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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • cable - A nautical unit of horizontal distance defined as 0.1 nautical mile = 185.2 m. Historically, a cable was defined as equal to 600 ft (100 fathoms).

  • calm - 1. The absence of apparent motion of the air. In the Beaufort wind scale, this condition is reported when smoke is observed to rise vertically, or the surface of the sea is smooth and mirrorlike. The National Weather Service reports a wind as calm when it is determined to have a speed of less than three knots. 2. See calm belt.

  • cap - (Also called lid.) A region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain its ascent. This tends to inhibit the development of convection until some physical mechanism can lift a parcel to its LFC. The intensity of the cap is measured by its convective inhibition. The term capping inversion is sometimes used, but an inversion is not necessary for the conditions producing convective inhibition to exist.

  • CAPE - Abbreviation for convective available potential energy.

  • CAPPI - (Abbreviation for constant-altitude plan position indicator.) A composite radar display constructed by assembling radar data from many PPIs at successive elevation angles to obtain the pattern of the data at a specified constant altitude.

  • capping inversion - A statically stable layer at the top of the atmospheric boundary layer. Although the word "inversion" implies that temperature increases with height, the word "capping inversion" is used more loosely for any stable layer (potential temperature increasing with height) at the top of the boundary layer. This inversion is a ubiquitous feature of the atmospheric boundary layer, formed because the troposphere is statically stable on the average, and because turbulence homogenizes air within the boundary layer, which by conservation of heat requires that a stable layer form at the top of the boundary layer. This inversion traps surface-induced turbulence and air pollutants below it, and causes the free atmosphere to not "feel" the earth's surface during fair weather (i.e., no drag, free slip, no heat or moisture from the surface, and winds are nearly geostrophic). See lid.

  • carrier - The unmodulated fundamental output of a radio or radar transmitter, which is capable of being modulated with information to produce a communications signal.

  • CAT - Abbreviation for clear-air turbulence.

  • catch - The amount of precipitation captured by a rain gauge.

  • CCL - Abbreviation for convective condensation level.

  • cell - 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.

  • channel - Specific wavelengths of onboard satellite sensors.

  • chart - A map showing, principally, the pressure pattern, height pattern, or any meteorological parameter at a specified time.

  • CIN - Abbreviation for convective inhibition.

  • circulation - 1. The flow or motion of a fluid in or through a given area or volume. 2. A precise measure of the average flow of fluid along a given closed curve. Mathematically, circulation is the line integral v · dr about the closed curve, where v is the fluid velocity and dr is a vector element of the curve. By Stokes's theorem, the circulation about a plane curve is equal to the total vorticity of the fluid enclosed by the curve. The given curve may be fixed in space or may be defined by moving fluid parcels. See circulation theorem.

  • cirrostratus - (Abbreviated Cs.) A principal cloud type (cloud genus), appearing as a whitish veil, usually fibrous but sometimes smooth, that may totally cover the sky, and that often produces halo phenomena, either partial or complete. Sometimes a banded aspect may appear, but the intervals between the bands are filled with thinner cloud veil. The edge of a veil of cirrostratus may be straight and clear-cut, but more often it is irregular and fringed with cirrus. Some of the ice crystals that compose the cloud are large enough to fall and thereby produce a fibrous aspect. Cirrostratus occasionally may be so thin and transparent as to render it nearly indiscernible, especially through haze or at night. At such times, the existence of a halo may be the only revealing feature. The angle of incidence of illumination upon a cirrostratus layer is an important consideration in evaluating the identifying characteristics. When the sun is high (generally above 50o elevation), cirrostratus never prevents the casting of shadows by terrestrial objects; and a halo might be completely circular. At progressively lower angles of the sun, halos become fragmentary and light intensity noticeably decreases. Cirrostratus may be produced by the merging of elements of cirrus (Cs cirromutatus); from cirrocumulus (Cs cirrocumulogenitus); from the thinning of altostratus (Cs altostratomutatus); or from the anvil of cumulonimbus (Cs cumulonimbogenitus). Since cirrostratus and altostratus form from each other, it is frequently difficult to delineate between the two. In general, altostratus does not cause halo phenomena, is thicker than cirrostratus, appears to move more rapidly, and has a more even optical thickness. When near the horizon, cirrostratus may be impossible to distinguish from cirrus. See cloud classification, cirriform.

