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Three of the most powerful hurricanes of 2005 were filled
with mysterious lightning.
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January 9, 2006: The boom of thunder and crackle
of lightning generally mean one thing: a storm is coming.
Curiously, though, the biggest storms of all, hurricanes,
are notoriously lacking in lightning. Hurricanes blow, they
rain, they flood, but seldom do they crackle.
 Surprise:
During the record-setting hurricane season of 2005 three of
the most powerful storms--Rita, Katrina, and Emily--did have
lightning, lots of it. And researchers would like to know
why.
Richard
Blakeslee of the Global Hydrology and Climate Center (GHCC)
in Huntsville, Alabama, was one of a team of scientists who
explored Hurricane Emily using NASA's ER-2 aircraft, a research
version of the famous U-2 spy plane. Flying high above the
storm, they noted frequent lightning in the cylindrical wall
of clouds surrounding the hurricane's eye. Both cloud-to-cloud
and cloud-to-ground lightning were present, "a few flashes
per minute," says Blakeslee.
Above:
The eye of Hurricane Emily photographed from the International
Space Station. [More]
"Generally
there's not a lot of lightning in the eye-wall region,"
he says. "So when people see lightning there, they perk
up -- they say, okay, something's happening."
Indeed,
the electric fields above Emily were among the strongest ever
measured by the aircraft’s sensors over any storm. "We
observed steady fields in excess of 8 kilovolts per meter,"
says Blakeslee. "That is huge--comparable to the strongest
fields we would expect to find over a large land-based 'mesoscale'
thunderstorm."
The
flight over Emily was part of a 30-day science data-gathering
campaign in July 2005 organized and sponsored by NASA headquarters
to improve scientists' understanding of hurricanes. Blakeslee
and others from NASA, NOAA and 10 U.S. universities traveled
to Costa Rica for the campaign, which is called "Tropical
Cloud Systems and Processes." From the international
airport near San Jose, the capital of Costa Rica, they could
fly the ER-2 to storms in both the Caribbean and the eastern
Pacific Ocean. They combined ER-2 data with data from satellites
and ground-based sensors to get a comprehensive view of each
storm.
Rita
and Katrina were not part of the campaign. Lightning in those
storms was detected by means of long-distance sensors on the
ground, not the ER-2, so less is known about their electric
fields.
Above:
The ER-2 en route to a hurricane. [More]
Nevertheless,
it is possible to note some similarities: (1) all three storms
were powerful: Emily was a Category 4 storm, Rita and Katrina
were Category 5; (2) all three were over water when their
lightning was detected; and (3) in each case, the lightning
was located around the eye-wall.
What
does it all mean? The answer could teach scientists something
new about the inner workings of hurricanes.
Actually,
says Blakeslee, the reason most hurricanes don't have lightning
is understood. "They're missing a key ingredient: vertical
winds."
Within
thunderclouds, vertical winds cause ice crystals and water
droplets (called "hydrometeors") to bump together.
This "rubbing" causes the hydrometeors to become
charged. Think of rubbing your socked feet across wool carpet--zap!
It's the same principle. For reasons not fully understood,
positive electric charge accumulates on smaller particles
while negative charge clings to the larger ones. Winds and
gravity separate the charged hydrometeors, producing an enormous
electric field within the storm. This is the source of lightning.
A
hurricane's winds are mostly horizontal, not vertical. So
the vertical churning that leads to lightning doesn't normally
happen.
Above:
An infrared GOES 11 satellite image of Hurricane Emily. Yellow
+ and - symbols mark lightning bolts detected by the North
American Lightning Detection Network. The green line traces
the path of the ER-2. Click
to view electric fields measured by the aircraft during the
flight.
Lightning
has been seen in hurricanes before. During a field campaign
in 1998 called CAMEX-3, scientists detected lightning in the
eye of hurricane Georges as it plowed over the Caribbean island
of Hispaniola. The lightning probably was due to air forced
upward -- called "orographic forcing" -- when the
hurricane hit the mountains.
"Hurricanes
are most likely to produce lightning when they're making landfall,"
says Blakeslee. But there were no mountains beneath the "electric
hurricanes" of 2005—only flat water.
It's
tempting to think that, because Emily, Rita and Katrina were
all exceptionally powerful, their sheer violence somehow explains
their lightning. But Blakeslee says that this explanation
is too simple. "Other storms have been equally intense
and did not produce much lightning," he says. "There
must be something else at work."
It's
too soon to say for certain what that missing factor is. Scientists
will need months to digest reams of data gathered in this
year's campaign before they can hope to have an answer.
Says
Blakeslee, "We still have a lot to learn about hurricanes."
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Authors: Patrick L. Barry
and Dr. Tony Phillips
| Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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