]> Occurrence of fog or haze in which considerable amounts of acidic material have been taken up from the gas phase, resulting in pH values less than approximately 3 in the liquid phase. Acid Fog Acid Haze also called acid fog A popular expression for the deposition by rainfall of various airborne pollutants (especially SO2 and NO2) that have harmful effects on vegetation, soils, buildings and other external structures. AdvectionFog A type of fog caused by the advection of moist air over a cold surface, and the consequent cooling of that air to below its dewpoint. The vector difference between the real (or observed) wind and the geostrophic wind Ageostrophic Wind Air Mass A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region. A principal cloud type (cloud genus) in the form of a gray or bluish (never white) sheet or layer of striated, fibrous, or uniform appearance. Ambient Temperature The temperature that is characteristic of the atmosphere surrounding a small-scale feature such as a cumulus cloud. In mountain meteorology, an upslope wind driven by heating (usually daytime insolation) at the slope surface under fair-weather conditions. An atmospheric anticyclonic circulation, a closed circulation. The wind in an anticyclone is in the clockwise direction in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Anvil Cloud The anvil-shaped cloud that comprises the upper portion of mature cumulonimbus clouds a type of cloud The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the ?column? of air lying directly above the point in question. A rare and randomly occurring bright ball of light observed floating or moving through the atmosphere close to the ground. A cloud plume often observed to extend downwind from isolated, sharp, often pyramid-shaped mountain peaks, even on otherwise cloud-free days. A synoptic-scale cloud pattern frequently observed in satellite imagery just prior to the onset of cyclogenesis A jet on the windward side of a mountain barrier, blowing parallel to the barrier. A particular aspect of a normal lightning flash occasionally seen when the observer happens to view end-on a number of segments of the irregular channel (zigzag lightning) and hence receives an impression of higher luminosity at a series of locations along the channel. A cloud variety composed of merged or separate elements that are elongated and parallel, either suggestive of ocean waves or arranged in ranks and files. Colloquial expression for a cumulonimbus anvil that spreads upwind into relatively strong winds aloft. A squall accompanied by dark clouds and generally by heavy rain. When dust, sand, snow, and/or spray is raised by the wind to a height of 6 feet or more Weakly luminous upward propagating discharges, blue in color, emanating from the tops of thunderstorms. Mesoscale vortices observed at the ends of a line segment of convective cells, usually cyclonic on the northern end of the system and anticyclonic on the southern end, for an environment of westerly vertical wind shear A fall wind with a source so cold that, when the air reaches the lowlands or coast, the dynamic warming is insufficient to raise the air temperature to the normal level for the region; hence it appears as a cold wind A bow-shaped line of convective cells that is often associated with swaths of damaging straight-line winds and small tornadoes. Phenomenon particularly associated with smog episodes in cities such as Denver, Colorado. The mountain wind of a canyon, that is, the nighttime down-canyon flow of air caused by cooling at the canyon walls. A statically stable layer at the top of the atmospheric boundary layer. In radar usage, a local maximum in radar reflectivity that undergoes a life cycle of growth and decay. The rising portion of the reflectivity maximum is indicative of updraft, and the later descending portion is indicative of a precipitation downdraft. Cells in ordinary convective storms last from 20 to 30 min, but often form longer-lasting multicell convective storms. Cells in supercell storms are more steady and last considerably longer. See also thunderstorm cell. The region of dense cloud near the core of a tropical cyclone. An east or southeast desert wind in Morocco (North Africa), especially in the north. The name given to the foehn in western North America, especially on the plains to the lee or eastern side of the Rocky Mountains in the United States and Canada. A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface. In popular terminology, any sudden and heavy fall of rain, almost always of the shower type. In cloud physics, the merging of two water drops into a single larger drop after collision. Any nonoccluded front, or portion thereof, that moves so that the colder air replaces the warmer air; that is, the leading edge of a relatively cold air mass. Level at which water vapor to condense Condensation Level An organized unit of convection within a convecting layer. It is isolated by a stream surface, with ascending motion in the center and descending motion near the periphery, or vice versa. On a thermodynamic diagram, the point of intersection of a sounding curve (representing the vertical distribution of temperature in an atmospheric column) with the saturation mixing ratio line corresponding to the average mixing ratio in the surface layer. A buoyant jet stream in which the buoyancy is supplied steadily from a point source; the buoyant region is continuous. A principal cloud type (cloud genus), exceptionally dense and vertically developed, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions. A principal cloud type (cloud genus) in the form of individual, detached elements that are generally dense and posses sharp nonfibrous outlines. Cumulus A cold low that has grown out of a trough and become displaced out of the basic westerly current and lies equatorward of this current. A cold low that has grown out of a trough and become displaced out of the basic westerly current and lies equatorward of this current. The piling up, or complete blocking, of cold air approaching the slopes of a mountain barrier. A decrease in the central pressure of a pressure system as depicted on a constant- height chart, or an analogous decrease in height on a constant-pressure chart; the opposite of filling. A crystal, particularly a planar ice crystal, with its macroscopic form (crystal habit) characterized by intricate branching structures of a treelike nature. In meteorology, an area of low pressure; a low or trough. A widespread convectively induced straight-line windstorm. A wind blowing from the desert. An area of strong, often damaging winds produced by a convective downdraft over an area from less than 1 to 10 km in horizontal dimensions. Small-scale downward moving air current in a cumulonimbus cloud. A wind directed down a slope, often used to describe winds produced by processes larger in scale than the slope. A very strong, usually gusty, and occasionally, violent wind that blows down the lee slope of a mountain range, often reaching its peak strength near the foot of the mountains and weakening rapidly father away from the mountains. Very small, numerous, and uniformly distributed water drops that may appear to float while following air currents. A low-level mesoscale boundary or transition zone hundreds of kilometers in length and up to tens of kilometers in width separating dry air from moist air. A migratory wavelike disturbance of the tropical easterlies. A squall or thunderstorm in the Mediterranean. After U.S.weather observing practice, the ceiling classification applied to a ceiling height that is determined in any of the following ways: 1) by means of a convective-cloud height diagram or dewpoint formula; 2) from the known heights of unobscured portions of natural landmarks, or objects more than one and one-half nautical miles from any runway of the airport; 3) on the basis of observational experience, provided the sky is not obscured by surface-based hydrometeors or lithometeors, and other guides are lacking or considered unreliable; or 4) determined by ceilometer or ceiling light when the penetration of the light beam is in excess of normal for the particular height and type of layer, or when the elevation angle of the clinometer or ceilometer-detector scanner exceeds 84?. In the classification of Jeffreys, a wind motion only in response to the pressure force. Fog formed as a result of evaporation of water that is warmer than the air. Precipitation (generally in the form of rain) of an unusually high rate. Any cyclonic-scale storm that is not a tropical cyclone, usually referring only to the migratory frontal cyclones of middle and high latitudes. A wind that accelerates as it moves downslope because of its low temperature and greater density. The sector, in a horizontal plane, between the occluded front and a secondary cold-front of an occluded cyclone. A sequence of long-lived tornadoes produced by a cyclic supercell storm. Tornadoes touch down at quasi-regular intervals (typically 45 min). Usually a new tornado develops in a new mesocyclone just after an old tornado has decayed in an old, occluded neighboring mesocyclone. Sometimes, two successive tornadoes may overlap in time for a few minutes. The two mesocyclones may rotate partially around each other. If the damage tracks of the tornadoes appear to form a wavy broken line, the family is classified as a series mode. In the more common parallel-mode family, the damage tracks are parallel arcs with each new tornado forming on the right side of its predecessor. The parallel mode is subcategorized into left turn and right turn, according to the direction in which the paths curve. An organized lifting zone of cumulus and towering cumulus clouds, connected to and extending outward from the mature updraft tower of a supercell or strong multicell convective storm. A warm, dry, downslope wind descending the lee side of the Alps as a result of synoptic-scale, cross-barrier flow over the mountain range. Any cloudform associated with the foehn, usually referring to standing clouds of two types, orographic clouds and mountain wave clouds. Orographic clouds may include crest clouds and the foehn wall. Wave clouds may consist of lenticular (including altocumulus standing lenticular, or ACSL) clouds, lee-wave clouds and cloud bands, and rotors. See also Bishop wave, chinook arch, contessa di vento, Moazagotl. The leeward edge of the orographic stratiform cap cloud as seen from the lee side of a mountain barrier, preceding or during a foehn or chinook event. The edge is generally abrupt and resembles a wall of cloud (?foehnwand? in German). This cloud often signifies the occurrence of orographic precipitation, especially snowfall in the cold season, over the peaks. See foehn cloud. Mountain waves, lee waves, or trapped lee waves in the air stream flowing over the mountain barrier that occur in association with foehn conditions. The Moazagotl is one example of a foehn wave made visible by lee-wave clouds. Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility. The humid east wind that crosses the divide of the Andes east of Lake Titicaca and descends on the west in violent squalls; probably the same as puelche. A sea breeze of Naples in Italy. Small cumulus clouds that are everywhere negatively buoyant compared to the surrounding environment, but that exist because the inertia of the rising thermals feeding them from underneath is sufficient to penetrate the lifting condensation level (LCL). Morphologically, these are often cumulus humilis clouds and are typically found at the top of the convective boundary layer during daytime over land, when a strong temperature inversion aloft prevents the clouds from growing deeper. Compare active cloud, passive cloud. A fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze. Rain that falls in liquid form but freezes upon impact to form a coating of glaze upon the ground and on exposed objects. In the Beaufort wind scale, a wind with a speed from 17 to 21 knots (19?24 mph) or Beaufort Number 5 (force 5). In the Beaufort wind scale, a wind with a speed from 34 to 40 knots (39?46 mph) or Beaufort Number 8 (force 8). In meteorology, generally, the interface or transition zone between two air masses of different density Fog associated with frontal zones and frontal passages. It is usually divided into three types: warm-front prefrontal fog; cold-front post-frontal fog; and frontal-passage fog. The first two types are a result of rain falling into cold stable air and raising the dewpoint temperature. Frontal-passage fog can result from the ?mixing of warm and cold air masses in the frontal zone? or by ?sudden cooling of air over moist ground.? A temperature inversion (temperature rising with height) in the atmosphere, encountered upon vertical ascent through a sloping front (or frontal zone). A horizontal wavelike deformation of a front in the lower levels, commonly associated with a maximum of cyclonic circulation in the adjacent flow. It may develop into a wave cyclone. The forced ascent of the warmer, less dense air at and near a front, occurring whenever the relative velocities of the two air masses are such that they converge at the front. A condensation cloud, typically funnel-shaped and extending outward from a cumuliform cloud, associated with a rotating column of air (a vortex) that may or may not be in contact with the ground. If the rotation is violent and in contact with the ground, the vortex is a tornado. Funnel clouds can occur through a variety of processes in association with convection. For example, small funnel clouds are infrequently seen extending from small, dissipating cumulus clouds in environments with significant vertical wind shear in the cloud-bearing layer. A squally northwesterly wind, cold, humid, and showery, that occurs in the rear of a low pressure area over the English Channel and off the Atlantic coast of France and northern Spain. A strong, low-level wind through either a relatively level channel between two mountain ranges or a gap in a mountain barrier; originally applied to strong (10?20 m s-1) easterly winds through the Strait of Juan de Fuca between the Olympic Mountains of western Washington State and the mountains of Vancouver Island, British Columbia, Canada. There they have been defined as ?a flow of air in a sea level channel that accelerates under the influence of a pressure gradient parallel to the axis of the channel.? As in the case of mountain- gap winds, this term has also been applied to pressure-gradient winds accelerating through a gap in a mountain barrier. The pressure gradient often results from a stable, post-cold-frontal anticyclone approaching the barrier and being partially blocked (see blocking) as it ascends the barrier, except for the flow through the gap or channel. The tehuantepecer of Central America is a well- known gap wind by this definition. These flows have sometimes been referred to as jet-effect wind and canyon wind. A sea breeze. In southwest France it refers to a southwesterly sea breeze that sets in about 9 A.M., reaches it maximum towards 2 P.M. and ceases about 5 P.M. A dense fog and/or drizzle from low stratus on the west coast of South America. It creates a raw, cold atmosphere that may last for weeks in winter and supplies a limited amount of moisture to the area. A local dry wind in the northern plains of Java, resembling the foehn. It is caused by a wind crossing the mountains near the south coast and pushing between the volcanoes. In the Beaufort wind scale, a wind with a speed from 7 to 10 knots (8?12 mph) or Beaufort Number 3 (Force 3). A fresh westerly wind of oceanic origin in Morocco. Hard squalls from the northeast in Libya and Africa. They are sudden and frequent and are accompanied by heavy rain and thunder. A hot dust-bearing desert wind in Tripolitania (northwestern Libya), similar to the foehn. A shallow gravity wind, along the icy surface of a glacier, caused by the temperature difference between the air in contact with the glacier and free air at the same altitude. The glacier wind does not reverse itself diurnally as do mountain and valley winds, but it reaches its maximum intensity in the early afternoon. The glacier wind is characterized by strongly turbulent flow. See katabatic wind. A gap wind or canyon wind through a gorge. A sudden squall of rain or sleet in England. Gosling blast Any disturbed state of the atmosphere, especially as affecting the earth's surface, implying inclement and possibly destructive weather. A wind resulting from cold air running or flowing down a slope, caused by greater air density near the slope than at the same altitude some distance horizontally from the slope. An Italian name for the northeast wind. It was given by Roman sailors to the northeast wind in the Gulf of Lions because it came from the direction of the Greek colony of Marsala (Marseilles). Wind names of similar origin are common in the western Mediterranean, for example, gregale. Any thunderstorm that is perceived by observers to be green. The perceptually dominant wavelength of light from green thunderstorms ranges from blue- green to yellow-green. The purity of the color is generally low and the physical mechanism that causes the green appearance is not understood. Although green clouds often occur in conjunction with severe weather, there is no evidence to support anecdotal attributions of the cause of this green to specific characteristics of severe storms, such as hail or tornadoes. The Maltese and best-known variant of a term for a strong northeast wind in the central and western Mediterranean and adjacent European land areas (stronger than the levante). In the Tirol, an east wind during March and April. 1. According to U.S. weather observing practice, a fog that hides less than 0.6 of the sky and does not extend to the base of any clouds that may lie above it. As an obstruction to vision in an aviation weather observation, ground fog is encoded GF. 2. See radiation fog. An air layer with its base at the ground surface and in which temperature increases with height. These often form at night over land under clear skies and are statically stable. See inversion, lapse rate. A nautical term for a violent squall of wind from mountain ravines on the Pacific side of Central America. 1. A sudden, brief increase in the speed of the wind. It is of a more transient character than a squall and is followed by a lull or slackening in the wind speed. Generally, winds are least gusty over large water surfaces and most gusty over rough land and near high buildings. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 s. 2. With respect to aircraft turbulence, a sharp change in wind speed relative to the aircraft; a sudden increase in airspeed due to fluctuations in the airflow, resulting in increased structural stresses upon the aircraft. 3. (Rare.) Same as cloudburst. A name applied to a wet sea fog or very fine drizzle that drifts in from the sea in coastal districts of eastern Scotland and northeastern England. It occurs most frequently in summer. A strong wind and sandstorm or duststorm in northern and central Sudan, especially around Khartoum, where the average number is about 24 a year. The name comes from the Arabic word habb, meaning ?wind.? Haboobs are most frequent from May through September, especially in June, but they have occurred in every month except November. Their average duration is three hours; they are most severe in April and May when the soil is driest. They may approach from any direction, but most commonly from the north in winter and from the south, southeast, or east in summer. The average maximum wind velocity is over 13 m s-1 (30 mph) and a speed of 28 m s-1 (62 mph) has been recorded. The sand and dust form a dense whirling wall that may be 1000 m (3000 ft) high; it is often preceded by isolated dust whirls. During these storms, enormous quantities of sand are deposited. Haboobs usually occur after a few days of rising temperature and falling pressure. Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus. An individual unit of hail is called a hailstone. By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin, formerly called small hail, may be classed as either ice pellets or snow pellets. Thunderstorms that are characterized by strong updrafts, large liquid water contents, large cloud-drop sizes, and great vertical height are favorable to hail formation. The destructive effects of hailstorms upon plant and animal life, buildings and property, and aircraft in flight render them a prime object of weather modification studies. In aviation weather observations, hail is encoded A. Any storm that produces hailstones that fall to the ground; usually used when the amount or size of the hail is considered significant. Particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog. A wind that opposes the intended progress of an exposed, moving object, for example, rendering an airborne object's airspeed greater than its groundspeed; the opposite of a tailwind. In popular terminology, a thunderstorm of the air mass type that develops near the end of a hot, humid summer day; this term has no precise technical meaning. Rain with a rate of accumulation exceeding a specific value that is geographically dependent. In the United States, the frequent fog on the slopes of the coastal mountains of California, especially applied when the fog overtops the range and extends as stratus over the leeward valleys. Generally, a thunderstorm based at a comparatively high altitude in the atmosphere, roughly 2400 m or higher. These storms form most strikingly over arid regions, and frequently their precipitation is evaporated before reaching the earth's surface. General term for winds characterized by intense heat and low relative humidity, such as summertime desert winds or an extreme foehn. A hurricane radar band of circular or spiral shape associated with a tropical cyclone (hurricane or typhoon). Made evident by radar observations, hurricane bands typically curve cyclonically inward toward the center of the storm. The bands may be classified as primary if they merge into the eyewall encircling the eye of the storm, or secondary if they are disconnected from the eyewall. Hurricane bands generally move slowly around the center of the storm in the direction of the hurricane circulation. See banded structure. A local strong wind that blows through the Dzungarian Gate (in western China), a gap in the mountain ridge separating the depression of Lakes Balkash and Ala Kul from that of Lake Ebi Nor. The wind resembles the foehn and brings a sudden rise of temperature, in winter from about -26? to about -1?C. A type of fog, composed of suspended particles of ice, partly ice crystals 20 to 100 m in diameter, but chiefly, especially when dense, droxtals 12?20 m in diameter. It occurs at very low temperatures, and usually in clear, calm weather in high latitudes. The sun is usually visible and may cause halo phenomena. Ice fog is rare at temperatures warmer than -30?C, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45?C in the vicinity of a source of water vapor. Such sources are the open water of fast-flowing streams or of the sea, herds of animals, volcanoes, and especially products of combustion for heating or propulsion. At temperatures warmer than -30?C, these sources can cause steam fog of liquid water droplets, which may turn into ice fog when cooled (see frost smoke). See ice-crystal haze, arctic mist. A storm characterized by a fall of freezing liquid precipitation. The attendant formation of glaze on terrestrial objects creates many hazards. A circulation similar to a sea breeze, except not at a shore. The inland sea breeze is a very weak thermal circulation caused by temperature contrast between different land surfaces and is sometimes observed between cool irrigated farm land and neighboring dry desert land. This phenomenon is observed only when the synoptic-scale winds are very light. In meteorology, a departure from the usual decrease or increase with altitude of the value of an atmospheric property; also, the layer through which this departure occurs (the inversion layer), or the lowest altitude at which the departure is found (the ?base of the inversion?). A local wind created by acceleration of the airflow through a gap, constriction, or channel in a mountain range or between ranges. The region of a jet stream axis with the greatest winds. Relatively strong winds concentrated within a narrow stream in the atmosphere. The axis of maximum wind speed in a jet stream. The process by which negative charge centers at successively more distant locations in a thundercloud are tapped for discharge by successive strokes of cloud-to-ground lightning. In Scotland, a heavy fall of snow. A hot, dry, west or southwest wind of foehn type in the lee of the Sri Lanka hills during the southwest monsoon in June and July. In India, a short-lived dusty squall at the onset of the southwest monsoon (April? June) in Bengal. A violent northeast wind of Central Asia occurring during spring and summer. On the Bulgarian coast, a west wind that usually follows rain and persists for one to three days. A violent east wind on Lake Tanganyika in Africa. A strong southwest wind on the southern shore of the Gulf of Aden, especially at Berbera, Somaliland, during the southwest monsoon. A front (usually a cold front) at which the warm air descends the frontal surface (except, presumably, in the lowest layers). Frontal surface above which air is descending. Most widely used in mountain meteorology to denote a downslope flow driven by cooling at the slope surface during periods of light larger-scale winds; the nocturnal component of the along-slope wind systems. A point of maximum decrease in atmospheric pressure over a specified interval of time; on synoptic charts, a point of greatest negative pressure tendency; opposed to a pressure- rise center. A moderate to gale-force southeasterly wind in the Persian Gulf; it is accompanied by gloomy weather, rain, and squalls. In England, a heavy fall of rain. 1. Cloud forms that arise from Kelvin?Helmholtz waves. 2. Vortical structures that result from the growth and nonlinear development of unstable waves in a shear flow. The billows get their name from the instability responsible for the growth of the unstable waves, Kelvin?Helmholtz instability. (Also spelled camsin, chamsin, kamsin, khamasseen, khemsin.) A dry, dusty, and generally hot desert wind in Egypt and over the Red Sea. It is generally southerly or southeasterly, occurring in front of depressions moving eastward across North Africa or the southeastern Mediterranean. The deep khamsins occur in spring with depressions traveling east-northeast across the northern Sahara. They are preceded by a heat wave lasting about three days and are followed by a duststorm. The passage of the depression is marked by a cold front bringing Mediterranean air and a sudden drop in temperature. See ghibli, chili, sirocco. A cold southwest wind of Simons Bay, South Africa. Local name for a strong southeast wind in the vicinity of Palmer in the Matanuska Valley of Alaska. The knik wind blows most frequently in the winter, although it may occur at any time of year. In winter the knik winds are accompanied by very pronounced temperature rises; cases of more than 10?C in 24 hours have been observed. These winds may last from one to ten days. They result from a pressure gradient normal to the Chugach Mountains, causing a pronounced foehn effect in the Matanuska Valley. A dry foehnlike wind from southeast or south in Cheribon and Tegal in Indonesia. It is caused by the east monsoon that develops a jet effect in passing through the gaps in the mountain ranges and descends on the leeward side. A stormy, rain-bringing wind from the southwest or south-southwest in Hawaii. It blows about five times a year on the southwest slopes that are in the lee of the prevailing northeast trade winds. Kona is the Polynesian word for ?leeward.? It is associated with a southward or a southeastward swing of the Aleutian low and the passage of a secondary depression (kona cyclone) from northwest to southeast, north of the islands. (Also spelled kosava, koschawa.) A cold, very squally wind, descending from the east or southeast in the region of the Danube ?Iron Gate? through the Carpathians, continuing westward over Belgrade, thence spreading northward to the Rumanian and Hungarian borderlands and southward as far as Nish. In winter it brings temperatures down to below -29?C and it is cool even in summer, when it is also dusty. It usually occurs with a depression over the Adriatic and high pressure over southern Russia, a frequent situation in winter. It is usually explained as a jet-effect wind through the Iron Gate, giving speeds well above the gradient wind, but J. K?ttner (1940) regards it rather as a katabatic wind intermediate between foehn and bora. The kossava has a marked diurnal variation, with its maximum occurring between 5 A.M and 10 A.M. (Also spelled Krakatau; formerly called overtrades.) A layer of easterly winds over the Tropics at an altitude of about 18?24 km. This layer tops the midtropospheric westerlies (the antitrades), is at least 6 km deep, and is based at about 2 km above the tropopause. This easterly current is more prominent and better defined in the summer hemisphere. It derives its name from the observed behavior of the volcanic dust carried around the world after the great eruption of Krakatoa (6?S, 105?E) in 1883. A long strip of cloud that sometimes lies against the southern base of Mount Etna in Sicily. It is said to herald rain. A moderate to strong southwest wind in Provence (southeastern France), mild, humid, and very cloudy or rainy. On the coast it raises a rough sea. It is not frequent, occurring only in March. In the Swiss? French Alps it is locally termed labech, and is squally with thunder, hail, and brief torrential downpours; it comes mainly in autumn and winter. Severe squalls during the change of seasons in October and November in Arabia. A wind, similar in origin to the sea breeze but generally