Helicity is a weather model parameter associated with changing rotation in the vertical direction.
It is a property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).
High helicity, with thunderstorms, is associated with tornadic activity.