S-Wave - A seismic wave that moves material it encounters perpendicular to its direction of travel. This type of seismic wave causes shear stress in the material it moves through. Also called a secondary wave or a shear wave. 

Saffir-Simpson Scale - A tropical storm and hurricane intensity classification system developed in 1971 by civil engineer Herbert Saffir and meteorologist Bob Simpson. This system has seven levels of tropical storm and hurricane intensity. The first two categories describe two stages of storm development before technically becoming a hurricane: TD – tropical depression and TS – tropical storm. Hurricanes are categorized according to levels of intensity that range from 1 to 5. A major hurricane is considered to be categorized 3 and above. These levels are based on the speed of the storm's sustained winds and its lowest surface atmospheric pressure.

Salcrete - A relatively thin surface deposit common to marine beaches that consists of sand particles cemented together by salt. This deposit forms when the evaporation of seawater leaves behind salt.

Salinity - The concentration of dissolved salts found in a sample of water. Measured as the total amount of dissolved salts in parts per thousand. Seawater has an average salinity of about 34 parts per thousand.

Salinization - A pedogenic process that concentrates salts at or near the soil surface because evapotranspiration greatly exceeds water inputs from precipitation.

Salt - (1) Compound mainly made of the mineral sodium chloride (NaCl). (2) Compounds that are produced as a result of a metal atom replacing a hydrogen atom in an acid.

Salt Dome - A convex-shaped layer of salts embedded in the horizontal layers of rock strata. Salt domes begin their formation when evaporation causes the precipitation and deposition of salt in an enclosed marine basin. Once covered by a significant layer of clastic sediment, the salt layer becomes buoyant relative to the surrounding sediment, causing the formation of the dome. This dome causes deformation of adjacent sedimentary and other rock strata, often producing a hill on Earth's surface. The vertical growth of large salt domes, anywhere from 1 to 10 kilometers (0.6 to 6.0 miles) in diameter, can even cause the folding and faulting of nearby rock layers. These features are important in oil exploration as they can trap petroleum. 

Salt Flat - An extensive flat area of salt crust formed from the drying out of a saline lake or inland sea.

Salt Marsh - A type of coastal wetland ecosystem that is inundated for some period of time by seawater. The plants in this community have a special adaptation that allows them to survive in environments with high salinity. Generally, found poleward of 30° North and South latitude. Compare with mangrove.

Saltation - The transport of sediment that is initiated by moving air or water. In this process, particles move from a resting surface to the transport medium in quick, continuous, repeated cycles.

Saltwater Intrusion - The invasion of seawater into freshwater aquifers in coastal and inland areas. This condition can occur when groundwater, which charges the aquifer, is withdrawn faster than it is replenished by precipitation.

Sample - A sample is a subset of data selected from a larger population. Most samples are drawn at random to ensure equal representation in the data for statistical analysis.

Sand - A mineral particle with a size between 0.063 and 2.0 millimeters (0.0025 to 0.079 inches) in diameter (with the Wentworth Scale and in the USA, between 0.0625 and 2.0 millimeters). Compare with clay, silt, gravel, cobble, and boulder. 

Sand Dune - A hill or ridge of aeolian sand deposits with a minimum height of less than one meter (a couple of feet) and a maximum height of about 50 meters (164 feet). Sand dunes first begin their lives as stationary piles of sand that form behind a vertical obstacle. When they reach a certain size threshold, continued growth may also be associated with active surface migration of sand. In a migrating dune, grains of sand are transported by wind from the windward to the leeward side and begin accumulating just over the crest. When the upper leeward slope reaches an angle of about 30 to 34 degrees, the accumulating pile becomes unstable, and small avalanches begin to occur, moving sand down the leeward slope. As a result of this process, the dune migrates over the ground as sand is eroded from one side and deposited on the other. This process also causes the appearance of the dune to take on a wave shape. Active movement of sand particles across the dune causes the windward slope to become shallow, while the leeward slope maintains a steep slip-face. Normally found in hot deserts and along sandy coastlines. Many different types of sand dunes occur, including: barchan, barchanoid ridge, transverse, parabolic, longitudinal, seif, star, dome, and reversing.

Sand Lens - A sporadic lens-shaped deposit of sand found in a sedimentary sequence. Such features are produced by the past existence of unique processes common in fluvial or aeolian environments, which form dunes, ripples, and other stream channel deposits.  These features can be used to determine the conditions that may have produced these sedimentary deposits.

Sand Ripples - Another term used for wind ripples.

Sand Sea - A large region of sand and sand dunes in a desert. Common to erg deserts.

Sand Sheet - A deposit of sometimes stratified, less well-sorted sand that almost resembles dunes. These geomorphic features are common in northern Europe. Sand sheets are believed to form when wind-blown materials settle on areas of patchy snow coverage

Sand Wedge - A form of ice wedge that contains accumulations of wind-blown sand in long vertical layers. A form of periglacial ground ice.

Sandbank - (1) A mound-shaped deposit of sand found on the Earth's surface, or submerged or partially submerged within a water body. (2) A significant deposit of sand found in coastal regions, along the continental shelf. These submerged features can be up to several kilometers (miles) wide, 80 kilometers (120 miles) long, and tens of meters (feet) tall. Sandbanks form when consistent subsurface currents are available for the transport and deposition of sand available from a source area. Can also be called an ocean bank.

Sandstone - A type of sedimentary rock that contains a large quantity of weathered quartz grains.

Sandstorm - A windstorm that has a significant quantity of sand, silt, and clay particles entrained in the wind of the storm.

Sandur (Sandar plural) - An Icelandic term used to describe a glacial outwash plain.

Santa Ana Wind - A warm, dry chinook-like wind that occurs in the southern third of California. This unique type of regional wind originates from the east, off an elevated desert plateau. Santa Ana Winds often greatly intensify brush and forest fires, especially when drought conditions prevail. 

Sapping - A term that describes lateral erosion occurring at the base of a cliff. The process often causes the cliff to fail over time.

Saprolite - A mass of bedrock that has become weathered or partially weathered in its original location.

Sapropel - A dark-colored deposit primarily composed of organic compounds and sediments found at the bottom of fresh and saline water bodies like lakes, coastal basins, estuaries, and lagoons. Formed when organic matter is mainly decomposed anaerobically. 

Saprophyte - A heterotrophic organism that gets its nutrition from consuming the biomass of dead organisms or the discarded tissues of living organisms. Many species of fungi and insects are saprophytes.

Sastrugi - Sharp ridges and hollows created by wind erosion of the surface made of snow and ice. 

Satellite - (1) A term used in astronomy to describe a planet's moon(s). (2) An artificial human-constructed mechanical device that has been placed into orbit. Satellites and the scientific instruments they carry are used for a wide range of purposes, including remote sensing, communications, navigation, weather forecasting, and meteorological data collection. 

Saturated Adiabatic Lapse Rate (SALR) - The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is lower than the dry adiabatic lapse rate (9.8°C per 1,000 meters, or 5.4°F per 1,000 feet) because of the heat added to the ascending air parcel by condensation and deposition processes. Also called wet adiabatic lapse rate and moist adiabatic lapse rate. Compare with the dry adiabatic lapse rate.

Saturation - (1) An atmospheric condition where water is changing its phase to liquid or solid. At saturation, relative humidity is 100% unless there is a shortage of deposition nuclei or condensation nuclei. Generally, this process is driven by atmospheric cooling. (2) A condition where the pore spaces in a soil are close to being filled with water.

Saturation Coefficient - A measure of a rock's ability to absorb moisture relative to the pore space it contains. The saturation coefficient is used to gauge the potential mechanical disintegration of rocks by freeze-thaw action.

Saturation Deficit - (1) The amount of added water vapor required to saturate an air mass (relative humidity equals 100%), holding air temperature and pressure constant. (2) The amount of added surface water required to raise the saturation level in a soil to its surface.

Saturation Mixing Ratio - The mass of water vapor that a kilogram of dry air can hold at saturation. Measured in grams and varies with air temperature.

Saturation Zone - See zone of saturation.

Savanna - A tropical or sub-tropical plant community characterized by trees and shrubs scattered among a ground cover of grasses and herbs. The climate of a savanna is tropical, with a dry season during the low-sun period of the year. Also spelled savannah.

Scabland - An area where frequent episodes of erosion remove surface vegetation cover, leaving a ground surface composed of soil, sediment, and rock.

Scale - The specific relative or proportional size or extent of a phenomenon as measured through space and/or time. Objects are often scaled down in maps to facilitate easy comprehension.

Scarification - The extensive movement of soil, sediment, and rock material caused by human actions.

