Weather Words - L

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"Likely" precipitation: As used in weather forecasts issued by the Chicago National Weather Service, a precipitation event whose probability of occurrence is believed to be either 60 percent or 70 percent.

La Niña: A widespread cooling of the surface waters of the eastern tropical Pacific Ocean -- the opposite of El Nino. The influences of La Nina and El Nino on trade winds form extremes of the atmospheric trade wind fluctuations termed the Southern Oscillation.

Labor Day Hurricane: On Sept. 2, 1935, perhaps the most intense hurricane ever to strike the United States ravaged the Florida Keys with 200 m.p.h. winds and a 15-foot storm surge. The death toll: 400.

Lag of seasons: The period between the shortest day (about Dec. 21) and the coldest part of winter (late January), or between the longest day (June 21) and the hottest part of summer (late July).

Lagoon: A shallow lake or pond connected to a larger body of water. Locally, the Lincoln Park lagoons on Chicago's lakefront are examples. Other examples are ocean lagoons behind sand dunes and coral reefs.

Lahar: A mudflow composed of volcanic debris and water.

Lake breeze: (1) 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 day. It is caused by the temperature difference when the lake surface is colder than the adjacent land. (2) A coastal local wind that blows from the surface of a lake onto the shores during the day when the lake surface is colder than the adjacent land; a type of sea breeze, but usually weaker.

Lake-effect snow: Localized snow that forms on the downwind side of large lakes. Common in the late fall and winter (November to February) in the Great Lakes region when cold, dry air picks up moisture from the unfrozen lake surfaces.

Lake Michigan tides: One-half inch to one and one-half inches in height. Like all bodies of water, Lake Michigan is affected by the gravitational pull of the sun and the moon and it experiences twice-daily tides.

Lake Michigan wind categories: The Chicago National Weather Service issues a small craft advisory for sustained winds 17-33 knots; a gale warning for 34-47 knots; a storm warning for 48 knots or higher.

Lake Vostok: The world's largest known subglacial lake (170 by 30 miles, 1,640 feet deep); located in Antarctica, covered by 2.5 miles of ice, and the site of the world's lowest air temperature (-129 degrees F).

Land: The solid part of the Earth's surface. More generally, the entire complex of surface and near-surface attributes of the Earth's surface that are significant to man.

Land breeze: A coastal local wind blowing from land to sea, caused by the temperature difference when the sea surface is warmer than the adjacent land. It blows by night and alternates with the daytime sea breeze.

Landfall: The intersection of the center of a tropical cyclone with a coastline. Because the storm's strongest winds are not precisely at its center, its highest winds can occur over land even without landfall.

Landscape: In geography, a vista comprised of all the natural features such as fields, hills, forests and water -- even clouds -- that the eye can comprehend from a single vantage point.

Landslide: Material slipping downhill under the influence of gravity, frequently occurring when the material is saturated with water.

Landslide damage: The annual U.S. damage from landslides (material slipping downhill under the influence of gravity, usually when the material is saturated with water) is estimated at $1 billion.

Landspout: A tornado. The landspout is so named because it looks like a weak Florida waterspout, except that it occurs over land.

Lapham, Increase: (1811-1875) Milwaukee naturalist, engineer and geologist, and the father of what would become the Weather Bureau (now the National Weather Service); issued the Bureau's very first weather forecast from Chicago on Nov. 8, 1870.

Lapse rate: (1) The rate at which air temperature decreases, or lapses, in a vertical ascent into the atmosphere. The lapse rate is termed steep when the decrease with height is large, small when the decrease is slight, negative when temperature increases with height. (2) The rate at which air temperature decreases, or lapses, as air rises into the atmosphere. If a cloud is not forming, air cools at 5.4 degrees per 1,000 feet of vertical ascent.

Late-April snow: 6.4 inches of snow at Chicago during April 22-26, 1910, established the city's latest significant snow event. Daily snow totals for the period were 0.1 inch, 2.1 inches, 0.9 inches, 2.5 inches and 0.8inch.

Latent heat: The quantity of heat absorbed or released without change of temperature when material undergoes a change of state. For example, heat is absorbed when ice at 32 degrees melts into water at 32 degrees, and that heat is latent heat.

Latin wind words: Boreas - the north wind; auster - the south wind; eurus - the east wind; zephyrus - the west wind. Wind direction is always stated as the direction from which the wind is blowing.

Lee: The side of a ship, or a shore location, away from the direction from which the wind is blowing; the side of a ship that is sheltered from the wind.

Leeward: (1) The direction or side of any object or land mass (e.g. island, mountain or region) that faces away from the prevailing wind.(2) The direction facing away from the wind; downwind.

Lenard, Philipp: (1862-1947) German physicist who, in 1904, was the first to describe the true shape of raindrops as spherical for small drops and flat for large drops -- not teardrop-shaped.

Lenticular clouds: Lens-shaped clouds which form mainly in the disturbed, wave-like wind motion downwind of mountains. Lenticular clouds are nearly stationary with respect to the ground.

Levanter: An easterly wind in the Straits of Gibraltar, most frequent in March and from July to October. It causes complex and dangerous eddies downwind from the Rock of Gibraltar and, when moisture is sufficient, a spectacular banner cloud extends from the Rock's summit downwind a mile or more.

Lift: The component of the force of air on an aircraft that is directed upward (positive lift) or directed downward (negative lift).

Light frost: A thin and more or less patchy deposit of frost.

