Eluding Aeolus: Turbulence, Gusts, and Wind Shear

Kristen Starr

Since the earliest days of American aeronautical research, NASA has studied the atmosphere and its influence upon flight. Turbulence, gusts, and wind shears have posed serious dangers to air travelers, forc­ing imaginative research and creative solutions. The work of NASA’s researchers to understand atmospheric behavior and NASA’s deriva­tion of advanced detection and sensor systems that can be installed in aircraft have materially advanced the safety and utility of air transport.

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EFORE WORLD WAR II, the National Advisory Committee for Aeronautics (NACA), founded in 1915, performed most of America’s institutionalized and systematic aviation research. The NACA’s mission was "to supervise and direct the scientific study of the prob­lems of flight with a view to their practical solution.” Among the most serious problem it studied was that of atmospheric turbulence, a field related to the Agency’s great interest in fluid mechanics and aerody­namics in general. From the 1930s to the present, the NACA and its suc­cessor—the National Aeronautics and Space Administration (NASA), formed in 1958—concentrated rigorously on the problems of turbulence, gusts, and wind shear. Midcentury programs focused primarily on gust load and boundary-layer turbulence research. By the 1980s and 1990s, NASA’s atmospheric turbulence and wind shear programs reached a level of sophistication that allowed them to make significant contribu­tions to flight performance and aircraft reliability. The aviation industry integrated this NASA technology into planes bought by airlines and the United States military. This research has resulted in an aviation transportation system exponentially safer than that envisioned by the pioneers of the early air age.

An Unsettled Sky

When laypeople think of the words "turbulence” and "aviation” together, they probably envision the "bumpy air” that passengers are often

subjected to on long-duration plane flights. But the term "turbulence” has a particular technical meaning. Turbulence describes the motion of a fluid (for, our purposes, air) that is characterized by chaotic, seem­ingly random property changes. Turbulence encompasses fluctua­tions in diffusion, convection, pressure, and velocity. When an aircraft travels through air that experiences these changes, its passengers feel the turbulence buffeting the aircraft. Engineers and scientists charac­terize the degree of turbulence with the Reynolds number, a scaling parameter identified in the 1880s by Osborne Reynolds at the University of Manchester. Lower numbers denote laminar (smooth) flows, inter­mediate values indicate transitional flows, and higher numbers are characteristic of turbulent flow.[1]

A kind of turbulent airflow causes drag on all objects, including cars, golf balls, and planes, which move through the air. A boundary layer is "the thin reaction zone between an airplane [or missile] and its exter­nal environment.” The boundary layer is separated from the contour of a plane’s airfoil, or wing section, by only a few thousandths of an inch. Air particles change from a smooth laminar flow near the leading edge to a turbulent flow toward the airfoil’s rear.[2] Turbulent flow increases friction on an aircraft’s skin and therefore increased surface heat while slowing the speed of the aircraft because of the drag it produces.

Most atmospheric circulation on Earth causes some kind of turbu­lence. One of the more common forms of atmospheric turbulence expe­rienced by aircraft passengers is clear air turbulence (CAT), which is caused by the mixing of warm and cold air in the atmosphere by wind, often via the process of wind shear. Wind shear is a difference in wind speed and direction over a relatively short distance in Earth’s atmosphere. One engineer describes it as "any situation where wind velocity varies sharply from point to point.”[3] Wind shears can have both horizontal and vertical components. Horizontal wind shear is usually encountered near coastlines and along fronts, while vertical wind shear appears closer to Earth’s surface and sometimes at higher levels in the atmosphere, near frontal zones and upper-level air jets.

Large-scale weather events, such as weather fronts, often cause wind shear. Weather fronts are boundaries between two masses of air that have different properties, such as density, temperature, or mois­ture. These fronts cause most significant weather changes. Substantial wind shear is observed when the temperature difference across the front is 9 degrees Fahrenheit (°F) or more and the front is moving at 30 knots or faster. Frontal shear is seen both vertically and horizontally and can occur at any altitude between surface and tropopause, which is the lowest portion of Earth’s atmosphere and contains 75 percent of the atmosphere’s mass. Those who study the effects of weather on aviation are concerned more with vertical wind shear above warm fronts than behind cold fronts because of the longer duration of warm fronts.[4]

The occurrence of wind shear is a microscale meteorological phe­nomenon. This means that it usually develops over a distance of less than 1 kilometer, even though it can emerge in the presence of large weather patterns (such as cold fronts and squall lines). Wind shear affects the movement of soundwaves through the atmosphere by bend­ing the wave front, causing sounds to be heard where they normally would not. A much more violent variety of wind shear can appear near and within downbursts and microbursts, which may be caused by thun­derstorms or weather fronts, particularly when such phenomena occur near mountains. Vertical shear can form on the lee side of mountains when winds blow over them. If the wind flow is strong enough, turbu­lent eddies known as "rotors” may form. Such rotors pose dangers to both ascending and descending aircraft.[5]

