• Weather Glossary (L)

    From Daryl Stout@57:57/10 to All on Sat Nov 7 00:08:39 2020
    This weather glossary contains information on more than 2000 terms,
    phrases and abbreviations used by the National Weather Service (NWS)...
    the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its
    territories.

    Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
    before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.

    ***

    La Nia
    La Nia, a phase of ENSO, is a periodic cooling of surface ocean waters in
    the eastern tropical Pacific along with a shift in convection in the
    western Pacific further west than the climatological average. These
    conditions affect weather patterns around the world. The preliminary CPC definition of La Nia is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal.

    Lag

    1) The measure of the time between the center of mass of precipitation to
    the center of mass of runoff (on the hydrograph); basin lag is a function
    of not only basin characteristics, but also of storm intensity and
    movement. Some hydrologic texts define lag from the center of mass of
    rainfall to the hydrograph peak.

    2) The time it takes a flood wave to move downstream.

    Lake Breeze
    A thermally produced wind blowing during the day from the surface of a
    large lake to the shore, caused by the difference in the rates of heating
    of the surfaces of the lake and of the land.

    Lake Effect Snow
    Snow showers that are created when cold, dry air passes over a large
    warmer lake, such as one of the Great Lakes, and picks up moisture and
    heat.

    Lake Effect Snow Advisory
    This product is issued by the National Weather Service when pure lake
    effect snow (this is where the snow is a direct result of lake effect
    snow and not because of a low pressure system) may pose a hazard or it
    is life threatening. The criteria for this advisory varies from area to
    area.

    Lake Effect Snow Squall
    A local, intense, narrow band of moderate to heavy snow squall that can
    extend long distances inland. It may persist for many hours. It may also
    be accompanied by strong, gusty, surface winds and possibly lightning. Accumulations can be 6 inches or more in 12 hours.

    Lake Effect Snow Warning
    This product is issued by the National Weather Service when pure lake
    effect snow (this is where the snow is a direct result of lake effect
    snow and not because of a synoptic storm or low pressure system) may pose
    a hazard or it is life threatening.

    Lake Effect Storm
    A fall or winter storm that produces heavy but localized precipitation as
    a result of temperature differences between the air over snow-covered
    ground and the air over the open waters of a lake.

    Lakeshore Flood Advisory


    Lakeshore Flood Watch


    Lakeshore Flooding


    LALs
    (L)ightning (A)ctivity (L)evels.
    LAL 1 - No thunderstorms.

    LAL 2 - Few building cumulus with isolated thunderstorms.

    LAL 3 - Much building cumulus with scattered thunderstorms. Light to
    moderate rain.

    LAL 4 - Thunderstorms common. Moderate to heavy rain reaching the ground.

    LAL 5 - Numerous thunderstorms. Moderate to heavy rain reaching the
    ground.

    LAL 6 - Dry lightning (same as LAL 3 but without the rain).

    Laminar
    Smooth, non-turbulent. Often used to describe cloud formations which
    appear to be shaped by a smooth flow of air traveling in parallel layers
    or sheets.

    Laminar Flow
    Streamline flow in which successive flow particles follow similar path
    lines and head loss varies with velocity to the first power.

    Land Breeze
    A coastal breeze at night blowing from land to sea, caused by the
    difference in the rates of cooling of their respective surfaces.

    Landfall
    The intersection of the surface center of a tropical cyclone with a
    coastline. Because the strongest winds in a tropical cyclone are not
    located precisely at the center, it is possible for a cyclone's
    strongest winds to be experienced over land even if landfall does not
    occur. Similarly, it is possible for a tropical cyclone to make
    landfall and have its strongest winds remain over the water. Compare
    direct hit, indirect hit, and strike.

    Landspout
    [Slang], a tornado that does not arise from organized storm-scale
    rotation and therefore is not associated with a wall cloud (visually)
    or a mesocyclone (on radar). Landspouts typically are observed beneath
    Cbs or towering cumulus clouds (often as no more than a dust whirl),
    and essentially are the land-based equivalents of waterspouts.

    Lapse Rate
    The rate of change of an atmospheric variable, usually temperature,
    with height. A steep lapse rate implies a rapid decrease in temperature
    with height (a sign of instability) and a steepening lapse rate implies
    that destabilization is occurring.

    Large Scale
    (Synoptic Scale) Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and
    low pressure areas seen on weather maps are synoptic-scale systems.

    Last Update
    The time and date in which the forecast was issued or updated. The
    forecast may be updated at any time as weather conditions warrant.

