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About Wind Shear
Shear luck hindering hurricanes - so far
By Maya Bell
The Orlando Sentinel
Posted on Tue, Aug. 22, 2006
MIAMI - They're just two little words, but they
can be music to the ears: wind shear.
The weather phenomenon perhaps best known for
knocking planes out of the sky decades ago also
can rip apart hurricanes or even prevent the
storms from forming. This year's shearing winds
have been a tad stronger than prognosticators had
expected, so they deserve partial credit for the
relative quiet of the 2006 hurricane season so
far.
They also can take a bow for keeping a persistent
low-pressure system that was drifting off the
coast of Jacksonville, Fla., last week from
sprouting into the season's fourth tropical storm.
By this time last year, the Atlantic basin had
produced nine named storms, four of which became
hurricanes.
Unfortunately, forecasters do not expect the calm
to last. Wind shear is a big reason.
"Wind shear is very strong for about six months;
then it drops off in June and July and reaches a
minimum in August and September," said Chris
Landsea, chief science officer at the National
Hurricane Center west of Miami. "That's a big
factor in why ... half of all major hurricanes
occur in September, and 90 percent of all
hurricanes occur in August, September and
October."
Though wind shear hasn't been as low as Landsea
and other federal forecasters anticipated when
they predicted another very busy hurricane season
in May, it's still about average, setting the
stage for what they still think will be another
active year. Earlier this month, they reduced
their seasonal outlook, predicting 12 to 15 named
storms, seven to nine of which will become
hurricanes.
If so, expect low wind shear to play a starring role.
Low wind shear was, after all, one of the key
reasons July 2005 was the busiest July on record,
spawning five named storms, three of which turned
into hurricanes. Coupled with warmer-than-usual
sea-surface temperatures, relaxed wind shear
created the perfect petri dish for hurricane
formation.
Here's how it works:
In the tropics, wind shear is created by
different wind flows at different altitudes. At
the low levels, predominant winds are from the
east. They're called easterlies, or the trade
winds.
But at the upper level, winds are more variable.
"You can get an easterly flow. You can get a
westerly flow. It can go strong; it can go weak,"
said Chris Velden, a research scientist at the
Cooperative Institute for Meteorological
Satellite Studies at the University of Wisconsin.
"It all depends on environmental weather systems
in the neighborhood."
Plop a budding cyclone amid these dueling winds,
and they will help determine how, or if, it will
grow. That's because hurricanes, which develop
from thunderstorms, are enormous heat machines
that extend from the surface of the sea to more
than eight miles high. Their key energy source is
the latent heat released when water vapor
condenses into rain.
With low wind shear, wind speeds are fairly weak
and uniform from the surface to the upper
atmosphere, enabling thunderstorms to grow
vertically and contain the heat to a smaller
area. That increases the chances for development
because it provides more power for the storm.
With high wind shear, winds in the upper
atmosphere are stronger, and sometimes blowing in
a different direction, than those closer to the
surface. That skews budding thunderstorms,
dispersing their latent heat over a larger area
and inhibiting development.
When predicting the viability and intensity of a
potential hurricane, forecasters either add or
subtract the difference between the winds at the
two levels to determine how much impact wind
shear might have.
If, for example, winds are blowing out of the
east at 10 mph near the ocean, and out of the
west at 15 mph eight miles up, they add the
opposing wind speeds, calculating shear at 25
mph. If the winds are 15 mph aloft and 10 mph
near the ocean, and both out of the east, they
subtract, calculating shear at 5 mph.
"Usually, if it's 10 mph or less, that's low or
weak wind shear. That's an environment that a
hurricane can form or develop," Landsea said. "If
it's 25 mph or more, that's considered strong,
and it's difficult for a hurricane to form or to
intensify."
That rule of thumb, though, is not foolproof.
Research studies have shown that wind shear is
more of an inhibiting factor for smaller and
newer storms, and those that form closer to the
equator, Landsea said.
In past decades, high wind shear was almost as
deadly for pilots and air travelers as it was for
budding hurricanes. In the 1970s and `80s,
several airline crashes and hundreds of deaths
were blamed on abrupt changes in wind at very
short distances and low altitudes.
Today, better training, Doppler radar and the
installation of low-level wind-shear-alert
systems at airports have all but eliminated such
accidents, according to Tom Haueter, deputy
director of aviation safety at the National
Transportation Safety Board.
So that leaves high wind shear something to love,
and to loathe losing as the peak of hurricane
season arrives.
"It'd be nice if the wind shear was strong, and
we had fewer hurricanes than we projected,"
Landsea said. "I don't think anyone on the
seasonal-forecast team would be disappointed if
we were wrong."
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© 2006, The Orlando Sentinel (Fla.).
My source:
http://www.kansascity.com/mld/kansascity/news/nation/15331988.htm
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