Scattering (Atmospheric) - This is an atmospheric process where small particles and gas molecules diffuse part of the incoming solar radiation in random directions without any alteration to the wavelength of the electromagnetic energy. Scattering, however, reduces the amount of incoming solar radiation reaching Earth's surface. As a result, a significant proportion of scattered shortwave solar radiation is redirected back to space. The amount of scattering depends on two factors: the wavelength of the incoming radiation and the size of the scattering particle or gas molecule. In Earth's atmosphere, the presence of a large number of particles about 0.5 µm in size results in shorter wavelengths being preferentially scattered. This factor also causes our sky to look blue because this color corresponds to those wavelengths that are best diffused. Three types of scattering occur in the atmosphere: Mie scattering, non-selective scattering, and Rayleigh scattering.

Scavenger - A heterotrophic organism that mainly consumes dead animals or the parts of dead animals for food. These organisms do not kill their food. Examples of scavengers include flies, various species of vultures, crabs, and hyenas. Also see herbivore, detritivore, omnivore, and carnivore.

Schist - A medium to coarse-grained metamorphic rock with well-developed bedding planes derived from the foliated recrystallization of platy minerals like mica. 

Science - Science is a way of acquiring knowledge. To do science, one must follow a specific universal methodology. The central theme in this methodology is the testing of hypotheses (hypothesis testing) and the ability to make predictions. The overall goal of science is to better understand nature and our Universe through rational thought.

Scientific Method - The general rational approach science uses to acquire knowledge. This method tries to be unbiased and neutral. Involves inductive and deductive reasoning, hypothesis testing and falsification, and predictive model building and evaluation.

Scientific Reductionism - The scientific study of a phenomenon in isolation from other phenomena that normally have cause-and-effect relationships with it.

Sciophyte - A plant adapted to survive best under shaded conditions.

Sclerophyllous Vegetation - Term used to describe drought-resistant vegetation common in Mediterranean climates. Some common adaptations present in this type of vegetation include deep roots, reduced leaf area exposed to the atmosphere, and thick, waxy leaves with closing stomata that reduce water loss.

Scoria - A type of igneous volcanic rock that is composed of basalt or andesite. Scoria is often colored dark brown to black and has a low mass because it contains many small visible pores. The numerous pores in this rock came from gas bubbles found in the lava before it solidified. This rock differs from pumice in that it will sink in water. 

Scour - (1) Refers to the erosive power of water. (2) Abrasive effects of rocks and sediments incorporated in the ice base of a glacier.

Scree - A significant accumulation of weathered rock fragments at the base of a steep rock slope or cliff.

Sea - (1) A body of saline water found on the Earth's continental surface. (2) A portion of an ocean that is close to a continent.

Sea Arch - A coastal landform composed of rock that resembles an arch. These landforms are created when waves erode through a thin headland from both sides. 

Sea Breeze - A local thermal circulation pattern found at the interface between land and water. In this air circulation system, surface winds blow from water to land during the daytime. The diagram below shows the three-dimensional air circulation pattern associated with a sea breeze. Note that low atmospheric pressure over land occurs because it heats faster than a water surface, which is cooler in terms of air temperature. Compare with a land breeze.

Sea Cliff - A cliff that is located at the shoreline.

Sea Floor - See ocean floor.

Sea Ice - Ice that forms when seawater freezes on the surface of an ocean or a sea. Because of the salts dissolved in seawater, sea ice forms when temperatures are lower than -1.9°C (28.6°F). Sea ice covers about 12% of our planet's oceans. Most of the sea ice is found in the Southern Ocean around Antarctica and in the Arctic Ocean. Measurements over the last few decades suggest recent human-induced global warming is significantly reducing the areal coverage of sea ice.

Sea Level - The average surface elevation of our planet's oceans.

Sea Level Pressure - Average atmospheric pressure at sea level. Currently, this value is 1013.2 millibars.

Sea Mount - See seamount.

Sea Salt - The salt derived from the evaporation of seawater.

Sea Smoke - A type of evaporation fog that forms over marine water bodies. 

Sea Stack - A steep pillar of rock located in the ocean, a short distance from the coastline. These coastal landforms are created when the action of waves erodes through a thin headland from both sides. 

Sea Surface Temperature - Usually, refers to the temperature of the topmost few meters (several feet) of the ocean.

Sea-Floor - See ocean floor.

Sea-Floor Spreading - The process of oceanic crust creation and seafloor movement that occurs at the mid-oceanic ridge.

Seafloor - See ocean floor. 

Seamount - A volcanic mountain found on an ocean basin with an origin not related to a mid-oceanic ridge or a tectonic subduction zone.

Seasons - Time periods generally defined by changes in the intensity and duration of sunlight received in the middle and high latitudes. The four seasons are typically recognized as Spring, Summer, Fall, and Winter. The astronomical definition is more precise and suggests the following times for the four seasons: Spring - March 22 to June 21; Summer - June 22 to September 22; Fall - September 23 to December 22; and Winter - December 23 to March 21.

Seaward - Something that is positioned or located away from land but towards an ocean or sea.

Seawater - The mixture of water and various dissolved salts found in Earth’s oceans and seas.  The average salinity of seawater is about 35 parts per 1000 (3.5%, 35 g/L or 599 mM). The salinity of seawater is not uniform across the world's oceans and varies between 31 and 39 parts per 1000. Seawater contains mainly the dissolved ions of sodium (Na⁺) and chloride (Cl^-). Minor dissolved ions include (in order of abundance): magnesium (Mg₂⁺), sulfate SO₄^2-), calcium (Ca₂⁺), potassium (K⁺), and bromine (Br^-). Seawater has a pH between 7.7 and 8.4. The density of seawater is 1.025 g/ml (pure freshwater is 1.000 g/ml). The freezing point of seawater becomes lower as salinity increases. At 35 parts per 1000, seawater freezes at -1.9°C (28.6°F).

Second Law of Motion - A simple idea first suggested by Sir Isaac Newton in 1687. This law states that the forces acting on an object do not just make it move; they also change how it moves. Further, acceleration only happens when there is a net force. If all forces cancel each other out, motion does not change. It also implies that heavier objects (with greater mass) necessitate more force to attain the same acceleration, and that the direction of the acceleration aligns with the direction of the net force. The following two equations are used to describe this law:

Force = Mass x Acceleration

and

Acceleration = Force / Mass 

Second Law of Thermodynamics - This law states that heat can never pass spontaneously from a colder to a hotter body. As a result, natural processes that involve energy flow must have a single direction, and all natural processes are irreversible. This law also predicts that the entropy of an isolated system always increases with time. 

Second-Growth Forest - A stand of forest that is the result of secondary succession. 

Secondary Atmosphere - The Earth’s atmosphere that existed between 4.0 and 2.5 billion years ago. At the beginning of this stage of the atmosphere's development, it was mainly composed of the gases water vapor (H₂O), carbon dioxide (CO₂), and nitrogen (N₂). By about 3.3 billion years before present, CO₂, H₂O, and N₂ are dominant. Molecular oxygen (O₂) begins to accumulate slowly because of the presence of ancient life, mainly in Earth's oceans. Compare with the early atmosphere and the living atmosphere.

Secondary Carnivore - Organisms that occupy the fourth trophic level in the grazing food chain. These organisms are carnivores. Also called the tertiary consumer.

Secondary Consumer - A Carnivore-type organism that occupies the third trophic level in a grazing food chain.  Also known as a primary carnivore.

Secondary Flow - A term in fluid mechanics that describes a minor flow in a gas or liquid that is moving in a direction transverse to the primary flow.

Secondary Minerals - Minerals that form as byproducts of weathering or hydrothermal alteration. The most common secondary minerals are hydrated silicates. Compare with primary minerals.

Secondary Productivity - Is the production of organic chemical energy or biomass in an ecosystem by way of heterotrophic or consumer organisms consuming dead or living organic matter from plants and animals. Compare with primary productivity. 

Secondary Pollutant - Atmospheric pollutants that are created chemically in the atmosphere when primary pollutants and other components of the air react. Compare with a primary pollutant.

Secondary Substance - Organic chemical produced by a plant that has no direct function in its metabolism. Many of these chemicals are toxic and are believed to be created by the plant to reduce herbivore damage or the negative effects of competition by other plants.

Secondary Succession - Plant succession on a previously vegetated surface. The soil or substrate at this location contains an active seed bank from which the plants grow.

Secondary Wave - See S-wave or shear wave.

Sedge - A type of grass-like plant that has adaptations to its morphology and physiology to allow it to survive successfully in moist habitats.

Sediment - (1) Solid material particles derived from the weathering of rock. (2) Solid material particles that have been or are being subject to the geomorphic processes of erosion, transportation, and deposition. Transport can be due to fluvial, marine, glacial, or aeolian agents.

Sediment Rating Curve - A numerical expression or curve on a graph that describes the quantitative cause and effect relationship between stream discharge and the sediment transported by a particular stream.

