La Niña - La Niña occurs when stronger than normal tropical Pacific trade winds cause an accumulation of cold surface seawater along the central and eastern Pacific Ocean near the equator. La Niña events normally occur around Christmas and last from a few weeks to a few months. Sometimes an extremely strong La Niña event can develop, lasting up to 2 years. The opposite of an El Niño. 

Laccolith - A mass of subsurface intrusive igneous rock that becomes embedded in between two layers of sedimentary rock and has its origins from mantle magma (see figure). Usually, either the top or bottom of the laccolith will be horizontal, while the opposite side will be curved.  Compare with batholith, dyke, sill, lopolith, and phacolith.

Lacuna - In geology, this term refers to the missing time interval associated with an unconformity within a series of chronologically laid-down beds.

Lacustrine - A feature or process involving a lake.

Lacustrine Deposit - A sediment deposit that was laid down in a lake.

Lacustrine Plain - A flat and extensive area of sediment deposits that originate from the past presence of a lake. 

Lag - A term that describes a situation where the timing of an event or the development state of a process is delayed because of some circumstance.

Lag Gravel - A type of sediment that consists of an accumulation of gravel-sized rock fragments found at the ground surface. Lag gravel is created when erosion by wind and water selectively removes smaller-sized particles and leaves the coarser rock fragments behind. 

Lag Time - The time delay that occurs for some causal mechanism and its coupled processes take place. For example, torrential rainfall from a thunderstorm can cause flooding when water enters a stream network and overflows the stream channel.

Lagoon - (1) A body of seawater that is almost completely cut off from the ocean by a barrier beach. (2) The body of seawater that is enclosed by an atoll.

Lahar - A very rapid type of downslope mass movement involving mudflows from volcanic ash.

Lake - A body of standing water found on the Earth's landmasses. The water in a lake is normally fresh, as the water input is usually the product of precipitation. Also see eutrophic lake, mesotrophic lake, and oligotrophic lake.

Lake Breeze - A local thermal circulation pattern found at the interface between a large lake and the surrounding area of land. In this air circulation system, winds near the ground surface blow from the lake to the land during the day. Lake breezes are best developed in the afternoon. Similar to sea breeze but on a smaller spatial scale.

Lake Rampart - A minor ridge found along the shore of the lake that is created by the shoreward movement of lake ice during the winter season.

Lamina (Laminae plural) - A thin layer of deposit in a sedimentary rock. Typically, less than 1 centimeter (half an inch) in thickness. Compare with bed.

Laminar Flow - The movement of water within a stream that occurs as uninterrupted parallel flows. Laminar flow generally occurs in areas where the frictional effects on the flowing water are minimal.

Land Breeze - Local thermal circulation pattern found at the interface between large surfaces of land and water, like where the ocean meets the coastline of a continent. In this air circulation system, surface winds blow from land to water during the night. Note that the high atmospheric pressure over land occurs because it cools faster than a water surface, which is warmer in terms of air temperature. Compare with sea breeze.

Land Bridge - A land connection that occurred in the past between now-separate continental masses isolated by the presence of an ocean. The Bering Land Bridge once connected the Kamchatka Peninsula (northeast Asia) with Alaska (northwest North America). This land bridge formed because of the accumulation of glacier ice on land during the Pleistocene Age, causing sea levels to lower substantially, more than 50 meters or 160 feet. Land bridges are important for biogeographical patterns of plants and animals, as they allow the movement of species between continents.

Land Classification - A system that classifies and groups units of land based on measured properties and/or the use of this land for a defined purpose. For example, the type of soil that a unit of land has may be used to classify its use for a particular agricultural purpose.

Land Capability - The relative value of land for agricultural use, based usually on an analysis of soil information.

Land Drainage - The process of draining excess water from an area of land. We often do this to improve the quality of land for other uses, such as agriculture and urban development.

Land Systems - A classification system used in land-use management and planning where similar areas are grouped together in a broader landscape based on measurable geological, ecological, pedological, agricultural, and geographical characteristics. 

