Clear-air turbulence, or CAT, is the turbulent movement of air masses in the absence of any visual clues such as clouds. The atmospheric region most susceptible to CAT is at altitudes of around 23,000 to 39,000 feet. This is the region of jet streams — fast-moving, narrow bands of wind high in the atmosphere that blow from west to east.
Wind shear refers to a sudden change in wind speed or direction across a short distance. When an aircraft encounters wind shear, it may experience abrupt vertical or horizontal movements, resulting in unanticipated changes in altitude or velocity. While wind shear is commonly observed in the vertical dimension, it can also manifest horizontally.
A primary theory explaining CAT posits that vertical wind shear generates atmospheric gravity waves, which subsequently break in a manner similar to ocean waves approaching shore. The disturbance produced during the breaking of these gravity waves is analogous to the foam created by ocean waves, the “foam” representing atmospheric turbulence.
CAT is a destroyer and even a killer on occasion. On Dec. 28, 1997, United Airlines Flight 826, carrying 393 people, hit heavy turbulence over the Pacific Ocean. Passengers who happened to be wearing their seat belts at the time described floating “like we were in an elevator falling down.” Those not wearing seat belts crashed into the cabin ceiling. One woman was killed as the result of severe head trauma and at least 102 people were injured.
CAT is a particularly difficult challenge for weather forecasters. The chances of encountering CAT increase when: a) the vertical wind speed shifts by at least 5 knots every 1,000 feet; b) the horizontal wind speed changes by 40 knots or more across 150 miles; or c) two air masses moving at significantly different speeds come together.
Steve Ackerman and Jonathan Martin, professors in the uW-Madison department of atmospheric and oceanic sciences, are guests on Wha radio (970 aM) at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