Sediment Yield - The amount of sediment exported from a particular drainage basin. Often measured yearly and expressed in terms of some unit area (kilograms per kilometer squared per year: kg km^-2 y^-1). The magnitude of sediment yield for a drainage basin is influenced by bedrock type, plant cover, relief, climate, land-cover change, and land-use. Two human activities that can significantly increase sediment yield are agriculture and forestry.

Sedimentary Rock - Rocks formed by the deposition, alteration, and/or compression and lithification of weathered rock debris, chemical precipitates, or organic sediments. Also see clastic vs non-clastic sedimentary rocks.

Sedimentation - A geomorphic process where sediment particles, once entrained and transported in air, water, or ice, settle onto some surface because of the reduction of forces keeping them suspended, chemical precipitation, electromagnetic binding of particles, and gravity. 

Seed - The fertilized ovule of a plant that contains an embryo and food products for germination. Once germinated, the embryo can grow into a mature individual.

Seed Bank - All of the seeds available for germination in the soil.

Seed Dispersal - The transport of a plant seed away from the parent plant by way of active movement or passive movement.

Seepage - (1) The gradual movement of water into the soil layer. (2) Slow movement of sub-surface water to the surface. Water flow from seepage is not great enough to be called a spring.

Seepage Lake - A lake that gets its water primarily from the seepage of groundwater.

Segregated Ice - A form of periglacial ground ice that consists of almost pure ice that often exists as an extensive horizontal layer. The ice layer grows due to the active migration of water around the feature. These features are found just below the active layer.

Seiche - An oscillating wave found within a body of water like a river, pond, lake, sea, estuary, or bay that produces a change in the water level. Seiches can be caused by changes in wind speed, atmospheric pressure, tsunamis, earthquakes, etc. These features can be found embedded in tidal fluctuations.

Seif - (1) A large sand dune that is elongated in the general direction of the dominant winds. (2) A sand dune formed by winds from multiple directions.

Seismic - The shaking displacement usually caused by an earthquake.

Seismic Wave - Successive wave-type displacement of rock usually caused by an earthquake.

Seismograph - An instrument that measures the energy contained in seismic waves from an earthquake or other type of ground displacement.

Seismology - A branch of science focused on the study of earthquakes and seismic activity.

Selective Absorber - A substance that absorbs only specific wavelength bands of electromagnetic radiation.

Selective Emitter - A substance that emits only specific wavelength bands of electromagnetic radiation.

Self-Regulation - The ability of some systems to maintain a steady state equilibrium through positive feedback and negative feedback.

Semi-Diurnal Tide - Tides that have two high and two low waters per tidal period.

Sensible Heat - Heat energy that can be measured by a thermometer and thus potentially sensed by humans.

Sensible Heat Flux - A process where excess heat energy is transferred into the atmosphere. The process first involves the movement of heat energy from the Earth's surface to the atmosphere by conduction and convection. The heat energy can then move horizontally by advection (atmospheric circulation).

Sensible Temperature - A measure of thermal comfort or discomfort by a human determined by the effect that humidity, wind, and solar radiation have on perceived air temperature. For example, most humans perceive that an increase in wind speed makes air temperature feel cooler.

September Equinox - One of two days during a year when the declination of the Sun is at the equator. The September Equinox denotes the first day of fall in the Northern Hemisphere. In the Southern Hemisphere, this date marks the first day of spring. During the September Equinox, all locations on the Earth (except the poles) experience equal (12-hour) day and night. The September Equinox occurs on either September 22 or 23.

Seral Community - An identifiable community of plants that occurs as a succession changes temporally from a non-vegetated site to a climax community.

Serclimax - An identifiable community of plants that forms a relatively long-lived stable stage during a succession. Despite its stable appearance, this stage is not the true climax community for this site.

Sere - See seral community.

Serir - A type of desert landscape characterized by a ground surface covered with gravel.

Severe Thunderstorm - This term is used to designate a thunderstorm that has reached a particular level of severity. This is an intense storm with frequent lightning and local wind gusts of 97 kph (60 mph), or hail that is 2 cm (3/4 of an inch) in diameter or larger.  Severe thunderstorms can also have tornadoes. In Canada, thunderstorms are also considered severe if their rainfall exceeds 50 millimeters (2 inches) in one hour, or 75 millimeters (3inches) in three hours.

Sexual Reproduction - Any process of reproduction that involves the fusion of the genes from two different individuals of the same species to create a new individual. Compare with asexual reproduction.

Shakehole - See sinkhole.

Shale - A fine-grained sedimentary rock composed of lithified clay particles.

Shared Socioeconomic Pathways (SSPs) - Are a variety of global greenhouse gas emission scenarios employed in human-caused climate change research with climate models to depict the potential change in economics, technology, and society's values. These scenarios serve as the foundation for the IPCC’s contemporary climate projections, influencing greenhouse gas emissions and climate risk. These scenarios replaced Representative Concentration Pathways.

Shear Box - An instrument used to measure the shear strength of a soil or some other material.

Shear Strength - A quantitative measure of the ability of soil or some other material to withstand shear stress. 

Shear Stress - A type of stress caused by forces operating parallel to each other but in opposite directions. Compare with normal stress.

Shear Wave - A seismic wave that creates wave-like motion perpendicular to the direction of seismic energy propagation. Also called S-Wave.

Sheet Flow - See overland flow.

Sheeting - A form of physical weathering of rock where surface sheets of material fracture and exfoliate because of pressure release. Also see exfoliation dome.

Sheetwash - The removal of loose surface materials by overland flow. A process of erosion.

Shield - A large stable area of exposed ancient (more than 600 million years) igneous and metamorphic rock found on continents. This rock forms the core of the continents.

Shield Volcano - A very large volcano created from alternating layers of numerous viscous lava flows. Shield volcanoes are slightly convex and have a slope with a gradient of between 6 and 12°. Their height can reach 9,000 meters (29,500 feet). The magma chemistry of these volcanoes is basaltic. Mauna Loa, a Hawaiian volcano, is a classic example of a shield volcano. 

Shoal - A region of shallow water found in a lake, sea, or ocean. A shoal can form around a sandbank and create navigational problems for boats.

Shore - The land area bordering a relatively large water body like a lake, sea, or ocean.

Shore Platform - See a wave cut platform.

Shoreline - The line that separates the land surface from a water body.  It is important to note that shoreline elevation can vary over time due to changes in water levels. Changes in water level can be caused by tides, atmospheric pressure, inflow of water, winds, climate change, etc. Also see coastline.

Short Wave - A small wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with long waves.

Shortwave Radiation - Electromagnetic radiation with a wavelength between 0.1 and 0.7 micrometers (µm). Commonly used to describe the radiation emitted from the Sun.

Shrub - A woody plant species that is smaller than a tree. Shrubs usually do not have trunks that elevate branches well above the ground surface. A shrub's branches are found close to ground level.

Shrubland - A plant community characterized by a dominance of shrub plant species. Shrublands can occur naturally or result from human activity. Shrublands are common in Mediterranean and desert climates. Often, natural shrublands are maintained by disturbances such as fire or herbivore browsing, which prevent trees from establishing themselves. Also called scrubland, scrub, and brush. 

Sial Layer - The part of the Earth's crust that forms the continents and is composed of relatively light, granitic rocks.

Siberian High - An atmospheric high pressure system that develops in winter over northern central Asia.

Side Looking Airborne Radar (SLAR) - An imaging instrument that remotely senses the environment using pulsating transmissions of microwave radiation. Objects are sensed when their surfaces reflect the radiation back to a receiver also located on the instrument. SLAR systems are often mounted on airplanes or satellites. SLAR systems are commonly used for remotely sensing soil moisture content, landscape relief, forest tree yields, oil spills, and sea ice location.

Sidereal Day - Time it takes to complete one Earth rotation relative to the position of a fixed star. This measurement takes 23 hours, 56 minutes, and 4.09 seconds. Compare with the mean solar day.

Sieve Deposits - A flood caused stream deposit. These deposits take the form of a lobe of sediment composed mainly of gravel and pebbles located on an alluvial fan. Sieve deposits are only produced when the stream load is deficient in finer-textured sediments.

Silcrete - (1) Sedimentary rock created by the chemical precipitation of silica. (2) A layer found in soils or sediments that is concentrated with silica. This layer is often compact, quite dense, hard, and sometimes impervious. These layers are common in arid and semiarid climates, where silica found in the soil dissolves into the soil solution and then precipitates out, cementing a narrow band of soil particles. Also called a hardpan.

Silica - A mineral that is composed of silicon dioxide, SiO₂.

Silicate - A group of minerals that have crystal structures based on a silica tetrahedron (SiO₄).

Silicate Magma - A magma that is felsic in composition.

Silicate Tetrahedron - A simple molecule that consists of a silicon (Si) atom surrounded by four oxygen (O) atoms. This molecule is a primary component of most silicate-type minerals found in the Earth's crust. The chemical formula of a silicate tetrahedron is SiO₄.