Land-Cover - Refers to the natural and human-constructed things that cover the Earth's land surface. Often, we describe the natural land-cover of an area by reporting on the ecosystem or biome dominating the surface.

Land-Cover Change - Describes the conversion of land-cover from one type to another. For example, the conversion of deciduous forest into cropland.

Land-Use - Describes the way humans use an area of land. Major uses of land include forestry, agriculture, livestock raising, industry, settlement, recreation, mining, and transportation.

Land-Use Change - The conversion of land-cover for some type of human use. Humans use land for forestry, agriculture, livestock raising, industry, settlement, recreation, mining, transportation, or other purposes associated with human social and economic activity.

Landfall - The coastline location where a tropical storm or hurricane moves from the ocean onto land.

Landfill - The human use of an area of land to dispose of and store various forms of human garbage and waste. This is normally done to minimize negative environmental impacts and to use the least amount of land possible. 

Landform - The physical and morphological characteristics of an area of the Earth's surface that are the result of geomorphic and geologic processes. These characteristics result from processes associated with tectonic uplift, subsidence, folding, faulting, weathering, volcanism, plate tectonics, glaciation, stream processes, climate, wave action, and wind. Landforms can be categorized according to the features observed.

Landform Classification - The grouping of landforms based on the processes that acted to create them or their morphological characteristics.

Landsat - A series of American satellites launched by NASA to remotely monitor resources and human activities on the surface of our planet. The first Landsat satellite was launched by the United States in 1972. Landsat 8 was launched on February 11, 2013, while Landsat 9 is scheduled to launch in September 2021. Landsat satellite systems use two types of sensors to monitor the Earth: Thematic Mapper and Multispectral Scanner. The Landsat satellite system has collected images of Earth's surface for more than 40 years. This historical archive of data is unsurpassed in quality, detail, and coverage. See the following website for more information - https://landsat.gsfc.nasa.gov.

Landscape Ecology - A field of ecology that studies the cause-and-effect relationships, processes, and patterns that exist between human systems, the natural abiotic environment, and life. One critical aspect of landscape ecology is that it investigates phenomena at a variety of different spatial scales. See the website https://www.landscape-ecology.org.

Landscape Evaluation - A system of rural landscape assessment for the purpose of management and planning.

Landslide - A general term used to describe the downslope movement of soil, sediments, rock, and other weathered materials because of gravity.

Landward - Something positioned or located away from a water body but towards the land.

Langley - Unit of the intensity of radiation measured per minute and equal to one calorie.

Lapse Rate - The rate at which air temperature decreases with an increase in altitude. Represents the vertical temperature gradient in the atmosphere. Meteorologists routinely measure the atmospheric lapse rate at weather stations via radiosondes. Also see environmental lapse rate, dry adiabatic lapse rate, and saturated adiabatic lapse rate.

Last Glacial Maximum - The period between 20,000 and 26,500 years ago, when most of the ice sheets and glaciers found in North America, Europe, and Asia reached their most recent maximum extent. During this time, Earth's climate was generally cooler and drier, areas of desertification were quite extensive, and the sea level was about 120 meters lower than today.  

Late Glacial Maximum - Time between 10,000 and 13,000 years ago when the Earth's global climate warmed substantially, causing rapid deglaciation in the Northern Hemisphere.  The end of this period marks the beginning of the Holocene. Also called the Tardiglacial.

Latent Heat - Is the energy required to change a substance to a higher state of matter (solid ➜ liquid ➜ gas). This energy is released from the substance when the change of state is reversed (gas ➜ liquid ➜ solid).

Latent Heat Flux - Latent heat flux is the global movement of latent heat energy through circulations of air and water. Atmospheric circulation transports latent heat energy vertically and horizontally to cooler regions where it is condensed as rain or is deposited as snow, releasing the heat energy stored within it.

