Vertical wind shear is a change in wind speed and/or direction with height in the atmosphere. It is what separates and stretches an updraft from its downdraft, and it is the single most important kinematic ingredient for organized convection. The American Meteorological Society defines it simply as the condition produced by a change in wind velocity with height.
The physics
When wind changes across a layer, the air acquires horizontal vorticity, which is spin oriented sideways rather than around a vertical axis. A rising updraft tilts that horizontal spin into the vertical, and the storm becomes a rotating column rather than a symmetric bubble.
Shear also tilts the storm. In a sheared environment, the updraft leans downshear and the precipitation core falls out on the downshear side rather than raining back into the inflow. That geometric separation is why sheared storms can sustain themselves for hours while unsheared storms collapse under their own rain.
How forecasters read it
Forecasters look at shear at multiple depths because each layer does a different job. Deep-layer shear, most often the 0 to 6 km bulk wind difference, discriminates supercells from disorganized modes. Low-level shear, in the 0 to 1 km layer, is a much stronger tornado signal because it acts on the inflow air the storm is actually ingesting near the ground.
The full picture lives on a hodograph, which plots wind vectors at successive heights and lets a forecaster see both the total magnitude of the change and the curvature of the profile. A long, curved hodograph in the lowest kilometer, combined with adequate deep-layer shear above, is the signature of a classic tornado environment.
Important limits
High shear over a dry, capped, or unstable-only-aloft column produces nothing. Any shear value has to be interpreted against the moisture and instability profile in the same sounding, and against the presence of a lifting mechanism to fire storms in the first place.
Model shear can also mislead. A short-fuse mesoscale feature, such as a boundary-layer outflow or a subtle backing near a warm front, can enhance the low-level shear a storm actually sees by a factor of two or more above what the model column shows. Storm-scale mesoanalysis is where those enhancements are caught.