Sill - Horizontal planes of igneous rock that run parallel to the grain of the original rock deposits. They form when magma enters and cools along bedding planes found within the Earth's crust. Also see intrusive igneous rock. Compare with batholith, dyke, laccolith, lopolith, and phacolith.

Silt - A mineral particle with a size between 0.002 and 0.063 millimeters (0.00008 to 0.0025 inches) in diameter (in the USA between 0.004 and 0.06 millimeters or 0.00016 to 0.0024 inches, Wentworth Scale 0.0039 to 0.0625 millimeters). Compare with clay, sand, gravel, cobble, and boulder.

Silt Drape - A thin deposit of silt that forms over existing surface features. Silt drape develops when silt particles drop out of the water that once held them in suspension.

Siltstone - A fine-grained sedimentary rock composed of lithified silt particles.

Silurian - One of six geologic periods that occurred from 444 to 416 million years ago during the Paleozoic Era. During this period, bony fish became common in oceans, and the first vascular plants appeared on land. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Sima Layer - The part of the crust that forms the ocean basins and lower layers in the Earth's crust and is composed of relatively heavy rocks that formed from basaltic magma.

Sink - (1) A location where some type of material is stored. (2) Another name for a sinkhole.

Sinkhole - A pit like hole found in areas of karst. These features are caused by the weathering of limestone or dolomite by subsurface drainage. Also called a sink, sink-hole, swallet, swallow hole, shakehole, or doline.

Sinter - A type of rock or deposit formed when calcium carbonate or silica precipitates out from hot water associated with hot springs and geysers.

Sinuosity - A measure of a stream channel's tendency to meander back and forth across the floodplain. It is measured as the ratio of the distance along a channel’s centerline and the straight-line distance down the stream valley.

Sinusoidal Equal-Area Map Projection - A map projection that represents areas in their true form on a two-dimensional map. Distances on this type of map are only correct along parallels and the central meridian. As a result, shapes become more distorted away from the central meridian and closer to the poles.

Siphon - (1) A sporadically flowing spring typical of karst landscapes. Thought to form when groundwater flows into an underground cavern that is connected to an inverted U-shape tunnel. One end of the tunnel is exposed to the ground surface, producing the spring, while the other end is connected to the cavern reservoir. When the water level in the reservoir reaches the high point of the tunnel, it creates a siphon, drawing water out of the cavern. The spring stops flowing when there is no more water to siphon. This process repeats itself when groundwater once again fills the cavern. Also called a rhythmic spring, ebb-and-flow spring, periodic spring, and intermittent spring. (2) A tunnel filled with water that connects two air-filled caverns in a cave system.

Skewness -  A statistical measure of the shape of data relative to the normal distribution. Data that is negatively skewed has a higher proportion of observations that are larger than the mean when graphed on a histogram. Data is positively skewed has a greater proportion of observations with values less than the mean.  Also see kurtosis.

Slab Failure - A type of hard rock failure that is caused when weathering weakens the structure of a rock mass along joints.

Slack - The low-lying area found between adjacent sand dunes in a dune field.

Slaking - See wetting and drying.

Slate - A type of fine-grained metamorphic rock with well-developed bedding planes derived from the minor recrystallization of shale.

Sleet - See ice pellets.

Slickenside - The smoothed and grooved rock surfaces sometimes found on the opposing sides of a fault plane. Caused by the friction and grinding associated with the rock surfaces sliding past each other in opposite directions along the fault plane.

Slide - See landslide.

Sling Psychrometer - A scientific instrument used to measure dew point and relative humidity. This type of psychrometer consists of two thermometers (a dry-bulb thermometer and a wet-bulb thermometer) encased in a housing. It uses a rotating handle and a twirling motion to ventilate its wet-bulb thermometer. 

Slip - Another term used to describe a landslide or mudslide.

Slip Face - The leeward side of a dune where material accumulates and slides or rolls downslope.

Slope - (1) The geometric angle of the ground surface as measured from horizontal. (2) A generic term used in Geomorphology to describe any topographic component of the Earth's solid surface above or below sea level.

Slope Aspect - The main compass direction that a slope faces (North, North East, East, South East, South, South West, West, and North West).

Slope Failure - The downslope movement of soil and sediment by processes of mass movement.

Slump - See rotational slip.

Small Circle - A circle on the globe's surface that does not bisect the center of the Earth. Parallels of latitude are examples of small circles.

Smog - A generic term used to describe mixtures of pollutants in the atmosphere. Also see industrial smog and photochemical smog.

Snout - The front end of a glacier. Also called the terminus.

Snow - A type of solid precipitation that forms in clouds with an air temperature below freezing. Snow forms when water vapor deposits directly as a solid on a deposition nucleus.  Snowflakes begin their life as very tiny crystals developing on a six-sided hexagonal deposition nucleus. The developing snowflake then grows fastest at the six points of the nuclei as these surfaces are more exposed to the atmosphere's water vapor. Snowfall is most common during fall, winter, and spring months, when air temperatures are below freezing, with frontal lifting associated with mid-latitude cyclones.

Snow Gauge - A meteorological instrument designed to measure the amount of snow falling to the ground surface accurately. In this device, the snow is heated until it melts, and a water equivalent is then measured.

Snow Line - Altitudinal or latitudinal limit separating zones where snow does not melt during the summer season from areas in which it does. Similar to the concept of firm limit, except that it is not limited to glaciers.

Snow Patch - An isolated region of snow that has survived the spring/summer melting process. A surviving snow patch may initiate nivation. 

Snow Pellets - A form of precipitation also known as graupel. Snow pellets are white, spherical bits of ice with a maximum diameter of 5 millimeters (0.20 inches). Snow pellets develop when supercooled water droplets freeze on snowflakes. Snow pellets often fall for a brief period when precipitation transforms from ice pellets to snow. Snow pellets can be easily distinguished from packed snowflakes as they tend to bounce when they strike the ground. Packed snowflakes are not dense enough to cause them to bounce.

Snowfield - An area with permanent snow accumulation. Usually found seasonally or permanently at high altitudes or middle to high latitudes on the Earth's terrestrial surface.

Snowmelt - The conversion of snow into runoff and groundwater flow with the onset of warmer temperatures and melting.

Snowpack - An accumulation of layers of snow on the ground surface that is compressed with depth by its own overlying weight. In some regions of the world, snowpack is an essential source of water, especially in spring.

Soil - A layer of unconsolidated material found at the Earth's surface that has been influenced by the soil-forming factors: climate, relief, parent material, time, and organisms. Soil normally consists of weathered mineral particles, dead and living organic matter, air space, and the soil solution.

Soil Aggregate - Many soil mineral and/or organic particles held together in a single mass. This natural property of soils improves the movement of air and water throughout the soil

Soil Colloids - Very small organic and inorganic particles found distributed throughout a soil, many of which are found floating in the soil solution. Inorganic colloids are often clay particles. Soil colloids carry a negative electrical charge and are the primary sites for cation exchange. Soil colloids hold large quantities of elements and compounds, which are used by plants for nutrition.

Soil Compaction - A process where a force is applied to the ground surface, causing the compression of the near-surface soil layer. This compression reduces the size of soil pore spaces, displaces soil water, and increases the soil's bulk density. Soil compaction can have negative consequences, such as reduced rainfall infiltration, increased runoff and erosion, and reduced plant growth. 

Soil Creep - Slow mass movement of soil downslope. Soil creep occurs when the stresses on the slope material are too low to cause rapid failure.

Soil Density - Refers to the weight of a sample of soil per unit volume of this substance. The density of soil is mainly influenced by the relative quantity of pore space, organic matter particles, and mineral particles. Soil mineral particles have a density that is between 2.60 and 2.75 grams per cubic centimeter (g cm^-3), while particles of organic matter have a density varying between 1.1 and 1.4g cm^-3. See bulk density.

Soil Erosion - The erosional transport of soil mineral particles and organic matter by wind, flowing water, or both. Human activities that disturb the soil surface or remove vegetation can significantly enhance this natural process.

Soil Fertility - The ability of a soil to provide the required nutrients for plant growth.

Soil Formation Factors - Five factors are recognized as having a major role in the formation and development of soils. These five factors are: climate, relief, parent material, time, and organisms.

Soil Horizon - A layer within a soil profile that differs physically, biologically or chemically from layers above and/or below it.

Soil Moisture Deficit - A condition that occurs when evapotranspiration and plant root consumption remove all of the soil water available for plant growth. Determining soil moisture deficit is important for managing plant growth with irrigation water. Two instruments are normally used to assess soil water conditions: a lysimeter and a neutron probe. 

Soil Moisture Recharge - The process of water filling the pore space found in a soil (creating water storage).