    LAT
    Latitude- The location north or south in reference to the equator,
    which is designated at zero (0) degrees. Lines of latitude are parallel
    to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude.

    Latent Heat
    Heat absorbed or released during a change of phase at constant
    temperature and pressure.

    Latent Heat Flux
    The flux of heat from the earth's surface to the atmosphere that is
    associated with evaporation or condensation of water vapor at the
    surface; a component of the surface energy budget.

    Latitude
    (abbrev. LAT) The location north or south in reference to the equator,
    which is designated at zero (0) degrees. Lines of latitude are parallel
    to the equator and circle the globe. The North and South poles are at
    90 degrees North and South latitude.

    LAWEB
    (Great Lakes Weather Broadcast) - A National Weather Service product
    containing an observation summary prepared to provide Great Lakes
    mariners with a listing of weather observations along or on the Lakes.

    Layer Composite Reflectivity Average
    This WSR-88D radar product displays the average reflectivities for a
    layer. Data is taken from all elevation angles contained in a given
    layer for each grid box. It is available for 3 layers (low, mid, high).
    It is used to aid in determining storm intensity trends by comparing
    mid level layer composite products with a low level elevation angle
    base reflectivity product and aid in routing air traffic.

    Layer Composite Reflectivity Maximum
    This WSR-88D radar product displays the maximum reflectivities for a
    layer. Data is taken from all elevation angles contained in a given
    layer for each grid box. It is available for 3 layers (low, mid, high). Currently, the low layer extends from the surface to 24,000 feet, the
    mid layer extends from 24,000 feet to 33,000 feet, and high layer
    extends above 33,000 feet. It is used to aid in determining storm
    intensity trends by comparing mid level layer composite products with
    a low level elevation angle base reflectivity product and aid in
    routing air traffic.

    Layered Haze
    Haze produced when air pollution from multiple line, area or point
    sources is transported long distances to form distinguishable layers
    of discoloration in a stable atmosphere.

    LCD (Local Climatological Data)
    This National Climatic Data Center (NCDC) publication is produced
    monthly and annually for some 270 United States cities and it's
    territories. The LCD summarizes temperature, relative humidity,
    precipitation, cloudiness, wind speed and direction observation.

    LCL
    1. Abbreviation for "local" or "locally"

    2. Lifting Condensation Level - the level at which a parcel of moist
    air becomes saturated when it is lifted dry adiabatically.

    LDS
    Lightning Detection System

    LDT
    Local Daylight Time.

    Leader
    The streamer which initiates the first phase of each stroke of a
    lightning discharge. The first stroke is led by a steeped leader, which
    may be preceded by a pilot streamer. All subsequent strokes begin with
    a dart leader.

    Leader Spot
    In solar-terrestrial terms, in a magnetically bipolar or multipolar
    sunspot group, the western part precedes and the main spot in that part
    is called the leader.

    Lee
    The side or part that is sheltered or turned away from the wind, such
    as with a mountain.

    Lee Wave
    The wavelike effect, characterized by severe updrafts and downdrafts,
    that occurs in the lee of a mountain range when rapidly flowing air is
    lifted up the steep front of a mountain range. Compare mountain wave.

    Leeside Low
    Extratropical cyclones that form on the downwind (lee) side of a
    mountain chain. In the United States, they frequently form on the
    eastern side of the Rockies and Sierra Nevadas.

    Leeward
    The side away from the wind. Compare windward.

    Left Exit Region
    Used interchangably with Left Front Quadrant; the area downstream from
    and to the left of an upper-level jet max (as would be viewed looking
    along the direction of flow). Upward motion and severe thunderstorm
    potential sometimes are increased in this area relative to the wind
    speed maximum. See also entrance region, right rear quadrant.

    Left Front Quadrant
    Used interchangably with Left Exit Region; the area downstream from and
    to the left of an upper-level jet max (as would be viewed looking along
    the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum.
    See also entrance region, right rear quadrant.

    Left Mover
    A thunderstorm which moves to the left relative to the steering winds,
    and to other nearby thunderstorms; often the northern part of a
    splitting storm.

    Length
    In hydrologic terms, the distance in the direction of flow between two
    specific points along a river, stream, or channel.

    Lentic System
    In hydrologic terms, a nonflowing or standing body of fresh water, such
    as a lake or pond.

    Lenticular Cloud
    A very smooth, round or oval, lens-shaped cloud that is often seen,
    singly or stacked in groups, near or in the lee of a mountain ridge.

