# Wind Shear in Aviation: Definition, Causes and Hazards

> ATPL-focused explanation of wind shear in aviation, including definition, causes, microbursts, and low-level wind shear hazards.

- **Subject:** 050 — Meteorology
- **Section:** 050-09 — Flight Hazards
- **Topic code:** 050-09-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/flight-hazards/wind-shear/

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Wind shear is defined as a sudden change in wind speed and/or direction over a short distance. It can occur in both the horizontal and vertical planes.

![Wind Shear Effect on Aircraft](https://upload.wikimedia.org/wikipedia/commons/b/b9/Downdraft_Wind_shear.jpg)

## Types and Causes
*   **Vertical Wind Shear**: Change in wind with height (knots per 100 feet).
*   **Horizontal Wind Shear**: Change in wind in the horizontal plane (knots per 1000 feet).

The main causes include:
1.  **Thunderstorms and Microbursts**: The most common and dangerous cause at low altitude.
2.  **Fronts**: In the transition zone between air masses.
3.  **Temperature Inversions**: Especially strong low-level inversions with strong winds above the stagnant cold air layer.
4.  **Jet Streams**: High-altitude wind shear (CAT).
5.  **Topography**: Wind flowing around obstacles or terrain.

## Microbursts
A microburst is a localized column of sinking air (downdraft) within a thunderstorm, which upon hitting the ground spreads out violently in all directions.
*   **Dimensions**: Typically 1 to 4 km in diameter.
*   **Duration**: Short-lived, generally less than 5 minutes.
*   **Intensity**: Downdrafts of up to 6000 fpm and horizontal wind changes of 50 to 90 knots.

### Types of Microbursts
*   **Wet**: Associated with heavy rain.
*   **Dry**: Associated with **Virga** (rain that evaporates before touching the ground). Evaporation cools the air, making it denser and accelerating its fall. A common visual sign is a ring of dust rising from the ground.

### Effect on Aircraft (Encounter Sequence)
When traversing a microburst (e.g., during landing):
1.  **Entry**: The aircraft encounters a strong **headwind**.
    *   Effect: Sudden increase in Indicated Airspeed (IAS) and lift.
    *   Aircraft Reaction: Tends to rise ("balloon") above the glide path.
2.  **Center**: The aircraft enters the strong **downdraft**.
    *   Effect: The aircraft is pushed down, losing altitude rapidly.
3.  **Exit**: The aircraft encounters a strong **tailwind**.
    *   Effect: Sudden decrease in IAS and dramatic loss of lift.
    *   Danger: If the aircraft is close to the ground and has low energy, it may impact the terrain before it can recover.

## Effects on Approach (General Rules)
*   **Sudden increase in headwind (or decrease in tailwind)**:
    *   Increases IAS and lift.
    *   The aircraft rises above the glide path (Glideslope).
    *   Risk: Long landing (overrun).
*   **Sudden decrease in headwind (or increase in tailwind)**:
    *   Decreases IAS and lift.
    *   The aircraft drops below the glide path (Glideslope).
    *   Risk: Short landing (underrun) or impact with terrain.

## Detection and Avoidance
*   **LLWAS (Low-Level Wind Shear Alert System)**: A system of sensors at airports that compares wind speed and direction at different points to detect wind shear.
*   **Pilot Reports**: Vital information on actual conditions encountered.
*   **Action**: If severe wind shear (especially microbursts) is reported or suspected, the recommended action is to **delay takeoff** or **go-around** and wait for conditions to improve.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.