Air turbulence is a disturbance of the air that is often felt by the passengers and crew of aircraft. Air turbulence may produce only a slightly bumpy ride. But in some cases, luggage and even passengers have been tossed about the cabin.
A major source of air turbulence is convective heating. This phenomenon begins when the sun heats the ground. The ground, in turn, heats the air just above it. Because warm air is less dense than cool air, parcels of warm air rise in a convection current. Cooler air then moves downward and inward, filling the space beneath the rising air. We can see an up-and-down movement of air parcels in puffy cumulus clouds. Air travelers who fly through even a small cumulus cloud usually feel some turbulence.
Much air turbulence is not readily visible and so is known as clear-air turbulence. Convective heating can create clear-air turbulence where there is not enough moisture to form clouds. Clear-air turbulence also occurs at extremely high altitudes, where clouds are rarely present.
An example of turbulence at high altitudes is a disturbance created by a jet stream. Jet streams are bands of fast-moving air that circle the earth at altitudes of about 30,000 to 45,000 feet (9,000 to 15,000 meters). Airflow in the core of the jet stream is usually smooth. However, near the stream’s outer edge, the high-speed air rubs against the surrounding low-speed air. This interaction creates pockets of air turbulence.
In the Northern Hemisphere, aircraft flying in an easterly direction often ride in a jet stream to reduce travel time. As an airplane crosses the stream’s outer boundary, the occupants of the craft usually feel turbulence.
Air turbulence is also associated with weather fronts, narrow zones between huge volumes of air that differ in temperature or humidity. These air masses move over the earth’s surface much like blobs of molasses on an apple. Where air masses bump and rub against one another, pockets of air turbulence form.
Airflow over mountain ranges creates air turbulence at virtually all altitudes. Certain cloud formations sometimes indicate the presence of low-altitude disturbances. But clear-air turbulence also occurs over mountain ranges. The movement of an air mass over a range may create several disturbances at the same time. This turbulence can be felt at altitudes as high as 60,000 feet (18,000 meters).