weaker, blowing from the surface of a large lake onto the shores during the afternoon; it is caused by the difference in surface temperature of land and water as in the land and sea breeze system. In addition to area, the depth of the lake is an important factor; a shallow lake warms up rapidly and is less effective as the source of a lake breeze in summer than is a deep lake. Lake breezes are well developed around the Great Lakes of North America, where they temper the summer heat. Generally, the effect of any lake in modifying the weather about its shore and for some distance downwind. In the United States, this term is applied specifically to the region about the Great Lakes or the Great Salt Lake. More specifically, lake effect often refers to the generation of sometimes spectacular snowfall amounts to the lee of the Great Lakes as cold air passes over the lake surface, extracting heat and moisture, resulting in cloud formation and snowfall downwind of the lake shore. Localized, convective snow bands that occur in the lee of lakes when relatively cold airflows over warm water. In the United States this phenomenon is most noted along the south and east shores of the Great Lakes during arctic cold-air outbreaks. Snowstorm occurring near or downwind from the shore of a lake resulting from the warming (destabilization) and moistening of relatively cold air during passage over a warm body of water. (Also called lamb-blasts, lamb-showers, lamb storm.) A slight fall of snow in the spring in England. A coastal breeze blowing from land to sea, caused by the temperature difference when the sea surface is warmer than the adjacent land. Therefore, it usually blows by night and alternates with sea breeze, which blows in the opposite direction by day. See puelche, karif. In England, a heavy fall of rain, accompanied by a high wind. (Rare.) A tornado. 2. Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer. Stratus cloud; a continuous cloud sheet capped by an inversion. 1. Any wave disturbance that is caused by, and is therefore stationary with respect to, some barrier in the fluid flow. Whether the wave is a gravity wave, inertia wave, barotropic wave, etc., will depend on the structure of the fluid and the dimensions of the barrier. 2. A mountain wave occurring to the lee of a mountain or mountain barrier. These waves can become visible in the form of lenticular or trapped lee-wave clouds. Region of convergence, often a line, downwind of a mountain or mountain ridge during fair-weather daytime conditions that are favorable for the formation of thermally forced upslope flow and deep convective mixing. Convergence forms between upslope (or sometimes light and variable) flow at lower elevations of the lee slopes and downslope flow at higher elevations, which results from the downward convective mixing of ambient momentum from the flow above ridgetops. With moist upslope flow and favorable conditions, updrafts produced by the convergence can lead to mountain cumulus formation, or trigger thunderstorm or severe weather activity. A commonly used term for clouds of the species lenticularis. Spanish nautical term for east wind. The name is given to a hot, dry, dusty easterly, or southeasterly wind that blows from the Atlantic coast of Morocco out to Madeira and the Canary Islands. It is a form of sirocco and occurs in front of depressions advancing eastward. Compare levanto. The Spanish and most widely used term for an east or northeast wind occurring along the coast and inland from southern France to the Straits of Gibraltar. It is moderate or fresh (not as strong as the gregale), mild, very humid, overcast, and rainy; it occurs with a depression over the western Mediterranean Sea. In summer it is rare and weak; in January it is inhibited by the Iberian anticyclone. It is most frequent from February to May and October to December. A levant (French spelling) with fine weather is a levant blanc; in the Roussillon region of southern France (where, as along the Catalonian coast of Spain, it is called llevant) it often brings floods in the mountain streams. The levanter of the Gibraltar Straits is a related phenomenon. Compare leste, lombarde, levantera. A persistent east wind in the Adriatic, usually bringing cloudy weather. It is a hot, sand- and dust-laden wind from between southeast and southwest that blows in front of a depression on the southeast coast of Spain but extends only a few miles inland. Level of free convection LFC Italian name for a southwest wind; used especially in northern Corsica for the west or southwest wind that blows throughout the year, and especially in winter when it is often stormy. On windward slopes it brings rain, with thunderstorms in summer and autumn. After crossing the mountains it is warm and dry, but may be very turbulent. A name sometimes given the west wind through the Straits of Gibraltar. In the Beaufort wind scale, a wind with a speed from 1 to 3 knots (1 to 3 mph) or Beaufort Number 1 (Force 1). In the Beaufort wind scale, a wind with a speed from 4 to 6 knots (4 to 7 mph) or Beaufort Number 2 (Force 2). The occurrence of air temperature below 0?C (32?F) that kills some, but not all, annual vegetation. This often occurs in the 0? to -1?C (32??30?F) range. See freeze. A thin and more or less patchy deposit of hoarfrost on surface objects and vegetation. Lightning is a transient, high-current electric discharge with pathlengths measured in kilometers. The most common source of lightning is the electric charge separated in ordinary thunderstorm clouds. A squall that occurs along a squall line. This term is now confined mostly to nautical usage. A storm of mesometeorological scale; thus, thunderstorms, squalls, and tornadoes are often put in this category. 1. Winds that, over a small area, differ from those that would be appropriate to the general large-scale pressure distribution, or that possess some other peculiarity. Often these winds have names unique to the area where they occur. Local winds may be classified into three main groups. The first includes diurnally varying airflows that are driven by local gradients of surface heat flux (e.g., near the shore of a sea or lake) or by diurnal heating or cooling of the ground surface in areas of sloping or mountainous terrain. These include land and sea breezes, mountain?valley circulations, and drainage and slope winds. The second group consists of winds produced by the interaction of a synoptic-scale flow with orography. These may be further subdivided into barrier jets, gap winds, downslope windstorms, and include such local phenomena as the tehuantepecer, Santa Ana, foehn, mistral, and bora. The third group includes those winds accompanying convective activity, more specifically individual thunderstorms or mesoscale convective systems. These are generally the surface manifestations of precipitation- cooled diverging outflow and in some locations are given special names due to the distinctive character of the weather associated with them (e.g., the haboob). 2. Local or colloquial names given to frequently occurring or particularly noteworthy winds (sometimes because of the bad weather associated with them), usually from a certain direction. Often these names reflect the direction from which the wind comes (e.g., sou'wester, nor'easter). An easterly wind (from Lombardy) that predominates along the French?Italian frontier. It comes from the High Alps. In winter it is violent and forms snowdrifts in the mountain valleys. In the plains it is gentle and very dry. It is associated with an anticyclone over France and central Europe, or with high pressure to the southeast of Europe and low pressure to the northwest along with falling pressure over western France. 1. A damp unpleasant wind that blows from the south in Madras (India). 2. A wind from the northeast at night in Sri Lanka. A mesoscale zone of anticyclonically turning winds that develops downstream of the Cheyenne Ridge in northeast Colorado and southeast Wyoming, and is often centered just east of the foothills of the Rocky Mountains near the town of Longmont, Colorado. The cause of the feature is the interaction of the ambient low level northwest flow with the east?west terrain feature known as the Cheyenne Ridge. See also Denver convergence?vorticity zone. (Also called mackerel gale.) A wind that ruffles the water, favoring the catching of mackerel. Same as a mackerel breeze A northwesterly wind with fine weather that blows, especially in summer, in the Adriatic; it is most frequent on the western shore and is equivalent to the etesians of the eastern Mediterranean. It is also found on the coasts of Corsica and Sardinia. Compare mistral. Maestro A wind, named after the Maloja Pass between the Engadine and Bergall, Switzerland, that blows down the valley of the Upper Engadine by day and either up or down by night. This deviation from the usual nature of mountain and valley winds is attributed to the fact that the stronger daytime valley wind from the south overtops the ridge and continues down the Engadine. A northwesterly wind with fine weather that blows, especially in summer, in the Adriatic; it is most frequent on the western shore and is equivalent to the etesians of the eastern Mediterranean. It is also found on the coasts of Corsica and Sardinia. Compare mistral. (Also called mamaliti, mamatili.) A light northwest wind of Sicily; a form of mistral. (Also called mammatus.) Hanging protuberances, like pouches, on the undersurface of a cloud. This supplementary cloud feature occurs mostly with cirrus, cirrocumulus, altocumulus, altostratus, stratocumulus, and cumulonimbus; in the case of cumulonimbus, mamma generally appear on the underside of the anvil (incus). See cloud classification. A warm moist southeast wind from the sea on the French Mediterranean coast and in the Maritime Alps, especially frequent in spring and autumn. In the Rh?ne delta it blows also from the south. The marin is associated with depressions that cross southern France or northern Spain and the Gulf of Lions. Generally, it