Latent Heat of Condensation - The amount of heat energy released to the environment when a gas changes its state to a liquid. For 1 gram of water, the amount of heat energy released is 540 calories at 100°C (212°F).

Latent Heat of Vaporization - The amount of heat energy required from the environment to change the state of a liquid to a gas. For 1 gram of water, the amount of heat energy required is 540 calories at 100°C (212°F).

Lateral Accretion - Stream channel process where sediments are deposited vertically because of migrating point bars. In contrast, most other types of deposits formed by stream processes are laid down horizontally.

Lateral Dune - A small sand dune found alongside a much larger dune. Common landform in sand-rich deserts.

Lateral Erosion - A form of horizontal stream erosion that occurs along the outside of meander bends. Lateral erosion occurs because stream flow is directed more towards the stream bank at the outside of meander bends of the stream channel.

Lateral Flow - A type of subsurface water flow that occurs in soil and sediments below the ground surface but above the water table. Involves the movement of water in a horizontal direction, usually along a more permeable subsurface zone. Compare with throughflow. 

Lateral Migration - A term that describes the movement of a stream channel across the floodplain over time due to bank erosion and sediment deposition.

Lateral Moraine - Moraine that is found along the sides of a glacier. Commonly found on glaciers that occupy a valley.

Laterite - A hard subsurface deposit of oxides of aluminum and iron found in tropical soils where the water table fluctuates with seasonal changes in precipitation. The formation of laterite layers involves the intense chemical weathering of soil parent material over time. Most laterites have a rusty red color because of the presence of iron oxide. Laterites are commonly mined for aluminum ore.

Laterization - A soil-forming process that creates a laterite layer.

Latitude - Latitude is a north-south measurement of position on the Earth's surface. It is defined by the angle measured from a horizontal plane located at the Earth's center, perpendicular to the polar axis. A line connecting all places of the same latitude is called a parallel. Latitude is measured in degrees, minutes, and seconds. Measurements of latitude extend from 0° (the equator) to 90° North and South. Compare with longitude.

Latosol - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. This soil is characterized by a thin O horizon, the presence of a laterite layer, and a deeply weathered profile.

Laurasia - A supercontinent that existed mainly in the Northern Hemisphere between 175 and 335 million years ago. Shown is the configuration of the Earth's landmasses 200 million years ago. The tectonic separation of this landmass created North America and Eurasia. Gondwana and Laurasia together formed the supercontinent Pangaea during the Carboniferous (359 to 299 million years ago). 

Lava - Molten magma released from a volcanic vent or fissure during a volcanic eruption (Image Source: U.S. Geological Survey). When first released at the Earth's surface, lava is a liquid and can have a temperature between 700°C and 1,200°C (1,300°Fand 2,200°F). Lava is about 100,000 times more viscous than water.

Lava Cone - A volcanic cone constructed from many superimposed lava flows. Most lava cones are composed of basalt-rich lava, which is more viscous and less explosive. Slope profiles of lava cones are typically low-angled and convex. Lava cones look like miniature shield volcanoes.

Lava Dome - See shield volcano.

Lava Field - An extensive, relatively flat area with many lava flows found at the base of one or more volcanic cones.

Lava Flow - A flow of lava coming from a volcanic vent.

Lava Fountain - A concentrated upward spray of molten lava from a volcanic vent or volcanic fissure.

Lava Lake - A huge lake-like accumulation of molten lava inside a volcanic crater. 

Lava Plateau - An elevated plateau composed of numerous solidified lava flows coming from many localized volcanic vents created over a long period of time.

Lava Tunnel - A below-ground passageway within a solidified lava flow.

Law of Basin Areas - The morphometric relationship observed in the mean basin area size of stream segments of a particular classification order in stream channel branching. This law was proposed by pioneering hydrologist R.E. Horton in the first half of the 20th century.

Law of Conservation of Energy - This law of thermodynamics states that energy can be transferred from one system to another in many forms, and it cannot be created or destroyed. Thus, the total amount of energy available in the Universe is constant.

