Wind shear & downbursts

What changes vertically or horizontally?

Compare wind vectors at nearby points before judging their effect on the flight path.

Vertical, horizontal and low-level wind shear

Wind shear is a spatial change of wind velocity: speed, direction or both, over a horizontal or vertical distance. Vertical shear usually means a change of the horizontal wind vector with height. Horizontal shear means a change across horizontal distance at a similar level. Neither term is simply another name for a vertical draught. A smooth directional change can create shear without violent turbulence, although sufficiently strong shear can generate turbulence.

For example, a 30 kt wind from west and a 30 kt wind from north-west have equal speeds but different vectors. An aircraft moving between those regions encounters a change in headwind/crosswind components. Conversely, a uniformly strong wind with no spatial change is not itself wind shear. Gustiness describes rapid fluctuations with time at an observation point; turbulence describes irregular eddy motion. They can accompany shear but are not identical definitions.

Low-level wind shear is shear near the surface affecting take-off, approach and landing. In the ICAO aerodrome context the lowest roughly 500 m or 1,600 ft above runway level is particularly important, including runway operations; relevant terrain-related effects can extend higher. Treat that as a practical low-level context, not a height above which shear becomes harmless. Limited height, speed margin and time make near-ground changes especially consequential.

Which situations can put changing wind near a runway?

Connect each source to a shallow layer or horizontal boundary.

Convective and non-convective sources

Thunderstorms, squall lines, gust fronts and downbursts create strong horizontal and vertical wind changes, often beyond the heaviest rain. Fronts can place contrasting winds across a sloping zone, and a sea-breeze or land-breeze boundary can produce a shallow sharp wind shift. A night-time inversion can decouple weak surface flow from a stronger low-level wind or jet above, leaving smooth but substantial shear across the stable layer.

Friction slows and turns near-surface wind relative to the free flow, creating shear through the boundary layer. Surface roughness changes, coastlines, buildings and obstacles can alter it locally. Relief can accelerate flow through gaps, create lee eddies, rotors and waves, or shelter one part of an aerodrome while another remains exposed. Strong background wind crossing nearby hills can create significant terrain-induced shear even without a thunderstorm.

The same surface mean wind can therefore coexist with very different approach-path conditions. Compare the upstream profile, inversion height, low-level jet, fronts and sea-breeze timing, terrain orientation and reports from recent aircraft. A calm or moderate wind at one surface sensor does not measure the full three-dimensional corridor. Shear forecasting and detection have spatial and temporal limits, so recent relevant observations are valuable.

What happens when the headwind component suddenly decreases?

Separate ground-relative motion, air-relative velocity and vertical air motion.

Immediate tendencies and performance

When an aircraft crosses a rapid wind change, its ground-relative velocity cannot instantaneously follow the new air mass. An increasing headwind or decreasing tailwind tends initially to increase airspeed; a decreasing headwind or increasing tailwind tends to reduce it. At a similar attitude and configuration, an airspeed loss can reduce lift and flight-path performance. Crosswind changes also alter drift and can affect the aircraft response. The full result depends on the wind vector, encounter path and aircraft dynamics.

A strong downdraught adds downward air motion and can increase sink relative to the ground even when the pilot maintains attitude and applies the approved response. In a classical microburst crossing, an initial headwind increase can be followed by a central downdraught and then an increasing tailwind/headwind loss. The initial gain is not a reliable sign that conditions are benign; reacting to it without accounting for the developing hazard can leave less margin for the subsequent loss.

Real outflows are often asymmetric, and a large initial headwind gain may be absent. Near the ground there may be insufficient height or performance to recover from a severe encounter. A steady groundspeed or a fixed wind addition cannot guarantee a safe path through it. Aircraft flight-path and control response must follow approved type/operator wind-shear procedures; the meteorological sequence is an explanation of the exposure, not a generic control prescription.

What accelerates the downdraught?

Follow precipitation loading and cooling into the near-surface outflow.

Downburst definition, development and weather

A downburst is an intense downdraught that reaches the ground or near-surface layer and spreads outward, producing strong often damaging horizontal winds. It is associated with convective cloud, showers or thunderstorms, but need not contain a tornado. The spreading flow can be strongly asymmetric, with local vortices and intense shear around its edges.

The weight of suspended precipitation and drag from falling particles contribute to the downward tendency, while evaporation of drops into drier air cools the descending parcel and increases its negative buoyancy. Melting or sublimation of frozen particles can add cooling under suitable conditions. The air accelerates downward, impinges near the surface and turns outward as a cold-pool outflow. The strongest surface winds can develop quickly after impact and move or evolve with the storm.

Wet downbursts have substantial precipitation reaching the surface and may be signalled by a dense rain shaft. Dry downbursts can occur with a high cloud base and deep dry sub-cloud layer: much precipitation evaporates as virga before reaching the ground, yet the cooling drives a strong downdraught. Dust rings or walls, rapidly spreading surface material, a gust front and reports can be clues. Absence of local rain or lightning does not exclude a dangerous convective outflow.

How do horizontal scale and duration differ?

Use the outflow footprint rather than cloud diameter to classify the burst.

Size, lifetime and repeated events

A microburst is a small intense downburst with an outflow horizontal extent of about 4 km or less; a macroburst has a larger footprint, over about 4 km. The boundary is a classification of horizontal scale, not a safe-distance rule or an intensity ranking. The cloud and rain shield can be far larger than the strongest surface outflow. Severe local winds and downdraughts can occur in the smaller event.

An individual microburst commonly has a lifetime of roughly 5–15 minutes from surface development to dissipation, with its most intense winds often lasting only a few minutes, of order 2–4 minutes. Larger downburst/outflow events can persist for tens of minutes, commonly described on a 5–30 minute scale or longer for an organised system with successive bursts. These are typical time scales, not a timer after which the runway is automatically safe.

Several bursts can develop within one convective system, overlap or regenerate along an outflow boundary. One report or alert can therefore be followed by another event in a nearby corridor. Changes in wind direction, speed and vertical motion may differ greatly between aircraft on different paths or only minutes apart. Current reports, alerts and observed evolution matter more than the nominal duration of a textbook single burst.

Which information can change the take-off or approach decision?

Combine warnings and reports with the full weather and terrain context.

Planning and in-flight avoidance

During planning and arrival/departure assessment, check convective forecasts, fronts, inversion/low-level-jet conditions, surface and upper wind, terrain exposure and recent wind-shear reports. Use aerodrome warnings and alerts where available, together with relevant aircraft reports. Ground radar, lidar and wind-sensor networks can improve detection but each has limitations; a conventional precipitation image alone does not measure every non-convective shear field.

In flight, recognise visual cues such as strong virga, spreading dust, rain shafts and gust fronts, but do not rely on them as compulsory precursors. Onboard predictive/reactive wind-shear systems, ATC information and actual flight indications add evidence. A predictive warning of a hazardous corridor is intended to support avoidance; the absence of an alert is not proof that all possible shear is absent, especially outside the installed system’s envelope or for terrain-related flow.

Avoid or delay exposure to a hazardous take-off/approach corridor using the applicable aircraft/operator criteria, and select alternatives when required. If encountered, execute the approved type-specific response and communicate/report the event with position, time, height, aircraft type and observed effects. A generic model cannot prescribe one pitch/thrust sequence for all aircraft, and no recovery technique guarantees escape from every severe low-level burst.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.