Soil Moisture Retention Curve - The mathematical description of a particular type of soil's ability to store water. Such information helps predict the amount of soil water available for plant growth. 

Soil Organic Matter - The living and dead organic constituents of a soil. The various constituents that make up organic matter are often at various levels of decomposition. Soil organic matter has many positive effects on the soil's physical and chemical properties.

Soil Permeability - The rate at which water and air move vertically through a soil.

Soil pH - A measure of the acidity or alkalinity of a soil as determined by pH. It is also a measurement of the concentration of hydrogen ions in a soil. Soil pH normally varies from 3.5 to 9.0. The optimum pH for plant growth is between 6.0 and 8.0.

Soil Porosity - The volume of water that can be held in a soil. It also refers to the ratio of the volume of voids to the total volume of the soil.

Soil Profile - The vertical arrangement of layers or horizons in a soil as determined visually or by some chemical or physical analysis.

Soil Science - The scientific study of soils from an interdisciplinary perspective, incorporating ideas and concepts from Biology, Chemistry, Physics, and Earth Science.

Soil Solution - The aqueous liquid found within a soil. This liquid typically contains ions released from mineral particles, organic matter, or plant roots and leaves.

Soil Structure - A general term that describes how mineral particles and organic matter are organized and clumped together in a soil.

Soil Taxonomy - The classification of a soil in a hierarchical system based on its various properties. This grouping system goes from general to specific.

Soil Texture - The relative quantities of the different types and sizes of mineral particles in a soil.

Soil Water - The water found occupying the pore spaces between soil particles.

Soil-Heat Flux - The rate of flow of heat energy into, away from, or through the soil.

Solar Altitude - The height of the Sun above the horizon as measured from either True North or True South.

Solar Constant - A term used to describe the average quantity of solar radiation (insolation) received by a horizontal surface at the edge of the Earth's atmosphere. This value is approximately 1,370 Watts per square meter.

Solar Day - The time required for the Earth to complete one rotation on its axis relative to the Sun.

Solar Energy - See insolation and solar radiation.

Solar Flare - A sudden and brief release of large quantities of electromagnetic energy from a localized area on the Sun's surface. These flares release electromagnetic radiation across a broad spectrum of wavelengths, from long radio waves to extremely short x-rays and gamma rays. The energy released from a typical flare is immense, about 1020 Watts.

Solar Noon - The point of time during the day when the Sun is aligned with True North and True South.

Solar Radiation - A form of electromagnetic radiation that originates from the Sun. Most of the Sun's radiation is emitted at wavelengths between 1.0 and 0.1 microns (µm). Also see insolation, direct solar radiation, and diffused solar radiation. 

Solar System - The collection of celestial bodies that orbit around a star. The Earth is one of many celestial bodies that form a Solar System around the Sun.

Solar Wind - A mass of ionized gas emitted into space by the Sun. The solar wind plays a role in the formation of auroras.

Solar Year - The time it takes the Earth to make one orbit around the Sun. This is approximately 365.2422 days.

Solfatara - Small vents that remain active many years after a significant volcanic event. These vents often emit sulfuric gases mixed with hot water vapor. 

Solid - A state of matter where molecules found in a substance do not have the property of flow.

Solifluction - A form of mass movement in environments that experience freeze-thaw action. It is characterized by the slow movement of soil material downslope and the formation of lobe-shaped features. Also see gelifluction.

Solonchak - A type of soil that drains poorly, is grey in color, and is found in arid to semi-arid environments. One important defining characteristic of these soils is that they often accumulate salts in the upper soil horizon. These soils are similar to those of the Aridisol order (United States Natural Resources Conservation Service Soil Classification System) and Solonetzic order (Canadian System of Soil Classification). This term has a Russian/Ukrainian origin.

Solonetz - A type of soil recognized in the United Nations' Food and Agriculture Organization (FAO) soil classification system. These soils have a significant clay content in the upper soil horizon and 15% exchangeable sodium. These soils are similar to those of the Aridisol order (United States Natural Resources Conservation Service Soil Classification System) and Solonetzic order (Canadian System of Soil Classification). This term has a Russian/Ukrainian origin.

Solonetzic Soil - Soil order (type) of the Canadian System of Soil Classification. This soil type is associated with grassland habitats where high levels of evapotranspiration cause the accumulation of salts at or near the soil surface. These soils are common in the dry regions of the prairies, where evapotranspiration greatly exceeds precipitation input. 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.

Solstice - Dates when the declination of the Sun is at 23.5° North or South of the equator. For the Northern Hemisphere, this date falls on June 21 or 22 (Summer Solstice). In the Southern Hemisphere, the date is December 21 or 22 (Winter Solstice).

Solum - The part of a soil that is capable of supporting living organisms.

Solutes - Organic or inorganic substances dissolved in a solution in some liquid. 

Solution - (1) A form of chemical weathering where rocks and minerals are dissolved by water. Materials entering the mixture can alter the solution's chemical nature and increase the strength of this weathering agent. For example, the mixing of carbon dioxide and water can form carbonic acid. (2) The dissolving of a substance into a liquid.

Solvent - A substance that can chemically dissolve other substances.

Source Region - Region on the Earth where air masses originate and come to possess their particular moisture and temperature characteristics.

Southeast Trade Winds - Southern hemisphere surface winds found in the tropics that blow from about 30° South latitude (Subtropical High Pressure Zone) to the equator (Intertropical Convergence Zone). These winds blow from southeast to northwest. Also see Northeast Trade Winds.

Southern Ocean - The Southern Ocean surrounds Antarctica, extending to the latitude 60° South. This ocean region covers about 4% of the Earth's surface or about 20,327,000 square kilometers (7,846,000 square miles).  Compared with other ocean regions, the floor of the Southern Ocean is quite deep, ranging from 4,000 to 5,000 meters (13,100 to 16,400 feet) below sea level over most of its area. 

Southern Oscillation - The reversal of atmospheric circulation typical in the tropical Pacific Ocean. This change often results in the development of an El Niño.

South Magnetic Pole - Location in the Southern Hemisphere where the lines of force from Earth's magnetic field are at right angles to the Earth's surface. This point on the Earth gradually changes its position with time.

South Pole - Surface location defined by the intersection of the polar axis with Earth's surface in the Southern Hemisphere. This location has a latitude of 90° South.

Space - (1) A distance, area, or volume. (2) An infinite three-dimensional area in which objects have relative coordinates to each other. (3) The region beyond the outer limits of the Earth's atmosphere that extends to the edge of the Universe.

Spatial Analysis - The study of the spatial patterns of natural and human-made phenomena using numerical analysis and statistics.

Spatial Isolation - The reproductive isolation of two or more populations of a species by distance or physical barriers. Over long periods, geographic isolation can lead to speciation through divergent evolution driven by environmental heterogeneity. Also called geographic isolation.

Spatial Tradition - An academic tradition in modern Geography that investigates geographic phenomena from a strictly spatial perspective.

Specialist Species - A species that has a relatively narrow ecological niche and a very limited geographic range. These species are often able to live in only one type of habitat, can tolerate only specific environmental conditions, and can consume only a few types of food. Compare with endemic and cosmopolitan species.

Speciation - The process by which new species originate through mutations, natural selection, and evolution.

Species - (1) The different kinds of organisms found on the Earth as defined by taxonomic and/or phylogenetic classification. (2) A group of interbreeding organisms that do not ordinarily breed with members of other groups.

Species Association - A particular grouping of species in a defined area. This term is commonly used in community ecology to describe a predictable mix of species types that is controlled by the specific environmental conditions operating in a habitat.

Species Diversity - The number of different species in a given region. Also see biodiversity, ecosystem diversity, and genetic diversity.

Species-Area Curve - The mathematical relationship between habitat size and the number of different species living in that space. The following mathematical formula describes the species-area curve:

S = cAZ

where S is the number of species, c is a constant which varies with the unit used to measure area, A is the habitat area, and z is the slope of the species-area relationship. 

Specific Gravity - The ratio of the mass of a body to the mass of an identical volume of water at a specific temperature.

Specific Heat - This is the heat capacity of a unit mass of a substance or heat needed to raise the temperature of 1 gram (g) of a substance 1 degree Celsius.

Specific Humidity - One of several different ways to measure atmospheric humidity. Specific humidity is the mass of water vapor in a given mass of air. Typically expressed in grams of water vapor per kilogram of air at a specific temperature.

Specific Retention - The ratio of the quantity of water that a mass of soil or rock can hold on to compared to its total volume after gravity drainage has occurred. In soils, specific retention decreases with the size of the soil particles.

Specific Yield - The amount of stored water drained from a saturated mass of soil or rock under the influence of gravity. In soils, specific yield typically increases with particle size. Specific yield can be used to estimate the amount of water available from an aquifer.

