Turbulence: Rocking and Rolling

Turbulence is a familiar phenomenon to most of us. Virtually everyone who has made an airline flight is familiar with the stomach-churning bumpiness caused by turbulence. But what causes turbulence?

Turbulence can be traced to a couple of main causes—heat and wind shear.

Heat- driven turbulence occurs primarily over regions such as the desert Southwest or the wide plains of the Midwest. Summer sunshine can boil up thick thermals that reach altitudes above 10,000 feet before losing upward energy and settling back toward the ground. Whether on their way up or down, they can buffet an airplane violently.

Wind-shear turbulence comes in several forms, including mountain waves that act similarly to the ripples in a stream formed as it flows around a rock. As with the water, winds flow up and over a mountain range, perpetuating ripples many miles long for a hundred miles or more.

Turbulence: Rocking and Rolling

By the Book

Wind shear is the turbulent air movement caused by a difference! in wind speed or wind direction between two or more bodies of і converging air—for example,

1 between a region of northwest­erly wind colliding with a region of southwesterly wind. In other cases, winds can blow one direc­tion at one altitude, and a differ­ent direction just a few hundred feet higher. Between the two layers, the air roils in powerful currents that create turbulence.

Clear-air turbulence, or CAT, is a rare but potentially deadly form of turbulence. CAT occurs near a fast-flowing river of air called the jet stream, and has been blamed for serious injuries to airplane passengers and crew. In 1998, a Japan Airlines jumbo jet near Tokyo flew into CAT so severe that three people on board were injured.

These kinds of reports are not uncommon. Weather forecasters try to pinpoint the location of the jet stream, which in North America is found at the boundary between the high-pressure warm air over most of the United States and the low-pressure cold air over most of Canada. The jet stream ranges in altitude from 10,000 feet to 30,000 feet or more.

The jet stream can reach speeds of nearly 200 m. p.h. at its fastest. When an airplane flies through an area where the speed of the jet stream changes rapidly in a short distance, severe turbulence is possible. CAT is a good reason to wear your seatbelt throughout a flight, particularly because this type of turbulence is notoriously unpredictable.

Turbulence can be forecast by meteorologists based on measurements of atmospheric stability. But there’s a second, more reliable technique—the informal exchange of information among pilots. Air-traffic controllers all over the country keep an informal tally of turbulence by polling airline pilots at high altitudes and small-airplane pilots at lower altitudes. What emerges is a mosaic portrait of turbulence in a region that other pilots can use to make their passengers comfortable and safe. It’s not as scientific as the detailed meteorological charts, but it’s first-hand and easy to use.

Severe Weather: Thunderstorms and Tornadoes

Once in a while, the atmosphere turns downright nasty. Thunderstorms, and their raging offspring, tornadoes, possess enough power to shred an airplane, and that includes large airliners and powerful military planes.

Thunderstorms

Thunderstorms grow best when the atmosphere is unstable and turbulent, and air temperature near the ground is hot and air temperature higher up in the atmosphere is cold.

Turbulence: Rocking and Rolling

Plane Talk

Lightning is the visible discharge of stored electrical energy created by friction between water droplets. Because air h a poor conductor of electricity, the electric charge builds up until it bridges the air between cloud and ground, setting off an upward moving flash of light and heating the nearby air to 43,000°F. The explosive heating causes the deafening thunder, which, if it occurs close enough, can create a bomblike concussion that can break windows. Because airplanes in flight are not grounded, lightning strikes rarely cause anything more than surface damage, lightning has never been blamed for an airplane accident

Thunderstorms begin when warm, humid air is carried aloft by convective currents. A passing weather front can begin the process; so can thermals caused by very hot weather.

Turbulence: Rocking and Rolling

Turbulence

One of the most common forms of turbulence is the "wake turbu­lence" that trails back from the wingtips of very large airplanes. Most powerful during takeoff and landing, wake turbulence comes from rapidly spinning wingb’p vor­tices that are an unavoidable by­product of lift Though only a few inches across, wingbp vortices can spin dangerously for many min­utes and seriously buffet any air­craft that follows too closely. Air-traffic controllers put plenty of space between planes to keep things sofe.

As the humid air rises and cools, the vapor condenses into clouds, releasing a small amount of heat as it does. Heat, which is released whenever water condenses, pushes the turbulent air even higher, for the same reason hot air inside a balloon floats. The higher the warm air goes, the more vapor condenses, which adds more heat to the air, sending it still higher. It’s a cycle that stops only when the top of the growing cloud towers to heights of 50,000 feet or more. Some massive thunderstorm clouds have even approached 60,000 feet.

When water droplets inside the growing cloud become too large to be carried aloft any longer, they begin to fall. The falling droplets start pulling air down with them, initiating a downward draft of air. As the droplets reach warmer air below, they evaporate. Evaporating water cools the air around it, and cooler air is denser and heavier, making it descend still faster.

Not all the droplets evaporate, though, and some of them reach the ground in the form of a rain shower.

Turbulence: Rocking and Rolling

By the Book

Hail is formed when the updrafts inside a thunderstorm push up­ward into freezing air. An ice core forms in the cold air. but is pre­vented from falling by the force of the updrafts. Water droplets carried by the updrafts strike the ice chunk and freeze onto it. Depending on the strength of the updraft, a hailstone can grow as large as a grapefruit, though most are the size of rice grains.

The cool air that accompanies the downward-rushing air of a thunderstorm explains the sometimes dramatic temperature drop as a thunderstorm approaches.

These two cycles—rising, condensing air and descending, evaporating air—take place simultaneously inside a developing thunderstorm for a time. After about an hour, the downdrafts begin to smother the updrafts, and the thunderstorm dies. However, the downdrafts from one storm can start nearby warm air rising, triggering another storm, which triggers still another storm, and so on for hours at a time, if conditions are right.

Thunderstorms can be dangerous, even deadly, to pilots. Pilots know that thunderstorms can throw massive chunks of hail as far as five miles away from the storm itself, and the violent shifts of wind inside a thunderstorm and for miles around it can send planes crashing to the ground. Pilots are warned to steer as far as 20 miles around a thunderstorm.

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