Eerie flashes of green lightning happen often in thunderstorms--but usually stay hidden in clouds.
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Green lightning strikes the ash cloud of Chaiten's erupting volcano on May 3, 2008, in Chile.
Photograph by Carlos Gutierrez, UPI Photo/Landov
Ιn May 2008, Chile's Chaiten volcano violently erupted,
spewing out clouds of dirty ash and illuminating the dark sky with a
most unusual kind of lightning.
Photographer
Carlos Gutierrez captured the dramatic nighttime display—in which green
lightning emerged from the ash cloud—in the striking image above. (See
also "PHOTOS: Chile Volcano Erupts With Ash and Lightning.")
The origin of the bright green lightning bolts remained a mystery until atmospheric scientist Arthur Few,
of Rice University in Houston, became curious about the phenomenon. "I
thought, 'That's funny; why don't we see this in lightning storms?'"
said Few at the meeting of the American Geophysical Union (AGU) in San Francisco on Monday.
Although green lightning seems unusual, Few now suspects it occurs during all thunderstorms but is concealed inside clouds.
Secret Origins
The
concealment results from the structure of storm clouds. On the inside,
the clouds contain ice crystals that are either positively or negatively
charged. Surges of electricity occur between positively and negatively
charged regions within the cloud—lightning—but they remain inside,
unseen by even the most committed storm chasers.
In
contrast, volcanic ash clouds carry their electrical charges on the
outside, where they are sparked by fragments of rock forcefully ejected
into the air during an eruption.
Few thinks that the
bright green bolts seen at the Chaiten volcano are simply "positive
streamers," or a current-transferring electrical surge from a positively
to a negatively charged region on the outside of the cloud.
If only we could see inside thunderclouds, he suggests, we would see green lightning more often.
Green Color Explained
But
why green? The green hue is given off by electrically excited oxygen
atoms, says Few. He thinks the same process paints the sky green during
the vivid light shows of the aurora borealis that can dominate northern
skies in winter.
This phenomenon may have also colored the tail of Comet Lovejoy as it passed overhead in November.
For the time being, however, Chaiten in Chile is the only volcano where green lightning has been photographed.
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