Almost anyone who flies frequently as a passenger or a pilot has a tale to tell of an encounter with clear air turbulence (CAT). CAT by definition is turbulence that is not associated with clouds and other visual weather like thunderstorms.

Most stories told by passengers about CAT encounters include accounts such as “it came out of nowhere,” and “we dropped immediately at least a thousand feet.” Anything unpredicted in terms of turbulence can be frightening to both passengers and pilots.

“CAT is turbulence that is not associated with clouds and other visual weather like thunderstorms.”

The human cost

Clear air turbulence is not only frightening and potentially dangerous to airline operators, it is very expensive. Passenger injuries often happen and claims for them can run into the millions. Commercial aircraft report at least 5,000 encounters with severe or greater turbulence every year with most of them occurring above 10,000ft.

Aircraft are rarely seriously damaged by CAT but any encounter with severe turbulence must be reported in the aircraft’s maintenance log and a detailed inspection of the airframe and systems has to be made by mechanics before the aircraft can fly again.

Passengers and crew are usually the victims of a CAT encounter. Broken arms and ankles along with concussions and lacerations are common in these encounters as well as injuries from items like carry-on baggage that tend to fly around in severe turbulence.

Airline pilots and dispatchers dislike CAT more than any other weather phenomenon because it is difficult to predict and as a result, almost impossible to avoid.

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Current turbulence predictive tools can give operators a general idea of where turbulence encounters might happen but can’t specifically say if they will happen at all.

Pilots reporting turbulence to ATC and to each other are usually the best way for aircraft to avoid or at least prepare for a turbulence encounter. Most often, an altitude change can mitigate the severity of the bumps but on oceanic routes these altitude changes are very difficult to get from controllers and most international pilots resolve to ride out turbulence when they are flying on the tracks.

Most often, CAT avoidance is an esoteric combination of data and forecasts provided the pilot from the dispatcher and a pilots experience and preferred techniques. In other words, CAT avoidance has always been more art than science, and the whole thing has been costing the world’s airlines untold millions in additional expenses.

A new CAT prediction technique

Recently, scientists from the University of Georgia in the United States have developed a new predictive technique for clear air turbulence. It is based on gravity waves.

Gravity waves are phenomena in the atmosphere that look like ocean waves but can occur in clear air. They can be created by airflow over mountains, frontal boundaries or by other causes. Gravity waves are also typically found trailing behind large rain shields that are produced by frontal activity. The type of gravity waves that the new technique measures are spontaneously generated and are associated with jet streams at airline cruising altitudes.

“Passengers and crew are usually the victims of a clear air turbulence encounter.”

The new method announced by John Knox, an assistant professor of the University of Georgia is based on something called the Lighthill-Ford theory of spontaneous imbalance, developed by a British theoretician in the early 1900s.

The important feature of this new technique is that it finally gives operators a single, consistent theory of imbalance that should make predicting CAT more accurate and reliable.

Improvements in predicting clear air turbulence can reduce risk, discomfort and monetary losses by airlines. If this new technique is viable and becomes as accurate and reliable as they hope, it will be as important to aviation safety as the introduction of airborne weather radar was years ago.