Lacunosus: Thin cloud patches, sheets, or layers marked by round holes with fringed edges. Applies mainly to cirrocumulus and altocumulus; may also apply, although very rarely, to stratocumulus.

Lake-Effect Snow: Snow showers associated with a cP air mass to which moisture and heat are added from below as it traverses a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.

Land-Breeze: A local wind, usually occurring during the night and early morning, when the cooler, denser air from the land moves out to the warmer sea. It is the reverse of a Sea-Breeze.

Lapse Rate: The rate at which temperature decreases with increasing height. The standard atmosphere assumes a lapse rate of 6.5°C per km up to the base of the stratosphere. The Dry Adiabatic Lapse Rate is 9.8°C/km.

Latent Heat: The heat absorbed or emitted without change of temperature during a change of state. heat is given out when water vapor condenses to form droplets and also when water freezes to form ice. Melting and evaporation absorb latent heat.

Law of Adiabatic Expansion/Compression: Any gas will cool that is allowed to expand freely from a higher pressure to a lower pressure without the transfer of external energy to the gas. Similarly, a gas will heat if compressed from a lower to a higher pressure in the absence of a transfer of energy from the gas.

Law of Buoyancy: Any material or object immersed in a fluid will tend to rise through the fluid if the fluid density is greater than the material density. The force associated with buoyancy is the difference between the weight of the displaced fluid and the weight of the immersed material. (The fluid we are most concerned with here is Earth's atmosphere!).

Law of Conservation of Angular Momentum: The product of the velocity of an object around a center of rotation (axis) and the distance squared of the object from the axis is constant.

Law of Hydrostatic Balance: The pressure at any height in the atmosphere is equal to the total weight of the gas above that level, when expressed as a weight per unit area (for example, kilograms per square meter).

Law of Photons: The smallest increment of energy that is carried in a beam of radiation having a given frequency and/or wavelength is proportional to the frequency of the radiation or inversely proportional to the wavelength of the radiation. In the first case, the proportionality constant is Planck's constant; in the second case, it is Planck's constant multiplied by the speed of light.

Law of Reflection: For the reflection of light from a smooth surface, the angle of incidence equals the angle of reflection.

Law of Refraction: A bit more complicated than the Law of Reflection, the Law of Refraction involves the index of refraction of a material. Every bulk material has an index of refraction, which measures the speed at which light can propagate through that medium relative to the speed of light in free space, c. If the index of refraction is ŋ, then the velocity of light in the medium, v, is given by
v = c/ŋ
Since the index of refraction is always greater than 1, the velocity of light is always reduced in a medium (compared with free space). The reduction in speed is negligible for gases; the index of refraction for air for example is only 1.0001. Water on the other hand has an index of refraction of 1.33 and glass, 1.5. When the indices of refraction of 2 adjacent media are different, the light rays are bent in crossing from one medium into the other. The angle by which the transmitted ray is deviated from the incident ray depends on the angle of incident ray on the interface and the ratio of the indices of refraction.

Layer Clouds: Clouds without vertical protuberances, such as Stratus, Altostratus or Cirrostratus.

Leader: The conductive path of ionized air that forms near a cloud base prior to a lightning stroke. The initial conductive path is referred to as a step leader because it extends itself Earthward in short, nearly invisible bursts. A Dart Leader, which is continuous and less branched than a step leader, precedes each subsequent stroke along the same path.

Lee Depression: A non-frontal depression formed to the lee of a mountain barrier which is high enough to obstruct the airflow. It is also called an Orographic Depression.

Leeside Low: Storm systems (extratropical cyclones) that form on the downwind (lee) side of a mountain chain. In the USA leeside lows frequently form on the eastern side of the Rockies and Sierra Nevada.

Leeward: The region downwind.

Lee Waves: Standing waves which can form when the airflow crosses a range of hills. Air ascends on the upwind side and descends on the downwind side of each wave.

Lenticular (Lenticularis): A lens-shaped cloud usually associated with lee waves; Clouds shaped like lenses or almonds that are occasionally iridescent. Lenticularis clouds are usually orographic; however, they may also occur in regions with level terrain. Applies mainly to cirrocumulus, altocumulus, and stratocumulus.