  • cirrus - (Abbreviated Ci.) A principal cloud type (cloud genus) composed of detached cirriform elements in the form of white, delicate filaments, of white (or mostly white) patches, or of narrow bands. These clouds have a fibrous aspect and/or a silky sheen. Many of the ice crystal particles of cirrus are sufficiently large to acquire an appreciable speed of fall; therefore, the cloud elements have a considerable vertical extent. Wind shear and variations in particle size usually cause these fibrous trails to be slanted or irregularly curved. For this reason, cirrus does not usually tend, as do other clouds, to appear horizontal when near the horizon. Because cirrus elements are too narrow, they do not produce a complete circular halo. Cirrus often evolves from virga of cirrocumulus or altocumulus (Ci cirrocumulogenitus or Ci altocumulogenitus), or from the upper part of cumulonimbus (Ci cumulonimbogenitus). Cirrus may also result from the transformation of cirrostratus of uneven optical thickness, the thinner parts of which dissipate (Ci cirrostratomutatus). It may be difficult at times to distinguish cirrus from cirrostratus (often impossible when near the horizon); cirrostratus has a much more continuous structure, and if subdivided, its bands are wider. Thick cirrus (usually cirrus spissatus) is differentiated from patches of altostratus by its lesser extension and white color. The term "cirrus" is frequently used for all types of cirriform clouds. See cloud classification, cirriform.

  • clear - 1. After U.S. weather observing practice, the state of the sky when it is cloudless or when the sky cover is less than 0.1 (to the nearest tenth.) In aviation weather observations, a clear sky state is denoted by the symbol "O." 2. The character of the sunrise or sunset when the disk of the sun is visible at these times. Compare cloudy. 3. To change from a stormy or cloudy weather condition to one of no precipitation and decreased cloudiness. 4. In popular usage, the condition of the atmosphere when it is very transparent (as opposed to hazy, foggy, etc.) and accompanied by negligible cloudiness. In weather forecast terminology, the maximum cloudiness considered is about 0.2. Compare cloudy, fair.

  • clear air - 1. Air that is devoid of clouds or fog. 2. In some contexts, air that is devoid of any solid or liquid particles that would reduce visibility.

  • clear sky - A sky free of clouds and other obscurations as observed from the point of observation.

  • clearing - A process of clouds, fog, or other obscurations becoming less prevalent with time.

  • clo - A unit of thermal insulation, usually applied to clothing or bedcovers. It is defined as the amount of insulation necessary to maintain comfort and a mean skin temperature of 33°C (92°F) for a person who is producing heat at the standard metabolic rate (50 Kcal m-2 of body surface per hour; one met) in an indoor environment characterized by a temperature of 21°C (70°F), relative humidity of less than 50%, and air motion of 6.1 m min-1. If if is assumed that 76% of the metabolic heat is lost through the clothing, the unit can be defined in physical terms as the insulation that will restrict heat loss to 1 Kcal m-2h-1 with a temperature gradient of 0.18°C across the fabric. In the first approximation, an insulation of one clo is provided by clothing material with a total thickness of 0.64 cm and air layers (between skin and clothing and between inner and outer garments) of about 0.51 cm.

  • CLOS - Abbreviation for clear line of sight.

  • cloud - 1. A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface. Cloud differs from fog only in that the latter is, by definition, close (a few meters) to the earth's surface. Clouds form in the free atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest stratum of fog. For condensation to occur at a low degree of supersaturation, there must be an abundance of cloud condensation nuclei for water clouds, or ice nuclei for ice-crystal clouds, at temperatures substantially above -40°C. The size of cloud drops varies from one cloud type to another, and within any given cloud there always exists a finite range of sizes. Generally, cloud drops (droplets) range from 1-100 m[&mgr;]m in diameter, and hence are very much smaller than raindrops. See cloud classification. 2. Any collection of particulate matter in the atmosphere dense enough to be perceptible to the eye, as a dust cloud or smoke cloud.

  • cloud band - A nearly continuous cloud formation with a distinct long axis, a length-to-width ratio of at least four to one, and a width greater than one degree of latitude.

  • cloud cap - Same as cap cloud.

