Wind shear is a rapid, localized change in wind velocity, involving a shift in speed or direction over a short distance. This meteorological phenomenon is a serious hazard to aircraft, particularly during the low-speed, low-altitude phases of takeoff and landing. Sudden wind changes can drastically affect lift and airspeed. Hazardous wind shear is anticipated in several distinct environments, from the airport surface to cruising altitude.
Specific Low-Altitude Scenarios
Hazardous wind shear is frequently expected near the ground, typically below 2,000 feet, when the wind changes are non-convective, meaning they are not caused by thunderstorms. One of the most common low-altitude scenarios is the passage of a frontal system, such as a cold or warm front. Significant shear is expected when the temperature difference across the front is 5 degrees Celsius or more and the front is moving at a speed of at least 30 knots.
Temperature inversions also create conditions ripe for hazardous wind shear, especially during clear, calm nights. In this stable atmosphere, a layer of warm air sits above colder air near the ground, decoupling the wind layers. The resulting friction with the surface slows the lower layer while the air aloft maintains a faster speed, creating a pronounced wind gradient. This effect is sometimes magnified by a nocturnal low-level jet, a narrow band of fast-moving air that develops just above the inversion layer.
Mechanical turbulence is another expected source of low-level shear, often occurring when strong surface winds encounter large structures or complex terrain. Obstacles like terminals, hangars, hills, or mountain ridges disrupt the smooth flow of air, causing irregular eddies and currents. This mechanical disruption of the airflow creates localized, short-distance changes in wind speed and direction that can be challenging for aircraft operating close to the ground.
Wind Shear Associated with Convective Activity
Hazardous wind shear is expected whenever convective activity, such as thunderstorms, is present. Thunderstorms generate intense localized columns of sinking air known as downbursts, with microbursts being a smaller, highly concentrated type. A microburst, typically spanning only 2.5 to 4 kilometers in diameter, is dangerous because the air can descend at rates up to 6,000 feet per minute.
When this rapidly sinking air hits the ground, it spreads outward in all directions, creating extreme horizontal wind shear. An aircraft flying through this area will initially experience a strong headwind, followed by a sudden shift to a tailwind and a powerful downdraft. This rapid sequence causes an immediate loss of airspeed and lift, which can exceed the aircraft’s performance capability, especially during landing or takeoff.
The leading edge of this cool, spreading air is called the gust front, and hazardous shear is expected along its boundary. Gust fronts can travel miles away from the parent thunderstorm, providing a sudden wind shift that may be encountered even when the sky overhead appears clear. Wind shear is also expected with both wet microbursts, which are accompanied by heavy rain, and dry microbursts, where precipitation evaporates before reaching the ground, often indicated by visible streaks of rain called virga.
High-Altitude and Clear Air Turbulence Triggers
Hazardous wind shear is not limited to low altitudes but is also expected in the upper atmosphere, where it manifests as Clear Air Turbulence (CAT), which is not associated with visible clouds. One of the most common triggers for high-altitude shear is the jet stream, a ribbon of fast-moving air in the upper troposphere. Strong shear is expected along the edges, or “walls,” of the jet stream, where wind speed changes rapidly over short lateral distances.
The most intense CAT occurs on the side of the jet stream where the wind speed change is greatest, often affecting aircraft cruising at altitudes between 20,000 and 40,000 feet. Mountain wave activity also generates significant high-altitude wind shear, especially when strong winds blow perpendicular to a mountain range. These conditions cause standing atmospheric waves that can extend far downwind and high into the stratosphere.
The resulting standing waves produce strong vertical currents and localized horizontal wind shear, which can cause sudden airspeed oscillations of 15 knots or more. Strong wind shear from mountain waves is expected on the leeward side of the mountains, sometimes producing rotor action near the surface and turbulence at altitude. Forecasting these waves requires looking for strong winds at the ridge crest, exceeding 15 knots, coupled with an atmospheric temperature inversion.
Meteorological Indicators and Reporting
The expectation of hazardous wind shear is managed using specialized forecasting tools and real-time reporting systems. Meteorologists look for conditions such as steep lapse rates and strong temperature contrasts, which indicate a high potential for instability or frontal wind shear. Specialized radar systems, including Terminal Doppler Weather Radar (TDWR), are deployed at airports to detect the localized wind velocity changes characteristic of microbursts and gust fronts.
Ground-based systems, such as the Low-Level Wind Shear Alert System (LLWAS), use a network of anemometers across an airport to compare wind speeds and directions. This comparison provides air traffic control with real-time warnings when a hazardous wind shear is detected near the runways. Modern aircraft are also equipped with Predictive Wind Shear (PWS) systems that use onboard Doppler radar to scan the air ahead for velocity shifts, providing the pilot with an alert before an encounter.
Pilot Reports (PIREPS) are an important reporting mechanism for observed wind shear and turbulence at all altitudes. Official aviation weather products, such as Significant Meteorological Information (SIGMETs), are issued by forecasters to alert pilots to the expected presence of turbulence or other significant weather phenomena. These indicators, both technological and observational, are used to determine when a hazardous encounter is most likely.