Scientists don't know everything they want to know about lightning, to be sure, or about the atmosphere over the tropical Atlantic, but on the face of it, it is fair to say that nothing about an airliner flying through the region would jump out as a particularly life-threatening circumstance to a researcher in the field.
"There are very few instances of lightning causing huge problems with aircraft," observed Cathy Kessinger, a research meteorologist at the National Center for Atmospheric Research in Boulder, CO, in an interview. "It doesn't penetrate typically. It's not seen as a big hazard all by itself."
Neither Kessinger or NCAR lightning specialist Wiebke Deierling were speaking directly to the circumstances of the Air France flight that disappeared over the Atlantic this week, but more generally about the state of the atmosphere and the state of lightning science.
For all its ancient manifestation, lightning still is not a very well understood phenomenon. According to satellite estimates, there are more than 3 million lightning strikes per day. Yet only since the 1990s, when scientists had the technical wherewithal to study large mesoscale storm systems with new radars and from the sky downward, rather than the ground up, were they able to see "upward discharges," such things as "blue jets" shooting up from cloud tops and "sprites" flashing above storms.
Research by Paul R. Krehbiel of New Mexico Tech and colleagues published in the March 2008 issue of the journal Nature Geoscience sought to establish a coherent explanation for the jets and sprites and other upward discharges from electrified clouds.
"Upward discharges above the tops of thunderstorms appear to be more diffuse than ordinary lightning at lower altitude," Krehbiel wrote in an email. Because of higher pressure, lower altitude lightning travels in "an arc-like channel that produces pinhole or sometimes larger holes in the skin of an aircraft." The New Mexico Tech researcher also noted: "It is not unusual for aircraft to get hit by lightning; when this happens it is usually the airplane that triggers the discharge." When it happens, the internal parts of the aircraft are usually unaffected. In the few documented cases of aircraft accidents caused by storms, Krebiel noted, either lightning somehow caused an explosion of a fuel tank, or the crash was blamed on severe wind shear or turbulence.
More basically, Deierling at NCAR noted that "there are a lot of open questions about lightning still. There is still some questions about how it actually gets initiated. That is still very heavily under discussion. Also, the charging processes, what actually charges a thunderstorm, is still to an extent under investigation," although researchers agree that you need a good mixture of liquid water and ice crystals to charge a thunderstorm.
The Air France flight may have been passing through a region of ocean known for its storminess, called the Intertropical Convergence Zone, where northern and southern hemisphere trade winds converge, but still, airliners pass through the region all the time. And while the thicker atmosphere of the Tropics makes for taller thunderstorms, the more violent of them would typically be found over land rather than the open ocean, researchers say.
- John D. Cox
IMAGE: National Center for Atmospheric Research
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