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Glossary of Meteorological Terms

Albedo - Reflectivity; the fraction of radiation striking a surface that is reflected by that surface.

Anticyclonic Rotation - Rotation in the opposite direction of cyclonic rotation. In a meteorological sense, cyclonic rotation is generally associated with low pressure, and anticyclonic rotation is associated with high pressure.

Arctic Front - The boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air. The overrunning of air above this boundary often provides a mechanism for lift.

Baroclinic Leaf - A cloud pattern on satellite images - frequently noted in advance of formation of a low pressure center

Baroclinic Zone - A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems. Also, wind shear is characteristic of a baroclinic zone.

Barrier Jet - A jet-like wind current that forms when a stably-stratified low-level airflow approaches a mountain barrier and turns to the left to blow parallel to the longitudinal axis of the barrier.

Black Ice - Slang reference to patchy ice on roadways or other transportation surfaces that cannot easily be seen.

Boundary Layer - This term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface.

Chinook - Also called Foehn Winds, chinooks are warm, dry winds that occur in the lee of high mountain ranges.

Closed Low - A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly.

Cold Advection - Transport of cold air into a region by horizontal winds. This transport often acts as a forcing mechanism, particular in the winter, and upward vertical motion may ensue.

Cold Front - A zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.

Confluence - A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

Convergence - A contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels.

Coriolis - A fictitious force used to account for the apparent deflection of a body in motion with respect to the earth, as seen by an observer on the earth. The deflection (to the right in the Northern Hemisphere) is caused by the rotation of the earth.

Cutoff Low - A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression).

Cyclogenesis - The formation or intensification of a cyclone or low-pressure storm system.

Deformation Zone - The change in shape of a fluid mass by variations in wind, specifically by stretching and/or shearing. Deformation is a primary factor in frontogenesis and frontolysis.

Diabatic Heating - A process which occurs with the addition or loss of heat. The opposite of adiabatic. Meteorological examples include air parcels warming due to the absorption of radiation or release of latent heat.

Difluence - A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence.

Downslope Flow - A thermally driven wind directed down a mountain slope. Downslope flow tends to inhibit precipitation on the leeward side of the mountain range, as is often the case in the Anchorage bowl.

Dry Slot - A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs.

ECMWF (European Center for Medium-Range Weather Forecasts) - Operational references in forecast discussions typically refer to the ECMWF's medium-range numerical forecast model, which runs out to 10 days.

Ensemble Forecasting - Multiple predictions from an ensemble of slightly different initial conditions and/or various versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.

Frontogenesis - The initial formation of a front or frontal zone.

Frontolysis - The dissipation of a front or frontal zone.

Gale Warning - A warning of sustained surface winds, or frequent gusts, in the range of 34 knots (39 mph) to 47 knots (54 mph) inclusive, either predicted or occurring, and not directly associated with a tropical cyclone.

Gap Wind - Strong winds that are enhanced by the constriction of narrow passes in complex terrain. Turnagain Arm and the Matanuska Valley are two such gaps that enhance winds.

Geostrophic Wind - A theoretical wind that is affected by coriolis force, blows parallel to isobars and whose strength is related to the pressure gradient.

GFS (Global Forecast System) - One of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 240 hours. The current operational run has a horizontal resolution of 40 km.

Gust Front - The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud.

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. Higher values of helicity favor the development of mid-level rotation.

High Pressure - An area of a relative pressure maximum that has diverging winds and rotates in the opposite direction as the earth. This is generally indicative of fair weather, with the possibility of low clouds and fog in large water sources like the Bering Sea.

Hurricane Force Warning - A warning for sustained winds, or frequent gusts, of 64 knots (74 mph) or greater, either predicted or occurring, and not directly associated with a tropical cyclone.

Infrared Satellite Imagery - This satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects.

Instability - The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.

Inversion - Refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. Such inversions inhibit mixing between lower and upper layers, and the presence of a surface inversion is often indicative of radiational fog.

Isentropic Lift - Lifting of air that is traveling along an upward-sloping isentropic (line of equal potential temperature) surface. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation.

Jet Streak - Same as Jet Max; a point or area ("streak") of relative maximum wind speeds within a jet stream. Jet streaks can often enhance foul weather conditions at the surface.

Jet Stream - Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams).

Katabatic - A wind that is created by air flowing downhill. It is often (though not always) used to characterize a wind that warms adiabatically as it descends, but remains relatively cold compared to its surroundings.

Lifted Index (LI) - A common measure of atmospheric instability. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.

Longwave Trough - A trough in the prevailing westerly flow aloft which is characterized by a large amplitude and (usually) slow movement.

Low Pressure - An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.

Mesocyclone - A storm-scale region of rotation, typically around 2-6 miles in diameter. The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.

