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"  (Photograph courtesy of NASA Langley Research Center) It&amp;#039;s an unpleasant but familiar experience: You&amp;#039;re cruising along at 30,000 feet when turbulence seems to yank the airplane out from under you. If you&amp;#039;re like some of the 26 injured passengers on Continental Airlines Flight 128 last Monday (or the two people on Delta Airlines Flight 2871 last Tuesday) and you&amp;#039;re not buckled in when this happens, you could meet the ceiling with unpleasant results. The Federal Aviation Administration says 58 airliner passengers are injured every year by turbulence. In fact, turbulence is the number one cause of injuries to passengers and flight attendants in nonfatal accidents. Two-thirds of those injuries happen above 30,000 feet\u2014just when you&amp;#039;re told you can get up and move about the cabin. We checked in with Virgin America chief pilot Rob Bendall to find out what causes turbulence and what, if anything, pilots and passengers can do to avoid the worst of its effects. There are three kinds of turbulence:   1. Turbulence during storms   Convective weather patterns, i.e., thunderstorms, represent the only form of turbulence that pilots, and the meteorologists who back them up, can actually see. Strong updrafts and downdrafts in the heart of a storm can shove an airplane up or down as much as 6,000 feet. &amp;quot;You can&amp;#039;t go through them, you&amp;#039;ve got to go around them.&amp;quot; Bendall says of thunderstorms. &amp;quot;That&amp;#039;s just the rules of the road.&amp;quot; While the worst turbulence occurs in the middle of a storm\u2014typically between 12,000 to 20,000 feet\u2014storms and the turbulence they create can rise as high as 50,000 feet, well above the 30,000 to 40,000-foot ceiling of most airliners. Fortunately, weather forecasts, radar, and updates from the ground and other aircraft can help pilots steer clear of the worst weather. But rough air is not the most dangerous element of flying through a thunderstorm. Storms bring other dangers, such as lightning and hail that can break cockpit windows or damage engines.  2. Turbulence over mountains    When strong winds blow perpendicular to mountain ranges, air flowing over the top of a mountain produces turbulence in the form of waves when it reaches the other side, just as ocean waves break on the lee side of a submerged reef. Although they can&amp;#039;t see the turbulence itself, pilots can anticipate so-called &amp;quot;mountain waves&amp;quot; as they fly over mountains because they are so common there. A further tipoff when conditions are right for mountain waves is the presence of lens-shaped lenticular clouds in the vicinity.  3. Unexpected turbulence   The most insidious kind of turbulence, clear-air turbulence, is invisible, comes without warning and occurs any time during a flight. One of the main culprits of clear-air turbulence is the boundary between the jet stream\u2014that aerial river that forms where arctic air masses meet warmer air from the south\u2014and the slower-moving air adjacent to it. This invisible boundary shifts unpredictably, and woe to any unstrapped passenger in a jet that crosses it. &amp;quot;If you&amp;#039;re flying in clear air, you have no indication at all,&amp;quot; Bendall says. If an aircraft has passed through the area ahead of your airplane, your pilot might get an advance warning of turbulence ahead. &amp;quot;But if you&amp;#039;re an early morning flight and you&amp;#039;re going through an area first, you&amp;#039;re going to be &amp;#039;Probe One.&amp;#039;&amp;quot; Even the worst turbulence is no cause for alarm\u2014by itself. &amp;quot;I don&amp;#039;t think an airplane has ever broken up in flight because of turbulence,&amp;quot; Bendall says. &amp;quot;These airplanes are built for so much more than even a severe [turbulent] event.&amp;quot; Which makes passenger safety when an airplane hits turbulence\u2014especially without warning\u2014primarily the responsibility of the passengers themselves. That means buckling your seatbelt, just as the pilots and stewardesses recommend, anytime you&amp;#039;re seated. Bendall advises air travelers not to get complacent. &amp;quot;The best thing to do is to not loiter around in the aisles of the airplane,&amp;quot; he says. &amp;quot;Do what you need to do, then get back to your seat and put on your seatbelt. You&amp;#039;re still hurdling through the air at 500 miles an hour; things can happen.&amp;quot; "^^<http://www.w3.org/2001/XMLSchema#string>
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