  • cloud classification - 1. A scheme of distinguishing and grouping clouds according to their appearance, and, where possible, to their process of formation. The one in general use, based on a classification system introduced by Luke Howard in 1803, is that adopted by the World Meteorological Organization and published in the International Cloud Atlas (1956). This classification is based on the determination: 1) genera¾[—]the main characteristic forms of clouds; 2) species¾[—]the peculiarities in shape and differences in internal structure of clouds; 3) varieties¾[—]special characteristics of arrangement and transparency of clouds; 4) supplementary features and accessory clouds¾[—]appended and associated minor cloud forms; and 5) mother-clouds¾[—]the origin of clouds if formed from other clouds. The ten cloud genera are cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus, and cumulonimbus. The fourteen cloud species are fibratus, uncinus, spissatus, castellanus, floccus, stratiform, nebulosus, lenticularis, fractus, humilis, mediocris, congestus, calvus, and capillatus. The nine cloud varieties are intortus, vertebratus, undulatus, radiatus, lacunosus, duplicatus, translucidus, perlucidus, and opacus. The nine supplementary features and accessory clouds are incus, mamma, virga, praecipitatio, arcus, tuba, pileus, velum, and pannus. (Note: Although these are Latin words, it is proper convention to use only the singular endings, e.g., more than one cirrus cloud is cirrus, not cirri.) 2. A scheme of classifying clouds according to their usual altitudes. Three classes are distinguished: high, middle, and low. High clouds include cirrus, cirrocumulus, cirrostratus, occasionally altostratus, and the tops of cumulonimbus. The middle clouds are altocumulus, altostratus, nimbostratus and portions of cumulus and cumulonimbus. The low clouds are stratocumulus, stratus and most cumulus and cumulonimbus bases, and sometimes nimbostratus. 3. A scheme of classifying clouds according to their particulate composition, namely, water clouds, ice-crystal clouds, and mixed clouds. The first are composed entirely of water droplets (ordinary and/or supercooled), the second entirely of ice crystals, and the third a combination of the first two. Of the cloud genera, only cirrostratus and cirrus are always ice-crystal clouds; cirrocumulus can also be mixed; and only cumulonimbus is always mixed. Altostratus is nearly always mixed, but can occasionally be water. All the rest of the genera are usually water clouds, occasionally mixed; altocumulus, cumulus, nimbostratus, and stratocumulus.

  • cloud cover - (Also called cloudiness, cloudage.) That portion of the sky cover that is attributed to clouds, usually measured in tenths or eighths of sky covered.

  • cloud height - 1. In weather observations, the height of the cloud base above local terrain. Compare ceiling. 2. (Rare.) The height of the cloud top above local terrain or above mean sea level. 3. (Rare.) The vertical distance from the cloud base to the cloud top; more commonly referred to as the "thickness" or "depth" of the cloud.

  • cloud layer - An array of clouds, not necessarily all of the same type, with bases at approximately the same level. It may be either continuous or composed of detached elements.

  • cloud shield - 1. In general, a broad cloud formation that is not more than four times as long as it is wide. 2. In synoptic meteorology, the principal cloud structure of a typical wave cyclone, that is, the cloud forms found on the cold-air side of the frontal system. The maximum areal coverage is usually found over the region in advance of the warm front, and the minimum behind the cold front. Within the area of the cloud shield, there is an idealized but smaller precipitation shield.

  • cloud system - (Or nephsystem.) An array of clouds and precipitation associated with a cyclonic-scale feature of atmospheric circulation. Cloud systems display typical patterns and continuity, the analysis of which is termed nephanalysis.

  • cloud top - For a given cloud or cloud layer, the highest level in the atmosphere at which the air contains a perceptible quantity of cloud particles.

  • cloudiness - Same as cloud cover.

  • cloudy - 1. The character of the sunrise or sunset when the disk of the sun is hidden at these times by clouds or an obscuring phenomenon. Compare clear. 2. In popular usage, the state of the weather when clouds predominate at the expense of sunlight, or obscure the stars at night. In weather forecast terminology, expected cloud cover of about 0.7 or more warrants the use of this term. Compare clear, partly cloudy.

  • col - (Also called saddle point, neutral point.) In meteorology, the point of intersection of a trough and a ridge in the pressure pattern of a weather map. It is the point of relatively lowest pressure between two highs and the point of relatively highest pressure between two lows.

  • cold-core low - Same as cold low.

  • cold front - 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. Compare cold type occlusion.

  • complex low - An area of low atmospheric pressure within which more than one low pressure center is found.

  • confluence - The rate at which adjacent flow is converging along an axis oriented normal to the flow at the point in question. It is the opposite of diffluence. In natural coordinates the confluence may be measured by where V is the speed of the wind, the n axis is oriented 90° clockwise from the direction of the wind vector, Vn is the wind component in the n direction, and y[&psgr;] is the wind direction, measured in degrees clockwise from the reference direction.

  • conjunction - In astronomy, the juxtaposition of the earth, sun, and one of the other planets or the moon, in which the angle subtended at the earth between the sun and the third body, in the plane of the ecliptic, is 0° (i.e., the third body lies either between the sun and the earth, or on the opposite side of the sun from the earth). Compare opposition, quadrature.