Law of Stream Lengths - The morphometric relationship observed in the cumulative size of stream segment lengths in stream channel branching. This law was proposed by pioneering hydrologist R.E. Horton in the first half of the 20th century.

Law of Stream Number - The morphometric relationship observed in the number of stream segments of a particular classification order in stream channel branching. This law was proposed by pioneering hydrologist R.E. Horton in the first half of the 20th century.

Law of Superposition - An underlying principle in the study of sedimentary stratigraphy that states that in any set of deposited layers of sediment or rock, the relative age of the layers normally decreases from bottom to top of the sequence. 

Law of the Minimum - A principle developed initially in agricultural science to explain the fact that most crops seem to be limited in growth by a single nutrient (like nitrogen, phosphorus, or potassium). This principle suggests that organisms are typically limited by only one single physical factor that is in shortest supply relative to demand. Originally proposed by Carl Philipp Sprengel in 1828, but later popularized by Justus von Liebig.

LDC - See Less Developed Country.

Leachate - A solution containing material leached from a soil.

Leaching - A soil process where water removes and transports humus and inorganic nutrients in solution downward through the soil profile. Two important terms associated with leaching are eluviation and illuviation. Eluviation refers to the movement of humus, chemical substances, and mineral particles from the upper layers of a soil to lower layers. Illuviation is the deposition of humus, chemical substances, and fine mineral particles in the lower layers of a soil from the upper layers.

Lead - A metallic, dark grey, dense, and relatively heavy chemical element identified by the symbol Pb. Lead is commonly extracted from ores that contain the natural mineral galena (lead sulfide or PbS). Lead is used in a variety of human-made products, including white paint, batteries, pewter, solder, fishing weights, buckshot, shielding against harmful radiation, and plumbing. Lead is also quite toxic to life, causing blood and neurological disorders.

Leaf Drip - The rainwater that falls to the ground surface from plant leaves after it has been intercepted by these structures.

Least Concerned - One of the categories used by the International Union for Conservation of Nature's (IUCN) Red List to describe the status of a species. This category suggests the species has been evaluated and it was determined that it will not become critically endangered, endangered, or vulnerable in the near future.

Lee - Side of a slope that is opposite to the direction of the flow of ice, wind, or water. The opposite of stoss.

Lee Depression - An area of surface low atmospheric pressure located on the leeward side of a mountain range. Lee depressions form because large vertical obstacles to horizontal atmospheric flow cause the continual development of anticyclonic divergence in the upper atmosphere, which leads to cyclonic convergence at the ground surface.

Lee Eddy - A small converging airflow (eddy) that frequently develops on the leeward side of vertical obstacles or steep drop-offs.  

Lee Waves - Are large sequential gravity waves found embedded in the horizontal flow of air in the atmosphere. These features form when airflow encounters a significant vertical obstruction, creating a series of oscillations in the atmosphere. Lee waves are often seen as wave clouds downwind of a mountain. 

Leeward - The downwind side of an object, like a mountain. The opposite of windward.

Legume - A plant species that is a member of the Fabaceae subfamily (for example, peas or beans). These angiosperm plants form symbiotic relationships with specific species of bacteria to acquire nitrogen for growth. 

Lentic - A habitat or group of organisms found in still, freshwater. Lentic habitats can include ditches, ponds, marshes, and lakes. Compare with lotic.

Lenticular Cloud - Stationary, often saucer-shaped clouds that form in the troposphere. They commonly form when air in the upper atmosphere with relatively high humidity moves over mountains. On the leeward side of the topographical obstruction, the air rises and condenses, forming the lenticular cloud. The lenticular cloud's streamline shape is normally perpendicular to the wind direction at the altitude of its formation. The Latin name for this type of cloud is Altocumulus lenticularis. 

Less Developed Country (LDC) - A country characterized by minimal industrialization, low technological development, low per capita income, and high population growth rates. Many of these countries are found in Asia, Africa, and Central and South America. Also see More Developed Country.

