What is 'helicity'?
(thanks to Will Hand for much assistance with this answer)
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 is related to:
(a): speed shear from surface to 3 km (about 700 hPa) - how much the wind speed changes over this altitude band.
(b): directional change of the wind over the same altitude band.
(c): the strength of the low-level wind contributing to the speed / directional shear (as above).
The numerically-greater each of these elements is, the higher is the helicity available for ingestion into a developing storm complex. (It should be noted that the storm will modify the local wind-field, often quite markedly: this means that caution should be exercised when using standard radio-sonde sounding data, or broad-scale NWP output to assess the likelihood or otherwise of severe local storms.)
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 relative vorticity. Often [ Hr ] is calculated between expected cloud base and cloud top.
Studies in North America looked at the use of helicity (ignoring sign) for forecasting the risk of tornadoes. They found the following:
Helicity 150-299 ... weak tornadoes (possible 'supercell')
Helicity 300-499 ... strong tornadoes (favourable for 'supercell' development)
Helicity > 450 ... violent tornadoes
( These figures should be used with caution in the UK where helicity will normally lie between -200 and +200 m**2/s**2 )
(See also this FAQ entry)