An Invisible Danger: Clear-air Turbulence
Even though pilots are taught to avoid turbulent air by looking for cumulus clouds, turbulence can strike even in the absence of clouds. This type of turbulence—especially dangerous because of its invisibility—is known as clear-air turbulence. It accounts for most turbulence-related injuries, mainly because pilots have no time to warn passengers and flight attendants to get strapped into their seats. Nearly 7 out of 10 turbulence incidents are the result of encounters with the clear-air variety.
Clear-air turbulence most often occurs as the result of a surface cold front (a large, moving mass of cold air) encountering warmer air in its path. Colder air is denser than warm air and tends to remain closer to the ground. Therefore, as the leading edge of a cold front advances, it forces its way under the warmer, lighter air in its path. This effect can create a trail of turbulent air thousands of meters above and 80 to 160 kilometers (50 to 100 miles) behind the leading edge of the cold front.
Encounters with clear-air turbulence most often occur along the borders of the jet streams, enormous currents of air thousands of miles long flowing eastward at more than 100 kilometers per hour. The major jet streams are usually found at least 10 kilometers (6 miles) above the surface. There are six jet streams that meander around the Earth. Three are in the Northern Hemisphere and three in the Southern Hemisphere—one each near the North and South poles, two along each side of the equator, and two running roughly across the center of each hemisphere.
Airlines prefer to fly their eastbound aircraft in a jet stream, because the high-speed eastward tailwind allows a plane to fly faster while burning less fuel. But this benefit has a downside. As a jet stream races along, eddies of air swirl from its boundaries into the slower air around it, creating turbulence. Therefore, using a jet stream in this way often means risking an encounter with clear-air turbulence as an airplane enters and leaves the stream.
Clear-air turbulence also can swirl near a deep upper trough, an elongated area of low air pressure at an altitude of 3,000 meters (10,000 feet) or more. Like water running down a drain, air flows counter-clockwise around troughs in the Northern Hemisphere and clockwise in the Southern Hemisphere. This circulation pattern can create turbulence along the trough's edges. The most turbulent air is often found upwind of the trough's base and along its centerline. Upper-trough turbulence can develop in small areas of the sky. They may last only 30 minutes or persist for an entire day. Such an elusive culprit is as difficult for meteorologists (weather forecasters) to predict and study as it is for pilots to avoid.