Bulk Richardson Number Calculator

The Bulk Richardson Number is a dimensionless ratio, developed by Weisman and Klemp (1982), that compares the buoyant energy available to a storm (CAPE) against the square of the ambient 0-6 km bulk wind shear. It is used to classify whether an environment favors supercells, multicells, or pulse storms.

BRN = CAPE / (0.5 x U^2), where CAPE is Convective Available Potential Energy in J/kg and U is the magnitude of the 0-6 km bulk wind shear vector in m/s, the vector difference between the mean wind in the lowest 500 m and the wind at 6 km.

Per Weisman and Klemp (1982), BRN values roughly between 10 and 45 correspond to the classic supercell regime, where buoyancy and shear are balanced enough to sustain a steady, rotating updraft. Values below 10 indicate very strong shear relative to buoyancy, and values above 45 indicate weak shear relative to buoyancy.

A BRN below 10 means the 0-6 km shear is very strong relative to the available buoyancy. Storms in this regime can still be highly sheared and organized, but updrafts may be tilted so strongly by the shear that they struggle to sustain themselves, sometimes described as shear dominating over buoyancy.

A BRN above 45 means buoyancy (CAPE) is large relative to the 0-6 km shear. With comparatively weak shear, updrafts tend to remain more upright and less organized, favoring multicell clusters or individual pulse-type thunderstorms rather than sustained, rotating supercells.

0-6 km bulk shear is the magnitude of the vector difference between the mean wind in the lowest 500 m of the atmosphere and the wind at 6 km above ground level. It measures how much the wind changes across the depth of a typical deep convective storm, not just at one level.

BRN is a bulk index using only the magnitude of shear across one deep layer (0-6 km) alongside CAPE, while SRH captures the vertical distribution and turning of the wind within a shallower layer (commonly 0-1 or 0-3 km) relative to storm motion. They answer different questions, BRN about overall storm mode (supercell vs multicell vs pulse), SRH about low-level rotational potential, and are typically used together.

It can, especially when low-level shear and SRH are also strong, but a very low BRN with extremely strong deep-layer shear can sometimes indicate storms that struggle to organize a sustained updraft. Forecasters look at BRN together with SRH, low-level shear, and CAPE profile shape rather than BRN alone to judge tornado potential.

Two inputs: CAPE in joules per kilogram, and the 0-6 km bulk wind shear magnitude in meters per second. If you only have component winds at the surface layer and 6 km, first compute the vector difference of those winds to get the shear magnitude.

They are related concepts but not the same calculation. The general (gradient) Richardson Number in fluid dynamics compares local buoyancy and shear at a point, while the Bulk Richardson Number used in convective storm forecasting is a bulk, layer-averaged adaptation developed specifically by Weisman and Klemp for classifying thunderstorm mode from CAPE and deep-layer shear.

This calculator accepts CAPE from 0 to 8000 J/kg and 0-6 km bulk shear from just above 0 to 60 m/s, covering the typical range seen in convective soundings from weak to extreme severe weather environments.