Wind shear is a change in the wind over a short distance, and near the ground it is one of the few weather hazards that can overwhelm an aircraft faster than the crew can react. Understanding where it comes from is the first step to staying out of its way.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
What wind shear is
Wind shear is a change in wind speed, wind direction, or both, over a short distance. It can be vertical, between one height and another, or horizontal, across a boundary such as a front. It exists at all levels, including the clear-air turbulence near a jet stream, but the dangerous kind for most flying is low-level wind shear: shear close to the ground, where take-off and approach leave no height and little time to recover. The FAA Aviation Weather Handbook (FAA-H-8083-28) and the long-standing FAA Pilot Wind Shear Guide, AC 00-54, set out the hazard, and the international reference is ICAO Doc 9817, the Manual on Low-level Wind Shear.
Where it comes from
Low-level wind shear has several sources, and knowing them tells you when to expect it:
- Thunderstorms. The downdraft, gust front, and microburst of a thunderstorm are the most violent source of shear. A thunderstorm anywhere near the airfield is a shear warning.
- Microbursts and downbursts. A concentrated downdraft, often less than a couple of miles across, that hits the ground and fans out. It is the classic killer on approach.
- Frontal surfaces. A sharp front, especially a fast-moving cold front, separates two different wind regimes, and crossing it brings a change of wind.
- Temperature inversions. On a clear, calm night the air near the ground cools and decouples from a faster flow just above it, the low-level jet, producing marked shear at the top of the inversion that can persist into the morning.
- Terrain and obstacles. Hills, buildings, and tree lines disturb the wind and create mechanical shear and turbulence near the surface.
- Sea breezes. The boundary between cool sea air and warm land air is another shear line.
What it does to the aircraft
The reason shear is dangerous is that an aircraft flies on its airspeed, not its ground speed, and shear changes airspeed suddenly.
Picture an approach into a microburst. On the way in, the aircraft meets an increasing headwind. Airspeed rises, the aircraft tends to balloon above the glidepath, and the natural reaction is to reduce power and push down. Moments later the aircraft passes through the core and out the far side into the downdraft and a tailwind. Now the airspeed falls away, lift is lost, and the aircraft sinks, exactly when it is low, slow, and configured to land, and possibly with the power already reduced from the first part. That headwind-to-tailwind reversal, over a few seconds, is what makes the microburst so lethal. The trained response is to recognise it early and go around with maximum available thrust, not to chase the airspeed.
How it is warned about and forecast
Several systems and products flag wind shear:
- Ground-based detection. Larger airports use a Low Level Wind Shear Alert System (LLWAS) and Terminal Doppler Weather Radar (TDWR), and controllers pass the alerts to you, often through the ATIS or directly.
- Pilot reports. A crew that meets shear should and often does report it, and a PIREP of a gain or loss of airspeed on final is a direct warning to those behind.
- The wind shear group in a forecast. A TAF or METAR can carry a
WSgroup, for exampleWS R24for shear reported on the approach to runway 24, or a non-convective low-level wind shear line in a TAF. - AIRMET Tango and convective warnings. Non-convective low-level wind shear can be forecast in an AIRMET, while any convective SIGMET implies a shear risk too. The FAA Aeronautical Information Manual describes these products.
A worked cue: if the ATIS reports a thunderstorm in the vicinity, or the surface wind is strong and gusty while a different wind is forecast just above, or a preceding aircraft reports a loss of airspeed on short final, treat all three as shear until proven otherwise.
The microburst on approach, step by step
It is worth walking through a microburst encounter on approach, because the sequence is counter-intuitive and the right response runs against the instinct of the moment. The FAA Pilot Wind Shear Guide, AC 00-54, describes the pattern.
An aircraft on a stable approach enters the outflow of a microburst and meets, first, an increasing headwind. The airspeed rises, the aircraft tends to climb above the glidepath, and the natural reaction is to reduce power and lower the nose to get back down. This is the trap, because the headwind is the leading edge of the event, not the whole of it.
Moments later the aircraft reaches the downdraft in the core, which pushes it down regardless of pitch, and then passes out the far side into a decreasing headwind that becomes a tailwind. Now the airspeed falls away, the wing loses lift, and the aircraft sinks, often with the power already reduced from the first phase and the aircraft low, slow, and configured to land. The same wind that helped a few seconds ago is now taking energy away at the worst possible time.
The trained response is to recognise the situation early, from the airspeed excursions, the sink rate, and the deviation below the glidepath, and to go around with maximum available thrust, following the pitch guidance for a wind shear escape and not changing the aircraft configuration until safely clear. The priority is energy and climb performance, not chasing the airspeed needle. The deeper lesson is that the only reliably safe way through a microburst is not to be in one: if the cues point to shear on the approach, the decision is to break it off and wait, because once you are in the core your margins are already small.
A second point follows. Because the headwind-then-tailwind reversal depends on flying through the outflow, the hazard is greatest close to the ground on take-off and landing, where there is least height to trade for airspeed. The same microburst at altitude is uncomfortable; on short final it is an emergency.
The night inversion and frontal shear
Not all dangerous shear comes from thunderstorms. Two non-convective sources catch pilots out precisely because the weather looks benign.
The first is the nocturnal inversion and low-level jet. On a clear, calm night the air near the ground cools and stops mixing with the faster-moving air a few hundred to a couple of thousand feet above it. The two layers decouple, and a marked shear sits at the top of the inversion, where a near-calm surface wind can sit beneath a 30 or 40 knot flow aloft. A worked cue: a still, clear dawn with fog or mist forming, and a forecast wind well above the surface wind, is a classic setup for shear on an early climb-out or approach, even though nothing looks threatening.
The second is frontal shear. A front is the boundary between two air masses with different winds, so crossing it, especially a sharp, fast-moving cold front, brings a change of wind direction and speed. The shear is strongest close to the frontal surface and is worth anticipating when a front is near the airfield around your arrival or departure.
In both cases the FAA Pilot Wind Shear Guide, AC 00-54, and the Aviation Weather Handbook make the same point: the absence of a thunderstorm does not mean the absence of shear. The clues are in the difference between the surface wind and the wind just above it, and in the presence of a front or a strong inversion.
Common pitfalls
- Chasing airspeed in shear. Reacting to the first increase by reducing power can leave you short of energy when the loss arrives. The answer is usually a go-around.
- Trusting a gap near a storm. The gust front and outflow extend well beyond the visible cloud and rain.
- Forgetting the morning inversion. A calm, clear night can leave marked shear at the top of the inversion well after sunrise.
- Ignoring a single PIREP. One report of shear on final is enough to change your plan.
- Discounting terrain. A strong wind across nearby hills or buildings produces shear and turbulence on short final.
In Pilot EFB
Pilot EFB decodes the METAR and TAF for your airfields and keeps the raw text, so a wind shear group or a gusty surface wind that hints at shear is there in front of you, and it shows SIGMETs and AIRMETs, including the convective warnings that carry a shear risk. What it does not do is detect wind shear in real time: that comes from the airport's ground systems, from ATC, and from pilot reports, none of which a planning app receives. Use Pilot EFB to spot the risk while you plan, and rely on the live warnings and your own training in the air. Pilot EFB is offline-first and is not a certified Electronic Flight Bag; briefings you have pulled stay readable offline, while fetching fresh data needs a connection.