What is the difference between low-level wind shear and a microburst?

Low-level wind shear is any rapid change in wind speed or direction below 2,000 feet AGL, caused by fronts, temperature inversions, terrain, or convection. A microburst is a specific, extremely intense downdraft from a convective cell that spreads outward at the surface, producing severe performance loss; it is a subset of LLWS and is considered far more dangerous due to its intensity and the speed at which it develops.

How can a dispatcher detect microburst activity at a departure airport?

Dispatchers monitor ATIS and D-ATIS for LLWAS and TDWR-generated microburst or wind shear alerts, review current METARs for sudden wind shifts and gusts, evaluate PIREPs from departing and arriving aircraft, and track convective radar products for cells near the airport. A combination of multiple indicators — such as virga on radar, a gust-front wind shift in the METAR, and a LLWAS alert on ATIS — greatly increases confidence that a microburst hazard exists.

Can a microburst occur when there is no rain at the airport?

Yes. Dry microbursts form when precipitation evaporates before reaching the surface — a process called virga — and the evaporative cooling actually intensifies the downdraft. These are especially common in arid regions such as the Southwest and High Plains and are particularly dangerous because there may be no obvious surface precipitation to warn pilots or controllers.

Low-Level Wind Shear and Microburst Awareness for Departure Planning

Low-level wind shear (LLWS) and the extreme subset known as the microburst represent some of the most treacherous hazards an aircraft dispatcher must account for during departure planning. Unlike turbulence at cruise altitude, these phenomena occur in the critical phase of flight when the aircraft is close to the ground, slow, and has little energy margin to recover from an unexpected performance loss. A dispatcher who understands the meteorological triggers, the geographic and temporal windows of highest risk, and the operational tools available can make informed go/no-go judgments and communicate actionable guidance to the flight crew.

This article draws on the FAA Aviation Weather Handbook (FAA-H-8083-28B) and covers the mechanics of wind shear and microbursts, their operational impact, the key alerting systems dispatchers monitor, and the decision criteria used in professional departure planning.

What Is Low-Level Wind Shear?

Wind shear is defined as a rapid change in wind speed and/or direction over a short distance. When this change occurs below 2,000 feet AGL — the altitude band through which departing aircraft typically climb before establishing a stabilized profile — it is classified as low-level wind shear. The shear may be horizontal (wind changing over a horizontal distance, such as across a runway) or vertical (wind changing with altitude). Both forms can abruptly alter the aircraft's airspeed, lift, and flight path.

LLWS has several common causes. Thunderstorms produce intense shear through their outflow and gust fronts. Temperature inversions, especially the nocturnal radiation inversion that forms on calm, clear nights, create a boundary between a calm surface layer and a faster-moving air mass aloft, generating significant speed shear. Frontal passages bring abrupt shifts in wind direction and speed as the front crosses the airport surface. Terrain can also channel or deflect airflow, creating mechanical wind shear on the lee side of ridges or through mountain passes.

Microbursts: The Extreme Case

A microburst is a concentrated, intense downdraft produced by a convective cell — most often a thunderstorm or a virga-producing shower — that spreads outward at the surface in all directions. FAA-H-8083-28B describes microbursts as typically being less than 2.5 nautical miles in diameter and lasting only 5 to 15 minutes in their most intense form, though their ground-level outflow can persist somewhat longer. Despite their small size and brief lifespan, microbursts are extraordinarily dangerous because of how they interact with a departing aircraft.

As an aircraft rolls down the runway and begins its initial climb into a microburst, it first encounters a headwind increase from the outflow on the near side of the downdraft. Airspeed rises, the aircraft climbs better than expected, and the crew may feel the aircraft is performing well. Almost immediately, the aircraft then enters the core downdraft, which drives it toward the ground. Moments later, it flies into the outflow on the far side — now a tailwind — which sharply reduces airspeed and lift. The net result is a severe loss of performance precisely when the aircraft has the least altitude to spare. Microburst encounters often produce airspeed changes exceeding 30 knots, and in severe cases considerably more, with downdrafts on the order of several thousand feet per minute in the core — far beyond the climb capability of any transport-category aircraft at low altitude.

Microbursts can occur with or without rain reaching the surface. Dry microbursts, common in the arid Southwest and High Plains, are particularly insidious because virga — rain that evaporates before reaching the ground — provides the only visual cue, and that cue may be overlooked or dismissed. The evaporative cooling intensifies the downdraft even as no precipitation is visible at the surface.

