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Weather Glossary - L

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La Niņa

The extensive cooling of the central and eastern Pacific Ocean. In Australia (particularly eastern Australia), La Niņa events are associated with warmer ocean temperatures and increased probability of wetter conditions. See El Niņo.

Laminar

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

Land breeze

A coastal breeze which flows from land to sea, usually at night. It forms as continental air cools below the temperature of a nearby maritime airmass and sets up a thermally driven circulation. The land breeze is occasionally refered to as a katabatic wind. See also Sea breeze. The land breeze is generally lighter than a sea breeze, around 5-10 knots.

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 Cb's or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

LAPS

Limited Area Prediction System. One of the regional numerical forecast model operated by the BoM. LAPS is run twice daily (00Z and 12Z) and gives forecasts of 72 hours. LAPS has an operational horizontal resolution of around 35km and 29 vertical levels. LAPS does have the capability to be operated at around 5km resolution, but this is over much smaller domains and largely only for research purposes.

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. See Sounding, DALR, SALR.

Large scale

See synoptic-scale.

Latent heat

Latent heat is a form of energy released or absorbed by a phase change of water. It results from the individual molecules releasing a small amount of energy as they drop to a less energetic state.

LCL

Lifted Condensation Level. The level where condensation (saturation) occurs if one lifts an unsaturated surface parcel dry-adiabatically.

Graphically on the skew-T plot it is the point where the dry adiabat (originating at the parcel temperature) and mixing ratio lines (originating at the parcel dew point temperature) intersect.

Leeward

The side of a mountain, hill or range that is sheltered from the wind. The opposite of windward.

Left Entrance Region

(or Left Rear Quadrant) The area upstream 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 are sometimes increased in this area relative to the wind speed maximum. See also exit region, Right Front Quadrant.

Left mover

A thunderstorm that moves appreciably to the left relative to the main steering winds and to other nearby thunderstorms. Left movers typically are associated with a high potential for severe weather. (Supercells are often left movers.) See right mover, splitting storm.

Left Rear Quadrant

See Left Entrance Region.

Lenticular

Latin - lens, lentil
Clouds having a lens or almond shape, usually elongated and with well-defined outlines. Occurs when a moist airstream crosses a mountain barrier, resulting in waves in the flow. Lenticular clouds form at the crest of the wave when the moisture in the air condenses.

Lenticular clouds are most commonly seen in either stratocumulus or altocumulus.

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 is also increased with storms near the crest of a LEWP.

LFC

Level of Free Convection. The first level on an atmospheric sounding where the temperature of a parcel raised from the surface is greater than the surrounding environment. This means that the parcel is now free to continue rising without the need of any additional energy input from the environment - hence it is "free" to continue convecting.

Lifted index

(or 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 (usually 500mb) 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. See sounding.

Lightning

The flash of light accompanying a sudden electrical discharge which takes place from or inside a cloud, or less often from high structures, the ground or mountains.

Lightning is caused when the negative charge in the lower part of the cloud and the positive charge in the upper part of the cloud become so great the air within the cloud breaks down to allow an electrical current to flow through it and discharge. The mechanism of charge separation is still highly debated. See also thunder.

Loaded gun

[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 (the convective temperature). See sounding.

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 Southern 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).

Low clouds

Clouds with bases below 6000 feet and are stratiform or cumuliform in variety.

Low latitudes

The latitude belt between 30° and 0° North and South.

Low level jet

(abbreviation 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 (US) 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 correct term would be low-level jetstream.

Low pressure system

See Cyclone.

LP storm

(or LP Supercell) 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 a dry line, and thus are sometimes referred to as dry line storms.

LP supercell thunderstorms often have a small updraft base as seen in this thunderstorm near Hart, Texas.

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Stormy fortnight in NSW and QLD

14:24 EDT Thunderstorms are due to develop daily across New South Wales and Queensland for almost two weeks.

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