Dear Tom,

A recent flight was very bumpy and we had the seatbelt sign on for much of the trip. What causes turbulence?

Roger M.
Oak Lawn

Dear Roger,
Turbulence occurs when there is an irregularity in the way air is moving when compared to the motion of air around it. We see turbulence all the time when we watch water flowing over a waterfall or the chaotic wake behind a moving boat. The swirls in a stream or river are also examples of turbulence. But it’s harder to picture atmospheric turbulence because its component gases are largely invisible. Air turbulence is most obvious to us when we are in an aircraft. The disruptive effect of mountains and hills as air flows over them leads to the rough ride we sometimes experience. Around jet streams (fast moving rivers of air above earth) eddies in the air’s movement can jostle aircraft, sometimes significantly. These are examples of turbulence. Often the turbulence is associated with weather systems or thunderstorms and is easily predictable and subsequently avoidable. However, changes in wind speed and direction that cause turbulence can also occur in clear air and are both invisible and unexpected. This turbulence is called clear-air turbulence (CAT) and can cause serious problems for aircraft and their passengers.