Meteorological Indicators and LLWS Sources for Dispatchers

Dispatchers integrate multiple data sources to assess LLWS risk at departure airports. The most operationally significant include:

PIREPs (Pilot Reports): Real-time crew observations of wind shear on approach or departure remain the most direct evidence of an active hazard. FAA-H-8083-28B emphasizes that PIREPs of wind shear or microburst activity at or near an airport should be treated as immediate red flags.
METARs and Speci observations: Rapid wind direction shifts, significant gusts, or sudden drops in temperature and dewpoint spread can indicate convective outflow reaching the surface. A gust front passage is generally characterized by a sudden wind shift and gusty speed increase as the outflow boundary crosses the airport.
TAFs: Terminal forecasts that include TEMPO or PROB40 groups for thunderstorms signal elevated LLWS risk during those windows.
Low-Level Wind Shear Alert System (LLWAS): Installed at many larger airports, LLWAS uses a network of anemometers positioned around the airport to detect diverging or converging surface winds. When the system senses a threshold wind difference between sensors, it generates an alert that is broadcast on ATIS and communicated by ATC. A dispatcher reviewing ATIS or D-ATIS updates should note any LLWAS alerts.
Terminal Doppler Weather Radar (TDWR): Available at many major airports, TDWR is specifically designed to detect microburst and wind shear signatures with high resolution. Its output feeds into automated alerts — Microburst Alert, Wind Shear Alert — broadcast on ATIS.
WSR-88D (NEXRAD) and radar mosaic products: Useful for identifying convective cells near the departure airport and tracking storm movement. Reflectivity and velocity products can hint at outflow boundaries and bow-echo structures associated with severe low-level shear.
Convective SIGMETs and AIRMETs Sierra/Tango: These area forecasts highlight regions of intense convection or moderate turbulence and provide broader situational awareness for corridor planning.

Why It Matters for Departure Planning

The dispatcher's role in LLWS management begins well before the aircraft pushes back. During preflight release planning, the dispatcher must assess whether current and forecast conditions at the departure airport present an unacceptable risk for the planned departure window. Unlike enroute hazards that develop gradually, LLWS can appear with very little warning. The brief lifespan of a microburst means that a condition that was absent when the release was signed can become critical by the time the aircraft reaches V1.

From a regulatory perspective under 14 CFR Part 121, the dispatcher shares legal responsibility with the captain for the safety of the flight. Signing a release for a departure during a period of confirmed or strongly suspected microburst activity without a clear mitigation plan — such as a delay to allow the cell to dissipate or move — exposes both the carrier and the crew to unacceptable risk. The dispatcher must be prepared to withhold or amend a release, coordinate a delay, or direct the flight to a suitable alternate departure airport if conditions are untenable.

Once airborne, the dispatcher continues to monitor conditions and can communicate updated information to the crew via ACARS or radio. If a microburst alert develops after departure, crew awareness of the dispatcher's real-time monitoring is a critical safety layer.

Key Numbers and Rules

Microburst diameter: Typically less than 2.5 NM; can be as small as a few hundred feet across.
Microburst duration: Peak intensity lasts approximately 5 minutes; total lifespan rarely exceeds 15 minutes.
Airspeed change: Encounters often exceed 30 knots and can be considerably more severe.
Downdraft in core: On the order of several thousand feet per minute — far beyond the climb capability of any transport-category aircraft at low altitude.
LLWS definition altitude: Below 2,000 feet AGL.
LLWAS alert threshold: Triggered when wind speed differences between airport sensors exceed a set threshold; the exact threshold varies by system generation and site configuration rather than a single fixed value.
Virga: Precipitation that evaporates before reaching the surface — a key visual indicator of potential dry microburst activity, particularly in low relative-humidity environments.

Operational Mitigation Strategies

When LLWS or microburst risk is identified, dispatchers and flight crews apply the following strategies:

Delay departure until the convective cell producing the hazard has been confirmed to dissipate or move clear of the airport. Because microburst duration and risk are variable and unpredictable, dispatchers should wait for confirmed clearance of the hazard rather than relying on a fixed delay window.
Select an alternate runway that avoids the known outflow or places the aircraft in a headwind component relative to the outflow direction.
Increase departure performance margins by reducing payload, adding fuel for holding, or using a lower assumed-temperature derate (or full-rated thrust) to preserve actual climb performance margins.
Coordinate with ATC to receive real-time LLWAS and TDWR alerts and to request updated PIREPs from aircraft ahead in the departure sequence.
Brief the crew on current LLWS conditions, expected location of any hazard, and the recommended escape maneuver technique (maximum thrust, pitch for stick-shaker/performance schedule, gear up as appropriate — specific technique per the airline's SOPs and aircraft AFM).

Common Test Traps

Confusing headwind gain with safety: The initial airspeed increase on the near side of a microburst can fool both crews and inexperienced dispatchers into thinking conditions are fine — it is a warning sign, not a margin of safety.
Assuming dry weather means no microburst risk: Dry microbursts from virga-producing cells are a real and deadly hazard, especially in arid regions. The absence of rain at the surface does not rule out a microburst.
Underestimating brevity and intensity: Because microbursts last only minutes, they may not appear in PIREPs until the hazard is fully developed, making real-time monitoring of LLWAS and TDWR outputs essential.
Treating LLWAS alerts as advisory only: An LLWAS microburst alert is a serious, operationally significant warning that should prompt immediate dispatcher action — not passive monitoring.
Forgetting the dispatcher's legal co-responsibility: Under Part 121, the dispatcher cannot simply defer all LLWS decisions to the crew. Withholding or amending the release is an affirmative dispatcher responsibility when conditions warrant.