Subj : Weather Terms (L)
To : ALL USERS
From : DARYL STOUT
Date : Sat Feb 13 2016 12:03 am
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 Ni%a
La Ni%a, 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 Ni%a 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.
--- BgNet 1.0a12 - The Thunderbolt BBS wx1der.dyndns.org Little Rock, AR
