April 2009Departments

The Weather Never Sleeps

The unseen force


Understanding wind and its effects

For most new pilots, getting used to the random shoves and bumps of the air isn't as easy as becoming comfortable with the airplane. For instance, a student pilot can't always be sure that his or her control inputs rolled the airplane slightly to the right or whether a wind gust caused the movement. Except on an uncommonly calm day, the air seems to always be moving-some days more than others.

During the outburst stage of a microburst, the wind curls as the cold air of the downburst moves away from the point of impact with the ground.

These air movements create aircraft turbulence, which the American Meteorological Society's Glossary of Meteorology defines as "motion of an aircraft in flight, especially when characterized by rapid up-and-down motion, caused by a rapid variation of atmospheric wind velocities." Velocity is used here with the scientific meaning of the combination of the wind's speed and its direction, such as a 10-knot wind from 210 degrees.

While it's not a guaranteed way to become comfortable with the whims of the wind, understanding what's going on should help to erase vague fears that the wind might have some nasty surprises in store. Such know-ledge will also help you avoid the few times and places when the wind could bring a nasty jolt.

You can begin to understand the air's movements by imagining it as moving water. The more obstacles it encounters, the rougher the water. The faster the water is moving, the rougher it becomes when it hits obstacles. Water in a challenging whitewater stream is moving fast as well as hitting many rocks or going over ledges in miniature waterfalls, creating complex, violent movements. Slow-moving water in the same stream doesn't require much skill to navigate in a kayak or raft.

There's one important difference between a whitewater stream and a stream of fast-moving air: A cubic foot of fresh water weighs approximately 62 pounds per cubic foot. At sea level, a cubic foot of air weighs approximately 0.07 pounds per cubic foot and it becomes even lighter at higher altitudes. Any particular amount of force will impart a great deal more acceleration to air than to water.

An important part of any weather briefing is learning about adverse conditions, both current and forecast, and these include turbulence. In addition to ensuring that you receive all airmets or sigmets (National Weather Service alerts) for turbulence that might affect your flight, you should also mull over the reported and forecast wind speeds along your route.

Although this information isn't always part of your weather briefing, you should also be aware of any "rocks" that could disturb the stream of air in which you will be flying. Since the air is so light, the "rocks" that impart swirling motions to it can be mountains that are miles away. When conditions are right, winds forced up when they run into mountains create up-and-down waves on the downwind side of the mountains that can extend for many miles. If you're landing, the "rocks" could be buildings and trees along the runway that can create swirls in even a mild crosswind.

Winds aren't the only air motions you need to think about. Unlike water flowing down a stream, air often moves vertically without going over something like a waterfall. When the ground warms up it warms ground-level air, which becomes less dense and begins to rise. Depending on the conditions, the air might rise perhaps a few thousand feet, or more than 30,000 feet.

These streams of rising air are called thermals, which glider pilots use to stay aloft without the benefit of engines. But pilots of powered airplanes usually find thermals annoying. Thermals are most likely to become a challenge, maybe even dangerous, in very hot weather-such as during the summer in the Southwest deserts. But, you don't have to be over a desert for thermals to make a flight bumpy.

Air that rises eventually comes down somewhere, but most of the time the sinking air is spread out over a larger area than the rising thermals, and therefore isn't moving down as fast as thermals are moving up. Even so, descending air can complicate a pilot's life, especially during landings.

Winds above the ground generally blow faster than those 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. Descending air can bring faster-moving air aloft down to the ground, causing wind gusts. In the United States, weather observers report gusts when the wind speed in any 10-minute period varies by 10 knots or more between the wind's peak speed and a lull. Gusts also frequently blow in slightly different directions than the wind during a lull.

A fast but steady wind isn't bumpy. High-flying jets routinely hitch rides on 100-mph jet streams. The ride becomes bouncy only when the airplane flies between areas in which the wind is blowing at different speeds, or from different directions.

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 relatively short distance.

An image of flowing water can help you see how wind shear works. A deep, fast-moving stream of water moves faster in the middle than along the edges because drag (friction) with the stream's banks slows the water next to them. Air along the boundary between adjacent "streams" of wind blowing at different speeds, or in different directions, takes on a spinning motion. This motion creates turbulence for any airplane that flies into it.

The most dangerous kind of wind shear is known as a microburst and occurs near the ground. A microburst begins forming when dry air flows into the upper part of a thunderstorm or a shower that's growing into a thunderstorm. Rain evaporates as it falls into the dry air, which cools the air. The cooled air, now denser than the surrounding air, plunges to the ground. When it hits, the downward-moving air spreads out in all directions as a swirl of wind that can instantly change a headwind into a tailwind and cause an airplane to crash. Special radars alert air traffic controllers at major air carrier airports about microbursts, and they can warn pilots of the danger. Airline pilots also receive microburst training. For general aviation pilots, the best rule is not to land or take off under a shower or thunderstorm even if the rain is light, or if the rain is evaporating before it reaches the ground

Pilots need to become comfortable flying in the almost always moving air, but not so comfortable they aren't alert to indications that air movements may be crossing the line from annoying to dangerous.

Jack Williams, a freelance science writer specializing in weather and climate, is the author of six books. He is an instrument-rated private pilot. His latest book, The AMS Weather Book: The Ultimate Guide to America's Weather, is forthcoming from the University of Chicago Press.


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