What wind shear is
Wind shear is any rapid change in wind velocity — direction, speed, or both — over a short distance in the atmosphere. The change can be horizontal (across a front or gust front boundary), vertical (with altitude through a temperature inversion), or both simultaneously.
The threat is not the wind itself — it is the change in wind faster than the aircraft can respond. An aircraft's airspeed is what produces lift. Groundspeed is the relationship between airspeed and wind. If you're on final at 70 KIAS with a 15-knot headwind (85 knots of groundspeed) and the wind instantaneously reverses to a 15-knot tailwind, your indicated airspeed drops to 55 knots — with no input on your part, in the time it takes to cross the wind shear boundary. At 300 ft AGL in landing configuration, 15 knots of margin is gone before you can react.
FAA definition: The AIM and FAA define low-level wind shear (LLWS) as any rapid change in wind direction or velocity below 2,000 ft AGL. Events below 1,000 ft AGL during approach and departure are the most operationally hazardous because altitude for recovery is minimal.
Low-level wind shear
Low-level wind shear has four primary causes. Knowing the cause lets you anticipate it before you encounter it.
Frontal wind shear
At a frontal boundary, two air masses with different temperatures, densities, and wind fields meet. The shear zone at the boundary is sharp — wind direction can shift 60–90° and speed can change 15–30 kt almost instantaneously as a front passes through the airport. A VFR pilot who departs behind a cold front into clear, stable air and then turns around to land back at the departure airport may cross the frontal boundary on final with no warning. Check the surface analysis and METAR trends. A METAR showing a 70° wind shift between two consecutive observations means the front crossed the field in that window.
Temperature inversion shear
On calm clear nights, the surface cools by radiation. This creates a temperature inversion — a stable layer where temperature increases with altitude rather than decreasing. Below the inversion, air is light and calm. Above it, moderate winds blow undisturbed. The interface between the two is a shear zone. Departing aircraft climb through it; arriving aircraft descend through it. The transition is abrupt — one moment there's calm air, the next the aircraft is in a 20-knot headwind that shifts the airspeed sharply before the autopilot or pilot can trim out the change.
Thunderstorm gust front
A thunderstorm downdraft hitting the surface spreads outward as a gust front. This cold outflow can extend 10–15 nm ahead of the visible storm. At the gust front boundary, winds can shift 90° or more and increase by 20–40 kt in seconds — and the gust front can arrive before the rain, before the thunder is overhead, and while the airport is still in VMC. Check METAR remarks for WSHFT (wind shift) and FROPA (frontal passage), and look at the radar before descent. A cell 18 nm to your northeast moving toward the field is a gust front threat before it's an overcast threat.
LLWAS — Low-Level Wind Shear Alert System
About 110 major U.S. airports are equipped with a Low-Level Wind Shear Alert System (LLWAS). LLWAS uses an array of anemometers around the airport perimeter and centerfield to detect differential wind speed across the network. When a threshold differential is detected, ATC broadcasts the alert: "Wind shear alert, east boundary wind 040 at 28 knots, centerfield wind 180 at 10 knots."
At LLWAS airports, these alerts are issued in real time. At airports without LLWAS, you are relying on PIREPs from preceding aircraft, your own preflight products, and ATC advisories. If a wind shear alert is issued for your approach, treat it the same as a SIGMET: plan to execute a go-around before entering the shear zone, not after you're in it.
Microbursts
A microburst is an intense, small-scale downdraft that diverges radially outward at the surface. It is the most dangerous form of wind shear in aviation — not because it's the most common, but because it is compact (1–3 nm diameter), brief (10–20 minutes), can produce surface wind differentials exceeding 45 knots across the cell, and the phase sequence of the encounter is exactly backwards from the instinctive pilot response. The downdraft itself can reach 6,000 ft/min — an aircraft in a 500 ft/min descent on a stabilized approach has no realistic means of climbing out of a downdraft moving at 6,000 ft/min.
Wet vs. dry microbursts
Wet microbursts form under thunderstorm cells producing heavy precipitation. The downdraft is driven by rain-cooled, dense air. Heavy rain is visible and the hazard is co-located with obvious convective activity — at least the pilot knows something is wrong overhead.
