Storm-relative helicity measures how much horizontal spin the wind profile provides to an updraft that is moving with a storm, in the layer the storm is actually ingesting. Units are square meters per square second, which represent the integrated overlap of wind veering and shear inside a chosen depth of the storm's inflow.
The physics: streamwise vorticity
Wind that changes direction or speed with height carries horizontal vorticity, which can be pictured as a horizontal tube of spin lying across the flow. When that tube is aligned with the storm-relative inflow, meteorologists call it streamwise vorticity. An updraft that tilts a streamwise tube upward converts horizontal spin into vertical rotation, which is how a mesocyclone begins.
SRH quantifies how much of the ambient horizontal vorticity is streamwise from the storm's point of view. A hodograph that curves clockwise through the inflow layer places more spin along the storm-relative wind, and that geometry is what large SRH values are describing.
How SRH is computed
Calculating SRH requires a wind profile and an assumed storm motion, usually the Bunkers right-mover for right-moving supercells. The storm motion is subtracted from every wind vector in the chosen layer, and the resulting storm-relative winds are integrated with the vertical shear across that layer.
The choice of layer changes the story. The 0 to 3 km layer captures the broader inflow depth of the mesocyclone. The 0 to 1 km and 0 to 500 m layers isolate the near-ground shear that recent research ties most strongly to tornado potential. The effective layer, developed by Thompson and colleagues, replaces a fixed depth with the actual layer of buoyant inflow, which matters when a storm is elevated or when a shallow cold pool cuts off the surface.
How forecasters use it
SRH is a core ingredient in composite parameters like the Supercell Composite and Significant Tornado Parameter. Broad rules of thumb place 0 to 3 km SRH values of 150 square meters per square second and up in supercell environments, and 300 and up in strongly rotating regimes. For 0 to 1 km SRH, values above about 100 favor tornadic supercells and values in the 150 to 250 range and higher are associated with a greater risk of significant tornadoes.
These are guides, not switches. A modest SRH profile can still produce tornadoes if the low-level shear is highly streamwise and the storm remains surface-based. A large SRH profile can underperform if storm mode goes linear, if inflow is undercut by outflow, or if the actual storm motion diverges from what the parameter assumed.
Important limits
SRH is a diagnostic of the environment, not a diagnostic of any individual storm. It cannot describe how a storm interacts with a boundary, how its cold pool evolves, or how forcing distorts its low-level flow. Two storms in the same SRH field can behave very differently.
Sampling matters. Model soundings smooth the near-ground shear that recent research says is most important. A real hodograph built from a nearby VAD or observed sounding often changes the picture, especially in the lowest few hundred meters. Treat SRH as one piece of the environment, cross-checked against actual storm behavior and against related fields such as effective bulk shear and low-level moisture.