La nina-
The most common of several names given toa significant decrease in sea surface temperature (cold events) in the central and eastern equatorial Pacific. La Nina is the counterpart to the El Nino warm event, and its spatial and temporal evolution in the equatorial Pacific is, to a considerable extent, the mirror image of El Nino, although La Nina events tend to be somewhat less regular in their behavior and duration.
La serpe-
A long strip of cloud that sometimes lies against the southern base of Mount Etna in Sicily. It is said to herald rain.
Labbe-
(Also spelled labe.) A moderate to strong southwest wind in Provence (southeastern France), mild, humid, and very cloudy or rainy. On the coast it raises a rough sea. It is not frequent, occurring only in March. In the Swiss French Alps it is locally termed labech, and is squally with thunder, hail, and brief torrential downpours; it comes mainly in autumn and winter.
Laheimar-
Severe squalls during the change of seasons in October and November in Arabia.
Lake breeze-
A wind, similar in origin to the sea breeze but generally weaker, blowing from the surface of a large lake onto the shores during the afternoon; it is caused by the difference in surface temperature of land and water as in the land and sea breeze system. In addition to area, the depth of the lake is an important factor; a shallow lake warms up rapidly and is less effective as the source of a lake breeze in summer than is a deep lake. Lake breezes are well developed around the Great Lakes of North America, where they temper the summer heat.
Lake effect-
Generally, the effect of any lake in modifying the weather about its shore and for some distance downwind. In the United States, this term is applied specifically to the region about the Great Lakes or the Great Salt Lake. More specifically, lake effect often refers to the generation of sometimes spectacular snowfall amounts to the lee of the Great Lakes as cold air passes over the lake surface, extracting heat and moisture, resulting in cloud formation and snowfall downwind of the lake shore.
Lake effect snow-
Localized, convective snow bands that occur in the lee of lakes when relatively cold airflows over warm water. In the United States this phenomenon is most noted along the south and east shores of the Great Lakes during arctic cold air outbreaks.
Lake effect snowstorm-
Snowstorm occurring near or downwind from the shore of a lake resulting from the warming (destabilization) and moistening of relatively cold air during passage over a warm body of water.
Lake evaporation-
The evaporation from the surface of a lake.
Lambing storm-
(Also called lamb blasts, lamb showers, lamb storm.) A slight fall of snow in the spring in England.
Laminar boundary layer-
An interfacial region in which flow is smooth and nonturbulent. Above a surface, a laminar layer will develop and fluid velocity will increase with distance from the surface, but not indefinitely. At some point, flow will become turbulent, with the laminar sublayer separating the turbulent layer from the surface. In the real world, most laminar boundary layers are extremely thin (order of 1 mm), but can be of biological importance, for example, next to plant leaves or as invertebrate refuges in streams.
Laminar flow-
(Also called sheet flow, streamline flow.) A flow regime in which fluid motion is smooth and orderly, and in which adjacent layers or laminas slip past each other with little mixing between them. Exchange of material across laminar layers occurs by molecular diffusion, a process about 106 times less effective than turbulence. Laminar flow can be easily predicted as velocity increases at a steady rate from a boundary. This contrasts with the chaotic and random nature of turbulent flow. Laminar flow is not a common occurrence in the statically neutral and unstable atmosphere and is confined to a very thin layer (1 mm) adjacent to very smooth surfaces such as snow and ice. However, in strongly statically stable regions such as the the nocturnal boundary layer, the Richardson number can be large enough that turbulence is suppressed, and the flow is laminar over a layer many tens of meters thick.
Laminar sublayer-
A layer in which the fluid undergoes smooth, nonturbulent flow. It is found between any surface and a turbulent layer above. See laminar boundary layer, laminar flow.
Land breeze-
A coastal breeze blowing from land to sea, caused by the temperature difference when the sea surface is warmer than the adjacent land. Therefore, it usually blows by night and alternates with sea breeze, which blows in the opposite direction by day. See puelche, karif.
Land evaporation-
The actual evaporation from a region of land.
Land lash-
In England, a heavy fall of rain, accompanied by a high wind.
Landspout-
(Rare.) A tornado. 2. Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer.
Langmuir circulation-
Roll circulations approximately aligned with the surface stress vector that frequently occur in the upper boundary layer of oceans or lakes. Although similar in form to atmospheric longitudinal roll vortices, Langmuir circulations are thought to be driven by nonlinear interactions between the surface gravity wave field and the larger scale turbulent motions within the mixed layer. They are sometimes called windrows because they form lines of surface debris or bubbles in their surface convergence zones. Their spatial scale is related to the depth of the mixed layer and their characteristic velocity is on the order of 8u, where u is the friction velocity in water. As a result of this scaling, Langmuir circulations generally require surface winds of at least 8 m s 1 in order to form. See coherent structures, longitudinal rolls.
Large reynolds number flow-
The behavior of a fluid with a Reynolds number typically greater than 104 to 106, which usually occurs within the atmosphere. The main property of such flows is a constant friction stress within the surface layer that depends only on relative roughness but not on the Reynolds number itself. Thus, molecular viscosity and qualities occurring in flow descriptions that are dependent on the Reynolds number may be totally ignored.
Layer cloud-
Stratus cloud; a continuous cloud sheet capped by an inversion.
