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Those Shifty Winds

Gemini Sparkle

Key Takeaways:

  • The article defines "veering" (clockwise wind change) and "backing" (counter-clockwise) with height or time, attributing these shifts primarily to thermal advection (warm air causes veering, cold air backing with height) and friction.
  • Within the planetary boundary layer (lowest few thousand feet), friction causes winds to decelerate and back with decreasing height (turn left in the Northern Hemisphere), typically resulting in a 20-30 degree shift.
  • However, chaotic mixing and turbulence, especially in actively heated boundary layers, often produce rapid, unpredictable wind shifts of 20-40 degrees in mere seconds, frequently overwhelming the more orderly friction and thermal effects.
  • Pilots descending into a deep boundary layer should generally anticipate a decrease in wind speed and subtle backing, but must be prepared for significant, random fluctuations caused by turbulence and the complex interaction of atmospheric forces.
See a mistake? Contact us.

An excellent article by editor Frank Bowlin on ILS technique appeared in the February issue. A small error led to some confusion, and IFR embarked on a quest for accuracy. That quest got stymied with some faulty background. We thank astute readers for returning us to the righteous path. I’ll now clear away all confusion and help you understand in great detail which way the wind shifts, and why.

Definitions
When talking about wind shear, it’s important to be thoroughly familiar with the terms “veering” and “backing”. Veering is a clockwise change in the wind direction, normally in terms of increasing height or with time. Good practice is to carefully qualify all measures of veering to avoid confusion. For example if the winds are from the south at the surface and from the west aloft, that’s described as a wind profile that “veers with height.”

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