What is "wind shear" and why is it hazardous to sUAS operations?

Why →Wind shear is a sudden change in wind velocity (direction or speed) over a short distance, horizontally or vertically. In manned aviation, wind shear on final approach has caused accidents when airspeed suddenly dropped. For sUAS, a sudden reversal from headwind to tailwind or a large speed change can overwhelm flight controller compensation, causing altitude loss or loss of control. Thunderstorm outflows are the most common source of severe low-level wind shear.

The trap →Gradual wind increase with altitude is normal wind gradient, not shear. A daily speed difference between surface and 400 feet is a measurement, not a definition. Wind shear is specifically characterized by the sudden and localized nature of the change.

Field note →Wind shear near thunderstorms and microbursts is the most dangerous form for drone pilots. Visual cues: blowing dust, rapid cloud movement changes, or sudden strong gusts with calm conditions immediately prior. If a thunderstorm cell is within 20 miles, wind shear is a realistic threat even without rain at your location.