Dry microbursts are more treacherous. They occur in arid climates — the Rocky Mountain west, high desert — when a thunderstorm produces a heavy rain shaft that evaporates completely before reaching the ground. This evaporation cools the air dramatically, intensifying the downdraft. At the surface, the sky may be hazy but clear. There is no rain. The only visual clue is virga: a rain shaft that evaporates visibly before reaching the ground. Any virga plus a surface wind change or dust cloud beneath it means a dry microburst may be in progress. Do not approach.
Response to a microburst alert
The FAA-recommended action when a microburst is reported on the approach corridor: do not attempt the approach. If already on approach and an alert is issued: immediately execute a go-around. Rotate to the go-around attitude, apply maximum continuous thrust, and do not allow airspeed to become the priority — attitude and thrust first. Accept the airspeed deviation. The aircraft needs altitude and thrust, not an on-speed condition.
The AIM advises delaying the approach until the microburst dissipates (typically 10–20 minutes from first detection). A microburst strong enough to be detected by LLWAS or reported by an arriving aircraft is strong enough to make the approach unsurvivable.
Mountain wave
Mountain wave (also called orographic wave or lee wave) forms when stable air flows over a mountain ridge and is set into oscillation on the downwind side. The wave is stationary relative to the terrain — it doesn't move with the wind. This explains why lenticular clouds formed in mountain wave appear frozen in place even in 50-knot winds aloft.
The hazard extends far beyond the mountains themselves. Standing waves can persist 100–300 nm downwind of the generating ridge and can reach above 50,000 ft in severe events. A pilot 200 nm east of the Rockies on a clear day with no terrain in sight can still encounter severe mountain wave turbulence — and does, regularly.
Formation conditions
Three conditions are required for significant mountain wave:
- Wind speed ≥ 25 knots at or above ridge height. Speeds of 40+ kt produce severe wave; speeds below 15 kt rarely sustain meaningful wave activity.
- Wind direction within 30° of perpendicular to the ridge. Flow parallel to the ridge doesn't generate coherent standing waves — the wave must have a cross-ridge component.
- Stable layer at or above ridge height, with less stable air above and below. The stable layer acts as a "lid" that forces the oscillation to remain coherent rather than dispersing immediately.
No clouds ≠ no wave. In dry air with low relative humidity, there is no moisture to condense into lenticular or cap clouds. The wave is structurally identical to a moist wave event — same oscillation amplitude, same rotor zone turbulence intensity — but completely invisible. The only indicators are the winds aloft forecast, the AFD, and PIREPs. Never infer clear conditions from a clear sky when the setup conditions are present.
Zone 1 — The cap cloud and crest
Air forced up the windward side of the ridge produces orographic lifting. On the windward face this is relatively smooth. At the crest, descent begins steeply on the lee side. The cap cloud (foehn wall) forms at the peak as moisture condenses on the windward face and evaporates on the lee side. Turbulence at and just past the crest can be severe but is predictable in location — it's right over the ridge.
Zone 2 — The rotor zone
Below the first wave trough on the lee side, at or below ridge height, lies the rotor zone. This is the most dangerous zone for aircraft. The descending wave forces air to circulate back upwind near the surface while the main wave flow continues downwind above — creating a rolling, chaotic region of extreme turbulence. Turbulence intensities of extreme (beyond severe, capable of causing structural damage) are possible in strong wave events in the rotor.
The visual signature: below the smooth, sculpted lenticular clouds above, ragged, disorganized cumulus or stratocumulus clouds appear near the surface. These rotor clouds look as though they are being shredded. A pilot flying toward a mountain pass and seeing smooth lenticular clouds ahead should recognize that the rotor zone is directly below those clouds, near the surface, in the corridor they plan to transit.
Zone 3 — Upper-level wave turbulence
Above the rotor zone, the wave undulations themselves produce turbulence — typically moderate to severe at wave crests and in troughs. For GA aircraft, the primary concern is at cruise altitudes crossing major terrain: 10,000–18,000 ft over the Rockies or Sierra Nevada in active wave. The ride through a trough can be abrupt and structural, even when the flight levels above appear smooth.
