Low-level wind shear & microburst
Wind shear is a sudden, drastic change in wind speed or direction over a short distance, vertical or horizontal (AWH 19.2.4).
Low-level wind shear (LLWS), near the ground, is dangerous: no altitude left to trade for recovery.
- Non-convective LLWS expected within 1,500 ft AGL is labelled WS in a TAF (TC AIM MET 7.3).
- TC AIM puts the hazard zone within 1,000 ft AGL, hitting hardest on approach and takeoff (TC AIM AIR 2.8).
- Common causes: frontal passages, thunderstorms, temperature inversions, or upper winds over 25 kt (AWH 19.2.4.1, PHAK ch.12).
Microburst: intense, localized downdraft that spreads outward on ground contact (TC AIM AIR 2.8). Most severe type of wind shear (AWH 22.7.3).
| Property | Value | Cite |
|---|---|---|
| Size | About 1 NM diameter at 2,000 ft AGL; 2 to 2.5 NM at surface | TC AIM AIR 2.8 |
| Life cycle | Seldom over 15 min | AIR 2.8(d) |
| Peak winds | About 2 to 4 min | AIR 2.8(d) |
| Downdraft | Up to 6,000 fpm | AIR 2.8(b) |
| Horizontal shear | Up to 90 kt (45 kt at surface) | AIR 2.8(b) |
- Multiples: expect more once activity starts. They can line up, producing for up to an hour (TC AIM AIR 2.8(d)).
- Visible clues (TC AIM AIR 2.8(c), AWH 22.7.3):
- Humid air: usually heavy rain.
- Drier air: virga may show.
- Ring of blowing dust may be the only visible sign.
Trap: thinking wind shear needs a thunderstorm (fronts, temperature inversions and strong upper winds also cause it), or treating a microburst as one short burst (peak winds last 2 to 4 min, but a line of them can run up to an hour).
References
- TC AIM AIR 2.8 (low-level wind shear / microburst)
- TC AIM MET 7.3 (TAF WS group; significant non-convective wind shear guideline)
- FAA Aviation Weather Handbook (FAA-H-8083-28) 19.2.4, 19.2.4.1, 22.7.3
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), ch.12