The microburst phenomenon, discovered and identified in the late 1970s by T. Theodore Fujita of the University of Chicago, involves highly localized, short-lived vertical downdrafts of dense cool air that impact the ground and radiate outward toward all points of the compass at high speed, like a water stream from a kitchen faucet impacting a basin.[6]

Speed and directional wind shear result at the three-dimensional boundary’s leading edge. The strength of the vertical wind shear is directly proportional to the strength of the outflow boundary. Typically, microbursts are smaller than 3 miles across and last fewer than 15 min­utes, with rapidly fluctuating wind velocity.[7]

Wind shear is also observed near radiation inversions (also called nocturnal inversions), which form during rapid cooling of Earth’s sur­face at night. Such inversions do not usually extend above the lower few hundred feet in the atmosphere. Favorable conditions for this type of inversion include long nights, clear skies, dry air, little or no wind, and cold or snow-covered surfaces. The difference between the inversion layer and the air above the inversion layer can be up to 90 degrees in direction and 40 knots. It can occur overnight or the following morn­ing. These differences tend to be strongest toward sunrise.[8]

The troposphere is the lowest layer of the atmosphere in which weather changes occur. Within it, intense vertical wind shear can slow or prevent tropical cyclone development. However, it can also coax thun­derstorms into longer life cycles, worsening severe weather.[9]

Wind shear particularly endangers aircraft during takeoff and land­ing, when the aircraft are at low speed and low altitude, and particularly susceptible to loss of control. Microburst wind shear typically occurs during thunderstorms but occasionally arises in the absence of rain
near the ground. There are both "wet” and "dry” microbursts. Before the developing of forward-looking detection and evasion strategies, it was a major cause of aircraft accidents, claiming 26 aircraft and 626 lives, with over 200 injured, between 1964 and 1985.[10]

Another macro-level weather event associated with wind shear is an upper-level jetstream, which contains vertical and horizontal wind shear at its edges. Jetstreams are fast-flowing, narrow air currents found at cer­tain areas of the tropopause. The tropopause is the transition between the troposphere (the area in the atmosphere where most weather changes occur and temperature decreases with height) and the stratosphere (the area where temperature increases with height).[11] A combination of atmo­spheric heating (by solar radiation or internal planetary heat) and the planet’s rotation on its axis causes jetstreams to form. The strongest jet – streams on Earth are the polar jets (23,000-39,000 feet above sea level) and the higher and somewhat weaker subtropical jets (33,000-52,000 feet). Both the northern and southern hemispheres have a polar jet and a subtropical jet. Wind shear in the upper-level jetstream causes clear air turbulence. The cold-air side of the jet, next to the jet’s axis, is where CAT is usually strongest.[12]

Although most aircraft passengers experience clear air turbulence as a minor annoyance, this kind of turbulence can be quite hazard­ous to aircraft when it becomes severe. It has caused fatalities, as in the case of United Airlines Flight 826.[13] Flight 826 took off from Narita International Airport in Japan for Honolulu, HI, on December 28, 1997.

At 31,000 feet, 2 hours into the flight, the crew of the plane, a Boeing 747, received warning of severe clear air turbulence in the area. A few minutes later, the plane abruptly dropped 100 feet, injuring many pas­sengers and forcing an emergency return to Tokyo, where one passenger subsequently died of her injuries.[14] A low-level jetstream is yet another phenomenon causing wind shear. This kind of jetstream usually forms at night, directly above Earth’s surface, ahead of a cold front. Low-level vertical wind shear develops in the lower part of the low-level jet. This kind of wind shear is also known as nonconvective wind shear, because it is not caused by thunderstorms.

The term "jetstream” is often used without further modification to describe Earth’s Northern Hemisphere polar jet. This is the jet most important for meteorology and aviation, because it covers much of North America, Europe, and Asia, particularly in winter. The Southern Hemisphere polar jet, on the other hand, circles Antarctica year-round.[15] Commercial use of the Northern Hemisphere polar jet began November 18, 1952, when a Boeing 377 Stratocruiser of Pan American Airlines first flew from Tokyo to Honolulu at an altitude of 25,000 feet. It cut the trip time by over one-third, from 18 to 11.5 hours.[16] The jetstream saves fuel by shortening flight duration, since an airplane flying at high altitude can attain higher speeds because it is passing through less – dense air. Over North America, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jetstream but can increase by more than 30 minutes it must fly against the jetstream.[17]

Strong gusts of wind are another natural phenomenon affecting avi­ation. The National Weather Service reports gusts when top wind speed reaches 16 knots and the variation between peaks and lulls reaches 9 knots.[18] A gust load is the wind load on a surface caused by gusts.

Otto Lilienthal, the greatest of pre-Wright flight researchers, in flight. National Air and Space Museum.

The more physically fragile a surface, the more danger a gust load will pose. As well, gusts can have an upsetting effect upon the aircraft’s flightpath and attitude.