is strong and regular, sometimes violent and turbulent in hilly country as the ayalas in the Massif Central; it is very humid, cloudy with hill fog, and often rainy (unless unaccompanied by fronts, when it is the marin blanc). The heavy rains, which may continue for one or two days on the mountain slopes, cause dangerous river floods. On the western slope of the C?vennes it becomes the autan. In the southern C?vennes the marin is called the aygalas. On the coast of Catalonia (northeast Spain) and Roussillon (southern France) it is the marinada and generally occurs with a depression centered over or south of the Gulf of Gascony. Compare sirocco. A cloud forming in maritime air containing relatively low concentrations of cloud condensation nuclei. These clouds are characterized by a broader droplet size distribution and low droplet concentrations (some 100 cm-3). See continental cloud. The local name, taken from the Matanuska River, for a strong, gusty, northeast wind that occasionally occurs during the winter in the vicinity of Palmer, Alaska. The morning wind, that is, an east wind. In the Morvan Mountains and the center of the Massif Central in France, the matinal often blows for several days, especially in summer, and brings fine weather. On winter mornings a northeast or east wind descends the western slopes of the Alps (where it is known as the matini?re) bringing cold and generally fine weather. Compare solaire. A front that forms in the low pressure zone that covers the Mediterranean between the cold air over Europe and the warm air over the Sahara. (Also called baiu front). A quasi-persistent, nearly stationary, east?west-oriented weak baroclinic zone in the lower troposphere that typically stretches from the east China coast, across Taiwan, and eastward into the Pacific, south of Japan. The term ?mei-yu? is the Chinese expression for ?plum rains.? The mei-yu front generally occurs from mid- to late spring through early to midsummer. This low-level baroclinic zone typically lies beneath a confluent jet entrance region aloft situated downstream of the Tibetan Plateau. The mei-yu/baiu front is very significant in the weather and climate of southeast Asia as it serves as the focus for persistent heavy convective rainfall associated with mesoscale convective complexes (MCCs) or mesoscale convective systems (MCSs) that propagate eastward along the baroclinic zone. The moisture source is typically the South China Sea and sometimes the Bay of Bengal. The usual lifting mechanism is low-level warm-air advection in association with a low-level jet on the equatorward flank of the baroclinic zone. Deep ascent and resulting organized MCCs/MCSs are especially favored when the low-level warm-air advection is situated beneath the favorable equatorward jet entrance region aloft. 1. (Also spelled melt?mi.) A strong wind from the northeast or east that often sets in suddenly and blows during the day in summer on the Bulgarian coast and in the Bosporus. 2. Same as monsoon. The altitude interval throughout which ice-phase precipitation melts as it descends. The top of the melting layer is the melting level. The melting layer may be several hundred meters deep, reflecting the time it takes for all the hydrometeors to undergo the transition from solid to liquid phase. The temperature of the melting layer is typically 0?C or slightly warmer. See bright band. The altitude at which ice crystals and snowflakes begin to melt as they descend through the atmosphere. In cloud physics and in radar meteorology, this is the accepted term for the 0?C constant-temperature surface (see bright band). It is physically more apt than the corresponding operational term, freezing level, for melting of pure ice must begin very near 0?C, but freezing of liquid water can occur over a broad range of temperatures (between 0? and -40?C; see supercooling). See also freezing point, ice point, melting point. Northwest wind on Lake Maggiore, Italy. The wind or wind component along the local meridian, as distinguished from the zonal wind. In a horizontal coordinate system fixed locally with the x axis directed eastward and the y axis northward, the meridional wind is positive if from the south, and negative if from the north. An anticyclonically rotating vortex, around 210 km in diameter, in a convective storm. A cyclonically rotating vortex, around 2?10 km in diameter, in a convective storm. The vorticity associated with a mesocyclone is often on the order of 10-2 s-1 or greater. (It should be noted that a mesocyclone is not just any cyclone on the mesoscale; it refers specifically to cyclones within convective storms.) Mesocyclones are frequently found in conjunction with updrafts in supercells. Tornadoes sometimes form in mesocyclones. Persistent mesocyclones that have significant vertical extent are detected by Doppler radar as mesocyclone signatures. Tornado warnings may be issued when a mesocyclone signature is detected. The Doppler velocity pattern of a mesocyclone within a severe thunderstorm. In a storm-relative reference frame, the idealized signature is symmetric about the radar viewing direction with marked azimuthal shear across the core region between peak Doppler velocity values of opposite sign. Typical signatures consist of Doppler velocity differences of 25? 75 m s-1 across core diameters of 2?8 km, with resulting azimuthal shear values of 5 ? 10-3 s-1 to 2 ? 10-2 s-1. A mesoscale wind maximum. It typically may have an along-flow length scale of tens to hundreds of kilometers and a cross- flow length scale of 100 km. Mesojets differ from planetary-scale jets, which can have length scales of several thousand kilometers, and synoptic-scale jets, which may have length scales of 1000? 2000 km and are commonly found in association with progressive synoptic-scale troughs and ridges. Larger mesojets may also sometimes be known as jet streaks. Mesojets can form adjacent to prominent orographic features in association with terrain-channeled flow. Mesojets are also seen in association with organized mesocale convective systems as typified by the evaporatively driven rear-inflow jet commonly found behind active squall lines lines. Mesojets may also be found in conjunction with prominent lower-tropospheric stable layers where the airflow can become decoupled from the planetary boundary layer, especially at night. An exceptionally well organized lower-tropospheric mesojet extending over hundreds of kilometers might be known as a low-level jet. A low pressure area on the mesoscale. It has been used to refer both to features observed within convective storms and features even larger in scale. Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers, including thunderstorms, squall lines, fronts, precipitation bands in tropical and extratropical cyclones, and topographically generated weather systems such as mountain waves and sea and land breezes. From a dynamical perspective, this term pertains to processes with timescales ranging from the inverse of the Brunt?V?is?l? frequency to a pendulum day, encompassing deep moist convection and the full spectrum of inertio-gravity waves but stopping short of synoptic-scale phenomena, which have Rossby numbers less than 1. A regular pattern of convective cells that can develop in an atmospheric boundary layer heated from below or radiatively cooled from cloud top. This phenomenon is readily observed in satellite imagery during cold air outbreaks when continental air passes over the relatively warm coastal ocean. Cloud lines, marking horizontal roll vortices, form initially in the developing marine atmospheric boundary layer. These lines evolve into open cells, which are defined by clouds in the upward motion along the edges of honeycomb- shaped cells, with less cloudy subsiding air in their centers. The convective structure further evolves into closed cells, which have cloudy centers and cloud-free edges. Mesoscale Convective Complex A subset of mesoscale convective systems (MCS) that exhibit a large, circular (as observed by satellite), long-lived, cold cloud shield. Alternatively, a dynamical definition of an MCC requires that the system have a Rossby number of order 1 and exhibit a horizontal scale comparable to the Rossby radius of deformation. In midlatitude MCS environments, the Rossby radius of deformation is about 300 km. A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction. An MCS exhibits deep, moist convective overturning contiguous with or embedded within a mesoscale vertical circulation that is at least partially driven by the convective overturning. Same as disturbance except that the atmospheric feature has a Rossby number of order 1 and exhibits a horizontal scale comparable to the Rossby radius of deformation. For such systems, both ageostrophic advection and rotational influences are important. A sharp horizontal temperature contrast of a few degrees Celsius within a width of tens of centimeters to a few meters along the trailing edge of a thermal plume in the surface layer of the atmospheric boundary layer. A warm, moist sea breeze from the south that sets in at midday in Provence, France, south of Mount Ventoux. In the Roussillon region the midday south wind (mitgjorn) is irregular and generally light, and is dry after crossing the Pyrenees. Convective storm that contains similar radar characteristics to those of a supercell (e.g., hook echo, WER, BWER), but is significantly smaller in height and width. The diameter of the radar-detected rotation is 1?8 km. This is a relatively new storm type, the existence of which has been confirmed by data from the recently installed WSR?88D radars in the United States. Mini-supercells occur in areas where the height of the equilibrium level is low, most often in the northern United States, but possibly under certain weather conditions in any area of the world. They are sometimes found in landfalling tropical cyclones. A ridge of smaller scale than a long-wave ridge. It ordinarily moves rapidly and is associated with a migratory anticyclonic disturbance in the