Wind shear is a rapid change in wind speed or direction over a short horizontal or vertical distance. Around Los Angeles, wind shear forms where sea breeze fronts meet inland heat, where terrain funnels and redirects wind, where Santa Ana winds pour through mountain gaps and where thunderstorm downdrafts spread out at the ground. Pilots care most. A jet on final approach into Burbank or Van Nuys can lose or gain airspeed in seconds if it flies through a sharp shear layer a few hundred feet above the runway.
What Is Wind Shear?
Wind shear is any significant change in wind speed, direction or both over a short distance. The distance can be horizontal, like the boundary between a cool sea breeze and warm inland air. It can also be vertical, like the difference between wind at the surface and wind a few hundred feet up.
Horizontal shear concentrates rotation along a line, which matters for storms. Vertical shear changes the wind an aircraft, balloon or tall structure feels as it moves up or down.
Wind shear ranges from gentle to violent. A slow overnight shift from onshore to offshore wind is technically shear; so is the violent wind reversal beneath a collapsing thunderstorm. Shear becomes a hazard when the change is fast compared with the size and speed of whatever moves through it. An airliner climbs or descends a long way in seconds during takeoff or landing, so aviation defines shear by airspeed loss rather than wind speed alone.
| Shear source | Typical trigger | Where it shows up locally | Typical impact |
|---|---|---|---|
| Sea-breeze front | Onshore flow meeting warm inland air | Coastal approach paths, LAX, Santa Monica | Sudden shift in headwind to crosswind or tailwind |
| Terrain-channeled flow | Mountain gaps and canyon funneling | Burbank, Van Nuys, Santa Ana wind events | Gusty, direction-variable low-level wind on approach |
| Frontal passage | Cold or warm front crossing the basin | Basin-wide, strongest near the surface | Rapid wind direction swing over minutes to an hour |
| Thunderstorm microburst | Collapsing convective downdraft | Inland valleys and basin during summer convection | Severe, short-lived headwind-to-tailwind reversal near the ground |
| Nocturnal low-level jet / inversion | Stable layer decoupling surface and above-surface wind | Basin mornings and evenings, light-wind periods | Vertical shear layer a pilot flies through on climb-out |
What Creates Wind Shear Around Los Angeles?
Wind shear around Los Angeles comes from several sources that often overlap on the same afternoon. The most common is the sea breeze front, the boundary where cool, moist Pacific air meets air that has heated over the basin and inland valleys all day. As that front pushes inland, usually in the afternoon, surface wind can swing 90 degrees or more within a few miles. The transition zone is a band of active shear, not a clean line.
A hot inland day pulls the sea breeze front through coastal airports earlier and with a sharper wind change than a mild day, because the front's strength depends on how much the basin heats compared with the ocean. For how wind, fog and storms delay flights, see LAX weather delays.
Terrain adds a second source. The San Gabriel and Santa Monica Mountains and passes like Cajon and Newhall channel and speed up the wind, so wind near them can differ sharply from open parts of the basin a few miles away. The effect is strongest during Santa Ana winds, when dry wind pours downhill from the mountains and deserts toward the coast, gusting near the foothills while the coast stays calmer. A ridge or canyon can also create a rotor, a band of turbulent, reversing wind on its downwind side.
Winter cold fronts add a third source, swinging the basin's wind direction within an hour or less as colder air pushes through. Thunderstorm downdrafts add a fourth and more violent one when storms develop, usually in the warm season over inland valleys and mountains. Low-level jets and nighttime temperature inversions add a fifth: a stable layer near the ground can separate the wind above it from the wind at the surface.
On a calm, clear night, surface wind can drop to almost nothing while a narrow band of much faster wind flows a few hundred feet up. That vertical shear is easy to miss from the ground but very real to an aircraft climbing or descending through it at dawn or dusk.
Why Is Low-Level Shear Dangerous to Aircraft?
Low-level wind shear is dangerous because an aircraft's lift depends on airspeed, and airspeed changes abruptly when the wind does, often near the ground where there is little room to recover. On approach, flying from a headwind into a tailwind cuts airspeed and lift at once, which can drop the plane below its glide path unless the crew reacts fast. Flying from a tailwind into a headwind suddenly raises airspeed and can push the plane above the glide path. Both changes are far more dangerous a few hundred feet above a runway than at cruising altitude.
