Definition
A change in wind speed or wind direction over a vertical distance. The wind at one altitude differs from the wind at another altitude close above or below it, and the difference is measured per unit of height (for example, knots per hundred feet).
Plain English
The wind at one altitude is doing something different from the wind just above or below it -- different speed, different direction, or both. Vertical wind shear is how sharply that change happens as you climb or descend through it.
Context Anchor
Pilots encounter vertical wind shear in weather reports, takeoff and landing planning, and discussions of thunderstorms, fronts, and strong winds near the surface.
Derivation
Shear comes from the Old English sceran, meaning to cut or divide. The wind is in effect 'cut' into layers that move differently. Vertical specifies that the cut is between layers stacked above one another, rather than side by side.
Why Pilots Care
It can suddenly reduce airspeed or lift, especially near the ground, requiring prompt power or configuration changes.
Grounding Statement
Imagine climbing through a layer where the wind below is calm but the wind a few hundred feet above is howling from the west -- the aircraft punches from one wind environment into another, and feels it.
Intuition Check
Vertical wind shear does not mean the wind is blowing straight up or down. It means the wind changes as altitude changes.
Example Sentence 1
The PIREP warned of strong vertical wind shear below three thousand feet, so the crew briefed an extra airspeed margin on final.
Example Sentence 2
During the climb the pilot noted a 15-knot drop in groundspeed caused by vertical wind shear.