Storm Relative Helicity Calculator

Storm relative helicity (SRH) measures the potential for a thunderstorm updraft to develop rotation by combining the vertical wind shear through a layer with the wind's motion relative to the storm itself. It is measured in m²/s² and is one of the primary ingredients forecasters use to assess supercell and tornado potential.

For a simple two-level layer, SRH = (u1 - cx)(v2 - cy) - (u2 - cx)(v1 - cy), where (u1, v1) and (u2, v2) are the wind components at the bottom and top of the layer, and (cx, cy) is the storm motion vector. This is equivalent to twice the signed area swept by the storm-relative wind vector between the two levels.

As a general guideline, SRH below 100 m²/s² is weak, 100 to 299 m²/s² makes supercells possible, 300 to 499 m²/s² is strong and favors organized rotating supercells, and above 500 m²/s² is considered extreme, an environment often associated with tornadic supercells. These bands are guidelines, not hard cutoffs.

SRH depends on the wind vectors relative to the storm, not the ground-relative wind. Because storm motion enters the formula directly, even a modest change in the assumed storm motion direction or speed, especially compared to the actual observed supercell motion, can swing the computed SRH substantially, which is why forecasters use motion-estimation methods like Bunkers rather than the raw mean wind.

0-1 km SRH focuses on the lowest kilometer of the atmosphere and correlates most strongly with low-level mesocyclone and tornado potential, while 0-3 km SRH covers a deeper layer and is used more broadly to assess overall supercell and mesocyclone potential. Both are computed with the same formula, just over different layer depths.

Yes. A negative SRH in this sign convention indicates the storm-relative wind turns in the opposite (anticyclonic) sense expected to support cyclonically rotating supercells, and is generally considered non-favorable for the classic right-moving supercell tornado threat, though it can favor left-moving (anticyclonic) supercells in rare cases.

No. SRH measures only the rotational shear ingredient. Tornado formation also requires sufficient instability (CAPE), adequate low-level moisture, and a mechanism to concentrate that rotation near the ground. High SRH with very stable air or no active thunderstorm produces no tornado at all.

Six inputs: the surface wind speed and direction, the top-of-layer wind speed and direction (for whichever layer you are assessing, commonly 0-1 km or 0-3 km), and the storm motion speed and direction. All speeds are entered in meters per second and all directions as compass degrees the wind is blowing from.

Using the standard meteorological convention, u = -speed x sin(direction) and v = -speed x cos(direction), where direction is the compass bearing the wind is blowing FROM, measured in degrees with 0 as north and 90 as east. The negative sign accounts for the wind vector pointing opposite to its "from" direction.

It uses the same underlying two-level discretized SRH formula that operational tools use for a single sounding layer. Operational SRH is typically computed from many sounding levels integrated over the full 0-1 km or 0-3 km depth, while this calculator simplifies that to one representative surface value and one representative top-of-layer value.

Because SRH is essentially a cross product between storm-relative wind vectors, rotating the storm motion vector (changing direction) swings the result through a full sine-like cycle from strongly positive to strongly negative, while changing only the storm motion speed along a fixed direction has a comparatively smaller, more linear effect on the result.