WeatherWise Scott Sumner

Wind shear comes into play at low altitudes

Wind shear is a change in wind speed or direction, either up or down, over a short distance. Turbulence, a form of wind shear, is present at high altitudes. Low-level wind shears, the focus of this article, is the phenomena that creates danger for pilots when they are landing or taking off.

Four factors that can initiate wind shear include new weather fronts, thunderstorms, temperature inversions and surface obstructions.

If a cold front is approaching a runway and is strong enough to create wind shear, it would have two characteristics: First, a 10-degree Fahrenheit (5-degree Celsius) or more temperature difference, and second, a speed of at least 30 knots, or 35 mph.

The appearance of wind shear is most common during thunderstorms. The typical afternoon or evening thunderstorms can blossom as high as 50,000 feet. As the temperatures climb throughout the day air rises. As the air cools to a certain point (called the dew point) condensation begins and clouds form. When the water droplets that form the cloud become big enough they fall, dragging down the air.

This dragging down of the air forms what is know as “downburst.” A downburst can change wind direction as much as 180 degrees. Further away from the initial downburst of a thunderstorm, lies a second problem – something called a “gust front” or “first gust.” As air flows downward from the thunderstorm and slams into the ground, it spreads out and can travel for miles at high rates of speed. This leading edge of cooler air forms a miniature cold front.

Temperature inversions (cold air aloft, warm air below it) coupled with strong winds from what is known as the low-level jet stream, can produce significant wind shear close to the ground.

Lastly, buildings and mountain ranges close to an airport can cause localized wind shear during takeoffs and landings. As air flows over a mountain range it cools as it rises and heats as it falls.

This constant temperature fluctuation creates what is called “turbulent eddies.” An “eddy” is defined as a change in the steady flow of a fluid (in this case, air), causing a vortex or swirling motion.

With the advance of today’s technology there have been fewer accidents related to wind shear.

Scott Sumner is a member of the American Meteorological Society.

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