Products, help and information

# Air motion maps

Several maps describe the state of the atmosphere, mainly relatively to the air motion. You can find more information about helicity and storm motion and CAPE below.

## Helicity / Storm motion

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 become more than likely a supercell and possibly spawn one or more tornadoes.

On the maps, the helicity is shown with colours, relatively to a colour scale, in m²/s², and through an atmospheric layer of several thousand metres (e.g. 3000-0 m).

Helicity is a variable that defines the amount of streamwise vorticity (i.e. directional shear). A steady storm updraft will ingest as a result of a given storm motion. For a straight hodograph, if the updraft moves off the hodograph to the right of the mean shear vector, it will tend to be correlated with cyclonic rotation; if the updraft moves to the left of the hodograph, the updraft rotation will tend to be anticyclonic. If the hodograph has clockwise (counterclockwise) curvature and the updraft moves within the curve of the hodograph, it will likewise be correlated with cyclonic (anticyclonic) rotation. This correlation between the mean shear vector and the updraft was derived analytically by Davies-Jones (1984) and the measure of this correlation has become known as the Storm-Relative Environmental Helicity (SREH), normally just referred to as "helicity." On a hodograph, the helicity value is proportional to twice the area swept out by the storm relative wind vectors from the surface to a specific height, usually 3 km.

The Storm motion is the average wind speed in knots at which a storm will move and the direction in which the storm will move from. The storm moves slower than the ambient wind speed, since a storm has a large mass of water that has to be pushed along. The turbulence within a storm also makes it more difficult to push along. Storms will move more quickly in cases where there is speed shear with height (wind speed increases with height).
The storm motion is given as the compass direction from which the storm will move from. The meteorological compass has 90 degrees being a wind from the East, 180 degrees being a wind from the South, 270 degrees being a wind from the West and 0 degrees / 360 degrees being a wind from the North.
Strong storms will veer (move to the right of the original path of motion) due to storm dynamics.
Storm motion gives insight into which direction supercells and tornadoes will move from on days in which supercell thunderstorms are favorable.
This information was taken in parts from the website of Meteorologist Jeff Haby.

## CAPE

CAPE stands for Convective Available Potential Energy. It is a measurement of the amount of energy available to a buoyant parcel of air during the process of convection. CAPE is measured in joules per kilogram (J/kg). The higher the amount the more productive is the atmosphere to severe weather i.e. the higher the figure the more unstable is the atmosphere.

The CAPE maps are often combined with streamlines and sometimes with wind barbs.