  • consequent - The THEN clause of an IF-THEN rule. See also antecedent.

  • contaminate - Introduce or increase the concentration of any physical, chemical, biological, or radiological substance in the water or soil.

  • convection - 1. In general, mass motions within a fluid resulting in transport and mixing of the properties of that fluid. Convection, along with conduction and radiation, is a principal means of energy transfer. Distinction is made between free convection (gravitational or buoyant convection), motion caused only by density differences within the fluid; and forced convection, motion induced by mechanical forces such as deflection by a large-scale surface irregularity, turbulent flow caused by friction at the boundary of a fluid, or motion caused by any applied pressure gradient. Free and forced convection are not necessarily exclusive processes. On a windy day with overcast sky, the heat exchange between ground and air is an example of forced convection. On a sunny day with a little wind where the ground temperature rises, both kinds of convection take place. 2. (Or gravitational or buoyant convection.) Motions that are predominantly vertical and driven by buoyancy forces arising from static instability, with locally significant deviations from hydrostatic equilibrium. Atmospheric convection is nearly always turbulent. Convection may be dry, that is, with relative humidities less than 100%, especially in the boundary layer, but is commonly moist, with visible cumuliform clouds. Most convective clouds are driven by positive buoyancy, with virtual temperature greater than the environment, but clouds with precipitation, evaporation, and/or melting can produce negatively buoyant convection. See slantwise convection. 3. As specialized in atmospheric and ocean science, a class of relatively small-scale, thermally (can be driven by salt concentration in the ocean) direct circulations that result from the action of gravity upon an unstable vertical distribution of mass. (In the case of slantwise convection, though, the motions are larger scale, and are driven by a combination of gravitational and centrifugal forces acting at an angle to the vertical.) Almost all atmospheric and oceanic convection is fully turbulent and is generally composed of a collection of convection cells, usually having widths comparable to the depth of the convecting layer. In the atmosphere, convection is the dominant vertical transport process in convective boundary layers, which are common over tropical oceans and, during sunny days, over continents. In the ocean, convection is prominent in regions of high heat loss to the atmosphere and is the main mechanism of deep water formation. Moist convection in the atmosphere is characterized by deep, saturated updrafts and downdrafts, and unsaturated downdrafts driven largely by the evaporation and melting of precipitation. This form of convection is made visible by cumulus clouds and, in the case of precipitating convection, by cumulonimbus clouds. Moist convection and radiation are the dominant modes of vertical heat transport in the Tropics. 4. In atmospheric electricity, a process of vertical charge transfer by transport of air containing a net space charge, or by motion of other media (e.g., rain) carrying net charge. Eddy diffusion of air containing a net charge gradient may also yield a convection current.

  • convective activity - General term for manifestations of convection in the atmosphere, alluding particularly to the development of convective clouds and resulting weather phenomena, such as showers, thunderstorms, squalls, hail, tornadoes, etc.

  • convective available potential energy - (Abbreviated CAPE.) The maximum energy available to an ascending parcel, according to parcel theory. On a thermodynamic diagram this is called positive area, and can be seen as the region between the lifted parcel process curve and the environmental sounding, from the parcel's level of free convection to its level of neutral buoyancy. It may be defined as CAPE = (a[&agr;]p - a[&agr;]e)dp, where a[&agr;]e is the environmental specific volume profile, a[&agr;]p is the specific volume of a parcel moving upward moist-adiabatically from the level of free convection, pf is the pressure at the level of free convection, and pn is the pressure at the level of neutral buoyancy. The value depends on whether the moist-adiabatic process is considered reversible or irreversible (conventionally irreversible) and whether the latent heat of freezing is considered (conventionally not). Compare convective inhibition.

  • convective cloud - A cloud that owes its vertical development, and possibly its origin, to convection.

  • convergence - 1. The contraction of a vector field; also, a precise measure thereof. Mathematically, convergence is negative divergence, and the latter term is used for both. (For mathematical treatment, see divergence).Compare confluence. 2. The property of a sequence or series of numbers or functions that ensures that it will approach a definite finite limit.

  • convergence band - Same as convergence line. Convergence bands in fair weather can form under thermal updrafts, and often form a fishnet or honeycomb pattern with the cell size of the same order as the depth of the mixed layer.

  • conveyer belt - See ocean conveyor belt.

  • counter - Generic term for instruments that measure radioactivity.

  • course - The direction of a line over the earth with reference to north (true or magnetic). Because the meridian lines converge at the poles, the true course of a line changes continuously except when it is actually meridional or when it is curved so as always to cross meridians at the same angle. Magnetic course also changes but in a nonuniform manner because of the lack of complete symmetry in the isogonic lines of the earth's magnetic field. Compare heading.

