Chapter 11: Weather Theory - FAA

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W I N DFigure 11-15. Turbulence caused by manmade obstructions.may “drop in” due to the turbulent air and be too low to clearobstacles during the approach.This same condition is even more noticeable when flying inmountainous regions. [Figure 11-16] While the wind flowssmoothly up the windward side of the mountain and theupward currents help to carry an aircraft over the peak ofthe mountain, the wind on the leeward side does not act ina similar manner. As the air flows down the leeward side ofthe mountain, the air follows the contour of the terrain andis increasingly turbulent. This tends to push an aircraft intothe side of a mountain. The stronger the wind, the greater thedownward pressure and turbulence become.Due to the effect terrain has on the wind in valleys or canyons,downdrafts can be severe. Before conducting a flight in ornear mountainous terrain, it is helpful for a pilot unfamiliarwith a mountainous area to get a checkout with a mountainqualified flight instructor.Figure 11-16. Turbulence in mountainous regions.11-10
Low-Level Wind ShearWind shear is a sudden, drastic change in wind speed and/ordirection over a very small area. Wind shear can subject anaircraft to violent updrafts and downdrafts, as well as abruptchanges to the horizontal movement of the aircraft. Whilewind shear can occur at any altitude, low-level wind shear isespecially hazardous due to the proximity of an aircraft to theground. Directional wind changes of 180° and speed changesof 50 knots or more are associated with low-level wind shear.Low-level wind shear is commonly associated with passingfrontal systems, thunderstorms, and temperature inversionswith strong upper level winds (greater than 25 knots).Wind shear is dangerous to an aircraft for several reasons. Therapid changes in wind direction and velocity change the wind’srelation to the aircraft disrupting the normal flight attitude andperformance of the aircraft. During a wind shear situation,the effects can be subtle or very dramatic depending on windspeed and direction of change. For example, a tailwind thatquickly changes to a headwind causes an increase in airspeedand performance. Conversely, when a headwind changesto a tailwind, the airspeed rapidly decreases and there is acorresponding decrease in performance. In either case, apilot must be prepared to react immediately to the changesto maintain control of the aircraft.In general, the most severe type of low-level wind shearis associated with convective precipitation or rain fromthunderstorms. One critical type of shear associated withconvective precipitation is known as a microburst. A typicalmicroburst occurs in a space of less than one mile horizontallyand within 1,000 feet vertically. The lifespan of a microburstis about 15 minutes during which it can produce downdraftsof up to 6,000 feet per minute (fpm). It can also produce ahazardous wind direction change of 45 degrees or more, ina matter of seconds.When encountered close to the ground, these excessivedowndrafts and rapid changes in wind direction canproduce a situation in which it is difficult to control theaircraft. [Figure 11-17] During an inadvertent takeoff intoa microburst, the plane first experiences a performanceincreasingheadwind (1), followed by performance-decreasingdowndrafts (2). Then, the wind rapidly shears to a tailwind(3), and can result in terrain impact or flight dangerouslyclose to the ground (4).Microbursts are often difficult to detect because they occurin relatively confined areas. In an effort to warn pilots oflow-level wind shear, alert systems have been installed atseveral airports around the country. A series of anemometers,placed around the airport, form a net to detect changes inwind speeds. When wind speeds differ by more than 15 knots,a warning for wind shear is given to pilots. This system isknown as the low-level wind shear alert system (LLWAS).It is important to remember that wind shear can affect anyflight and any pilot at any altitude. While wind shear may bereported, it often remains undetected and is a silent dangerto aviation. Always be alert to the possibility of wind shear,especially when flying in and around thunderstorms andfrontal systems.Wind and Pressure Representation on SurfaceWeather MapsSurface weather maps provide information about fronts, areasof high and low pressure, and surface winds and pressuresfor each station. This type of weather map allows pilots toStrong downdraftIntended PathIncreasing headwindIncreasing tailwind2OutflowOutflow134Figure 11-17. Effects of a microburst wind.11-11
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Chapter 11: Weather Theory - FAA

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