Light freeze: The occurrence of temperatures below 32 degrees F that kills some but not all annual vegetation.

Lightning: (1)Any and all of the forms of visible electrical discharge produced by thunderstorms. The lightning bolt itself can heat the air through which it travels to 54,000 degrees F. (2) A long spark that discharges regions of excess atmospheric charge. The total discharge, called a flash, is initiated by a downward (or upward) traveling spark known as the stepped leader.

Lightning 30/30 rule: If it takes less than 30 seconds to hear thunder after seeing the flash, lightning is near enough to pose a threat; after the storm ends, wait 30 minutes before resuming outdoor activities.

Lightning alley: A belt of land through central Florida that experiences more thunderstorms (and consequently more lightning) than any other place in the United States.

Lightning capital of the United States: The state of Florida. It receives about 1.4 million lightning ground strikes per year, or about 25 ground strikes per square mile annually.

Lightning capital of the world: The nation of Brazil, which receives about 70 million lightning ground strikes per year. The United States experiences about 22 million lightning ground strikes annually.

Lightning channel: The irregular path through the air along which a lightning discharge occurs.

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.

Lightning distance: Count the number of seconds between the lightning flash and its thunder, and divide by five. That will give the distance in miles of the closest part of the lightning bolt.

Lightning fact: (1) The United States experiences more than 20 million lightning ground strikes each year. Among the 50 states, Florida experiences the greatest number of them, Alaska the least.

Lightning flash: (1) The total observed series (lasting a second or less) of individual luminous electrical events that occur in such rapid succession in the atmosphere that the human eye cannot resolve them. (2) The total series (lasting a second or less) of individual electrical events by which charge is transferred along an atmospheric channel between positive and negative charge centers.

Lightning ground strikes: About 22 million cloud-to-ground lightning strikes occur annually in the United States. Florida experiences the greatest number, with about 1.4 million ground strikes per year.

Lightning polarity: Positive or negative, depending on the direction of current flow between the cloud and ground; negative when current flows from cloud to ground (95 percent of cases), positive (5 percent) when ground to cloud.

Lightning rod: A grounded metallic conductor with its upper extremity extending above the structure that is to be protected from damage due to lightning.

Lightning safety rule for (1) Outdoor athletic events: "If you can see it, flee it; if you can hear it, clear it." Once off the field, play should not resume until 30 minutes after the final rumble of thunder.(2) Outdoor events: "If you can see it, flee it; if you can hear it, clear it." Once indoors, outside activities should not resume until 30 minutes after the final rumble of thunder.

Lightning strikes: Only about one in four lightning bolts actually strikes the ground. Three of four bolts strike within cloud, from cloud to cloud, or from cloud to air.

Lightning-strike statistics: In the United States, males account for 84 percent of lightning fatalities and females only 16 percent; males account for 82 percent of lightning injuries, females only 18 percent.

Lightning width: The primary current-carrying portion of a lightning discharge is less than one-half inch wide (although it can be many miles in length).

Likely: As used in issuances of the National Weather Service, the probability term used to describe a weather event (like rain) whose probability of occurrence is believed to be either 60 percent or 70 percent.

Lithometeor: The general term for all dry particles in the atmosphere. Common lithometeors are dust, haze, smoke and sand.

Lithosphere: The solid portion of the Earth, as distinguished from the atmosphere and hydrosphere.

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.

Lolly ice: Ice crystals that form in supercooled sea water that is too turbulent to permit the formation of an ice sheet.

London smog disaster: Dec. 4-10, 1952, damp air, gray skies, no wind and a powerful inversion sealed London in a toxic mix of pollutants from power plants, coal stoves and the city's new fleet of 8,000 diesel buses. The toxic air brought death to 12,000 Londoners.

Looming: A mirage effect produced by greater-than-normal bending of light traveling through the lower atmosphere, permitting objects that are below the observer's horizon to be seen.

Los Angeles hurricane: On this Sept. 25, 1939, a rare West Coast hurricane moved onshore south of Los Angeles, California. Heavy rain drenched the area (5.66 inches in Los Angeles, 11.60 inches at Mount Wilson). Forty-five lives were lost at sea.

Low: (1) The minimum temperature observed over a fixed period of time, usually the calendar day; (2) On a weather map, an area of a local minimum of air pressure, often associated with inclement weather.

Low: (low pressure center): On a weather map, an area of a local minimum of air pressure; often associated with inclement weather. When viewed from above, winds spiral into a low pressure center in a counterclockwise sense.

Low clouds: The designation appled to clouds whose bases are no higher than 6,000 feet above the surface. Clouds within this group are stratus, stratocumulus, and most cumulus and cumulonimbus (thunderhead) bases.

Low pressure center: On a weather map, an area of a local minimum of air pressure; often associated with inclement weather. When viewed from above, winds spiral into a low pressure center in a counterclockwise sense.

Low pressure system: An atmospheric area of low air pressure, usually a few hundred miles in diameter, whose winds (in the Northern Hemisphere) spiral inward in a counterclockwise sense.

Lowest temperatures: World, -129 degrees (July 21, 1983, at Vostok, Antarctica); United States, -80 degrees (January 23, 1971, at Prospect Creek, Alaska); Chicago, -27 degrees (January 20, 1985, at O'Hare Airport).

Lunar eclipse: An alignment of the Earth, sun and moon in such a way that the Earth is in a direct line between the sun and moon, and the Earth's shadow falls upon the face of the moon.