Glossary of Meteorological Terms

Below is a list of terms and acronyms commonly used in the meteorological community.

• Advection: The horizontal transport of a quantity by the wind. Mathematically it is the dot product of the wind vector and the horizontal gradient.
• AFWA: Air Force Weather Agency
• Anticyclone: A region of high pressure or ridging. The flow around an anticyclone is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Anticyclones are typically associated with fair skies and dry conditions.
• Baroclinic: A cyclone or anticyclone whose lifecycle is governed by thermal advection. Most common type of systems observed in the Mid-Latitudes.
• Baroclinic Leaf: A leaf-shaped cloud pattern on satellite imagery that indicates the onset of mid-latitude cyclogenesis. Occurs when an approaching shortwave trough interacts with a baroclinic zone to initiate cyclogenesis.
• Barotropic: A cyclonic or anticyclonic circulation that receives no contribution from thermal advection. Barotropic systems are typically semipermanent (i.e., subtropical highs, subpolar lows), but can be seasonal (i.e., tropical cyclones, thermal lows, (ant)arctic highs).
• Bow Echo: A Doppler radar severe thunderstorm signature that appears as a bowed line in Reflectivity mode. Damaging straight-line winds often occur at the leading edge of the bow echo.
• Cold Front: The leading edge of an advancing cold air mass. With cold, dense air undercutting warm air ahead of the front, brief but occasionally intense convective precipitation is common with cold fronts.
• Convection: The vertical transport of heat and/or moisture within the atmosphere. Thunderstorms are an example of moist convection.
• Convergence: The act of air (or a fluid) moving toward a given location.
• Coriolis Force: The cyclonic force resulting from the earth's rotation that deflects fluid flows to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Cyclone [1]: A region of low pressure or troughing. The flow around a cyclone is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones are typically associated with clouds, precipitation, and occasionally strong winds.
• Cyclone [2]: A tropical cyclone with sustained winds of 65 knots (119 km/h) or greater in the Pacific Ocean south of the Equator or the Indian Ocean.
• Divergence: The act of air (or a fluid) moving away from a given location.
• Geostrophic Flow: The resultant flow when the pressure gradient force is balanced with the Coriolis force. Geostrophic flow travels parallel to the horizontal pressure gradient and to the right of the pressure gradient force.
• Hook Echo: A hook-shaped appendage from a thunderstorm that appears on Doppler radar in Reflectivity mode. A hook echo indicates very strong rotation within the thunderstorm. In most cases a tornado has already touched down if a hook echo appears.
• Hurricane: A tropical cyclone with sustained winds of 65 knots (119 km/h) or greater in the Atlantic Ocean or in the Pacific Ocean east of the International Date Line and north of the Equator.
• Law of Conservation of Energy: States that energy cannot be created nor destroyed, but it can change forms.
• Mesoscale: Scale used for analyzing and forecasting weather features on the order of 2 km to 2,000 km in length. The mesoscale is further divided into the meso-alpha which covers features of 200-2000 km, including smaller tropical cyclones, MCSs and MCCs. The meso-beta scale focuses on features on the order of 20-200 km, and the meso-gamma scale governs features on the order of 2-20 km.
• Mesoscale Convective Complex (MCC): A large, organized complex of thunderstorms whose area of -32^oC cloud tops covers and area of 100,000 km² and -52^oC covers 50,000 km² or greater (approximately the size of Iowa), and a eccentricity (minor axis:major axis ratio) of 0.7 or greater for at least six hours.
• Mesoscale Convective System (MCS): A large, organized cluster of thunderstorms that does not meet the criteria of an MCC, but is more concentrated than individual thunderstorm cells. Squall lines, and small, short-lived thunderstorm clusters are examples of MCSs.
• NCDC: National Climatic Data Center
• NCEP: National Center for Environmental Prediction
• NHC: National Hurricane Center
• NOAA: National Oceanic and Atmospheric Administration
• NSSL: National Severe Storms Laboratory
• NVA: Negative vorticity advection.
• Planetary Scale: Scale used for analyzing longwave pattern and Rossby waves on the order of 10,000 km or greater.
• PVA: Positive vorticity advection.
