hurricane.
