Published on 09/25/03

Scientist investigates mystery air turbulence

By Brad Haire
University of Georgia

You're sitting comfortably in a commercial airliner flying through clear blue skies. The flight attendant bends down to hand you a drink, and suddenly the plane jolts. The drink spills in your lap. What just happened?

Clear-air turbulence (CAT) is a phenomenon that has baffled the aviation industry for more than a half century, said John Knox, an associate research scientist with the University of Georgia College of Agricultural and Environmental Sciences.

But he's starting to find some answers to the mystery.

Airborne potholes

Even if you haven't flown in a plane before, no doubt you've heard people speak of experiencing turbulence during a flight.

"It's kind of like hitting a giant airborne pothole," said Knox, who studies atmospheric conditions. It can be scary at the least or cause injury to fliers.

Due to reroutes, cancellations or damage, turbulence costs the aviation industry an estimated $100 million and more than 300 injuries a year. It's caused by the up-and-down motions of the air a plane flies through.

"Air waves in the atmosphere are like the waves in the ocean or in a pond," Knox said. "When a plane flies through the waves, the tail can get lifted in the crest and nose caught down in the trough. (The plane) gets bumped around."

Highs and lows

Turbulence is most often caused by weather activity around thunderstorms, which are usually associated with low pressure systems in the atmosphere.

It's easier to predict this type of turbulence, he said. Forecasters can see or predict with relative accuracy where thunderstorms might develop. A pilot can fly around storms or be prepared for turbulence when flying through a low pressure region.

But turbulence can also happen out of the blue in clear skies. Sometimes it's caused by high pressure systems.

"Predicting turbulence in clear skies is much more tricky," he said, "because there are no visible signs."

The primary suspect for CAT, he said, has been a kind of atmospheric motion known as gravity waves, which can move through clear skies as easily as through cloudy ones.

Two types of gravity waves are those caused by winds blowing over mountains and those created by sharp changes in wind speeds near low pressure fronts.

Gravity waves do contribute to CAT, he said. But CAT has jostled planes far away from mountains and fronts, too.


Though high pressure systems are usually associated with clear skies, they can still cause quite a disturbance in the atmosphere. Winds in a high pressure system turn in a radius. If the system grows stronger, those winds can turn faster and tighter and cause what is known as inertial instability, he said.

"This instability is not well known in the aviation forecasting community," he said. But it could be the cause of certain kinds of CAT.

For the near future, CAT will still startle pilots and air travelers. But Knox feels he is one step closer to putting the puzzle together.

If you throw a rock into a still pond and you see where the rock hits the water and you know how waves behave, you can predict how and where the waves will release from the initial impact point of the rock.

"We think we know the waves," he said. "We just need to know more about the rocks and how many there are."

Knox hopes his research leads to computer models that can help aviation forecasters better predict CAT.

Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.