February 2005Departments

The Weather Never Sleeps

Lumps, bumps, and bruises

The why, when, and where of rough air

After flying into Hurricane Fran the evening of September 5, 1996, aboard a large, four-engine National Oceanic and Atmospheric Administration research aircraft, I wrote a story for USA Today that in one place read:

"At 5:45, the computer screen (in front of my seat) shows we are flying at 5,000 feet, toward the northwest about 150 miles southeast of Wilmington, North Carolina, in 102-mph winds.

"Three minutes later, rain begins hitting the windows and the wind outside is 115 mph, but the ride is still smooth."

After reading a draft of the story, an editor asked, "How can it be smooth when the wind is 115 mph?"

Good question. It prompted me to add a paragraph: "How can this be? Fran is throwing out 115-mph winds, but they're steady winds, having little effect on us. Quick changes in wind speed or direction cause turbulence. So while a drop in wind speed from 100 to 50 mph in a few seconds could severely bounce a plane, a steady 115-mph wind won't." That paragraph is probably as good a way as any for pilots to begin understanding turbulence.

Wind speed alone doesn't create turbulence. Changes in wind speed and direction are the culprits when your flight begins to feel like the air has potholes, maybe even craters. In the early days of flight, pilots talked about "air pockets." The term implies that the air has "pockets" of empty space, which at first glance might seem like a good way to explain the sudden "bumps" that airplanes hit. It's easy to imagine how this idea came about. As his biplane bounced around, Barry Barnstormer could have figured that the air must have potholes like the ones that jolted his 1919 Model R Hupmobile on the way to the flying field.

But the air isn't like the gravel or asphalt of a road; it's a fluid like water. If a "hole" somehow happens to develop in water or air, the pressure of the surrounding fluid fills it in. You can create a more accurate picture of many kinds of turbulence by thinking of the air being like water that's sometimes still, sometimes flowing. Moving air, like flowing water, is sometimes smooth, sometimes turbulent. Imagine the water in a white-water stream.

One important source of turbulence, however, doesn't fit into the usual mold of flowing water. Unlike water, air can go up to great heights without being confined to pipes like those that carry water to the tops of skyscrapers. In other words, the air moves up and down as well as roughly parallel to the Earth's surface.

When the ground warms up and heats the air adjacent to it, the air becomes less dense and begins to rise. Depending on several factors such as the temperature of the air at different heights above the ground, the air might rise perhaps a few thousand feet, or more than 30,000 feet.

Air that rises to 30,000 feet will create thunderstorms, which we can think of as turbulence machines that also produce other hazards to aircraft -- such as hail, lightning, and rain heavy enough to drown engines.

Streams of air rising from the ground are called thermals, and they are the friends of glider pilots, who use them to stay aloft without the benefit of engines. Pilots of powered airplanes usually find thermals annoying. Thermals are most likely to become a handful, maybe even dangerous, in very hot weather, such as during the summer on the Southwest's deserts. But, you don't have to be over a desert for thermals to make a flight bumpy.

A sunny, calm day with the air hardly stirring and just a few puffy clouds in the sky might seem like the perfect time to take a friend for a first airplane ride. And it could be. But it could also be one of those days with lots of bumps, which aren't dangerous but could be disconcerting to a first-time flyer. If this happens, climb above the small, puffy clouds, and the ride should smooth out because the clouds mark the tops of thermals.

But, if the clouds are growing like castles in the air, the thermals are soaring high and could become thunderstorms. They are to be avoided. If clouds are staying small and puffy because the thermals are not soaring toward the stratosphere, you can look for a smoother ride late in the day as the ground cools off and the thermals fade.

Air that goes up eventually comes down, but most of the time air descends so slowly that we don't notice it. When air does descend quickly, it's almost always from thunderstorms or even benign-looking rain showers. When such descending air is concentrated in an area of about 2.5 miles in diameter or less, it's called a microburst, and it is a serious danger to aircraft near the ground taking off or landing (see "The Weather Never Sleeps: Ill Wind," June 2003 AOPA Flight Training).