    Levee
    (Dike) In hydrologic terms, a long, narrow embankment usually built to
    protect land from flooding. If built of concrete or masonary the
    structure is usually referred to as a flood wall. Levees and floodwalls
    confine streamflow within a specified area to prevent flooding. The
    term "dike" is used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.

    Level of Free Convection
    (LFC) - The level at which a parcel of saturated air becomes warmer than
    the surrounding air and begins to rise freely. This occurs most readily
    in a conditionally unstable atmosphere.

    LEWP
    Line Echo Wave Pattern. A bulge in a thunderstorm line producing a
    wave-shaped "kink" in the line. The potential for strong outflow and
    damaging straight-line winds increases near the bulge, which often
    resembles a bow echo. Severe weather potential also is increased with
    storms near the crest of a LEWP.

    LFC
    An acronym for Level of Free Convection- the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

    LFT
    Lift

    LGT
    Light

    LGWV
    Long Wave

    LI
    Lifted Index. A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have
    if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level.
    Negative values indicate instability - the more negative, the more
    unstable the air is, and the stronger the updrafts are likely to be with
    any developing thunderstorms. However there are no "magic numbers" or
    threshold LI values below which severe weather becomes imminent.

    Lid
    (Also called cap.) 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.

    LIFR
    Low Instrument Flight Rules

    Lifted Index
    (abbrev. LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have
    if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level.
    Negative values indicate instability - the more negative, the more
    unstable the air is, and the stronger the updrafts are likely to be with
    any developing thunderstorms. However there are no "magic numbers" or
    threshold LI values below which severe weather becomes imminent.

    Lifting Condensation Level
    (LCL) - The level at which a parcel of moist air becomes saturated when
    it is lifted dry adiabatically.

    Light Bridge
    In solar-terrestrial terms, it is observed in white light, a bright
    tongue or streaks penetrating or crossing sunspot umbrae. The appearance
    of a light bridge is frequently a sign of impending region division or dissolution.

    Lightning
    (abbrev. LTNG) A visible electrical discharge produced by a thunderstorm.
    The discharge may occur within or between clouds, between the cloud and
    air, between a cloud and the ground or between the ground and a cloud.

    Lightning Channel
    The irregular path through the air along which a lightning discharge
    occurs. A typical discharge of flash between the ground and the cloud is actually a composite flash which is composed of several sequential
    lightning strokes, each of which is initiated by a leader and terminated
    by a return streamer.

    Lightning Discharge
    The series of electrical processes by which charge is transferred along
    a channel of high ion density between electrical charge centers of
    opposite sign. This can be between a cloud and the Earth's surface of
    a cloud-to-ground discharge.

    Lightning Stroke
    Any of a series of repeated electrical discharges comprising a single
    lightning discharge (strike). Specifically, in the case of a
    cloud-to-ground discharge, a leader plus its subsequent return streamer.

    Likely
    (abbrev. LKLY) In probability of precipitation statements, the equivalent
    of a 60 or 70 percent chance.

    Limb
    In solar-terrestrial terms, the edge of the solar disk.

    Limb Flare
    In solar-terrestrial terms, a solar flare seen at the edge (Limb) of the
    sun.

    Limnology
    In hydrologic terms, the branch of hydrology that pertains to the study
    of lakes.

    Line Echo Wave Pattern
    (abbrev. LEWP) A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate.

    Line Source
    An array of pollutant sources along a defined path that can be treated
    in dispersion models as an aggregate uniform release of pollutants along
    a line. Example: the sum of emissions from individual cars traveling
    down a highway can be treated as a line source. Compare area source and
    point source.

    Liquid Water Equivalent
    Same as Water Equivalent; the liquid content of solid precipitation that
    has accumulated on the ground (snow depth). The accumulation may consist
    of snow, ice formed by freezing precipitation, freezing liquid
    precipitation, or ice formed by the refreezing of melted snow.

    Lithometeor
    Atmospheric phenomena which affect the state of the atmosphere. They
    constitute dry particles that hang suspended in the atmosphere, such as
    dust, smoke, sand, and haze.

    Lithosphere
    In hydrologic terms, that part of the earth which is composed
    predominantly of rocks (either coherent or incoherent, and including the disintegrated rock materials known as soils and subsoils), together with everything in this rocky crust.

    Littoral Zone
    In hydrologic terms, the area on, or near the shore of a body water.

    Live Capacity
    In hydrologic terms, the total amount of storage capacity available in
    a reservoir for all purposes, from the dead storage level to the normal
    water or normal pool level surface level. Does not include surcharge,
    or dead storage, but does include inactive storage, active conservation
    storage and exclusive flood control storage.

