What is helicity?
(thanks to Will Hand for providing this answer) A. This is a derived parameter which quantifies the tendency for airflow in the lower levels of the troposphere to ‘corkscrew’ and thus encourage the formation of storms with strong mesoscale circulations, possibly leading to tornadic activity. Helicity has units of energy and can therefore be interpreted as a measure of wind shear energy that includes the directional shear. If there is no directional shear then the helicity is zero: if the wind backs with height then the helicity is negative; if it veers with height (more normal in storms in maritime NW Europe) then the helicity is positive. Helicity is usually derived in a storm frame of reference, the ‘storm relative’ helicity, [ Hr ] between the surface and a height, [ h ] and is calculated as an integral between those limits thus: (Vh – C) x Wh x dh [units=m**2/s**2 ] Where [ Vh ] is the environmental horizontal wind velocity , [ C ] is the storm velocity and [ Wh ] is the local rela