lower troposphere. See short wave. A pressure trough of smaller scale than a long-wave trough. It ordinarily moves rapidly and is associated with a migratory cyclonic disturbance in the lower troposphere. See short wave. A cloud containing both water drops (supercooled at temperatures below 0?C) and ice crystals, hence a cloud with a composition between that of a water cloud and that of an ice- crystal cloud. A type of atmospheric boundary layer characterized by vigorous turbulence tending to stir and uniformly mix, primarily in the vertical, quantities such as conservative tracer concentrations, potential temperature, and momentum or wind speed. The thickness, zi, of the mixed layer, defined as the location of a capping temperature inversion or statically stable layer of air. Often associated with, or measured by, a sharp increase of potential temperature with height, a sharp decrease of water-vapor mixing ratio, a sharp decrease in turbulence intensity, a sharp decrease in pollutant concentration, a change of wind speed to geostrophic, a minimum of turbulent heat flux, and a maximum of signal intensity from remote sensors such as sodars and wind profilers. Quite variable in space and time, the mixed-layer depth typically increases during fair-weather daytime over land from tens of meters shortly after sunrise to 1?4 km before sunset, depending on the location and season. The three-part change of the atmospheric boundary layer that typically occurs during fair weather over land on sunny days. In the early morning, the mixed layer is shallow, slowly deepening, cool (in a potential temperature sense), and is capped by the remains of the stable boundary layer from the previous night. In mid- to late morning, the top of the mixed layer exhibits rapid rise as heating eliminates the nocturnal inversion, and the mixed layer grows through the residual layer. The third stage in late morning and afternoon is that of a deep (order of 1?2 km) convective boundary layer of relatively constant depth. The statically stable layer of air at the top of the atmospheric boundary layer. Because the troposphere is statically stable on the average (i.e., potential temperature increases with height), and because turbulence in the boundary layer causes potential temperatures to become somewhat well mixed there, conservation of heat requires that there be a potential temperature increase (i.e., a temperature step or inversion) at the top of the boundary layer. It is this inversion that separates the boundary layer from the rest of the troposphere by limiting the domain of turbulence. It is also responsible for trapping pollutants near the ground during fair weather. Removal of pollutants out of the top of the atmospheric boundary layer through the mixed-layer capping inversion. Normally pollutants cannot escape through the capping inversion. However, penetrating cumulus clouds, thunderstorms, mountain circulations, and frontal circulations can force polluted air through the inversion to vent pollutants into the free atmosphere. A cloud formed when two subsaturated volumes of moist air with different temperatures and vapor pressures mix isobarically and adiabatically to form a volume of moist air with an intermediate temperature and vapor pressure above the saturation value at that temperature. Fog, light and of short duration, produced by the mixing of two moist but nonsaturated air masses with different temperatures. A stationary bank of cirriform cloud marking the upper portion of the system of lenticular clouds formed in the lee wave produced by flow across the Sudeten Mountains in southeastern Germany; a type of foehn cloud. The strong wind blowing across a mountain crest, responsible for the formation of the Moazagotl cloud. In the Beaufort wind scale, a wind with a speed from 11 to 16 knots (13 to 18 mph) or Beaufort Number 4 (Force 4). In the Beaufort wind scale, a wind with a speed from 28 to 33 knots (32 to 38 mph) or Beaufort Number 7 (Force 7). Atmospheric convection in which the phase changes of water play an appreciable role. All cumuliform clouds are manifestations of moist convection. The enthalpy exchange between condensing water vapor or freezing liquid water and air (see latent heat) is a major contributor to the positive buoyancy of updrafts, while the reverse exchange between air and evaporating water or melting ice contributes strongly to the negative buoyancy of downdrafts. An extension or protrusion of moist air into a region of lower moisture content. Cloudiness and precipitation are closely related to moist tongues. The development of an area in the boundary layer (e.g., often observed on surface or 850-mb charts) where moisture values become higher than in the surrounding region. Moisture pooling typically occurs in an area of low-level convergence during the warm (growing) season, and can have a significant effect on convection initiation and evolution. Breeze blowing from Arve toward Geneva in Switzerland. A depression that forms within the monsoon trough. The term is most frequently used to describe weak cyclonic disturbances that form over the Bay of Bengal and generally track northwestward over the Indian subcontinent. These occasionally intensify into tropical cyclones if they remain over warm ocean water long enough. The term is also used to describe depressions that form within the monsoon trough near Australia and in the western North Pacific region. The term has gained ascendancy in use to refer to a broad tropical cyclonic vortex characterized by 1) its large size, where the outermost closed isobar may have a diameter on the order of 600 n mi (1000 km); 2) a loosely organized cluster of deep convective elements, which may form an elongated band of deep convection in the east semicircle; 3) a low- level wind distribution that features a 100 n mi (200 km) diameter light-wind core, which may be surrounded by a band of gales or contain a highly asymmetric wind field; and 4) a lack of a distinct cloud system center. Most monsoon depressions that develop in the western North Pacific eventually acquire persistent central convection and accelerated core winds, marking their transitions into conventional tropical cyclones. An advection fog produced as a monsoon circulation that transports warm moist air over a colder surface. The temporary extension of deep monsoon flow into a region not normally dominated by persistent monsoon flow. This temporary extension or surge may last from a few days to three weeks. These surges most commonly occur eastward across the Philippine Sea into the western North Pacific and east of Australia into the western South Pacific. The establishment of a reverse-oriented monsoon trough is accompanied by an eastward surge in the monsoon flow. Monsoon surges are often precursors to the development of tropical cyclones. A convection of the summer monsoon circulation of the western North Pacific characterized by 1) a very large nearly circular low-level cyclonic vortex (not the result of the expanding wind field of a preexisting monsoon depression or tropical cyclone) that has an outermost closed isobar with a diameter on the order of 1200 n mi (2500 km); 2) a cloud band bordering the southern through eastern periphery of the vortex/surface low; and 3) a relatively long (two week) life span. Initially, a subsequent regime exists in its core and western and northwestern quadrants with light winds and scattered low cumulus clouds; later, the area within the outer closed isobar may fill with deep convective cloud and become a isobar or tropical cyclone. Note: a series of midget tropical cyclones may emerge from the ?head? or leading edge of the peripheral tropical cyclone of a monsoon gyre. The night land breeze on Lake Geneva, Switzerland. It blows from the north from 5-7 P.M. until 7-9 A.M. as a poweful breeze. In the late fall and winter it blows almost throughout the day. A nocturnal component of the mountain?plains or mountain?valley wind systems encountered during periods of light synoptic flow. Fog formed by orographic lifting to condensation of moist air up a mountain slope. A local wind blowing through a gap between mountains, a gap wind. This term was introduced by R. S. Scorer (1952) for the surface winds blowing through the Strait of Gibraltar. When air stratification is stable, as it usually is in summer, the air tends to flow through the gap from high to low pressure, emerging as a ?jet? with large standing eddies in the lee of the gap. The excess of pressure on the upwind side is attributed to a pool of cold air held up by the mountains. Similar winds occur at other gaps in mountain ranges, such as the tehuantepecer and the jochwinde, and in long channels, such as the Strait of Juan de Fuca between the Olympic Mountains of Washington and Vancouver Island, British Columbia. Compare jet-effect wind, canyon wind, mountain wind. The diurnal cycle of local winds in a mountain valley during clear or mostly clear periods of weak synoptic flow. The diurnal cycle of local winds between a mountain or a mountain range and the adjacent or surrounding plains during periods of weak synoptic flow. An atmospheric gravity wave, formed when stable air flow passes over a mountain or mountain barrier. A cloud that forms in the rising branches of mountain waves and occupies the crests of the waves. The most distinctive are the sharp-edged, lens-, or almond-shaped lenticular clouds, but a variety of stratocumulus, altocumulus, and cirrocumulus forms appear in both the main, vertically propagating waves and in the lee waves. See mountain wave, foehn cloud. A nocturnal, thermally forced wind from the direction of the mountains, generated by cooling along the mountain slopes; a downvalley wind, or the nighttime downslope (katabatic) component of a mountain?plains wind system. A convective storm system usually composed of a cluster of ordinary convective cells at various stages of their life cycle. New cells within the convective system 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. A multicell storm