Lidar: Stands for Light Detection and Ranging. A system of pulsed light produced by a laser. It is used like radar to study clouds and dust particles in the atmosphere.

Lifting Condensation Level (LCL): The height at which rising air that is cooling at the dry adiabatic rate becomes saturated and condensation begins.

Lightning: An electrical discharge from a Cb which causes intense ionization and brilliant flash of light often accompanied by a crack of thunder. Lightning from a mature thunderstorm struck the seaplane Cassiopeia during September, 1938 burning the hat off the pilot's head and singeing his hair!

Light Pillars: A light pillar can sometimes be seen above the sun when it is setting or rising. It is caused by reflection of light off the base of horizontally aligned plate ice crystals in the atmosphere. The extend of the pillar is usually only a few degrees. More rarely, it is as much as 20 degrees or more. Light pillars are possible above and below the sun or moon; however, for earth-bound observers, the upper light pillar is most common, while the lower pillar is more likely when you are in an airplane flying above a cloud of ice crystals.
The upper and lower light pillars at the sun can be present together with the parhelic circle and then form a giant cross in the sky, which was considered a much feared omen by ancient and medieval folklore.

Linear System: Equations where alterations in the initial state result in proportional alterations in any subsequent state. Compare with non-linear systems, where alterations in the initial state need not produce proportional changes in a subsequent state.

Line Squall (Squall Line): A long line where there is a rapid increase of wind, usually associated with cold fronts or severe thunderstorms. It may be marked by an arch or line of low black cloud, a rapid rise of wind speed and veer of wind, a rapid drop in temperature and rise in pressure.

Liquid-in-Glass Thermometer: A devise for measuring temperature that consists of a tube with a liquid-filled bulb at one end. The expansion or contraction of the fluid indicates temperature.

Lithosphere: Rigid outer layer of Earth that includes the uppermost part of the mantle and the crust (both basaltic oceanic and granitic continental).

Little Ice Age: The period from about 1550 to 1850 when average global temperatures were lower, and alpine glaciers increased in size and advanced down mountain canyons.

Long-Range Forecasting: Estimating rainfall and temperatures for a period beyond 3-5 days, usually for 30-day periods. Such forecasts are not as detailed or reliable as those for shorter periods obviously.

Longwave Radiation: A term most often used to describe the IR energy emitted by Earth and the atmosphere. Terrestrial radiation.

Longwaves in the Westerlies: A wave in the major belt of westerlies characterized by a long length (thousands of kilometers) and significant amplitude. Also called Rossby Waves.

Looming: A mirage that allows objects that are below the horizon to be seen!

Low: Another term for a depression. A deep low pressure area with winds gusting to 150 miles per hour during January, 1921, destroyed eight times as many Douglas fir trees in Washington and Oregon as did the great volcanic explosion of Mount St. Helens in May 1980! Though we should consider the fact that Mt. St. Helen's eruption was considered a tiny burp compared to other eruptions!

Low Cloud: A cloud that forms below a height of about 2,000m.

Lowitz Arcs: Lowitz arcs are formed when horizontally aligned plate ice crystals (responsible for the 22-deg. left and right parhelia), are spinning along their horizontal axes through the sky. (This effect is the same metastable state as that of a playing card, for example, which you hold at its long edge and then let it fall). As the spinning ice crystals have one more degree of freedom of rotation than those responsible for the parhelia, an arc is formed rather than a spot of light. Lowitz arcs extend from the 22-deg parhelia down to the 22-deg halo (the lower Lowitz arcs), and up between the circumscribed halo and the 22-deg. halo (the upper Lowitz arcs). The arcs are very rare, with only a few sightings per year by a team of halo observers. Lowitz arcs can also form in a 120-deg.parhelion, as these are caused by the same type of plate ice crystals. Then, the 120 deg parhelion has two crossing arcs passing through it, and the combination takes the shape of an hourglass.

Low-Level Jet Streams: Jet streams that typically form near Earth's surface below an altutude of about 2km and usually attain speeds of less than 60 knots.