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Sept.
13, 2006: Here's something fun to try in your kitchen:
Go to the freezer, open the door and pry loose an ice cube.
Next, look around the freezing compartment for some frost—the
crystalline fuzz that loves to coat your frozen English peas.
Found it? Rub the ice cube gently across the frost.
Nothing
happens.
Well,
what did you expect, a bolt of lightning?
Actually,
that's just how lightning gets started. Miles above Earth
in cumulonimbus clouds, tiny ice crystals are constantly bumping
against larger ice pellets. The two kinds of ice rubbing together
act like socks rubbing against carpet. Zap! Before you know
it, the cloud is crackling with electric potential—and a bolt
of lightning explodes to the ground.
Right:
Lightning, photographed by William Biscorner of Memphis, Michigan.
[Larger image]
It
may seem hard to believe that a powerful bolt of lightning,
which heats the air in its path three times hotter than the
surface of the sun, could spring from little pieces of ice.
But that's how it is, according to theory, and indeed laboratory
experiments have confirmed that you can generate electricity
from ice-ice collisions.
Still,
it does sound fantastic. So, "we decided to check it
out," says Walt Petersen, a lightning researcher at the
National Space Science and Technology Center in Huntsville,
Alabama.
Over
a three year period, Petersen and his colleagues used the
Tropical Rainfall Measurement Mission (TRMM) satellite to
look inside more than one million clouds. "TRMM has a
radar onboard to measure the amount of ice in a cloud. And
it has an optical detector called LIS (lightning imaging sensor)
to count lightning flashes." By comparing the ice content
of a cloud to its flashes, they could tell if ice and lightning
really go together.
They
do. "We found a strong correlation between ice and lightning
in all environments—over land, over sea and in coastal areas."
On global scales, the correlation coefficient between lightning
"flash density" (flashes per square-kilometer per
month) and "ice water path" (kilograms of ice per
square-meter of cloud) exceeded 90%. Even stronger correlations
were found on the smaller scale of individual storm cells
where, for example, about 10 million kilograms of ice would
produce one lightning flash per minute.
10
million kilograms. No wonder you couldn't get a spark going
in your freezer. A great deal more ice is required to make
lightning.
In
a real thundercloud, millions of pieces of ice are constantly
bumping together, pushed by updrafts ranging in speed from
10 to 100 mph. Tiny ice crystals become positively charged
and waft to the top of the cloud, while bulkier ice pellets
(called "graupel") become negatively charged and
plummet to the bottom. This separation creates mega-volts
of electrical tension--and hence the lightning.
Above:
Lightning rates vs. ice mass measured in thunderstorm cells
over Kansas/Colorado (black) and Alabama (red). [More]
Now
that the correlation between ice and lightning is so well
established, it can be put to good use. Petersen explains:
"Computer
programs we write to predict weather and climate need to know
how much ice is in clouds. The problem is, ice is hard to
track. We can't station a radar over every thundercloud to
measure its ice content. To improve our computer forecasts,
we need to know where the ice is."
Lightning
can help. "Because there's such a strong correlation
between lightning and ice, we can get a good idea of how much
ice is 'up there' by counting lightning flashes." Sensors
like LIS, which are inexpensive and can be stationed on the
ground as well as in Earth orbit, make this easy to do.
Back
to your freezer: You might want do something about those English
peas.
A
complete account of Petersen's research may be found in the
proceedings of the LIS
International Workshop, being held this week in Huntsville,
Alabama.
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Author: Dr. Tony
Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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