Lee wave-
1. Any wave disturbance that is caused by, and is therefore stationary with respect to, some barrier in the fluid flow. Whether the wave is a gravity wave, inertia wave, barotropic wave, etc., will depend on the structure of the fluid and the dimensions of the barrier. 2. A mountain wave occurring to the lee of a mountain or mountain barrier. These waves can become visible in the form of lenticular or trapped lee wave clouds.
Leeside convergence-
Region of convergence, often a line, downwind of a mountain or mountain ridge during fair weather daytime conditions that are favorable for the formation of thermally forced upslope flow and deep convective mixing. Convergence forms between upslope (or sometimes light and variable) flow at lower elevations of the lee slopes and downslope flow at higher elevations, which results from the downward convective mixing of ambient momentum from the flow above ridgetops. With moist upslope flow and favorable conditions, updrafts produced by the convergence can lead to mountain cumulus formation, or trigger thunderstorm or severe weather activity.
Lenticular cloud-
A commonly used term for clouds of the species lenticularis.
Leste-
Spanish nautical term for east wind. The name is given to a hot, dry, dusty easterly, or southeasterly wind that blows from the Atlantic coast of Morocco out to Madeira and the Canary Islands. It is a form of sirocco and occurs in front of depressions advancing eastward. Compare levanto.
Levante-
The Spanish and most widely used term for an east or northeast wind occurring along the coast and inland from southern France to the Straits of Gibraltar. It is moderate or fresh (not as strong as the gregale), mild, very humid, overcast, and rainy; it occurs with a depression over the western Mediterranean Sea. In summer it is rare and weak; in January it is inhibited by the Iberian anticyclone. It is most frequent from February to May and October to December. A levant (French spelling) with fine weather is a levant blanc; in the Roussillon region of southern France (where, as along the Catalonian coast of Spain, it is called llevant) it often brings floods in the mountain streams.
Levantera-
A persistent east wind in the Adriatic, usually bringing cloudy weather.
Leveche-
It is a hot, sand and dust laden wind from between southeast and southwest that blows in front of a depression on the southeast coast of Spain but extends only a few miles inland.
Libeccio-
Italian name for a southwest wind; used especially in northern Corsica for the west or southwest wind that blows throughout the year, and especially in winter when it is often stormy. On windward slopes it brings rain, with thunderstorms in summer and autumn. After crossing the mountains it is warm and dry, but may be very turbulent.
Liberator-
A name sometimes given the west wind through the Straits of Gibraltar.
Light air-
In the Beaufort wind scale, a wind with a speed from 1 to 3 knots (1 to 3 mph) or Beaufort Number 1 (Force 1).
Light breeze-
In the Beaufort wind scale, a wind with a speed from 4 to 6 knots (4 to 7 mph) or Beaufort Number 2 (Force 2).
Light freeze-
The occurrence of air temperature below 0 degree C (32 degree F) that kills some, but not all, annual vegetation. This often occurs in the 0 degree to minus 1 degree C (32 degree to 30 degree F) range.
Light frost-
A thin and more or less patchy deposit of hoarfrost on surface objects and vegetation.
Line squall-
A squall that occurs along a squall line. This term is now confined mostly to nautical usage.
Local storm-
A storm of mesometeorological scale; thus, thunderstorms, squalls, and tornadoes are often put in this category.
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
Lofting-
The phenomenon where the upper part of a smoke plume diffuses more rapidly upward than the bottom part diffuses downward. This generally occurs when the boundary layer near the ground is more stable than it is aloft. Compare coning, fanning, looping.
Lombarde-
An easterly wind (from Lombardy) that predominates along the French Italian frontier. It comes from the High Alps. In winter it is violent and forms snowdrifts in the mountain valleys. In the plains it is gentle and very dry. It is associated with an anticyclone over France and central Europe, or with high pressure to the southeast of Europe and low pressure to the northwest along with falling pressure over western France.).
Long crested wave-
Ocean surface waves that are nearly two dimensional, in that the crests appear very long in comparison with the wavelength, and the energy propagation is concentrated in a narrow band around the mean wave direction.
Long shore wind-
1. A damp unpleasant wind that blows from the south in Madras (India). 2. A wind from the northeast at night in Sri Lanka.
Longmont anticyclone-
A mesoscale zone of anticyclonically turning winds that develops downstream of the Cheyenne Ridge in northeast Colorado and southeast Wyoming, and is often centered just east of the foothills of the Rocky Mountains near the town of Longmont, Colorado. The cause of the feature is the interaction of the ambient low level northwest flow with the east west terrain feature known as the Cheyenne Ridge. See also Denver convergence vorticity zone.
Longshore current-
(Also called littoral current.) The resultant current produced by waves being deflected at an angle by the shore. In this case the current runs roughly parallel to the shoreline. The longshore current is capable of carrying a certain amount of material as long as its velocity remains fairly constant; however, any obstruction, such as a submarine rock ridge or a land point cutting across the path of the current, will cause loss of velocity and consequent loss of carrying power.
Love wave-
A type of seismic surface wave having a horizontal motion that is shear or transverse to the direction of propagation. Its velocity depends only on density and rigidity modulus, and not on bulk modulus. It is named after A. E. H. Love, the English mathematician who discovered it.