• Skew-T: A chart meteorologists use to plot temperature, dew point, and wind data from balloon soundings. Temperature/dew point is plotted on the horizontal axis, with the lines skewed to the right as one moves up on the chart. The vertical coordinate is pressure, which is plotted logarithmically. Derived values, such as mixing rations, potential temperature, and equivalent potential temperature are also plotted.
• SPC: Storm Prediction Center
• Supercell: A large, rotating severe thunderstorm. Rotation of the supercell is caused by a mesocyclone within the storm. Supercells often produce tornadoes, and the strongest supercells can produce families of tornadoes that can reach a mile (1.6 km) in width and devastate large areas. There are three type of supercells: Classic supercells feature intense precipitation in an arc from the northeast to the southwest associated with the forward and rear flank downdrafts, while the inflow region on the southeast side remains rain-free. A high-precipitation (HP) supercell occurs where a deeper layer of moisure is present and the rain-free area is much smaller. HP supercells are particularly dangerous because tornadoes are often rain-wrapped and can be difficult to see. Low-precipitation (LP) supercells occur over areas with limited moisture, and the precipitating region of the supercell is much smaller. However, LP supercells are capable of producing larger, more destructive hail than classic or HP supercells due to increased evaporational cooling with dry air entrainment.
• Synoptic Scale: Scale that focuses on features on the order of 2,000-10,000 km in length. The synoptic scale includes most mid-latitude cyclones and anticyclones, fronts, shortwave troughs and ridges, and large tropical cyclones.
• Tornado: A violently rotating column of air which is in contact with the ground and the overlying cloud. The visible part of the tornado is known as the "condensation funnel," due to the presence of condensed water droplets suspended in the tornado. In LP supercells the tornado may initially be invisible until a sufficient amount of debris is lifted to make the funnel visible. In HP supercells the tornado may be rain-wrapped, making it difficult to see. Most tornadoes are weak and short-lived, but most deaths and property damage occur with the small percentage of tornadoes that can grow to over 1 mile (1.6 km) in width, travel 100 miles (160 km) or more, and have winds in excess of 300 MPH (480 km/h) before dissipating.
• Tornado Vortex Signature (TVS): A signature on Doppler radar that indicates rotation within a thunderstorm. In Radial Velocity mode a TVS appears as strong winds moving toward the radar next to strong winds moving away from the radar ("gate-to-gate" shear). In Reflectivity mode a TVS appears as a hook-shaped appendage from the thunderstorm cell (see Hook Echo).
• Tropical Cyclone: A warm-core barotropic system that typically forms over warm ocean waters (27^oC/80^oF or higher). It usually originates as a disorganized mass of convection, but becomes a tropical disturbance when the convection becomes sufficiently organized to produce a low-level circulation and a decrease of surface pressure. The tropical disturbance becomes a tropical cyclone when its circulation closes off from the environmental flow.
• Typhoon: A tropical cyclone that has sustained winds of 65 knots (119 km/h) or higher in the Pacific Ocean west of the International Date Line and north of the Equator, including the South China Sea, Sea of Japan, Yellow Sea, or East China Sea.
• Tropical Depression: A tropical cyclone with winds less than 34 knots (61 km/h).
• Tropical Storm: A tropical cyclone with winds greater than 34 knots (61 km/h), but less than 64 knots (119 km/h).
• Vorticity: The amount of rotation within the atmopshere at a given location. There are three types of vorticity. Absolute vorticity is a measure of a fluid's rotation about a vertical axis with respect to the earth's rotation. Relative vorticity is the absolute vorticity minus the earth's rotation (Coriolis force). Horizontal vorticity measures the amount of rotation about a horizontal axis. Positive vorticity is associated with cyclonic circulation, troughs, and low pressure, while negative vorticity is associated with anticyclones and high pressure.
• WSR-88D: Weather Surveillance Doppler Radar, used in the United States. The WSR-88D is an S-band radar with a 10.7 cm wavelength and a maximum range of 248 nautical miles (460 kilometers). It became operational in 1988. During the 1990s, 143 WSR-88D radars were installed in the United States. The last WSR-88D site at North Webster, Indiana became operational on March 17, 1998.¹

Footnotes

1. 19980318: NEXRAD-Northern Indiana-NOUS (fwd)