    LIVV
    Lifted Index Vertical Velocity

    LKLY
    Likely- In probability of precipitation statements, the equivalent of a
    60 or 70 percent chance.

    LLJ
    Low Level Jet - A region of relatively strong winds in the lower part of
    the atmosphere. Specifically, it often refers to a southerly wind
    maximum in the boundary layer, common over the Plains states at night
    during the warm season (spring and summer). The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in
    this sense the more proper term would be low-level jet stream.

    LLWS
    Low Level Wind Shear

    LMTD
    limited

    LN
    Line

    Loaded Gun (Sounding)
    [Slang], a sounding characterized by extreme instability but containing
    a cap, such that explosive thunderstorm development can be expected if
    the cap can be weakened or the air below it heated sufficiently to
    overcome it.

    Local Convective Wind
    In fire weather terminology, local thermally driven winds arising over a comparatively small area and influenced by local terrain. Examples
    include sea and land breezes, lake breezes, diurnal mountain wind
    systems and columnar convective currents.

    Lofting
    A pattern of plume dispersion in a stable boundary layer topped by a
    neutral layer, in which the upper part of the plume disperses upward
    while the lower part of the plume undergoes little dispersion.

    Long Term Retention
    Retention of data for 5 years to satisfy requirements for local studies
    and to support litigation.

    Longitude
    The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are
    correlated to longitude.

    Longwave Radiation
    A term used to describe the infrared energy emitted by the earth and
    atmosphere at wavelengths between about 5 and 25 micrometers. Compare
    shortwave radiation.

    Longwave Trough
    A trough in the prevailing westerly flow aloft which is characterized
    by large length and (usually) long duration.

    Loop Prominence System
    (abbrev. LPS) In solar-terrestrial terms, a system of loop prominences associated with major flares.

    Looping
    A pattern of plume dispersion in an unstable atmosphere, in which the
    plume undergoes marked vertical oscillations as it is alternately
    affected by rising convective plumes and the subsiding motions between
    the plumes.

    LOPRES
    low pressure

    Loran
    Long Range Navigation, a system of long range navigation whereby
    latitude and longitude are determined from the time displacement of
    radio signals from two or more fixed transmitters.

    Lotic System
    In hydrologic terms, a flowing body of fresh water, such as a river or
    stream.

    Low
    A region of low pressure, marked as "L" on a weather map. A low center
    is usually accompanied by precipitation, extensive cloudiness, and
    moderate winds. See Cyclone.

    Low Frequency
    (abbrev. LF) The portion of the radio frequency spectrum from 30 to
    300 kHz.

    Low Level Jet
    (abbrev. LLJ)- A region of relatively strong winds in the lower part of
    the atmosphere. Specifically, it often refers to a southerly wind
    maximum in the boundary layer, common over the Plains states at night
    during the warm season (spring and summer).

    The term also may be used to describe a narrow zone of strong winds
    above the boundary layer, but in this sense the more proper term would
    be low-level jet stream.

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

    Lowland Flooding
    In hydrologic terms, inundation of low areas near the river, often
    rural, but may also occur in urban areas.

    LP Storm
    Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy
    precipitation core. LP storms often exhibit a striking visual
    appearance; the main tower often is bell-shaped, with a corkscrew
    appearance suggesting rotation. They are capable of producing tornadoes
    and very large hail. Radar identification often is difficult relative
    to other types of supercells, so visual reports are very important. LP
    storms almost always occur on or near the dry line, and thus are
    sometimes referred to as dry line storms.

    LPS
    Loop Prominence System- In solar-terrestrial terms, a system of loop prominences associated with major flares.

    LRG
    Large

    LSR
    Local Storm Report. A product issued by local NWS offices to inform
    users of reports of severe and/or significant weather-related events

    LST
    Local Standard Time

    LTD
    Limited

    LTL
    Little

    LTLCG
    Little Change

    LTNG
    Lightning- A visible electrical discharge produced by a thunderstorm.
    The discharge may occur within or between clouds, between the cloud
    and air, between a cloud and the ground or between the ground and a
    cloud.

    LTR
    Later

    LTST
    latest

    LVL
    Level

    LVLS
    levels

    LWR
    Lower

    Lysimeter
    In hydrologic terms, a device to measure the quantity or rate of
    downward water movement through a block of soil usually undisturbed,
    or to collect such percolated water for analysis as to quality.
    --- SBBSecho 3.11-Win32
    * Origin: The Thunderbolt BBS - tbolt.synchro.net (57:57/10)