Lessivage - The mechanical translocation of silicate clay particles by way of suspension in soil water to a location down the soil profile. This process does not alter the clay's chemical structure.

Levee - A ridge of coarse deposits found running alongside stream channels and elevated above the floodplain. Levees form from the deposition of sediment during floods.

Liana - A species of plant that uses the support of trees to elevate its leaves above the forest canopy.

Lichen - An organism that consists of a symbiotic joining of a species of fungi and a species of algae or cyanobacteria. In this symbiotic partnership, the fungus provides the algae or cyanobacteria with water and nutrients and protection from the environment.  In exchange, the algae or cyanobacteria supply the fungus with synthesized carbohydrates and other organic matter that are byproducts of their photosynthesis. Lichens have many different forms and colors and are found on our planet in a great variety of habitat types. Shown is a type of lichen with a shrubby and highly branched form that is often found growing on the branches or bark of conifer trees.

Lichenometry - The scientific technique of using lichens for dating things in the late Holocene. Lichenometry is often used to date past glacial events. Lichens grow at steady rates, and some species even grow as circular expanding patches. The size of these patches can be used to infer the time when a surface became free of glacial ice.

Light - See visible light.

Light Minerals - Any rock-forming mineral that has a specific gravity of less than 2.8 and is light in color. Some examples of light minerals include quartz, feldspars, and calcite.

Lightning - The visible instantaneous discharge of electricity through the atmosphere, normally associated with thunderstorms. The lightning event occurs between electrically charged regions between two thunderstorm clouds or a region in a cloud and the ground surface. The lightning bolt is composed of extremely hot plasma produced by the flow of electrons. Lightning also produces a deafening booming sound called thunder.

Light-Year (Lightyear or Light Year) - Distance that light travels in the vacuum of space in one year. Approximately 9.7 trillion kilometers (6.0 trillion miles).

Lignite - Low-grade coal with a carbon content of between 60 to 70%. Also called brown coal.

Lime Requirement - The amount of lime (CaCO₃) addition required to raise the pH of a soil to some desired level. Liming is used by farmers to raise the pH of ordinarily acidic soils, with the desired effect of increasing crop yields.

Limestone - Sedimentary rock composed of carbonate minerals, especially calcium carbonate. Limestone can form through clastic and non-clastic sedimentary processes. Clastic limestones are formed from the breakup and deposition of shells, coral, and other marine organisms by wave action and ocean currents. Non-clastic limestones can be formed either as a precipitate or by the lithification of coral reefs, marine organism shells, or marine organism skeletons.

Limestone Pavement - A karst landform that consists of relatively flat exposed limestone bedrock. Further, the feature appears to be paved blocks because the surface is intersected by regular cracks. 

Limiting Factor - An abiotic condition that most controls the growth of a species. For most terrestrial plants, this condition is the supply of the soil nutrient nitrogen. Also see the Law of the Minimum.

Limnic - Something associated with freshwater.

Limnology - This is an interdisciplinary field of knowledge that studies the science associated with inland waters. Limnology employs knowledge from Ecology, Environmental Science, Biology, Chemistry, Earth Science, and Physics. Also called freshwater science.

Limnophyte - A plant that lives submerged in a freshwater environment.

Limonite - A common yellow to brown colored iron ore made of a mixture of hydrous ferric oxides.

Lineament - A general term used to describe a large-scale linear surface landscape feature. Often, these landscape features result from the area's structural geology.

Lipid - Is an organic compound composed of carbon atoms that have two hydrogen atoms attached. Lipids are commonly known as fats and oils. They also belong to the family of molecules known as hydrocarbons.

Liquefaction - Temporary transformation of a mass of soil or sediment into a fluid mass. Liquefaction occurs when the cohesion of soil or sediment particles is lost. It can be triggered by seismic waves from an earthquake. For this condition to take place, the pore spaces between soil particles must be at or near saturation.

