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.
http://weatherfaqs.org.uk/node/69
Helicity. 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).
http://www.weather.gov/glossary/index.php?word=helicity
Helicity. 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).
http://forecast.weather.gov/glossary.php?word=HELICITY
(1). Note that three dimensional helicity is the product of the three wind components with the three components of vorticity. Also, note that the far right hand term is a product of the vertical velocity and the vertical relative vorticity. The units of helicity are m s -2 and of Storm Relative Helicity m 2 s -2 or J kg -1.
http://tornado.sfsu.edu/geosciences/classes/m500/Shear_Helicity/Helicity.htm
This has allowed models to operate on the time and space scales appropriate for the Storm Prediction Center (SPC). The SPC outlook time scale is 24 hours while the space scale for verification purposes is for a radius of 40-km.
http://www.caps.ou.edu/reu/reu10/papers/Hitchcock.pdf
E. Horizontal Storm Relative Helicity. Observations show that what is important in a thunderstorm developing a rotating updraft in its midlevels is not so much the helicity ingested (as suggested by equation (6)), but the STORM RELATIVE helicity ingested.
http://tornado.sfsu.edu/geosciences/classes/m500/Shear_Helicity/Helicity.htm
Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m 2 /s 2 or more) favor the development of mid-level rotation (i.e. mesocyclones).
http://www.weather.gov/glossary/index.php?word=helicity
. Helicity. Helicity is simply a measure of the amount of rotation found in a storm's updraft air. If there is significant rotation in a storm's updraft air, the storm will more than likely become a supercell and possibly spawn one or more tornadoes. In the movie below, we can see how wind with horizontal rotation can be converted into the vertical.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/modl/fcst/params/hel.rxml
Another view of this vertically oriented rotation is shown with the updraft region in red. If air can enter the storm in the direction of the orange arrow, then there will be rotation in the updraft -- possibly leading to a supercell. Image by COMET. To put this in perspective, we could see how directional wind shear can allow storm inflow air to enter a storm along these vortex rolls.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/modl/fcst/params/hel.rxml
Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m 2 /s 2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m 2 /s 2. You can either type in the word you are looking for in the box below or browse by letter.
http://forecast.weather.gov/glossary.php?word=HELICITY
what is storm helicity
Storm helicity is a  measure of the amount of rotation found in a storm's updraft air. Storm Helicity is a measure of the amount of rotation found in a storm's updraft air.