Statistically, turbulence resulting in injury is a rare occurrence. In 2013, the FAA reported that there were 11 passenger injuries and 13 crew injuries from turbulence. With U.S. carriers servicing approximately 50 to 70 million passengers per month, the chances of turbulence causing injury are on par with snagging that 600 million dollar lottery ticket. This fact alone should be enough to assuage the nervous flier’s turbulence anxiety. If not, read on.
Turbulence is caused by sudden changes in wind speed and/or changes in wind direction. Changes in direction could be in the vertical, lateral, or a combination thereof. Since aircraft move within air-masses, sudden changes in wind speed or direction impart an influence on aircraft motion in the axes of flight (Pitch, Roll, and Yaw). This perturbation of the aircraft is dependent upon the wind velocity and rate of change (wind gradient).
Known in pilot parlance as “Wake Turbulence”, this type is a result of the wing design of modern aircraft. If not given a wide enough berth, it can cause controllability problems for aircraft, mostly on the takeoff and landing phases where airplanes are closer together. Wake turbulence, if visible to the naked eye, would appear as tornadic vortices emanating from aircraft wingtips. The FAA has done much research into wake turbulence.
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This type of turbulence is caused by horizontal winds that come into contact with jutting ground features such as trees, buildings, and mountains. When the localized wind strikes surrounding impediments, it causes small, irregular, tumbling vortices called eddies. These eddies result in the sudden changes in wind speed and direction that perturb aircraft trajectory.
As an air-mass is heated by insolation, (A fancy word to describe the solar radiation) air at the surface heats and rises. These rising pockets of air can cause sudden changes in aircraft position, which is felt through the sensation of turbulence.
Shear turbulence is a result of adjacent air masses with differing winds over a horizontal or vertical distance. To remember this type of turbulence, think of garden shears. There are two different surfaces that meet when the garden shears are in motion. In shear turbulence where two air masses meet, there will be intermixing of the two, and eddy currents will form. The eddies will cause erratic changes in aircraft motion that will be perceptible from within the aircraft. This type of turbulence is common with frontal weather, jet-stream boundaries, and thunderstorms.
Turbulence can be very difficult to forecast and thus predict in aircraft operations, especially when it is turbulence of the clear air variety. Clear Air Turbulence (CAT) is turbulence that has no visible evidence of its existence. Because of the fickle nature of the atmosphere, the pilots can be as surprised as the passengers with a close encounter of severe turbulence. For this reason, heed crew member admonitions to keep your seat belts fastened when seated.
Fortunately, pilots are able to easily predict turbulence when particular weather conditions are present. Extant are certain cloud developments, such as cumulonimbus (thunderstorms) and building cumulus clouds, which indicate a very high likelihood of uncomfortable & dangerous turbulence. Pilots avoid these clouds judiciously and with zeal. As a final note and an aside, take a look at the Flight Crew. Do they look nervous? If not, then there is a very high likelihood that the aircraft will return to terra firma in the same condition that it left in.