may have a lifetime of several hours, and may also have supercells incorporated as a part of the system as well. See also cell, ordinary cell, supercell, thunderstorm. The Arabic name for a northeasterly wind that ocurrs in winter on the Iranian coast of the Persian Gulf, especially near the entrance to the gulf and also on the Makran coast. (Also spelled narbon?.) In France, a wind coming from Narbonne; a north wind in the Roussillon region of southern France resembling the tramontana. If associated with an influx of arctic air, it may be very stormy with heavy falls of rain or snow. It is especially violent in the region of Perpignan where it blows in a succession of squalls for several days. In Provence it is rarer and blows from the west. In lower Languedoc and the southern C?vennes, the narbonnais is an infrequent, mild, moist, moderate southwest wind in winter and early spring, sometimes bringing thunderstorms. Wind with a speed between 28 and 33 knots or force 7 on the Beaufort wind scale. Dry squalls that occur at night in southwest Africa and the Congo. It is likely that this term is loosely applied to other diurnal local winds such as mountain wind, land breeze, midnight wind, etc. A principal cloud type (cloud genus), gray colored and often dark, rendered diffuse by more or less continuously falling rain, snow, sleet, etc., of the ordinary varieties and not accompanied by lightning, thunder, or hail. (Rarely called luminous clouds.) Thin silvery-blue cirrus-like clouds frequently seen during summer twilight conditions at high latitudes (above 50?) in both hemispheres. They are the highest visible clouds in the atmosphere, occurring in the upper mesosphere at heights of about 85 km, and are closely related to the polar mesospheric clouds seen in satellite observations at similar altitudes over the summer polar cap. Noctilucent clouds are now known to consist of tiny ice particles with dimensions of the order of tens of nanometers, growing in the extreme cold of the summer polar mesopause region. The condensation nuclei on which the particles grow are thought to be either smoke and dust particles of meteoric origin or large hydrated positive ions. Strong upwelling of air from below, associated with a pole-to-pole meridional circulation in the upper mesosphere, is responsible for both the extreme cold and the upward flux of water vapor. Although water-vapor mixing ratios are very low (less than 10 parts per million by volume) in the region, the temperatures are also low enough to produce a high degree of supersaturation at times. Anomalously strong radar echoes from the region, known as polar summer mesospheric echoes, are also associated with the clouds. Compare nacreous clouds, polar stratospheric clouds. The cool layer of air adjacent to the ground that forms at night. At night under clear skies, radiation to space cools the land surface, which in turn cools the adjacent air through processes of molecular conduction, turbulence, and radiative transfer. This causes a stable boundary layer to form and grow to depths of a few hundreds of meters, depending on the season. Many interacting processes can occur within the statically stable nocturnal boundary layer: patchy sporadic turbulence, internal gravity waves, drainage flows, inertial oscillations, and nocturnal jets. The lowering of temperature during night time, due to a net loss of radiant energy. A tornado that occurs with a parent cloud in its growth stage and with its vorticity originating in the boundary layer. The parent cloud does not contain a preexisting midlevel mesocyclone. Landspouts and gustnadoes are examples of the nonsupercell tornado. A strong, persistent northerly wind in the Philippines. The winter north wind in Spain. 2. A strong cold northeasterly wind in Mexico and on the shores of the Gulf of Mexico. It results from an outbreak of cold air from the north; actually, the Mexican extension of a norther. See chocolatero. 3. A norther in Central America. A northerly foehn wind blowing down the Italian side of the Alps. The northern slopes are normally cooler than the southern slopes, and the dynamic warming is often insufficient to overcome the difference of temperature. Hence a warm dry northerly wind of foehnlike character occurs less frequently than the south foehn. A cyclonic storm off the east coast of North America, so called because the winds over the coastal area are from the northeast. A northeast wind, particularly a strong wind or gale. A northerly wind; in general, a cold windstorm from the north. (Often contracted nor'wester.) A northwesterly wind (as Canterbury northwester). In U.S. weather observing practice, the designation for the sky cover when the sky is completely hidden by surface-based obscuring phenomena. A front that forms as a cyclone moves deeper into colder air. Wind blowing from land to sea. During synoptic conditions of light winds, offshore winds near the surface often occur at night as a component of the land breeze. A wind blowing from water onto land; the wind may be a result of heating differences between land and water or related to synoptic weather patterns. Mountain clouds produced by orographic lifting of moist air to saturation. Fog formed as moist air blows up a mountain slope and becomes saturated. Ascending air flow caused by mountains. Mechanisms that produce the lifting fall into two broad categories: 1) the upward deflection of horizontal larger-scale flow by the orography acting as an obstacle or barrier; or 2) the daytime heating of mountain surfaces to produce anabatic flow along the slopes and updrafts in the vicinity of the peaks. For flow over a hill, the top layer in the boundary layer that accelerates relative to its upstream value due to the Bernoulli effect. A surface boundary formed by the horizontal spreading of thunderstorm-cooled air. Nocturnal cold-air jet flowing out of the mouth of a valley or canyon as it opens onto a plain. A domelike protrusion above a cumulonimbus anvil, representing the intrusion of an updraft through its equilibrium level. A cumulus cloud that is no longer dynamically connected with the atmospheric boundary layer via updrafts or downdrafts. Strong straight-line winds associated with nontornadic outflow from strong thunderstorms. A straight line vortex, the flow of which can be modeled in two dimensions, with a point concentration of vorticity surrounded by irrotational flow. An airmass thunderstorm that forms rapidly in an otherwise rain-free environment. This most often occurs on warm, humid days, in unstable meteorological conditions. The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels. A squall line less than about 100 km ahead of a cold front, in the warm sector, having an orientation more or less parallel to the cold front. Quantity Of Precipitation Amount of precipitation in all forms In meteorology, the result of radiative cooling of the earth's surface and adjacent air. Radiational cooling occurs, as is typical on calm, clear nights A common type of fog, produced over a land area when radiational cooling reduces the air temperature to or below its dewpoint. The complete cloud and precipitation structure associated with an area of rainfall sufficiently elongated that an orientation can be assigned. A region of sharply reduced precipitation on the lee side of an orographic barrier, as compared with regions upwind of the barrier. The middle portion of the nocturnal atmospheric boundary layer characterized by weak sporadic turbulence and initially uniformly mixed potential temperature and pollutants remaining from the mixed layer of the previous day. The intense luminosity that propagates upward from earth to cloud base in the last phase of each lightning stroke of a cloud-to-ground discharge. A low-level, horizontal, tube-shaped arcus cloud associated with a gust front of a convective storm or occasionally a cold front. A haze created by the presence of finely divided particles of sea salt in the air, usually derived from the evaporation of sea spray. Reduced visibility in the atmospheric boundary layer caused by suspended particles of soil, mixed into the air during strong winds. Removal of pollutants from the air by either rain or snow. Rainout (or snowout), which is the in-cloud capture of particulates as condensation nuclei, is one form of scavenging. A coastal local wind that blows from sea to land, caused by the temperature difference when the sea surface is colder than the adjacent land. The horizontal discontinuity in temperature and humidity that marks the leading edge of the intrusion of cooler, more moist marine air associated with a sea breeze. A special (and rare) case of the process of occlusion, where the point at which the cold front first overtakes the warm front (or quasi-stationary front) is at some distance from the apex of the wave cyclone. A low-level, horizontal, wedge-shaped arcus cloud associated with a convective storm's gust front. In general, any destructive storm, but usually applied to severe local storms in particular, that is, intense thunderstorms, hailstorms, and tornadoes. A natural fog contaminated by industrial pollutants, a mixture of smoke and fog. The amount of snow on a mountain; the depth of snow that has fallen. A layer in the middle or upper troposphere in widespread precipitation in which ice crystals form in small convective cells and fall to lower altitudes. The process by which a single convective cell splits into two supercells, one dominated by cyclonic rotation and the other by anticyclonic rotation, their paths then deviating substantially from each other and other nearby convective cells. A strong wind characterized by a sudden onset, a duration of the order of minutes, and then a rather sudden decrease in speed. A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms. A wind that assists the intended progress of an exposed, moving object, for example, rendering an airborne object's groundspeed greater than its airspeed; the opposite of a headwind. A temperature-measuring system in which the