Downdrafts; Deciphering them may make air travel safer
''The substance of the winds is too thin for human eyes, their written language is too difficult for human minds.'' So wrote naturalist John Muir.Skip to next paragraph
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Although the winds remain invisible, weather scientists are learning to decipher their message. It's a skill that may soon make air travel safer.
In fact, the brightly colored image on a radar display near here could be called a direct translation of wind writing. The display, mounted in a trailer on the plains east of Boulder, represents a special kind of radar which ''reads'' the strength and direction of the winds. Called Doppler radar, it tracks the winds by detecting the motion of airborne water droplets.
Meteorologists are using such radar in a three-year, $2.2 million research project called the Joint Airport Weather Study (JAWS). Its purpose is to uncover the special atmospheric conditions that create so-called microbursts. These are small-scale, explosive downdrafts which have been identified as the cause of several airplane crashes in the last decade.
The occurrence of these short-lived, violent winds was not even suspected in 1975 when an Eastern airliner crashed on landing at the Kennedy International Airport and a Continental jetliner hit the end of the runway while trying to take off from Denver's Stapleton International Airport.
But Theodore Fujita of the University of Chicago, who analyzed the accidents, concluded that violent gusts of wind blowing directly downward were involved. A world authority on severe storms, Dr. Fujita had seen a number of ''star burst'' patterns of wind destruction in the aftermath of severe storms. These were totally unlike the weaving signature of tornadoes. Just as a downward flow of water splatters on the ground, so too would a downward gust of air flow out in all directions. Dr. Fujita suggested that this would create the star burst patterns and explain what had happened to the two airplanes.
When first proposed, the existence of these ''downbursts'' was hotly debated among meteorologists. But once researchers began looking for them, they turned out to be a fairly common phenomenon. Meteorologists documented ten downbursts in Illinois during a large field study in 1977. The current JAWS project is a follow-on to that investigation. It is more narrowly focused on small-scale downbursts, which represent the greatest hazard to aircraft during takeoffs and landings.
The existence of downbursts has been confirmed. But the specific conditions that cause them remain a mystery. According to a report on the current study, ''It is unknown what special physical mechanisms cause downdrafts to reach downburst intensity; it is also unknown if these stronger downdrafts have a different origin.'' Dr. Fujita, one of the principal investigators on JAWS, and his coinvestigators John McCarthy and James W. Wilson of the National Center for Atmospheric Research, published the report in the Bulletin of the American Meteorological Society earlier this year.