by starroute » Mon Jan 09, 2006 10:13 pm
<!--EZCODE AUTOLINK START--><a href="http://science.nasa.gov/headlines/y2006/09jan_electrichurricanes.htm">science.nasa.gov/headline...icanes.htm</a><!--EZCODE AUTOLINK END--><br><!--EZCODE QUOTE START--><blockquote><strong><em>Quote:</em></strong><hr>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.<br><br>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. . . .<br><br>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." . . .<br><br>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.<br><br>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.<br><br>What does it all mean? The answer could teach scientists something new about the inner workings of hurricanes.<br><br>Actually, says Blakeslee, the reason most hurricanes don't have lightning is understood. "They're missing a key ingredient: vertical winds."<br><br>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.<br><br>A hurricane's winds are mostly horizontal, not vertical. So the vertical churning that leads to lightning doesn't normally happen.<br><br>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.<br><br>"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.<br><br>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."<hr></blockquote><!--EZCODE QUOTE END--><br> <p></p><i></i>
