One-half the scalar product of the velocity and vorticity vectors. It is a conserved quantity if the flow is inviscid and homogeneous in density, but is not conserved in more general viscous flows with buoyancy effects. The concept is useful in understanding severe convective storms and tornadoes, since in strong updrafts the velocity and vorticity vectors tend to be aligned, yielding high helicity. Three-dimensional turbulence containing a nonzero mean value of helicity may develop an inertial decay range, but the development is slowed by helicity. The reluctance of helical turbulence to cascade into an inertial range means that small-scale atmospheric flows with high helicity are less unstable and more predictable than small-scale flows with low helicity. See also storm-relative environmental helicity, streamwise vorticity.