NAM (North American Mesoscale) - One of the operational forecast models run at NCEP. The NAM is run four times daily, with forecast output out to 84 hours over the Northern Pacific. The current operational run has a horizontal resolution of 12 km.

Negatively Tilted Trough - An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

NVA (Negative Vorticity Advection) - The advection of lower values of vorticity into an area. This generally implies convergence in the upper levels and downward vertical motion.

Occluded Front - A composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.

Omega - A term used to describe vertical motion in the atmosphere. Omega is determined by the amount of spin (or large scale rotation) and warm (or cold) advection present in the atmosphere.

Orographic - Related to, or caused by, physical geography (such as mountains or sloping terrain).

Outflow Boundary - A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature.

Overrunning - A weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface. It refers to a pattern characterized by widespread clouds and steady precipitation on the cool side of a front or other boundary.

Overshooting Top - A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm.

Persistence - A forecast methodology that is often implemented when there is little synoptic change. Persistence presumes that there will be little change from day-to-day.

Polar Front - A semipermanent, semicontinuous front that separates tropical air masses from polar air masses.

Polar Jet Stream - Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.

Positively Tilted Trough - An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Potential Vorticity - This plays an important role in the generation of vorticity in cyclogenesis, especially along the polar front. It is also very useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks.

Pressure Gradient - The amount of pressure change occurring over a given distance. A tighter gradient (isobars are closer together) implies stronger wind.

PVA (Positive Vorticity Advection) - The advection of higher values of vorticity into an area. PVA often leads to divergence in the upper levels and upward vertical motion.

Radiational Cooling - The cooling of the lower atmosphere during the nighttime hours. This, in combination with light surface winds and relatively clear skies, will often set the stage for fog.

Radiational Heating - The warming of the lower atmosphere during the daylight hours. This is often a precursor for convective initiation and potentially thunderstorms.

Radiosonde - An instrument that is carried aloft by a balloon to send back information on atmospheric temperature, pressure and humidity by means of a small, expendable radio transmitter.

Retrograde Motion - Movement of a weather system in a direction opposite to that of the basic flow in which it is embedded, usually referring to a closed low or a longwave trough which moves westward.

Rex Block - A blocking pattern where there is an upper level high located directly north of a closed low.

Ridge - An elongated area of relatively high atmospheric pressure; the opposite of trough.

Right Mover - A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers).

Shear - 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.

Shortwave - A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. These waves are usually much smaller in scale than longwaves, and move much faster.

Sounding - A set of data measuring the vertical structure of an atmospheric parameter (temperature, humidity, pressure, winds) at a given time. In Anchorage, balloons is launched twice daily (early afternoon, early morning) to measure these parameters.

SREF (Short Range Ensemble Forecast - One of the operational forecast models run at NCEP. The SREF is run twice daily, with output out 87 hours. See ensemble for more details.

Stationary Front - A front between warm and cold air masses that is moving very slowly or not at all.

Storm Warning - A marine warning where forecasts call for 50 to 60 kts of wind and turbulent wave conditions.

Thermal Wind - A theoretical wind that blows parallel to the thickness lines, for the layer considered, analogous to how the geostrophic wind blows parallel to the height contours. The speed and direction of the thermal wind are determined by vector geometry where the geostrophic wind at the upper level is subtracted from the geostrophic wind at the lower level.

Triple Point - The intersection between an occluded, warm, and cold front. This point is often a focus area for a secondary area of low pressure to develop.

Trough - An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

UKMET - A medium-range (3 to 7 day) numerical weather prediction model operated by the United Kingdom Meteorological Agency. It has a resolution of 75 kilometers and covers the entire northern hemisphere. Forecasters use this model along with the ECMWF and GFS in making their extended forecasts (3 to 7 days).

Unstable Air - Air that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.

Vertically Stacked System - A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening, slow-moving, and often are occluded at the surface.

Visible Satellite Imagery - This type of satellite imagery uses reflected sunlight to see things in the atmosphere and on the Earth's surface. Because this imagery is dependent on sunlight, it cannot be used during night and for much of the winter.

Vorticity - A measure of the rotation of air in a horizontal plane. Positive (counter-clockwise or cyclonic) vorticity can be correlated with surface low development and upward vertical motion (in areas of positive vorticity advection).

Warm Advection - Transport of warm air into an area by horizontal winds. This often implies the presence of stratiform lifting in the lower levels of the atmosphere.

Warm Front - A transition zone between a mass of warm air and the colder air it is replacing.

Water Vapor Imagery - One of the major channels of the operational weather satellites. This channel detects upper level moisture, and can often be diagnosed for upward vertical motion, shortwaves, and potential vorticity.

Zonal Flow - Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal.

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