  • creep - 1. The movement of water under or around a structure built on permeable foundations that may lead to erosion. 2. The slow, downslope movement of surface soil or rock debris, usually imperceptible except when observed for long durations.

  • creeping - 1. The motion of the index of an aneroid barometer after it has been subjected to a large and rapid change in pressure. This movement is a slow adjustment of the index toward the correct pressure. The physical cause of creeping is not clearly understood. 2. See soil creep.

  • crystal - A more or less regular periodic array of atoms, molecules, or ions, usually forming a solid. In everyday parlance crystal is used in a bewildering variety of ways, sometimes contradictory. Fine glassware is called crystal, although glass, an amorphous solid, is the antithesis of a crystal. A solid with facets exhibiting external symmetry may be called a crystal, although a solid without such facets may still be a crystal. A pure liquid such as water is said to be crystal clear even though transparency is not an essential property of a crystal.

  • cumulus - (Abbreviated Cu.) A principal cloud type (cloud genus) in the form of individual, detached elements that are generally dense and posses sharp nonfibrous outlines. These elements develop vertically, appearing as rising mounds, domes, or towers, the upper parts of which often resemble a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their bases are relatively dark and nearly horizontal. Near the horizon the vertical development of cumulus often causes the individual clouds to appear merged. If precipitation occurs, it is usually of a showery nature. Various effects of wind, illumination, etc., may modify many of the above characteristics. Strong winds may shred the clouds, often tearing away the cumulus tops to form the species fractus. Under certain conditions cumulus clouds may be arranged in files, cloud streets, oriented approximately parallel to the wind direction. Changes in direction of illumination and in background cause modification of color and of apparent surface relief. Cumulus is composed of a great density of small water droplets, frequently supercooled. Within the cloud larger water drops are formed that may, as the cloud develops, fall from the base as rain or virga. Ice crystal formation will occur within the cloud at sufficiently low temperatures, particularly in upper portions as the cloud grows vertically. Occasionally the growth of ice crystals at the expense of water droplets will reduce the entire cloud to diffuse trails of snow. Cumulus most often forms directly in clear air as a result of convection in air of sufficiently high moisture content for a condensation level to be reached. As a result, a distant diurnal cycle of cumulus frequency is observed. Over a landmass, the cumulus maximum occurs after midday (for a horizontal extent, early afternoon; for vertical extent, somewhat later). Over a water surface, the cycle is reversed and much less obvious, with the cumulus maximum generally recognized as occurring after midnight. The vertical growth of a cumulus cell is restricted and modified by the existence and character of layers of relative static stability above the cloud base. Cumulus may also evolve from the convective transformation of stratus or stratocumulus (Cu stratomutatus or Cu stratocumulomutatus). Cumulus may be generated by altocumulus and, again, stratocumulus (Cu altocumulogenitus and Cu stratocumulogenitus).Cumulonimbus is the ultimate manifestation of the growth of cumulus; therefore, at a certain point, it is difficult to differentiate between the two. If a cloud in doubt reveals no fibrous structure, it is still cumulus; if still in doubt, cumulonimbus further differs in that it is accompanied by lightning, thunder, and sometimes hail. The elements of altocumulus are smaller and, along with those of stratocumulus, tend to be more merged than the separated units of cumulus. Cumulus has the unique ability to penetrate other preexisting cloud layers, sometimes partially dissipating, at other times apparently fusing with, the impaled layers. The cumulus, in this instance, retains its identity as long as it remains primarily vertically developed, is physically (although perhaps not visibly) separate from the other cloud, and has a tower- or dome-shaped summit. See cloud classification, trade-wind cumulus.

  • current - 1. Any movement of material in space. See air current, ocean current. 2. Any movement of electric charge in space, by virtue of which a net transport of charge occurs as, for example (in atmospheric electricity), in a conduction current, convection current, or precipitation current.

  • cut-off low - 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. See cutting-off process, cut-off high, cold pool.

  • cycle - 1. Any process or sequence of states in which the initial and final states of a system are the same. 2. A unit of wave frequency, actually one cycle per second. See kilocycle, megacycle, kilomegacycle.

  • cyclonic - Having a sense of rotation about the local vertical the same as that of the earth's rotation: that is, as viewed from above, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere, undefined at the equator; the opposite of anticyclonic.

  • cyclonic circulation - Fluid motion in the same sense as that of the earth, that is, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere, undefined at the equator.

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