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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • L band - See radar frequency bands.

  • LAC - Abbreviation for local area coverage.

  • lag - 1. That part of the difference between the output of an instrument and its input that is due to the failure of the instrument to respond instantaneously to variations of the input signal. It is a function of the instrument's time constant. See time lag. 2. A time displacement of a time series. See autocorrelation. 3. See delay.

  • large scale - In meteorology, a scale in which the curvature of the earth is not negligible. This is the scale of the high tropospheric long-wave patterns, with four or five waves around the hemisphere in middle latitudes. These waves are within the province of both the general circulation and synoptic meteorology, but the terminology should distinguish this scale from that of the migratory high and low pressure systems of the lower troposphere. Rossby waves and other long barotropic waves are large-scale disturbances. See cyclonic scale.

  • latitude - The angular distance along the meridian from the point in question to the equator. Latitude is normally described as so many degrees north or south of the equator. Compare longitude.

  • lead - A long fracture or separation between ice floes wide enough to be navigated by a ship. A lead may be covered by thin ice.

  • levant - Same as levante.

  • level surface - Same as geopotential surface.

  • LF - Abbreviation for low frequency. See radio frequency band.

  • LI - Abbreviation for lifted index.

  • lid - (Also known as cap, capping inversion, capping layer.) A thin layer with enhanced static stability separating a layer below possessing large convective available potential energy from a layer above with lower static stability. The presence of a lid is generally accompanied by substantial convective inhibition. Air parcels with insufficient kinetic energy rising into the bottom of a lid will be unable to penetrate it.

  • LIF - See laser-induced fluorescence.

  • lift - The component, perpendicular to the relative wind and in the plane of symmetry, of the total force of air on an aircraft or airfoil. It must be specified whether this applies to a complete aircraft or the parts thereof. In the case of a lighter-than-air craft, this is often called dynamic lift.

  • light - Often synonymous with visible radiation but sometimes applied to electromagnetic radiation well outside the visible spectrum.

  • lightning - 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 (cumulonimbus). Well over half of all lightning discharges occur within the thunderstorm cloud and are called intracloud discharges. The usual cloud-to-ground lightning (sometimes called streak lightning or forked lightning) has been studied more extensively than other lightning forms because of its practical interest (i.e., as a cause of injury and death, disturbances in power and communication systems, and ignition of forest fires) and because lightning channels below cloud level are more easily photographed and studied with optical instruments. Cloud-to-cloud and cloud-to-air discharges are less common than intracloud or cloud-to-ground lightning. All discharges other than cloud-to-ground are often lumped together and called cloud discharges. Lightning is a self-propagating and electrodeless atmospheric discharge that, through the induction process, transfers the electrical energy of an electrified cloud into electrical charges and current in its ionized and thus conducting channel. Positive and negative leaders are essential components of the lightning. Only when a leader reaches the ground does the ground potential wave (return stroke) affect the lightning process. Natural lightning starts as a bidirectional leader, although at different stages of the process unidirectional leader development can occur. Artificially triggered lightning starts on a tall structure or from a rocket with a trailing wire. Most of the lightning energy goes into heat, with smaller amounts transformed into sonic energy (thunder), radiation, and light. Lightning, in its various forms, is known by many common names, such as streak lightning, forked lightning, sheet lightning, and heat lightning, and by the less common air discharge; also, the rare and mysterious ball lightning and rocket lightning. An important effect of worldwide lightning activity is the net transfer of negative charge from the atmosphere to the earth. This fact is of great important in one problem of atmospheric electricity, the question of the source of the supply current. Existing evidence suggests that lightning discharges occurring sporadically at all times in various parts of the earth, perhaps 100 per second, may be the principal source of negative charge that maintains the earth-ionosphere potential difference of several hundred thousand volts in spite of the steady transfer of charge produced by the air-earth current. However, there also is evidence that point discharge currents may contribute to this more significantly than lightning. See also cloud-to-ground flash, intracloud flash lightning discharge.

  • linear - Confined to first-degree algebraic terms in the relevant variables. For example, a + bx + cy is linear in x and y; a sin x + b cos y is linear in the coefficients a and b, but nonlinear in x and y.

  • Lips - The ancient Greek name for the southwest wind, which is the sea breeze in Athens. On the Tower of the Winds it is represented by a bare-legged young man carrying a piece of a trireme. This may indicate either that the wind favored home-coming ships or that, when stormy, it caused wrecks. Today the name is applied to any hot wind, usually the sirocco.

  • LIS - Abbreviation for Lightning Imaging Sensor.

  • LOC - Abbreviation for limit of convection.

  • local winds - 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).

  • long wave - 1. (Or major wave; also called planetary wave.) With regard to atmospheric circulation, a wave in the major belt of westerlies that is characterized by large length and significant amplitude. The wave length is typically longer than that of the rapidly moving individual cyclonic and anticyclonic disturbances of the lower troposphere. The angular wavenumber of long waves is generally taken to be from 1 to 5. Compare short wave; see Rossby wave. 2. (Also called shallow water wave.) A wave with a relatively long wave length and period. For ocean waves, this is typically a wave of period greater than about 10 s and wave length greater than about 150 m.

  • longitude - The angular distance along the equator measured from the prime meridian to the meridian of the point in question. Compare latitude.

  • loo - (Also called lu, loo marna.) A hot wind from the west in India.

  • low - (Sometimes called depression.) In meteorology, an "area of low pressure," referring to a minimum of atmospheric pressure in two dimensions (closed isobars) on a constant-height chart or a minimum of height (closed contours) on a constant-pressure chart. Since a low is, on a synoptic chart, always associated with cyclonic circulation, the term is used interchangeably with cyclone. Compare high.

  • low aloft - Same as upper-level cyclone.

  • low cloud - See cloud classification.

  • lower atmosphere - Generally and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the troposphere and lower stratosphere); hence used in contrast to the common meaning for the upper atmosphere. In other contexts, the term implies the lower troposphere.

  • LTE - Abbreviation for local thermodynamic equilibrium.

  • lu - Same as loo.

  • lull - A momentary decrease in the speed of the wind.

  • lux - A photometric unit of illuminance or illumination equal to one lumen per square meter. A level of illumination between 200 and 1000 lux is generally considered to be adequate for homes and offices. Compare foot-candle.

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