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# 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, vorticity and CAPE below.

## Helicity / Storm motion

Helicity (3000-0 m) / Storm motion over Europe

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.

## Vorticity

Vorticity at 500 hPa and sea level pressure over Oceania

Relative vorticity is a measure of the rotation of fluids about a vertical axis relative to the Earth's surface. Colours indicate the strength of relative vorticity, red for positive (counterclockwise rotation) and blue for negative (clockwise rotation) vorticity, respectively. Isobars for air pressure are sometimes combined on the map. The maps are relative to a specific altitude (in hPa).

Positive vorticity due to shear

Positive vorticity due to curvature

Positive vorticity at the 500 hPa level is often associated with cyclones and troughs in the 500 hPa topography.
Positive vorticity develops in a wind field with counterclockwise curvature and/or due to shear with higher velocities on the right, as seen in flow direction.

Negative vorticity due to shear

Negative vorticity due to curvature

Negative vorticity develops in a wind field with clockwise curvature and/or due to shear with higher velocities on the left, as seen in flow direction.
Negative vorticity at the 500 hPa level is often associated with fair weather and ridges in the 500 hPa topography.

Vorticity is an important measure and used to locate dynamically active zones and fronts. The omega-equation, an equation used to diagnose vertical motion (or the so called omega, in pressure units) links vorticity and vertical motion. It says that: greater upward velocity occurs where there is greater advection of positive vorticity by the thermal wind.

Thermal wind

The geostrophic vorticity at the 700 hPa level is often used as a representative value for the omega equation. Now the thermal wind is only a mathematical construct (vector difference between geostrophic winds at two different heights or pressures) and not an actual wind. To examine the thermal wind, thickness maps are needed.

A thickness map between two different pressures (e.g 1000 and 500 hPa) is a measure of the average virtual potential temperature within that layer, where blue is cold and red is warm. The thermal wind is parallel to the thickness contours. Closer packing of thickness colors indicates a stronger horizontal temperature gradient and thus a stronger thermal wind. By the thermal wind relationship, the horizontal temperature gradient causes the geostrophic wind to change with altitude (how much is shown by a thermal wind vector).

Note that if thickness lines (layer temperature) cross pressure lines, there is a temperature advection (transport of a warm air mass by the wind). The wind is parallel to pressure lines and stronger if isobars (lines of constant pressure) are closer together.
Greater upward velocity favors clouds and heavier precipitation and that is another good reason to look for vorticity. It may be complicated to evaluate vertical motion from vorticity, but this has historical reasons. If you like it simple examine the plots of vertical velocity.

## CAPE

CAPE and streamlines at 10 m over Europe

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.