Air also descends from virga -- rain or snow that evaporates before it hits the ground. It can shake an airplane that flies under the innocent-looking virga. This is because the evaporating rain cools the surrounding air, making it like a cold-air balloon that plunges toward the ground.

While up-and-down air motions cause some turbulence, most of the bumpy air is caused by air that's flowing more or less parallel to the ground, with up-and-down motions adding complications.

One of the most common complications is gusting wind. Winds above the ground generally blow faster than near the ground because there's no friction with the ground to slow the wind. Winds aloft also tend to blow from slightly different directions than low-level winds.

When air is rising in thermals, air is also coming down nearby. While the descent is usually slow, the descending air keeps the velocity -- the speed and direction -- that it had above the Earth. For instance, say the wind is blowing from 270 degrees at 10 kt at the surface, and air right above, which was blowing from 240 degrees at 20 kt, descends. When it reaches the ground, the wind will momentarily blow from the southwest at around 20 kt.

By the way, a gust is defined as a sudden, brief increase in wind speed, usually lasting fewer than 20 seconds. In the United States, weather observers report gusts when the peak wind speed in any 10-minute period varies by 10 kt or more between the wind's peak speed and a lull.

Gusts that momentarily change wind speed and direction aren't the only effect of the ordinary difference in wind speeds and directions at different altitudes. Differences in wind velocity create wind shear, which is a common cause of turbulence. Wind shear refers to the change in wind speed or direction, or both, over a distance.

Imagine a stream of water that's flowing faster in the middle of the stream than along the edge. Now, put a paddle wheel in the stream with an axle that you hold on to, keeping the wheel from washing down the stream as it lies flat in the water with one side in the middle, and the other side near the edge. While the water is trying to push the entire wheel downstream, the wheel spins because the water is pushing with more force on the part in the middle of the steam.

In a similar way, air in the layer between winds blowing at different speeds, or directions, takes on a spinning motion like the wheel in the water.

As an airplane flies into the upward-moving air, it rises and then sinks a moment later in the downward-moving air. Wind shear, by the way, can be either horizontal or vertical and can occur at any altitude. For pilots, the most dangerous wind shear is the low-level shear created by microbursts. When wind blasts down from a thunderstorm or shower, it not only tends to push an airplane down, it also creates sudden changes in wind speed and direction when it hits the ground and spreads out.

In addition to turbulence created by up-and-down air movements and wind shear, wind becomes turbulent when it runs into things large and small. Some of the most dangerous turbulence is created by strong winds flowing over mountains. If conditions are right, winds forced up when they run into mountains create up-and-down waves on the downwind side of the mountains; these can extend downwind a considerable distance.

As with thermals, knowledgeable (and properly equipped) glider pilots can use such mountain waves to climb higher than most airliners fly. On the other hand, mountain waves have caused serious injuries and damage, even crashes, of both large and small airplanes.

A line of trees or the buildings at an airport can create turbulence, too. Thinking of the air as being a fluid that you can see, like water, can help you imagine the swirls and eddies created by winds from different directions hitting objects a few hundred feet from a runway you are planning to use.

The faster the wind is blowing, the more turbulence it can create. When you're planning a flight, the "adverse conditions" section of a standard weather briefing should include any reported or forecast turbulence along your route of flight.

In addition, you should expect at least some turbulence from thermals on any sunny day, winter or summer, especially in the late morning or early afternoon. For this reason, if no thunderstorm or larger-scale storm is around, the air is usually calmer early in the morning and late in the afternoon, making these usually the best time to take up a first-time flyer in a small airplane.

Jack Williams is the weather editor of usatoday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.

For more information, see the book Turbulence: A New Perspective for Pilots by Peter F. Lester, published by Jeppesen Sanderson Training Systems, Englewood, Colorado.