For high-altitude operations: mountain wave extends to the tropopause and in extreme events above it. Jet aircraft encountering CAT east of the Rockies at FL350 are often in mountain wave — there is no visual indicator. The GTG (Graphical Turbulence Guidance) product and AIRMET/SIGMET coverage are the primary tools at those altitudes.
Products to check
AIRMET Tango
AIRMET Tango covers moderate turbulence (non-convective), sustained surface winds ≥30 kt, and non-convective low-level wind shear. Mountain wave turbulence and LLWS from inversions and fronts are both covered under Tango. Valid 6 hours; updated every 6 hours or as needed. G-AIRMETs at aviationweather.gov provide graphical versions with more frequent updates and altitude-selectable overlays — use these to visualize the geographic and vertical footprint relative to your route.
If mountain wave is severe enough to be forecast as severe turbulence (rather than moderate), it escalates to a SIGMET. A SIGMET for severe turbulence over your planned route is a hard stop for most GA operations.
Winds aloft forecast (FB)
The FB is the setup product for mountain wave. Check the wind at the ridge-height altitude level for the terrain you're crossing. Winds ≥25 kt within 30° of perpendicular to the ridge create the conditions for wave formation. Winds ≥40 kt at ridge height with the right direction almost guarantee a significant wave event.
Compare the FB to the AFD for the NWS forecast office covering your route. When mountain wave is expected to be significant, the forecaster will say so explicitly in the AFD synopsis and aviation section — including their confidence level and which altitude layers are most affected.
PIREPs
PIREPs are the only real-time observations of wave turbulence from inside the affected airspace. A cluster of MOG TRB or SVR TRB PIREPs at 12,000 ft over a mountain corridor means the Tango is validated by actual aircraft. Absence of PIREPs does not mean clear conditions — it may mean no aircraft have flown through the area recently, which is itself meaningful in a popular corridor.
Read the full PIREP: where exactly (relative to terrain), what altitude, what time, and what the aircraft type was. A turbulence PIREP from a heavy turboprop at 18,000 ft may not be representative of the rotor zone a light single would encounter at 9,000 ft.
ATIS wind shear advisories
When LLWAS detects a threshold differential, or when PIREPs within the past 30 minutes report wind shear on the approach, ATC includes the advisory in the ATIS. The advisory typically includes the reported airspeed gain or loss: "Wind shear advisory, 20-knot loss on final." This is not a general caution — it means a pilot in a similar configuration reported a 20-knot loss. Plan for it explicitly on your approach. If you encounter it, execute the missed approach immediately.
METAR remarks
In the METAR remarks section, look for: WSHFT (wind shift at a specific time), FROPA (frontal passage), VRB winds (variable direction — possible frontal or outflow boundary nearby). A METAR with VRB winds and gusting to 30+ kt is not just turbulence — it's a sign of a nearby shear zone.
Red flags
Lenticular clouds visible
ACSL or CCSL clouds stationary in the lee of terrain confirm active mountain wave. The rotor zone is directly below those smooth, sculpted clouds, near the surface, in the corridor you may be planning to transit. Do not fly below lenticular clouds to get through a pass.
Virga over the airport
Virga — rain that evaporates before reaching the ground — indicates dry microburst conditions are possible. Any surface wind change, dust cloud, or gust front activity beneath virga means a microburst may be in progress. Hold, divert, or wait for the cell to dissipate. Do not attempt the approach.
Rapid METAR wind change
A wind direction shift ≥60° or speed change ≥15 kt between consecutive METARs (or in SPECI remarks) indicates a frontal passage or gust front at the field. Compare the most recent 2–3 METAR observations to understand the rate of change — a front moving fast enough to shift 60° in 30 minutes will shift again during your approach.
FB perpendicular to terrain
Winds aloft ≥25 kt at ridge height, within 30° of perpendicular to the ridge, are the setup condition for mountain wave. Cross-check with AIRMET Tango status and PIREPs. At ≥40 kt, treat significant wave as confirmed unless a recent PIREP along the route reports smooth conditions at your planned altitude.
