Storm-Relative Helicity 0-3 km

Storm-Relative Helicity 0-3 km (SRH) is a specialized atmospheric parameter quantifying the integrated horizontal vorticity available for storm-scale rotation in the 0-3 kilometer layer above ground, computed relative to expected storm motion vectors. Expressed in square meters per square second (m²/s²), SRH combines wind shear (which creates horizontal spinning of air) with storm-relative wind vectors (which determine how much of that spinning gets ingested into updrafts) to produce a single metric predicting supercell rotation potential. Unlike simple wind shear which measures only the magnitude of wind change with height, SRH accounts for the alignment between the wind profile and the direction storms will move, recognizing that rotation develops most efficiently when low-level winds blow from the right of storm motion (in the Northern Hemisphere), creating streamwise vorticity parallel to updraft inflow that can be tilted vertically into mesocyclonic rotation.

SRH interpretation for photography: values below 50 m²/s² indicate minimal low-level rotation potential—even well-organized storms will lack prominent mesocyclones or rotating cloud bases. SRH of 50-100 m²/s² suggests weak rotation potential with subtle mesocyclone development possible but not visually dramatic. SRH of 100-200 m²/s² indicates moderate rotation potential favoring supercell development with visible mesocyclones, wall cloud formation, and clear rotation signatures in low-level cloud motion—excellent for storm structure photography. SRH of 200-300 m²/s² represents strong rotational potential producing well-defined mesocyclones with dramatic rotating wall clouds, rapid cloud base motion, and prominent supercell visual characteristics. SRH exceeding 300 m²/s² indicates very high rotation potential associated with significant severe weather including tornado risk—creates the most visually spectacular storm structures but requires extreme caution and situational awareness.

For photographers, SRH provides advance warning of rotation-capable environments: when SRH exceeds 150 m²/s² combined with sufficient instability (CAPE > 1000 J/kg), supercell storms with photogenic rotating structure become likely. PhotoWeather's GFS-derived SRH forecasts enable multi-day planning for storm photography trips targeting environments with both the thermodynamic support for strong updrafts (instability) and the kinematic support for rotation (SRH)—the combination producing the most visually dramatic storm structures. The physical mechanism underlying SRH's predictive power involves vortex stretching: when horizontal vorticity (spinning air along a horizontal axis created by wind shear) is tilted vertically by a storm updraft and then stretched vertically as air accelerates upward, angular momentum conservation intensifies the rotation similar to a figure skater pulling in their arms. SRH quantifies how much horizontal vorticity is available in storm-relative coordinates and oriented favorably for this tilting process.