Takeoff carries a similar risk. A climbing aircraft that loses its headwind, or meets a tailwind, has little spare airspeed and engines already near full power. That is why low-level wind shear is the specific hazard aviation authorities watch near airports.
The National Weather Service and the Federal Aviation Administration treat it as its own forecast and alert category. The concern is lift during takeoff and landing, when aircraft are slowest and closest to the ground.
When Are Southern California Airports Vulnerable?
Southern California airports face different shear risks by season, time of day and location. Coastal airports, including LAX and Santa Monica, get the most sea breeze shear on warm afternoons, when the marine push inland can arrive as a sharp boundary. Airports near the mountains and passes, including Burbank and Van Nuys, get more terrain-driven shear. During Santa Ana events, wind pouring off the San Gabriels can gust and shift direction over the runways while nearby areas look calm.
San Fernando Valley communities such as Woodland Hills, directly in the path of that downhill wind, often feel the same gusty, shifting wind at ground level that an aircraft meets on approach.
Cool-season fronts bring shear across the whole basin as a storm's winds rotate through, usually over an hour or two. Santa Ana events cluster in fall and winter, when strong high pressure over the Great Basin pushes dry air through the passes toward the coast. Terrain-driven shear at Burbank and Van Nuys therefore varies through the year.
Summer and early-fall afternoons bring the least frequent but most intense shear: outflow from thunderstorms over inland valleys and mountains. A microburst's straight-line downdraft changes the wind far more abruptly than a sea breeze or front. No Southern California airport is immune, but each faces a different mix of risks at different times. Pilots and dispatchers treat coastal, basin and mountain-side airports as different operating environments.
How Is Shear Different From Turbulence or a Microburst?
Wind shear, turbulence and a microburst are related but different. Wind shear is the change in wind speed or direction across a distance. Turbulence is the bumpy ride an aircraft gets in disturbed air, which shear can cause but so can rising air currents, mountain waves or another plane's wake. A microburst is a short, concentrated column of sinking air from a thunderstorm that spreads out in straight lines at the ground, creating intense local shear.
Every microburst creates wind shear, but not all wind shear comes from a microburst, and not all turbulence involves shear. A sea breeze front creates real horizontal shear without a microburst or much turbulence. A mountain wave downwind of the San Gabriels can create strong turbulence with only a gradual change in average wind. Microbursts have their own detection systems and warnings because they are so sudden and severe, while low-level wind shear advisories cover the broader category.
For how thunderstorm downdrafts form locally, see microbursts in Southern California thunderstorms. For wind shear's role in tornadoes, see can tornadoes hit Southern California.
How Is the Hazard Detected?
Low-level wind shear is detected with ground sensors, radar and pilot reports together. Many large U.S. airports use a Low-Level Wind Shear Alert System (LLWAS), a network of wind sensors around the airfield that compares readings to flag developing shear or microbursts in real time. Terminal Doppler Weather Radar, where installed, detects an approaching storm's wind field before it reaches the airport, giving controllers and pilots more warning.
Pilot reports, called PIREPs, are one of the most valuable sources of shear information. A crew that feels a sudden airspeed change on approach radios it to air traffic control for the planes behind it.
Routine airport weather observations, called METARs, record surface wind at regular intervals and flag rapid changes. The National Weather Service Los Angeles/Oxnard office and the National Weather Service San Diego office issue forecasts and advisories for conditions that favor low-level shear, such as an approaching front, a Santa Ana setup or a thunderstorm outlook. The Federal Aviation Administration and airlines add their own thresholds and pilot training. During an event, sensors, radar, pilot reports and advisories together give controllers and crews a real-time picture.
Wind shear is invisible: the sky can look calm while the wind a few hundred feet up, or a few miles inland, changes fast enough to matter to an aircraft. If you are flying in or out of a Southern California airport during a Santa Ana event, an approaching front or a summer thunderstorm outlook, check the airport's observations and FAA or airline status pages. Shear is very local, and a basin-wide wind forecast cannot capture it.
WeatherLA's forecasts for Downtown Los Angeles, Pasadena, Burbank and Van Nuys give general wind conditions across the region. For a specific flight, use the specific airport's sources.