Convection within 20 nm
A thunderstorm cell within 20 nm of your route can place its gust front over your airport before the cell itself arrives. The outflow can precede the storm by 10–15 nm. Confirm current winds with ATIS, look at the radar for cell movement, and request a current wind check from ATC on approach if conditions are changing.
ATIS wind shear advisory
Any ATIS wind shear advisory should be treated as verified — it means either LLWAS detected a threshold differential or a preceding pilot reported the shear. Determine whether the source (LLWAS or PIREP) is still valid for your arrival time and plan the approach assuming the shear will be there. Have the go-around committed before you start the approach, not when the airspeed drops.
Checkride questions
Q: What is wind shear and why is it most dangerous at low altitude?
Wind shear is a rapid change in wind speed or direction over a short distance. It is most dangerous below 2,000 ft AGL because at low altitude the aircraft is in a low-energy state (low airspeed, flaps deployed on approach, or climbing out at Vx/Vy on departure) and there is insufficient altitude to recover from an upset or loss of control. A 20-knot headwind-to-tailwind reversal on short final at 300 ft AGL may leave no time or altitude for recovery even with immediate and correct pilot response. (AIM 7-1-26)
Q: Describe the sequence of events when an aircraft encounters a microburst on approach.
Phase 1 — headwind: entering the outflow boundary, the aircraft encounters a headwind and airspeed increases. The instinctive pilot response is to reduce power to recapture the glidepath. Phase 2 — downdraft: the aircraft enters the core of the microburst where there is a strong downdraft (up to 6,000 ft/min) — altitude is lost rapidly regardless of power setting. Phase 3 — tailwind: exiting the downwind side of the microburst, a tailwind causes a sudden, severe airspeed loss. The aircraft is now low, slow, near idle, and below the glidepath. Recovery from this energy state at low altitude may be impossible. The go-around must be initiated before entering the cell. (AC 00-6B §11.10; AIM 7-1-26)
Q: What is a dry microburst and where does it typically occur?
A dry microburst forms in arid environments when a thunderstorm produces a heavy rain shaft that evaporates completely before reaching the surface. The evaporation cools the air, intensifying the downdraft. At the surface there is little or no rain, and the sky above the airport may appear hazy but clear. The visual cue is virga — a visible rain shaft that tapers and ends above the ground. Any surface wind change, dust plume, or gust beneath virga means a dry microburst is possible. Common in the Rocky Mountain West, the high desert Southwest, and the Great Plains during summer afternoons. (AC 00-6B §11.10)
Q: What three conditions are required for significant mountain wave?
1. Wind speed of 25 knots or more at or above ridge height. 2. Wind direction within approximately 30° of perpendicular to the ridge. 3. A stable layer at or above ridge height, with less stable air above and below — this stable layer acts as a waveguide that keeps the oscillation coherent as it propagates downwind. If any of the three conditions is absent, the wave will be weak or will not form. (AC 00-6B Ch. 11)
Q: What is the rotor zone and where is it located?
The rotor zone is a region of violently turbulent, circulating air on the lee side of a mountain, at or below ridge height, beneath the first wave trough. Surface air in the rotor zone moves back toward the mountain (upwind) at low altitude while the main wave airflow above moves downwind — creating a rotating, chaotic circulation. It is the most dangerous zone in a mountain wave system. Turbulence intensity can be extreme. The visual indicator is ragged, irregular cumulus clouds below the lenticular clouds above; in dry air, there is no visual indicator. (AC 00-6B §11.5)
Q: What are lenticular clouds and what do they indicate?
Lenticular clouds (Altocumulus Standing Lenticular — ACSL) are smooth, lens-shaped clouds that appear stationary in the sky despite strong winds. They form at the crests of mountain waves — air continuously flows through the cloud, condensing on the upwind edge and evaporating on the downwind edge. The cloud appears stationary because the wave itself is stationary relative to the terrain. Their presence confirms active mountain wave. Rotor zone turbulence is directly below them, near the surface. In dry air, mountain wave can be equally severe with no clouds at all. (AC 00-6B §11.3)
Q: Which AIRMET covers mountain wave turbulence and LLWS?