Liquid - A state of matter where molecules can flow, and the surface of this mass displays the property of surface tension.

Liquid Limit - See Atterberg limits.

Liquid-In-Glass Thermometer - A thermometer design that uses a thermally sensitive liquid encased in a graduated clear glass cylinder. Changes in temperature cause the liquid to either expand (with warming) or contract (with cooling). The most commonly used liquids for this type of instrument are mercury and alcohol. These thermometers are often used for taking meteorological measurements of air temperature.

Lithification - A process by which sediments are consolidated into sedimentary rock.

Lithology - The physical characteristics and structure of a rock visible to the naked eye or using low-powered magnification. 

Lithosol - A shallow soil that lacks any significant soil horizon development.

Lithosphere - Is the solid inorganic portion of the Earth composed of rocks, minerals, and sediments. The lithosphere includes continental crust, oceanic crust, and the topmost part of the upper mantle. This layer is typically about 100-150 kilometers (60-90 miles) thick and can glide over the rest of the upper mantle. The lithosphere may be as thin as 40 km (25 miles) or as thick as 280 kilometers (175 miles). Because of high temperatures and extreme pressure, deeper portions of the lithosphere can undergo plastic flow over geologic time. The lithosphere is also the zone of earthquakes, volcanoes, mountain building, and continental drift.

Litter - The accumulation of leaves, twigs, and other forms of organic matter on the soil surface. In most soils, the surface litter layer is at various stages of decomposition.

Litterfall - Movement of leaves, twigs, and other forms of organic matter from plants to the litter layer found in soil.

Little Climatic Optimum - See Medieval Warm Period.

Little Ice Age - A climatically defined period roughly from 1550 to 1850 CE. During this period, global temperatures were at their coldest since the beginning of the Holocene.

Littoral - Something associated with a lake or ocean shoreline.

Littoral Drift - Loose sediment that is transported by waves and currents along the shoreline through beach drift and longshore drift in coastal areas.

Littoral Transport - The process of loose sediment moving along a coastline. This process has two components: longshore transport and onshore-offshore transport.

Littoral Zone - The zone along a coastline that is between the spring tide high water and low water marks.

Living Atmosphere - The Earth’s atmosphere that existed between 2.5 billion years ago and today. The development, evolution, and growth of life increased the quantity of oxygen in the atmosphere from <1% to 21%. By about 500 million years ago, the concentration of atmospheric oxygen levels off at about 21%. At the end of this stage, human activity began modifying the concentration of several greenhouse gases, and the lower atmosphere was mainly composed of the gases nitrogen (N₂) - 78%, oxygen (O₂) - 21%, argon (Ar) - 0.9%, and carbon dioxide (CO₂) - 0.042%. Compare with the early atmosphere and the secondary atmosphere.

Load - See stream load. 

Load Structures - A type of small-scale sedimentary structure that occurs in much larger sedimentary strata. These features form when sand is deposited on top of saturated clay or silt. The weight of this deposit then creates irregularly shaped lobes of sand that sink into the layers of clay or silt, deforming these layers downward. The lobes of sand often have intermittent tongues of clay or silt that have migrated up into the sediment under pressure. Also called load casts.

Loam - A soil that contains a roughly equal mixture of clay, silt, and sand. Very suitable for growing most crops.

Lobe - A tongue-like extension of some material in the natural environment. For example, the ice lobe of an alpine glacier.

Local Climate - Refers to the micro-climatic conditions of a relatively small area. For example, the local climate of an urban area or a hillside facing the Sun.

Local Group - Is a grouping of galaxies (galaxy group), caused by gravity, that includes the Milky Way Galaxy, Andromeda Galaxy, and more than 30 smaller galaxies located in the Universe. The Local Group is part of a much larger grouping of numerous galaxy groups known as the Virgo Supercluster. Also see supercluster. 

Location - A term used in geography that deals with the relative and absolute spatial position of natural and human-made phenomena.

