JetStream Glossary: L's

  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/Land Breeze
A lake breeze occurs when prevailing winds blow off the water, while a land breeze indicates winds blowing from land to sea. Both are caused by the difference in surface temperature (heating) of the land and water. As a result, a lake breeze occurs during the day while a land breeze happens at night.
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" (15 cm) 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 low pressure system) may pose a hazard or it is life threatening. The criteria for this warning varies from area to area.
Lakeshore Statement
The local National Weather Service Offices with Great Lakes responsibility will issue this product to alert the public when their is either a potential or actual reports of minor Great Lakes lakeshore flooding and erosion. This means that the lakeshore flooding or erosion would not cause too much damage to property, but it would be an inconvenience to living or driving in those areas.
Lakeshore Warning
The local National Weather Service Offices with Great Lakes responsibility will issue this product to alert the public when there is either a potential or actual reports of major Great Lakes lakeshore flooding and erosion. If precautions are not taken, this could pose a considerable threat to life and property.
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.
Slang for 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. The global average rate of temperature change with height in the atmosphere is 6.5°C/km.

The adiabatic lapse rate (or dry adiabatic lapse rate) is the normal rate of change (9.8°C/km) for a dry parcel of air that is moved up or down and cools or warms as the pressure changes. The wet (moist) adiabatic lapse rate (4.9°C/km) is the rate at which saturated air cools as it ascends.

See synoptic-scale.
An acronym for Lifted Condensation Level.
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.
Lee Effect
The effect of topography on winds to the lee (downwind) side of an obstacle such as a steep island, cliff, or mountain range.
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 Nevada.
Leeward is on the side facing the direction toward which the wind is blowing. On the other hand, windward is on the side facing the direction away from the wind.
Left Front Quadrant (or 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. Also, see entrance region and 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. Also, see right mover.
The distance in the direction of flow between two specific points along a river, stream, or channel.
Lentic System
A nonflowing or standing body of fresh water, such as a lake or pond. Compare lotic system.
Levee (Dike)
A long, narrow embankment usually built to protect land from flooding. If built of concrete or masonry 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)
It is the height at which a parcel of air lifted dry-adiabatically until saturated (this is the Lifting Condensation Level) and moist-adiabatically thereafter would first become warmer (less dense) than the surrounding air. At this point, the buoyancy of the parcel would accelerate upward without further need for forced lift.
An acronym for Line Echo Wave Pattern.
An acronym for Level of Free Convection.
An acronym for Lifted Index.
See Cap.
Lifted Index (LI)

It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet/5,500 meters) denotes the LI.

Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the Showalter Index (SWI). The greater negative values of LI indicate energy available for parcel ascent.

Lifted index values
Lifted Index (LI) Thunderstorm Indication
< -5 Very Unstable, Heavy/strong thunderstorm potential
-3 to -5 Unstable, Thunderstorms probable
0 to -2 Marginally Unstable, Thunderstorms possible
Light Icing
The rate of ice accumulation that may create a problem if the flight is prolonged in this environment (over one hour). Occasional use of de-icing equipment removes/prevents accumulation. It does not present a problem if de-icing/anti-icing equipment is used.
Lifting Condensation Level (LCL)
It is the height at which a parcel of air becomes saturated when lifted dry-adiabatically.
A sudden visible flash of energy and light caused by an electrical discharges from thunderstorms.
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 Flash
The total luminous phenomenon accompanying a lightning discharge. It may be composed of one to a few tens of strokes that use essentially the same channel to ground.
Lightning Ground Flash Density
The number of cloud-to-ground flashes per unit time per unit area.
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.
A National Weather Service precipitation descriptor for a 60% or 70% chance of measurable precipitation (0.01"). When the precipitation is convective in nature, the term numerous will occasionally be used. See Precipitation Probability (PoP).
The fear of tornadoes and hurricanes.
The branch of hydrology that pertains to the study of lakes.
The fear of lakes.
Line Echo Wave Pattern (LEWP)

A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate. A meso-high pressure area is usually present behind the accelerating thunderstorms.

