A radar capable of transmitting and receiving two orthogonal polarizations.

The transmitted polarization must be switchable at a rate that is fast compared with the timescale of changes in the scattering properties of the target and the propagation medium.

American Meteorological Society, cited 2013: Dual-Polarization Radar. Glossary of Meteorology. [Available online at http://http://glossary.ametsoc.org/wiki/Dual-polarization_radar]

Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except for the size convention that hail must have a diameter greater than 5 mm.

Sometimes distinguished by shape into conical, hexagonal, and lump (irregular) graupel.

American Meteorological Society, cited 2013: Graupel. Glossary of Meteorology. [Available online at http://glossary.ametsoc.or/wiki/Graupel]

Precipitation in the form of balls or irregular lumps of ice more than 5mm in diameter, always produced by convective clouds, nearly always cumulonimbus.

Any scattering produced by spherical particles whose diameters are greater than 1/10 the wavelength of the scattered radiation. This type of scattering causes the clouds to appear white in the sky. Often, hail exhibits in this type of scattering.

National Weather Service Glossary, cited 2013.

(Also called multicell, multicell thunderstorm). A cluster of ordinary cells and/or supercells at various stages of their life cycle in close enough proximity to at least share a common precipitation area and cold pool (gust front).

New cells are generated primarily by either low-level convergence along a preexisting boundary or by lifting at the leading edge of the system-scale cold pool that was produced by the previous cells. The cells move roughly with the mean wind. However, the storm motion usually deviates significantly from the mean wind due to discrete propagation (new cell development) along the gust front. The multicellular nature of the storm is usually apparent on radar with multiple reflectivity cores and maximum tops. Lifetime may be several hours.

Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance within the atmosphere. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

National Weather Service Glossary, cited 2013.

An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft (i.e., a mesocyclone), which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min).

Most rotating updrafts (in the Northern Hemisphere) are characterized by cyclonic vorticity (see mesocyclone). The supercell typically has a very organized internal structure that enables it to propagate continuously. It may exist for several hours and usually forms in an environment with strong vertical wind shear. Supercells propagate in a direction and with a speed other than indicated by the mean wind in the environment. Such storms sometimes evolve through a splitting process, which produces a cyclonic, right-moving (with respect to the mean wind), and anticyclonic, left-moving, pair of supercells. Severe weather often accompanies supercells, which are capable of producing high winds, large hail, and strong, long-lived tornadoes. American Meteorological Society, cited 2013: Supercell. Glossary of Meteorology. [Available online at http://glossary.ametsoc.or/wiki/Supercell]

Supercells comprise a spectrum, but are often sub-categorized based on the extent to which their mesocyclone is wrapped in precipitation as revealed by their radar and/or visual appearance.

A low precipitation (LP) supercell (also called a dryline storm) is dominated by updraft with little precipitation reaching the ground. It is visualized by an exposed updraft and a translucent to nearly transparent precipitation core. Low-level mesocyclones and tornadoes are rare owing to the lack of a well defined rear flank downdraft (RFD). Most of the precipitation is carried well downstream of the updraft by strong (>30 m/s or 58 kt) anvil-layer winds.

A high precipitation (HP) supercell (also called an HP storm) is a highly efficient precipitation producer that develops and maintains precipitation-filled rear flank downdrafts (RFDs) that often envelop the mesocyclones. This makes visual identification of any embedded tornadoes difficult and very dangerous. HP supercells often produce large damaging hail, extreme and prolonged downbursts, and flash flooding.

A classic supercell, which falls in between these two extremes, exhibits moderate precipitation production. While there may be some precipitation with a classic supercell's rear flank downdraft (RFD) (and hook echo), its radar reflectivities will be lower than its foward flank downdraft (FFD) precipitation core.

(Also called supercooled liquid water). Liquid water at a temperature less than the freezing point. Important in the formation of graupel and hail.

(Also called hail spike.) A radar artifact caused by radar microwave scattering associated with large hydrometeors, typically severe hail.

The TBSS is strictly an artifact of the electromagnetic radar beam being subject to “Mie scattering” instead of the usual “Rayleigh scattering” process. A TBSS forms as incident energy from the radar is reflected off the hail, down to the ground, then back up to the hail and back to the radar. Because of the delay in reception of the pulses, the radar circuitry displays the TBSS as downrange from the hail core.

The TBSS is characterized by a 10-30 km (5-16 nm) long, low reflectivity (< 25 dBZ), echo “spike” aligned radially downrange from a high reflectivity (usually > 63 dBZ) core. The TBSS signature also produces low radial velocities (V), high spectrum widths (SW), extremely low correlation coefficients (CC), and extremely positive Differential Reflectivity (ZDR) transitioning into lower positive or even negative values farther down-radial. The presence of a TBSS with reflectivities greater than 5 dBZ on a S-band (10 cm) radar (such as the WSR-88D) suggests that the thunderstorm possesses severe hail.

In general, a local storm, invariably produced by a cumulonimbus cloud and always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail.

A thunderstorm is a consequence of atmospheric instability and constitutes, loosely, an overturning of air layers in order to achieve a more stable density stratification. A strong convective updraft is a distinguishing feature of this storm in its early phases. A strong downdraft in a column of precipitation marks its dissipating stages. Thunderstorms often build to altitudes of 40 000–50 000 ft in midlatitudes and to even greater heights in the Tropics; only the great stability of the lower stratosphere limits their upward growth.

American Meteorological Society, cited 2013: Thunderstorm. Glossary of Meteorology. [Available online at http://glossary.ametsoc.org/wiki/Thunderstorm]

A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cumulonimbus.

Compare downdraft.

National Weather Service Glossary, cited 2013.

Weather Surveillance Radar - 1988 Doppler; NEXRAD unit. National Weather Service Glossary, cited 2013.