Loess - Unstratified, yet homogeneous, deposits of silt laid down by aeolian processes over extensive areas of the mid-latitudes during glacial and postglacial times.

Logarithmic Scale - A measurement scale based on logarithms. Values on this scale increase exponentially.

Longitude - Longitude is a west-east measurement of position on the Earth's surface. Longitude is defined by the angle measured from a vertical plane running through the polar axis and the Prime Meridian. A line connecting all places of the same longitude is called a meridian. Longitude is measured in degrees, minutes, and seconds. Measurements of longitude range from the Prime Meridian at 0° to 180° west and east from this point. Compare with latitude.

Longitudinal Dune - A sinuous sand dune that can be more than 100 kilometers long (60 miles) and 100 meters (300 feet) high. These dunes are created when there are strong winds from at least two directions. The dune ridge is symmetrical, aligned parallel to the net direction of the wind, and has slip-faces on either side. 

Longitudinal Valley - A long valley found between two almost parallel mountain chains. These features are common in areas where folding is responsible for the formation of mountains.

Longshore Current - A water current that moves parallel to the shoreline.

Longshore Drift - The movement and deposition of coastal sediments because of longshore currents.

Longshore Transport - The transport of sediment in moving water parallel to a shoreline.

Long Waves - See Rossby waves.

Longwave Radiation - A form of electromagnetic radiation with a wavelength roughly between 0.7 and 100 micrometers (µm). Also called infrared radiation.

Lopolith - A flat, saucer-shaped intrusive igneous rock mass with a concave center.  Compare with batholith, dyke, sill, laccolith, lopolith, and phacolith.

Lotic - A habitat or group of organisms found in moving freshwater. An example of a lotic habitat would be a stream. Compare with lentic. 

Lovelock, James E. - British scientist and naturalist best known for developing the Gaia Hypothesis. This theory suggests that life on Earth functions like a superorganism regulating its environment through biological processes that influence the atmosphere, lithosphere, and hydrosphere.

Low Pressure - An area of atmospheric pressure within the Earth's atmosphere that is below average. If this system is on the Earth's surface and contains circular wind flow and enclosed isobars, it is called a cyclone.

Low Tide - The lowest elevation of the ocean surface during a 24-hour and 50.4-minute tidal cycle. Same as the low water mark.  Compare with high tide.

Low Water - This term refers to the minimum elevation of the ocean surface during a 24-hour and 50.4-minute tidal cycle for diurnal or semi-diurnal tides. Same as high tide. Compare with high water.

Low Water Mark - The elevation of the ocean surface along a shoreline at low tide. Compare with the high water mark.

Lower High Water - Term used to describe the lower event of the two maximum ocean surface elevations that occur in a mixed tide during a 24-hour and 50.4-minute tidal cycle.

Lower Low Water - Term used to describe the lower event of the two minimum ocean surface elevations that occur in a mixed tide during a 24-hour and 50.4-minute tidal cycle.

Lower Mantle - Layer of the Earth's interior extending from 670 to 2,900 kilometers (415 to 1,800 miles) below the surface crust. The lower mantle is believed to be composed mainly of ultramafic rock. This layer is hot and plastic, and part of the mantle layer. Compare with the upper mantle.

Lunar Day - The interval of time it takes for the Earth's Moon to complete one rotation on its axis with respect to the Sun. This time interval is equal to approximately 24 hours and 50 minutes.

Lutite - A general term used to describe a fine-grained sedimentary rock composed of either silt or clay particles, or a mixture of both.

Luvisol Soil - Soil order (type) of the Canadian System of Soil Classification. This soil type is associated with forest vegetation. The most identifying traits of this soil are the presence of calcareous parent material, which results in a high pH and strong eluviation of clay from the A horizon. For more information on this soil type, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - https://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/index.html.

Lynchet - A human-made hillside terrace created by ploughing.

Lysimeter - A meteorological instrument used to measure potential evapotranspiration and actual evapotranspiration.