A meso-low pressure area is usually present at the crest of the wave. 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.

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
The area on, or near the shore of a body water.
Live Capacity
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.
Live Fuel Moisture - Greenness Maps
Four vegetation greenness maps are derived weekly from Normalized Difference Vegetation Index (NDVI) data observed by satellites and provided by the EROS Data Center, U.S. Geological Survey. These maps have a 1 kilometer (0.6 mile) spatial resolution. Maps with historical references (RG and DA) are based on the years 1989 through 1995. The derived maps are:
  • Visual Greenness Maps - These maps portray vegetation greenness compared to a very green reference such as an alfalfa field or a golf course. The resulting image is similar to what you would expect to see from the air. Normally dry areas will never show as green as normally wetter areas.
  • Relative Greenness Maps - These maps portray how green the vegetation is compared to how green it has been historically (since 1989). Because each pixel is normalized to its own historical range, all areas (dry to wet) can appear fully green at some time during the growing season.
  • Departure from Average Greenness Maps - These maps portray how green each pixel is compared to its average greenness for the current week of the year.
  • Live Moisture Maps - These maps portray experimental live vegetation moisture with values ranging from 50 to 250 percent of dry weight.
An acronym for Low Level Jet.
Loaded Gun (Sounding)
Slang for 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 Climatological Data (LCD)

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

Most monthly publications also contain the 3-hourly weather observations for that month and an hourly summary of precipitation. Annual LCD publications contain a summary of the past calendar year as well as historical averages and extremes.

The LCD contains 3-hourly, daily, and monthly values. The annual issue contains the year in review plus normals, means and extremes.

Local Flooding
Flooding conditions over a relatively limited (localized) area.
Local Storm Report (LSR)
A product issued by local NWS offices to inform users of reports of severe and/or significant weather-related events.
Log and Safety Boom
A net-like device installed in a reservoir, upstream of the principal spillway, to prevent logs, debris and boaters from entering a water discharge facility or spillway.
Long Term Storage Dams

Reservoirs used for recreational use or storage of irrigation, municipal or industrial water. Because water is impounded on a "permanent" basis, the design of these dams is more complex than for tailings or flood control detention dams.

A long term storage dam may include an impermeable core surrounded by shell material, have many types of drains and filters, outlet works, with gates and valves, seepage collection boxes, and possibly several spillways. The capacity of the spillway is dependant upon the downstream hazard potential.

Longwave Trough
A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time.

Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months. Smaller disturbances (e.g., shortwave troughs) typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).

Lotic System
A flowing body of fresh water, such as a river or stream. Compare lentic system.
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 Clouds
The bases of these clouds range from near the surface to about 6,500 feet (2,000 meters) in middle latitudes. These clouds are almost entirely of water, but the water may be supercooled at sub-freezing temperatures. Low clouds at sub-freezing temperatures can also contain snow and ice particles. The two most common members of this family are stratus and stratocumulus.
Low Drifting (DR)
A descriptor used to describe snow, sand, or dust raised to a height of less than 6 feet (2 meters) above the ground.
Lowland Flooding
Inundation of low areas near the river, often rural, but may also occur in urban areas.
Low Level Jet (LLJ)

It often forms at 1-1.5 km under the exit region (the place just ahead of a speed maximum) of an upper-level jet (ULJ) streak. It has a strong isallobaric component due to the pressure rise/fall pattern under the right exit/left exit region of the jet streak, respectively. It is usually at a significant angle with respect to the ULJ and has a strong southerly component.

This enhances the warm, moist air ahead of an upper level front-jet system. This is common over the Plains states at night during the warm season (spring and summer). Another type of a LLJ is called a nocturnal jet.

LP Storm - Low-Precipitation storm

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.

The fear of darkness.
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.