Science: Predicting CAT’s Claws

Professional airmen have long feared the way-up phenomenon of clear air turbulence—called CAT. But not until recent months, with at least one airline disaster attributable to CAT (TIME, April 11), has awareness of the danger struck home to the high-flying U.S. public. The worst thing about CAT was that even the experts have had little notion of how to avoid its claws—until now.

Clear air turbulence is exactly what it sounds like: an airplane speeding through a cloudless part of the sky can be ripped apart by an invisible tempest. CAT is most often met just above or just below the 30,000-35,000-ft. jet stream — and modern aircraft like best the levels near the great, racing jet stream. It has been agreed that CAT is caused by wind shear —the “friction” between adjacent air masses moving at different speeds. Last week Joseph J. George, chief meteorologist for Eastern Airlines, told how this knowledge might be put to work to predict CAT’s claws so that airliners can be warned to skirt the peril.

After studying the meteorological records of bad CAT occurrences, George and his research team concluded that not one, but two different sorts of wind shear must be present to cause dangerous clear air turbulence. One is horizontal shear, caused by friction between air masses moving side by side at different speeds. The other is vertical shear, caused by the friction between two air masses, the one above the other, moving at different speeds.

To forecast severe CAT, George studies weather data in search of above-and-below air layers that are being dragged violently past each other by the jet stream. He draws on a map the area where this vertical shearing is sufficiently intense. Then he scans the data a second time to see if any air masses on the same level are moving rapidly past each other. If this is the case, he marks another area on his map (see diagram). If the areas overlap, the overlap has the two necessary kinds of violent shear. It is therefore apt to be full of wing-wracking CAT. The troubled air is usually only a few thousand feet thick, and it slopes upward, its high end toward a slow-moving air mass that is being jostled by the speedy jet stream.

George is confident that his method can spot severe CAT in time to warn pilots not to fly through it. The only trouble is that present facilities cannot furnish enough weather data to track down CAT wherever it may exist.