Any Way the Wind Blows

What is atmospheric wind shear and how do atmospheric conditions result in the generation of turbulence? How does it affect pilots?

Wind shear is where the wind changes speed or direction abruptly. When the wind encounters a solid object, a whirl of air called an eddy forms on the object’s downwind side. The size and shape of the eddy often depend on the size and shape of the obstacle and on the speed of the wind. Roll eddies, which are usually produced from winds that move over a mountain range and have lots of vertical motion, resulting in the most turbulence. Eddies can also form in clear air and result in clear air turbulence. Air pockets are areas of descending air that cause turbulence, and cause aircraft to "drop" when they fly through one. Vertical wind shear is the change in wind speed or direction with altitude and horizontal wind shear is the change in wind speed with a change in lateral position at a certain altitude. In stable air with weak winds, the eddy is nonexistent or small. As wind speed and surface heating increase, instability develops, and the eddy becomes larger and extends through a greater depth... In stable air, light winds produce small eddies and little vertical mixing. Greater winds in unstable air create deep, vertically mixing eddies that produce strong, gusty surface winds (Ahrens and Henson, 2018).

Eddies that form on the downwind of a mountain in a wind shear zone result in horizontal disturbance of airflow and produce turbulence. If there is enough moisture present in the air, billow clouds or lenticular clouds can indicate the presence of turbulence. Clear air turbulence happens at higher altitudes, where the jet stream is, and there are usually no visual clues to help detect the presence of it. Clear air turbulence is generated by rising mountain-produced turbulence and by the collision of different bodies of air moving at different speeds.

Clear air turbulence can cause harm to jets and commercial aircraft because it generally is present at higher altitudes, which is the level at which those aircraft fly.

In Florida, an 84-year-old man died due to serious injuries that were caused by a turbulent flight. The clear-air turbulence struck at 31,000 feet when the DC-9 was approaching Palm Beach International Airport for a landing. Thirteen people required medical assistance and four of them were seriously injured. Passenger Betty Miller of Miramar said, “It was like a whiplash on the end of something. It felt like we bought the farm.” Another passenger, Alison Levins, 33, of Boston, said the turbulence lasted about 30 seconds (AP News, 1990).

References

Ahrens, C. D., & Henson, R. (2018). Meteorology Today: Introductory Weather Climate & Environment, 12th Edition. Mason, OH: Cengage Learning.

AP News. (1990, October 25). Man dies from turbulent flight injuries. https://apnews.com/article/3a3be4ab88697034003f3746600d1e28

Zhang, Y. (Director). (2020, May 13). Clear Air Turbulence 2020-5-10 [Video file]. https://youtu.be/qs6YCY31hXY