Weather

From time to time the atmosphere rattles an airplane with severe turbulence. If the airplane is an airliner and the turbulence is severe enough, the incident makes the news for a couple of days—as happened on July 20, 2010, to United Flight 967, a Boeing 777-200 en route from Washington Dulles International Airport to Los Angeles with 225 passengers and 10 crew members.

The airliner was flying at 34,000 feet southeast of Kansas City, Missouri, when it ran into the turbulence. It injured 17 passengers and four flight attendants seriously enough for them to be taken to hospitals after the flight diverted to Denver.

Incidents like this are the reason airline pilots ask passengers to keep seat belts buckled when they are in their seats, and announce that they are turning on the “Fasten Seat Belts” sign when they have a reason to suspect turbulence—including reports they may hear other pilots make to air traffic controllers.

Moving air causes turbulence. Think of air like water moving down a stream. When the water is moving slowly and the stream is free of obstructions such as rocks, the water flows smoothly. Add a few rocks to the stream, and it becomes slightly turbulent as water hits the rocks and flows over and around them. When the water flows over a rock it goes up, and then comes down, creating a wave downstream from the rock. The faster the water is moving, the bigger the waves.

In a similar way, wind flowing over a mountain, the top of a thunderstorm, or around buildings or trees near a runway moves in random ways at varying speeds. In addition to moving horizontally, air also rises when it’s heated and descends when it’s cooled. Meteorologists call such up-and-down air movements convection.

Air pockets. Some people believe “air pockets” cause turbulence. Those who talk about “air pockets” apparently think a vacuum somehow forms in the atmosphere and that an airplane falls when it runs into the pocket. This can’t happen. Even at 34,000 feet the air pressure is approximately 3.63 pounds per square inch. This means that if you had a box measuring one inch on each of its six sides, a pressure of 3.63 pounds would press against each of the sides. If a vacuum could somehow form in the atmosphere, the pressure of the surrounding air would fill it instantly.

Wind shear and turbulence. Wind shear is the most common cause of turbulence. Visualize winds that are close to each other blowing in opposite directions, which creates whirls or eddies in the zone between the opposing winds. Winds blowing at any angle to each other can create shear; so can winds blowing in the same direction at different speeds.

Low-level wind shear is the most dangerous because a pilot might not have enough time to recover to avoid crashing. Microbursts cause the most dangerous low-level wind shear and have caused several airliners to crash. Thanks to the work of scientists and the aviation industry that led to today’s detection systems and pilot and controller training, a microburst has not caused an airliner crash in the United States since 1994.

A good day for flying. Warm air currents rising from the ground are forming the clouds, with the flat bottoms showing where the rising air has cooled enough for water vapor to begin condensing into cloud drops. Air is slowly sinking in the clear areas between clouds.

The down and then up air motions as an airplane flies from under a cloud cause turbulence, probably light with an occasional moderate bump in an unusually vigorous updraft. When the atmosphere is relatively stable and the clouds aren’t growing into towering cumulus clouds or thunderstorms, the air above the clouds will be generally smooth. In the late afternoon, with strong sunlight no longer heating the ground, the updrafts fade away and the clouds evaporate. This is a good time to take someone for their first ride in a small airplane, because the trip likely will be smooth.

Thunderstorms.The morning after turbulence rattled United Flight 967, the AccuWeather Blog's weather expert, Henry Margusity, said that a thunderstorm had “exploded upwards into the jet’s flight path. The storm (top) climbed from about 25,000 feet to 45,000 feet in 30 minutes.”

The National Transportation Safety Board's investigation will determine if that is true, but it seems reasonable, because over the years many pilots have discovered that you don’t have to fly into a thunderstorm to run into its extreme turbulence. In fact, airline pilots are advised to stay at least 20 nautical miles away from thunderstorms in all directions and not to fly above small storms that are growing.

Thunderstorm turbulence is also felt on the ground as downdrafts hit the surface and travel as gust fronts, which can cause rapid changes in wind speed and direction as they move across an airport.