Bulk wind shear, or bulk wind difference, is the vector subtraction of the wind at the bottom of a chosen layer from the wind at the top. It expresses the net change in the wind across that layer as a single number and direction.
What the number is measuring
Because it is a vector subtraction, bulk shear captures both magnitude and turning across the layer. If the surface wind is 15 knots from the south and the wind at 6 km is 50 knots from the west-southwest, the vector difference is a large westerly component. That vector is what a storm has to work with when it lifts inflow air through the layer.
The layer chosen matters. The 0 to 6 km layer approximates the depth of a typical Plains thunderstorm and is standard for supercell diagnostics. The 0 to 1 km layer isolates the near-ground profile that most affects tornado potential. The 0 to 3 km layer is common in tornado composites, and 0 to 8 km is sometimes used for very tall storms.
How forecasters use it
Storm Prediction Center guidance treats 0 to 6 km bulk shear near 25 to 40 knots as the transition zone from non-supercell to supercell modes, with values of 40 knots or more clearly favoring supercells. These values are context-dependent guides, not hard thresholds, and the same shear can support very different outcomes depending on instability and low-level structure.
Bulk shear also feeds many composite parameters. It is one input to the significant tornado parameter and to the effective bulk shear on which modern supercell environmental checklists depend. On days when instability is high, forecasters accept somewhat lower bulk shear for supercells. On days when instability is limited, they need more shear to compensate.
Important limits
Bulk shear cannot tell the difference between a straight hodograph and a curved one. Two profiles with identical 0 to 6 km bulk wind difference can produce very different storm modes if one has all its turning in the lowest kilometer and the other has none. Reading the hodograph shape is not optional.
Bulk shear also ignores where in the layer the change occurs. A profile with all its shear packed above 3 km, with weak flow beneath, looks acceptable on paper but produces storms that struggle to tap the sheared layer. The effective-layer versions of bulk shear were developed to address exactly this limitation.