AIRMET Tango covers moderate turbulence (non-convective), sustained surface winds ≥30 kt, and non-convective low-level wind shear. Valid 6 hours; updated every 6 hours or as needed. If turbulence escalates to severe, a SIGMET is issued. G-AIRMETs at aviationweather.gov show the turbulence area graphically with altitude selectors. For airport-level LLWS, check the ATIS for LLWAS alerts and consult recent PIREPs from preceding arrivals. (AC 00-45H §7.5)
Q: What is LLWAS and how does it protect arriving aircraft?
The Low-Level Wind Shear Alert System (LLWAS) uses a network of anemometers positioned around the airport — at the perimeter and centerfield — to detect wind differentials across the field. When the system detects a threshold differential (typically 15 kt or more), ATC broadcasts a wind shear alert on ATIS and issues it to arriving aircraft: "Wind shear alert, northwest boundary wind 310 at 30, centerfield wind 200 at 8." Approximately 110 major U.S. airports are equipped. At airports without LLWAS, wind shear detection depends on PIREPs from preceding aircraft. (AIM 7-1-26)
Would-You-Fly scenario
Educational example only — this teaches the questions a pilot should ask, not a specific flight decision
You're planning a VFR cross-country from KDEN to KGJT (Grand Junction, CO) — approximately 175 nm crossing the Colorado Rockies. The route takes you over terrain up to 14,000 ft MSL. Your aircraft is a Cessna 172S with service ceiling of 14,000 ft. The briefing shows the following:
- Winds aloft at 18,000 ft: 290° at 54 kt. At 12,000 ft: 285° at 38 kt.
- AIRMET Tango active across the Colorado Rockies, surface to FL240. "Moderate to occasionally severe turbulence in mountain wave."
- AFD from BOU (Boulder NWS): "Strong upper-level flow nearly perpendicular to the Continental Divide. Significant mountain wave expected. Lenticular activity confirmed by GOES satellite this morning. Several PIREPs report turbulence in the rotor zone in the 9,000–13,000 ft band east of the Continental Divide."
- PIREPs from the KDEN–KGJT corridor (filed within 2 hours): two reports of MOG TRB at 12,000 ft; one report of SVR TRB at 9,500 ft with a 2,000-ft altitude deviation over the Sawatch Range.
- Current KGJT METAR: 10SM CLR, 320 at 26 kt gusting 44 kt, 18/M03.
What are the go/no-go considerations?
- Is this a severe wave event? 54 kt at 18,000 ft nearly perpendicular to the Continental Divide is the upper end of the wave-generation spectrum. The AFD explicitly says "significant mountain wave" and a PIREP documents severe turbulence with a 2,000-ft altitude deviation. For a light single, 2,000 ft of uncommanded altitude deviation is a structural threat, not just an uncomfortable ride. Severe mountain wave can exceed the structural limits of a Cessna 172.
- Is there a safe altitude available? The service ceiling is 14,000 ft. The terrain is up to 14,000 ft. The AIRMET extends to FL240. There is no altitude within this aircraft's range that is above the terrain and potentially outside the AIRMET coverage. In mountain wave, flying higher doesn't necessarily mean flying in smoother air — the wave can be most severe at mid-levels.
- Are the PIREPs recent enough to be valid? Filed within 2 hours, in a slowly evolving synoptic pattern (strong upper ridge), these PIREPs are still operationally current. They confirm the Tango is real and that the corridor has active wave turbulence right now.
- What does the destination METAR tell you? KGJT gusting 44 kt from 320° means mountain wave is reaching the surface at the destination. This is not a routine gusty afternoon — 44-knot gusts at a valley airport on the lee side of the Rockies indicate the wave is energetically active all the way to the surface, which is also where you plan to land.
No-go. This flight combines: severe turbulence documented by PIREPs in the exact corridor, no safe altitude available below the terrain ceiling, active mountain wave reaching the surface at the destination (44-knot gusts), and a light aircraft whose structural limits can be exceeded by severe wave turbulence. The correct decision is to wait for conditions to moderate — checking the next winds aloft issuance and the AFD update — or to reroute south through lower terrain via KTEX or KPUB where the perpendicular flow to the Rockies may be weaker. Mountain wave is a patience problem, not a technique problem.