Spectrum - (1) The complete range of wavelengths found in electromagnetic radiation. (2) A graph that describes the quantity and quality of radiation emitted from a particular object or substance at various wavelengths. (3) The range of colors seen in a rainbow.

Speed of Light - Velocity of light in a vacuum. This velocity is approximately 3 x 10⁸ meters per second (300,000 kilometers per second or 186,000 miles per second). It takes light from the Sun eight minutes and twenty seconds to reach the Earth.

Speleology - A scientific field of inquiry that studies caves. Also spelled spelæology or spelaeology.

Speleothem - A general term used to describe secondary mineral depositional features containing calcium, silica, and/or gypsum found in caves. 

Sphericity - A measure of the roundness of an object. It is usually determined by comparing the object's surface area to that of a sphere with the same volume.

Spheroidal Weathering - A type of below-ground chemical weathering where the corners of jointed rocks become rounded over time. Rock changes from a rectangular to a more round shape.

Spit - A long and narrow accumulation of sand and/or gravel that projects into a body of ocean water. These features form as a result of the deposition of sediments by longshore drift. 

Spodosols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. A spodosol soil has pronounced layers of illuviation characterized by accumulations of iron and aluminum oxides in the B horizon. These soils form in moist, cool climates. 

Sponge - Are simple invertebrate animals that do not have nervous, digestive, or circulatory systems. These multicellular organisms have pores and channels throughout their bodies that allow water to flow, providing them with food and oxygen and removing wastes. Most of the 5,000 to 10,000 species of sponge live in our planet's oceans and seas. A few species live in freshwater aquatic systems.  The distribution of this organism is worldwide, with species found from the Earth's polar regions to the tropics. Sponges have existed well before the Cambrian Explosion and possibly first evolved about 750 million years ago.

Sporadic Permafrost - A form of permafrost that exists as small islands of frozen ground in otherwise unfrozen soil and sediment.

Spore - A unicellular organic unit used for asexual reproduction by many species of plants, algae, fungi, and protozoa. Spores are often designed for easy dispersal and survival over extended periods in harsh environmental conditions. 

Sport Hunting - Is a type of hunting where humans kill animals for recreation. This type of hunting is typically done legally. Compare with subsistence hunting, commercial hunting, and poaching.

SPOT (Centre National d'Etudes Spatiales) - Series of satellites developed by the French Space Agency, in cooperation with Belgium and Sweden, to remotely monitor (remote sensing) resources and other objects on the Earth. The first SPOT satellite was launched in 1986. See the following website for more information - https://cnes.fr/en.

Spring - (1) The season between winter and summer. Astronomically, it is the period from the March Equinox to the June Solstice in the Northern Hemisphere and from the September Equinox to the December Solstice in the Southern Hemisphere. (2) A continuous natural flow of water from the sub-surface to the surface. It usually occurs when the water breaks the Earth's surface. Also see hot spring.

Spring Tide - A type of tide that occurs every 14 to 15 days and coincides with the new and full Moon. This tide has a large tidal range because the gravitational forces of the Moon and the Sun are complementary. Contrasts with a neap tide.

Squall Line - A long band of many adjacent thunderstorm cells found ahead of a mid-latitude cyclone cold front.

Stability - The capability of a system to tolerate or recover from disturbance or an environmental stress.

Stability Analysis - A group of methods used to determine the failure potential of a soil or rock slope. Central to this set of procedures is knowledge of the cohesion and frictional characteristics of the materials that make up the slope. 

Stable Atmosphere - A condition in the atmosphere where isolated air parcels have a tendency to sink. Stable parcels of air tend to be cooler than the surrounding air. Compare with a neutral atmosphere and an unstable atmosphere.

Stable Equilibrium - This type of equilibrium occurs when the system displays the tendency to return to the same measured state after a disturbance.

Stack - An isolated part of a coastal cliff that exists free-standing in the sea as a vertical column of rock. These unique features are the result of erosion by energetic waveaction along a headland. Shown is a stack known as the Old Man of Hoy, located on the coastline of Scotland. Also called sea stacks.

Stadial - A term that identifies a period of colder temperatures and glacial advance within a much larger period when widespread glaciation is dominant in a region's landscape.  Compare with interstadial. 

Stadial Moraine - See recessional moraine.

Staff Gauge - A measuring device used to determine the water depth for a particular location in a stream. 

Stage - The elevation of the water surface in a stream channel.

Stalactite - A ceiling deposit found in caves and mines that often forms from the precipitation of minerals found in water. It may also be composed of deposits of lava, clay, silt, sand, and bat excrement. Looks like an icicle. 

Stalagmite - A floor deposit found in caves and mines that often forms from the accumulation of precipitated minerals found in water. The mineral-rich water originates as ceiling drippings. Stalagmites may also be composed of deposits of lava, clay, silt, sand, and bat excrement. Looks like an icicle.

Standard Atmosphere (atm) - An international reference unit of pressure. 1 atm = 101325 pascals (Pa) = 101.325 kilopascals (kPa) = 1013.25 millibars (mb).

Standard Atmospheric Pressure - A pressure of 101325 pascals (Pa) or 101.325 kilopascals (kPa) or 1013.25 millibars (mb).

Standard Deviation - A statistical measure of the dispersion of observation values in a data set around the mean (average). Calculated by determining the square root of the variance.

Standard Rain Gauge - A meteorological instrument designed to accurately measure the amount of rain falling on the ground surface.

Standard Temperature and Pressure - Standard scientific reference equal to a temperature of 273.15Kelvins (0°C, 32°F) and an absolute pressure of 100,000 Pascals (1 bar, 14.5 pounds per square inch).

Standing Wave - A wave found in some medium of transport that appears to have a constant oscillation. A variety of causes can give rise to standing waves. One common cause is when two series of waves moving in the opposite direction interfere with each other. Also called a stationary wave.

Star - A large and very massive, self-luminous celestial body of gas that appears illuminated because of the emission of electromagnetic radiation derived from its internal source of energy.

Star Dune - A large pyramidal or star-shaped dune with three or more sinuous radiating ridges forms a central peak of sand. This sand dune has three or more slip-faces. This dune forms because of variable winds. This dune does not migrate along the ground, but it does grow vertically.

Starch - A complex carbohydrate composed of thousands of glucose units. Main compound that plants use to store their food energy.

State Change - See phase change.

State of Matter - A form of matter. Matter can exist in four different forms: gas, liquid, solid, and plasma.

Static Equilibrium - This type of equilibrium occurs where force and reaction are balanced, and the properties of the system remain unchanged over time. 

Stationary Front - A transition zone in the atmosphere where there is little movement of opposing air masses, and winds blow towards the front from opposite directions.

Steady Flow - (1) A condition of constant flow velocity in a stream over space or time. (2) Term used in fluid dynamics to describe a situation where the flow velocity in a fluid at a specific point does not change over time.

Steady-State Equilibrium - In this type of equilibrium, the average condition of the system remains unchanged over time.

Steam Fog - See evaporation fog.

Stefan-Boltzmann Law - This radiation law states that the amount of electromagnetic radiation emitted by a body is proportional to the 4th power of its temperature, measured in Kelvin units. The following simple equation can express this law: 

E* = σT⁴

where E* is the amount of radiation emitted by the body in Watts per square meter, σ is a constant equal to 0.0000000567, and T is the temperature of the body in Kelvins.

Stemflow - Is the process that directs precipitation down plant branches and stems. This process redirects water, causing the ground around the plant's stem to receive additional moisture. The amount of stemflow is determined by leaf shape and the architecture of stems and branches. In general, deciduous trees produce more stemflow than coniferous vegetation.

Step-Pool Stream - Is a form of stream channel morphology characterized by alternating channel-wide steps composed of cobbles and boulders and deep pools.  Common in steep mountain streams.

Steppe - A Russian term used to describe grasslands found in the mid-latitudes.

Stevenson Screen - A specially designed housing for meteorological instruments used to keep measurements standardized around the world. This housing consists of a white wooden box with double louvered sides. It is mounted on a stand 1.5 meters or 4.5 feet (this varies from country to country, between 1.2 and 1.8 meters or 3.9 and 5.9 feet) above the ground surface and contains a maximum thermometer, minimum thermometer, barometer, dry-bulb thermometer, and wet-bulb thermometer. 

Stick Slip - A situation where the transport of ice in an alpine glacier flowing over bedrock has sporadic variations in velocity.

Stillstand - (1) A period where the movement of a glacier appears to be stationary. (2) A time where tectonic activity in an area seems to be non-existent. (3) Period when the sea level does not change.

Stochastic Model - A mathematical model that simulates cause and effect processes as being randomly determined or controlled in a probabilistic fashion.  

Stochastic Process - A cause-and-effect process that is randomly determined or controlled by probability. 

Stock - A type of igneous intrusion of rock that is large and unevenly shaped.

