Speed shear is a change in wind speed with height while the wind direction stays roughly the same. On a hodograph it plots as a nearly straight line pointing outward from the origin along a single radial.
What it is measuring
When a column shows speed shear, the wind at the surface is one magnitude and the wind at a chosen upper level is a larger magnitude in the same direction. Vector subtraction between the two produces a shear vector aligned with that common direction.
Physically, speed shear creates horizontal vorticity whose axis is oriented perpendicular to the flow. When an updraft rises into that shear, it tilts the horizontal vortex tubes into the vertical and produces a couplet with equal-and-opposite rotation on the storm's flanks. That balanced couplet is the fingerprint of the classic splitting supercell in a straight-line hodograph.
How forecasters read it
Speed shear is easiest to read in the deep-layer 0 to 6 km bulk wind difference. Strong deep-layer speed shear, on the order of 35 to 40 knots or more, is enough to sustain organized convection even when the profile is mostly unidirectional. Squall lines and bow echoes in strongly sheared but weakly veered profiles are a common expression of that setup.
On a straight hodograph with only speed shear, the resulting storms often produce mirror-image left and right splits that move away from each other. Forecasters watching a splitting pair in real time treat the split itself as a sign that low-level directional shear was limited.
Important limits
Pure speed shear misses much of what makes tornadoes. Nearly all significant tornado environments have not just deep-layer magnitude but also curvature in the lowest kilometer, which is not visible in a straight-line hodograph.
The label is also somewhat artificial. Real profiles almost always contain some of both speed shear and directional shear, and the meaningful diagnostic is the combined shape of the hodograph rather than a clean classification into one type or the other. The distinction is a teaching tool more than an operational category.