thermally sensitive element is located at a distance from the indicating element. The mean wind-shear vector in geostrophic balance with the gradient of mean temperature of a layer bounded by two isobaric surfaces. The sound emitted by rapidly expanding gases along the channel of a lightning discharge. The relatively cool pool of air that results when a thunderstorm downdraft reaches the earth's surface and spreads horizontally as a density current. In synoptic meteorology, the inclination to the vertical of a significant feature of the circulation (or pressure) pattern or of the field of temperature or moisture. The wind system, occupying most of the Tropics, that blows from the subtropical highs toward the equatorial trough; a major component of the general circulation of the atmosphere. In meteorology, an elongated area of relatively low atmospheric pressure; the opposite of a ridge. Air in which static instability prevails. Long narrow sheets of warm air rising from a heated surface. A type of fog formed when air flows upward over rising terrain and is, consequently, adiabatically cooled to or below its dewpoint. A wind directed up a slope, often used to describe winds produced by processes larger in scale than the slope. A wind that ascends a mountain valley (upvalley wind) during the day; the daytime component of a mountain?valley wind system. In the Northern Hemisphere, a wind that rotates in a clockwise direction with increasing height; the opposite of backing wind. A local, often abrupt lowering from a cumulonimbus cloud base into a low-hanging accessory cloud, normally a kilometer or more in diameter. A narrow stream of air that transports large amounts of heat, moisture, and westerly momentum Warm Front That area, within the circulation of a wave cyclone, where the warm air is found. Traditionally, it lies between the cold front and warm front of the storm; in the typical case, the warm sector continually diminishes in size and ultimately disappears (at the surface) as the result of occlusion. 1. In general, any tornado over a body of water. 2. In its most common form, a nonsupercell tornado over water. A type of atmospheric boundary layer characterized by vigorous turbulence tending to stir and uniformly mix, primarily in the vertical, quantities such as conservative tracer concentrations, potential temperature, and momentum or wind speed. The removal of atmospheric gases or particles through their incorporation into hydrometeors, which are then lost by precipitation. An atmospheric optical phenomenon in which the observer appears to be engulfed in a uniformly white glow. Air in motion relative to the surface of the earth A strong, cold, dry, west wind of eastern Asia, especially northern China, that sweeps across the plains in winter carrying a fine yellow dust from the deserts. Any soft, gentle breeze. In meteorology, the flow of air along a latitude circle; more specifically, the latitudinal (east or west) component of existing flow. A measure of strength of the middle-latitude westerlies, usually expressed as the horizontal pressure difference between 35 and 55N latitude, or as the corresponding geostrophic wind. The kinetic energy of the mean zonal wind, obtained by averaging the component of the wind along a fixed latitude circle. The elevation band on a mountain or orographic barrier that receives the greatest precipitation for a seasonal or annual average. 5 5 5 5 5 5 5 5 5 5 The component of a three-dimensional velocity vector oriented along the radial direction from the origin point or axis in polar, cylindrical, or spherical coordinates. In connection with Doppler radar, the radial velocity component is called Doppler velocity. A process lapse rate of temperature, the rate of decrease of temperature with height of a parcel of dry air lifted by a reversible adiabatic process through an atmosphere in hydrostatic equilibrium. The rate of decrease of temperature with elevation Processes by which traces gases or particles are transferred from atmosphere to a surface. The generally small, localized atmospheric circulations. A moist-adiabatic process in which the liquid water that condenses is assumed to be removed as soon as it is formed, by idealized instantaneous precipitation. The wavenumber in the zonal direction, that is, along a line of constant latitude. Atmospheric State variables governing the atmospheric state The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the column of air lying directly above the point in question. The character and amount of atmospheric pressure change during a specified period of time, often a three-hour period preceding an observation. A measure of the mean thermodynamic stability in a layer beneath 700 mb. The state of a layer of unsaturated air when its lapse rate of temperature is less than the dry-adiabatic lapse rate but greater than the moist-adiabatic lapse rate. High Pressure Low Pressure The temperature that an unsaturated parcel of dry air would have if brought adiabatically and reversibly from its initial state to a standard pressure, p0, typically 100 kPa. The height within which some parameter, such as pressure or density, decreases by a factor 1/e in an isothermal atmosphere. The atmospheric pressure at mean sea level, either directly measured or, most commonly, empirically determined from the observed station pressure. The ratio of a region's total precipitation to the amount of ?external? precipitation originating as evaporation from the oceans as opposed to evapotranspiration from the land. The temperature an air parcel would have if cooled from its initial state adiabatically to saturation, and thence brought to 1000 mb by a moist-adiabatic process. Dry Adiabatic Process 1. An adiabatic process in a hypothetical atmosphere in which no moisture is present. 2. An adiabatic process in which no condensation of its water vapor occurs and no liquid water is present. Convection that originates from an atmospheric layer above the boundary layer. The evaporation from the surface of a lake. The actual evaporation from a region of land. >Heat flux averaged over a layer of air, such as the boundary layer; heat-flux divergence or difference between the top and bottom of a layer. Analogous to convective available potential energy, except that it is related to the negative buoyancy associated with evaporative cooling of liquid water within a sinking cloudy air parcel. The character and amount of atmospheric pressure change during a specified period of time, often a three-hour period preceding an observation. The environmental lapse rate of temperature in an atmosphere in which the density is constant with height (homogeneous atmosphere), equal to g/R, where g is the acceleration of gravity and R the gas constant. property describing unstable atmosphere Instability The atmospheric pressure at mean sea level, either directly measured or, most commonly, empirically determined from the observed station pressure. Aerosol having its origin over the continents with industrial, urban, agricultural, forest, and desert sources, with potential for high concentrations of hygroscopic aerosol. (Abbreviated MIBL.) An internal boundary layer caused by advection of air across a discontinuity in surface roughness. When the new surface is rougher than the old one, the MIBL depth grows roughly as the 0.8 power of the ratio of the two roughness lengths. In this example, the MIBL grows to include the whole surface layer. The presence of substances in the atmosphere, particularly those that do not occur naturally Air Pollution A pattern of smokestack plume dispersion in a statically stable atmosphere, in which the plume spreads out in the horizontal like an oriental fan and meanders about at a fixed height with little vertical spread. The phenomenon where the upper part of a smoke plume diffuses more rapidly upward than the bottom part diffuses downward. This generally occurs when the boundary layer near the ground is more stable than it is aloft. Compare coning, fanning, looping. Type of air pollution, such as Los Angeles smog, associated with the buildup of oxidation products formed from the degradation of hydrocarbons, etc. An effect due to heat sources within a local exhaust enclosure (stack) producing convective air currents with vertical velocities proportional to the rate of heat transferred to the surrounding air and to the height of rise of the heated air. A theoretical explanation of the process by which precipitation particles may form within a mixed cloud (composed of both ice crystals and liquid water drops). A law describing the relationship of the horizontal wind direction in the atmosphere to the pressure distribution. The path of an air parcel with absolute vorticity that remains constant in horizontal flow. The energy needed to lift an air parcel vertically and pseudoadiabatically from its originating level to its level of free convection (LFC). A relationship between surface fluxes and state of the air in the mid mixed layer, which applies to situations where convective thermals are causing nonlocal transport vertically in the atmospheric boundary layer. A line of constant potential temperature on a thermodynamic diagram. A set of different forecasts all valid at the same forecast time(s). A forecast of weather conditions for a period extending beyond three or more days from the day of issuance. The occurrence of air temperature below 0?C (32?F) that kills annual vegetation without formation of frost crystals on surfaces. See freeze, dry freeze, hard freeze, light freeze. The estimation of the amount of rainfall or rainfall rates based on radar measurements or satellite data. The representation in a numerical model of the turbulent transports of heat and moisture by nonprecipitating cumulus clouds with cloud tops below 3000 m above the surface. A means of quantifying the threat of rapid cooling during breezy or windy conditions that may result in hypothermia in cold conditions. A relationship between radar reflectivity factor Z (mm6 m-3) and rain rate R (mm h-1).