Stomata (singular Stoma) - Small openings on the surface of a plant that are used for gas exchange.

Stone Pavement - See desert pavement.

Storage - This refers to the ability of a component of the hydrologic cycle to act as a reservoir for water. In the broadest sense, some of the places where water is kept on our planet include glaciers, ice caps, lakes, oceans, streams, soil, the atmosphere, and bedrock.

Storm - A surface disturbance in the atmosphere normally associated with low atmospheric pressure, fast-moving winds, and heavy precipitation. Common types of storms include thunderstorms, mid-latitude cyclones, and hurricanes.

Storm Runoff - A rapid and intense form of runoff generated from precipitation associated with a thunderstorm, mid-latitude cyclone, or hurricane.

Storm Surge - A relatively rapid rise in the height of the ocean along a coastline. It is often caused by the winds associated with a tropical storm or hurricane pushing water towards land.

Storm Track - The actual path taken by a storm (thunderstorm, mid-latitude cyclone, or hurricane) or the average path taken by many storms.

Stoss - Side of a slope that faces the direction of flow of ice, wind, or water. The opposite of lee.

Strain - (1) An external force that causes the deformation of an object through pulling or stretching. Compare with stress. (2) The measurement of the external force that causes the deformation of an object through pulling or stretching. 

Stratified Drift - A type of glacial drift that has been partially sorted by glaciofluvial meltwater. 

Stratigraphic Unit - (1) A distinct and definable stratum of rock. (2) A sequential series of strata that have similar characteristics and properties. These characteristics and properties may include minerals, fossils, magnetic polarity, chemistry, and the presence of clasts.

Stratigraphy - A sub-discipline of Geology that studies the sequence, spacing, composition, and spatial distribution of sedimentary deposits and rocks.

Stratocumulus Cloud - Low altitude gray colored cloud composed of water droplets with a patchy appearance. Each cloud patch consists of a rounded mass. This cloud has a somewhat uniform base and usually covers the entire sky. Between the patches, the blue sky is visible. Found in an altitude range from the surface to 3,000 meters (9,843 feet). 

Stratopause - The stratopause is a relatively thin atmospheric transition layer found between the stratosphere and the mesosphere. This layer is about 50 kilometers (31 miles) above Earth's surface.

Stratosphere - Atmospheric layer found at an average altitude of 11 to 50 kilometers (6.8 to 31.1 miles) above the Earth's surface. Within the stratosphere exists the ozone layer. Ozone's absorption of ultraviolet sunlight causes air temperature within the stratosphere to increase with altitude.

Stratovolcano - See composite volcano.

Stratum (Strata plural) - A homogeneous or gradational distinct layer or bed found in consolidated rock or unconsolidated soil or sediment. Visually identifiable from layers above and below it.

Stratus Cloud - Low altitude gray colored cloud composed of water droplets (Image Source: Wikimedia Commons, image is in the public domain). This cloud has a uniform base and typically covers the entire sky. It is also quite thick and can obscure the Sun. Light precipitation often falls from it. Stratus clouds are found in an altitude range from the surface to 3,000 meters (9,843 feet).

Stream - A long, narrow channel of water that flows as a function of gravity and elevation across the Earth's surface. Most streams empty into a lake, sea, or ocean.

Stream Bank - The sides of the stream channel.

Stream Bed - The bottom of the stream channel.

Stream Channel - A long trough-like depression that is normally occupied by the water in a stream.

Stream Discharge - A stream's rate of flow over a particular period. Usually measured by a current meter and expressed in cubic meters per second. Stream discharge depends on the volume and velocity of the flow.

Stream Flow - A term used to describe the flow of water in a stream or stream channel.

Stream Gradient - The change in elevation from a stream's headwaters to its mouth expressed in degrees, percentage, or as a distance ratio (rise/run).

Stream Load - Refers to the material or sediment carried by a stream. Stream load normally consists of three components: bed load (pebbles and sand, which move along the stream bed without being permanently suspended in the flowing water), suspended load (silts and clays in suspension), and dissolved load (material in solution).

Stream Long Profile - The vertical and horizontal profile of a stream. Most streams have a concave profile.

Stream Order - The relative position, or rank, of a stream channel segment in a drainage network.

Stream Valley - A relatively long linear depression in the landscape that slopes down to a stream. This landscape feature is formed by erosion associated with flowing water.

Streamline - (1) A design of an object where its form offers the least resistance to fluid flow. (2) A line that is aligned perfectly with the direction of the fluid flow.

Stress - (1) The internal force within an object caused by strain. (2) In biology, this term refers to an organism's response to adverse abiotic or biotic environmental conditions. 

Stress-Tolerant Species - According to J.P. Grime's theory of plant strategies, these species have the greatest fitness in habitats with a high intensity of stress and a low intensity of disturbance. These plants live in habitats where water, light, heat, or nutrients are scarce, or where heat is excessive and limits growth. Stress-tolerant plants have morphological and physiological adaptations that allow them to thrive under stress.  However, these adaptations come at a cost to the plant, and consequently, it has less biomass to allocate to growth and reproduction. Stress-tolerant plant species tend to be long-lived, have tissues resistant to stress, organs that store nutrients and water for later use, are often evergreen, and time their flowering to particular environmental cues. Compare with competitive and ruderal species.

Striations - Deep scratches found on rock surfaces caused by glacial abrasion.

Strike - One of the directional properties of a geologic structure, such as a fold or a fault. Strike is the horizontal direction taken by an imaginary line drawn on the plane of the formation. The concept of strike is illustrated in the diagram where N, E, S, and W cardinal directions, the blue plane is an ideal horizontal plane, the green plane is an ideal vertical plane, the red plane is the plane of the stratum bed we are measuring, z is the strike, Z is the strike direction, σ is the strike angle (0-180°), F is the dip direction (0-360°), and φ is the dip angle (0-90°). Also see dip.

Strike-Slip Fault - A fault that displays mainly horizontal displacement along a vertical fracture. This type of fault has minimal vertical displacement of the fault blocks. Strike-slip faults can be further classified as being sinistral or dextral. Sinistral faults show left-lateral displacement, while dextral faults show right-lateral displacement. Also called a transform fault.

Stromatolite - A type of sedimentary deposit created by biochemical processes associated with bacteria, cyanobacteria, and lichens. Stromatolites are common in the  Precambrian fossil record. They are evidence of some of the earliest life on our planet. 

Structural Landform - This is a landform created by massive Earth movements due to plate tectonics. Includes landforms with the following geomorphic features: fold-created mountains, rift valleys, and volcanoes.

Subaerial - Something that is found or exists on the land surface.

Suballuvial Bench - A gradually sloping bedrock mass with surface deposits of alluvial sediments at its lower elevations.

Subatomic Particles - Are extremely small particles that make up the internal structure of individual atoms.

Subduction (Tectonic) - A process associated with plate tectonics where one lithospheric plate is pushed below another into the asthenosphere.

Subduction Zone - A linear zone where tectonic subduction occurs.

Subglacial - Refers to processes and conditions occurring under a glacier.  Compare with englacial, proglacial, and supraglacial.

Subglacial Stream - A long, narrow channel of water or stream that flows as a function of gravity beneath the main ice mass of an ice sheet or glacier.

Subhumid - An environment, habitat, or location that has a climate similar to a grassland or a savanna. Subhumid environments often have seasons with plentiful and scarce moisture. Compare with humid.

Sublimation - The process by which ice changes into water vapor without first becoming liquid. This process requires approximately 680 calories of heat energy for each gram of water converted.

Sublittoral - Refers to a zone of elevation between the low tide level at the shoreline and the edge of the continental shelf at approximately 200 meters (650 feet) below sea level.

Submarine Canyon - V-shaped canyons found beneath the ocean surface that are cut into the continental slope to a depth of up to 1,200 meters (3,940 feet). These features are typically associated with major streams that empty into the oceans along the edges of continents. 

Submerged Forest - A remnant dead forest that is submerged beneath a lake, ocean, or some other water body. It occurs when an environmental factor rapidly submerges a terrestrial area with forest cover in water or seawater. One common human activity that can create submerged forests is the construction of a dam that creates a flooded reservoir.

Submergent Coastline - An area along a coastline that has been influenced by a rise in sea level because of isostasy or eustasy. Landscape features associated with submergent coastlines include submerged stream valleys and fjords. Compare with an emergent coastline.

Subpolar Glacier - A type of glacier where the ice found from its surface to its base has a temperature as cold as -30°C (-22°F) throughout the year. This temperature is well below the pressure melting point. However, melting does occur in the accumulation zone in the summer. One of the three types of glaciers: cold glacier, temperate glacier, and subpolar glacier.

Subpolar Lows - Surface zone of low atmospheric pressure located at about 60° North and South latitude. These low pressure systems are produced by the frontal lifting of subtropical air masses over polar air masses.

