Vertical Wind Shear Calculator

Vertical wind shear is the vector change in the horizontal wind between two heights divided by the height interval. It has to be computed as a vector because wind carries direction: each level is decomposed into u and v components using the meteorological convention, in which the reported direction is the direction the wind blows from, so u = -V sin(dir) and v = -V cos(dir). The magnitude of the difference vector divided by the layer depth gives the shear. A layer in which speed is unchanged but direction turns 90 degrees has zero speed shear and substantial vector shear, so the distinction is not academic. Two sets of operational thresholds dominate. In severe convective forecasting, bulk shear across the surface to 6 km layer of about 20 m/s or more supports supercells, 12 to 20 m/s favors organized multicells, and below roughly 10 m/s storms tend to be short lived pulse cells; the bulk shear squared value shown here is the denominator of the Bulk Richardson Number, whose range of roughly 10 to 45 is associated with supercell modes. In tropical meteorology the relevant quantity is deep layer shear between 850 and 200 hPa, where values under about 5 m/s are favorable for intensification and values above 10 to 12 m/s usually inhibit it by ventilating the warm core. The main caveat is that a two-level calculation returns only a layer average and discards hodograph curvature, which is what actually governs storm-relative helicity and mesocyclone development.

Vertical shear was understood theoretically long before it could be measured routinely. The thermal wind relation, which ties the change of geostrophic wind with height to the horizontal temperature gradient, emerged from the dynamical meteorology associated with Vilhelm Bjerknes and the Bergen school in the early twentieth century, and it explained why jet streams sit above zones of strong temperature contrast. Winds aloft were first sampled with pilot balloons tracked by theodolite, then routinely with rawinsondes from the 1940s, which is when vertical wind profiles became a daily operational product rather than a research curiosity. Two research lines turned shear into a forecast parameter. In severe storms work, Joseph Klemp and Robert Wilhelmson and later Morris Weisman and Klemp used numerical cloud simulations in the late 1970s and early 1980s to show that shear organizes storm structure and produces splitting supercells, and the Bulk Richardson Number came out of that effort. In aviation, Tetsuya Theodore Fujita analyzed the Eastern Air Lines Flight 66 accident of 1975 and later cases and identified the downburst and microburst as small-scale low-level shear events; the JAWS field project near Denver in 1982 confirmed their structure, and the loss of Delta Flight 191 in 1985 accelerated deployment of Terminal Doppler Weather Radar and onboard shear detection systems. Mark DeMaria and colleagues quantified the shear-intensity relationship for tropical cyclones during the 1990s.