Weather

Almost all student pilots, especially in the United States, are experienced auto drivers when they take up flying. After an instructor explains how to control an airplane around the longitudinal, lateral, and vertical axes, most students quickly adapt to controlling a vehicle moving in three dimensions instead of two.

Air also moves in three dimensions. This is a challenge for a pilot learning how to adjust the airplane’s path over the ground, such as when the wind is pushing it to one side of a final approach to landing. It isn’t scary, and most student pilots enjoy mastering ground-reference maneuvers.

Rougher air movements cause the “bumps” called turbulence, which the American Meteorological Society’s Glossary of Meteorology defines as “a state of fluid flow in which instantaneous velocities exhibit irregular and apparently random fluctuations. This condition can be superimposed upon mean fluid flow.” “Velocity” is used here in the scientific meaning as the combination of speed and direction, such as a 10-knot wind from 210 degrees. And for some students this rather clinical definition masks what can be an unnerving phenomenon.

FORGET ‘AIR POCKETS.’ In the early days of flight, pilots talked about “air pockets,” and even now you hear people, such as airline passengers who have encountered turbulence, use that term. It implies that the air has pockets of empty space. At first glance it might seem a good explanation of the sudden bumps that airplanes experience. It’s easy to imagine an early pilot or passenger who didn’t know much about meteorology, thinking the bumps his airplane seemed to hit in the air were holes—just like those his car hit on poorly maintained roads.

But the air isn’t like the gravel or asphalt of a road; it’s a fluid, like water. If a hole somehow develops underwater or in the air, the pressure of the surrounding fluid instantly fills it in. Watching a white-water stream, maybe with the aid of a toy boat, is a good way to visualize how the random fluctuations of moving air can affect your airplane. If you’ve ever taken a kayak or raft through white water, you’ve experienced turbulence firsthand.

When the water hits a rock in the middle of a stream, it splashes up and often continues downstream as a wave. When water squeezes between two rocks, it speeds up. As water flows over and around rocks, it often develops tiny whirlpools that flow downstream. If you’ve gone white-water rafting, you’ve experienced “instantaneous velocities” that “exhibit irregular and apparently random fluctuations.” They are no longer abstract fluid dynamic terms.

While visualizing air as water will help you form a picture of air turbulence, keep one important difference in mind: A cubic foot of fresh water weights 62.46 pounds. A cubic foot of air at sea level weights approximately 0.075 pounds. As Isaac Newton’s second law of motion tells us, the acceleration that a force imparts to an object is inversely proportional to the object’s mass. In other words, a force deflecting a cubic foot of air is going to have a much bigger effect than a similar force deflecting a cubic foot of water since the mass of the water is so much more.

One important source of aircraft turbulence, however, doesn’t fit into the flowing-water model. Unlike water, air can go up to great heights without being pumped through pipes, such as to the tops of skyscrapers. Air naturally and normally moves up and down as well as roughly parallel to the Earth’s surface as wind.

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 can rise perhaps a few thousand feet, or more than 30,000 feet. When the weather is calm air may rise at speeds of one mile an hour or less. On the other hand, air under and inside a powerful thunderstorm can be rising faster than 50 mph.

One of meteorology’s rough rules of thumb is that thunderstorm downdrafts are about half the speed of the updrafts. This is one reason why even the most experienced pilots in the biggest airplanes avoid flying into or even too close to thunderstorms. Imagine flying into a 50-mph updraft and a second or two later encountering a 25-mph downdraft.

WIND SHEAR OFTEN THE CULPRIT. Downdrafts and updrafts are more broadly defined as wind shear. A good way to begin understanding it—and how it causes turbulence—is by imaging a stream of water that’s flowing faster in the middle of the stream than along the edge. This is common in both air and water because friction with the stream’s banks slows the water, as friction with the ground slows low-altitude winds.

Imagine putting a paddle wheel in the stream with an axle that you hold on to, keeping the axle vertical with one side near the bank. 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. An airplane that flies into the upward-moving air rises and then sinks a moment later in the downward-moving air. Wind shear can be either horizontal or vertical and can occur at any altitude.

Streams of warm air rising from the ground are called thermals and glider pilots love them because they are a good way to climb after releasing from the towplane. In contrast, thermals often irritate pilots of powered airplanes because they’re bumpy. On hot summer days over the Southwest’s deserts, thermal turbulence may often be more than most pilots want to handle. This is why many pilots crossing deserts fly in the early morning before the ground heats up, and land where they’ll have a cool place on the ground to spend the afternoon.

When you’re far from a desert, a sunny, calm day with the air hardly stirring on the ground and with a few puffy clouds in the sky might seem like the perfect time to take a friend for a first airplane ride. It could be. But it could also be one of those days with lots of bumps, which aren’t dangerous to an experienced pilot, but could be disconcerting to a first-time flyer. To avoid the bumps, climb above the small, puffy clouds where the ride should smooth out because the tops of the clouds show where thermals have run out of steam and air is no longer rising.

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.

GUSTS COMPLICATE LANDINGS. One of flying’s most common complications is gusting wind, especially at ground level when you’re taking off or landing.

Gusts and their turbulence begin with winds above the ground generally blowing faster than near the ground because friction is not slowing wind aloft. Faster winds aloft also 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 knots at the surface, as air aloft—which is blowing from 240 degrees at 20 knots—descends. When it reaches the ground, the wind will momentarily shift from the southwest at around 20 knots.

This change in wind speed and direction causes turbulence and briefly changes the wind’s crosswind component and the landing airplane’s groundspeed.

The National Weather Service defines a gust 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 knots or more between the wind’s peak speed and a lull.

Turbulence can be caused by manmade structures around the airport (FAA Pilot's Handbook of Aeronautical Knowledge).