Subsea Permafrost - A form of permafrost that exists beneath the sea surface in ocean sediments.

Subsequent Stream - A stream whose course was the outcome of headward erosion into easily eroded rock strata. 

Subsidence - The lowering or sinking of a portion of the Earth's terrestrial surface.

Subsistence Hunting - Is a type of hunting where animals are killed for food required for the hunter's survival. Compare with sport hunting, commercial hunting, and poaching.

Subsolar Point - The location on the Earth where the Sun is directly overhead. Also see declination.

Subtropical High Pressure Zone - An extensive region of surface atmospheric high pressure located at about 30° North and South latitude. These high pressure systems are produced by vertically descending air currents from the Hadley Cell.

Subtropical Jet Stream - Relatively fast, uniform winds concentrated within the upper atmosphere in a narrow band. The subtropical jet stream exists in the subtropics at an altitude of approximately 13 kilometers (8.1 miles). This jet stream flows from west to east and is usually a bit slower than the polar jet stream. Average wind speed is between 145 and 215 kilometers per hour (90 and 135 miles per hour). Also see the jet stream and the polar jet stream.

Subvolcanic Rock - See hypabyssal rock.

Succession - See plant succession or ecological succession.

Succulent Vegetation - A group of plants that can survive in deserts and other dry climates by having no leaves. Instead, the surface of their branches and stems is used for photosynthesis. This adaptation reduces the surface area for evaporation, thus reducing the loss of scarce water.

Suction - The movement of a fluid substance like air and water because of the creation of a partial vacuum. For example, in the atmosphere, air flows into a region when a low pressure is created. 

Suffosion - An erosional process that creates areas of surface subsidence, usually in areas with underlying limestone bedrock. It occurs when non-cohesive soil particles such as silt are gradually washed downward through the soil layer into fissures and cracks in the subsurface bedrock. 

Sugar - A type of simple carbohydrate chemically composed of carbon, oxygen, and hydrogen.

Sulfation - A chemical reaction that occurs between sulfur oxides and calcium carbonate in humid environments. Causes the conversion of calcium carbonate into gypsum and carbon dioxide.

Sulfate Aerosol - A type of solid compound commonly found in the atmosphere. These particles play an important role in reflecting, absorbing, and scattering incoming solar radiation. The source of these compounds is both natural and human-made. Most of the human-made particles come from the combustion of fossil fuels.

Sulfur Dioxide - A gas produced from volcanic eruptions, ocean spray, organic matter decomposition, and the burning of fossil fuels. Sulfur dioxide is a component in the creation of acid precipitation. This colorless gas has the chemical formula SO₂.

Sulfur Oxides - This term refers to a number of chemical compounds containing sulfur and oxygen, including sulfur oxide (SO), sulfur dioxide (SO₂), and sulfur trioxide (SO₃).

Sulfuric Acid - Acid with the chemical formula: H₂SO₄.

Summer - The season between spring and fall. Astronomically, it is the period from the June Solstice to the September Equinox in the Northern Hemisphere and from the December Solstice to the March Equinox in the Southern Hemisphere.  

Summer Solstice - The summer solstice denotes the first day of the summer season. In the Northern Hemisphere, the summer solstice occurs on either June 21 or 22 (depending on the year). December 21 or 22 is the summer solstice in the Southern Hemisphere. During the summer solstice, locations in their respective hemispheres experience the longest day of the year.

Sun - The luminous star around which the Earth and other planets in our solar system orbit. The Sun is a large celestial body with a diameter of about 1.39 million kilometers and a mass 10⁹ times that of the Earth. Because of its high surface temperature (5773 K or 5500°C), the Sun emits 63,000,000 Watts per square meter of solar radiation into space. The Sun is mainly composed of hydrogen (73.5%) and helium (24.9%). The Sun is about 150 million kilometers (93 million miles) from Earth. However, the Earth's orbit is not circular but elliptical, and as a result, the Sun is closest to the Earth on January 3 (perihelion) and farthest away on July 4 (aphelion). 

Sunshine Recorder - See Campbell-Stokes Recorder.

Sunrise - Moment in time when the Sun's edge first appears above the Earth's horizon.

Sunset - The moment in time when the Sun's edge completely disappears below the Earth's horizon.

Sunspot - Dark-colored region on the Sun that represents an area of cooler temperatures and extremely high magnetic fields. 

Supercluster - Are gravity-created aggregations of a large number of galaxy groups located somewhere in the Universe. Superclusters are the largest known celestial bodies in the Universe, with some spanning about 500 million light-years. 

Supercooled Water - The cooling of water well below 0°C (32°F) without freezing. Common in clouds where there is a deficiency of deposition nuclei.

Supercooling - A situation where a substance remains in a liquid state even though its temperature is below the freezing point. Water droplets in stratus and cumulus clouds are often supercooled to temperatures between 0°C and -40°C. This cloud phenomenon occurs because deposition nuclei are in short supply. Deposition nuclei are required for the formation of ice crystals in clouds. 

Supereon - The longest unit in the geologic time scale. There is only one supereon - the Precambrian. See the International Commission on Stratigraphy (stratigraphy.org) for more information on the geologic time scale.

Superimposed Drainage - Situation where a stream drainage pattern that developed in the past on a different landscape environment is superimposed onto a new landscape type because of weathering and erosion. Evidence for a superimposed drainage system is the observation that the drainage pattern does not seem to be properly hydrologically adjusted to the present landscape.

Superimposed Ice - An ice accumulation process on the surface of a glacier that occurs when meltwater refreezes. It is usually found between the equilibrium line and the firn line on a glacier.

Supersaturation - An atmospheric condition where saturation occurs at a relative humidity greater than 100% because of a shortage of deposition or condensation nuclei.

Supporting Services - One of four benefits humans receive from ecosystems, as described in the United Nations’ Millennium Ecosystem Assessment. This service makes the other three services (cultural, provisioning, and regulating) possible. For example, ecosystems produce biomass, create atmospheric oxygen, support nutrient cycling, and provide habitat for species. Compare with cultural services, provisioning services, and regulating services.

Supraglacial - Refers to processes and conditions occurring within the body of a glacier. Same as englacial. Compare with proglacial and subglacial. 

Surface Air Temperature - In meteorology and weather forecasting, this term refers to the temperature of the air about 1.5 meters (4.5 feet) above the ground surface, where it is routinely measured at land-based weather stations.

Surface Creep - The sliding and rolling movement of soil particles on the Earth's surface because of wind. Eolian process of soil particle movement. 

Surface Heat Flux - The process by which heat energy is transferred into land and ocean surfaces on the Earth. Much of this transfer occurs when solar radiation absorbed at the land or ocean surface is converted into heat energy. On land, surface heat is transferred down into the ground by conduction. Heat energy is transferred to greater depths in the ocean because liquids can mix by convection. Heat energy stored in ocean waters can also move quickly over large horizontal distances through ocean currents.

Surface Storage - Refers to the stored accumulation of water on the ground surface in the form of ponds, lakes, and other standing bodies of water. The term surface storage is often applied to a defined drainage basin or, more generally, to the concept of the hydrologic cycle. 

Surface Tension - The tension associated with a liquid's surface, caused by the forces of attraction between molecules.

Surface Wave - A type of seismic wave that travels across the Earth's surface. These earthquake-generated waves cause the Earth's surface to roll or sway, like ocean waves.

Surge - A large, destructive ocean wave caused by very low atmospheric pressure and strong winds. Hurricanes are often the source of surges.

Surging Glacier - A glacier that is experiencing flow rates that are up to 100 times faster than normal. This typically occurs as short-lived, sporadic events. Stretches of stagnation prior to the surging event can last from 10 to 200 years. Many glaciers in Norway, the Canadian Arctic, Alaska, and Iceland are currently experiencing surges, and some scientists have hypothesized that this is a response to climate change and warmer temperatures. 

Suspended Load - the portion of the stream load that is carried almost permanently suspended in flowing water.

Suspension - The erosional movement of sediment continually being held in the transport medium of air, water, or ice.

Sustainable Development - Forms of economic growth and other human activities that meet the requirements of the present without jeopardizing the ability of future generations of individuals to meet their own needs.

Sustained Yield - A concept applied to renewable resources that suggests there is a level of consumption that will not reduce the base quantity of the resource being used. However, higher consumption levels will cause the resource's base quantity to decline as use exceeds its ability to replenish itself.

Swale - (1) Areas of low elevation on the uneven surface of a glacial moraine. (2) Low elevation areas found between beach ridges. (3) The low elevation zones in between sand dune ridges. 

Swamp - A type of wetland ecosystem where woody plants dominate. Compare with marsh.

Swash - The thin sheet of water that moves up the beach face after a wave of water breaks on the shore.

Swell - A relatively smooth ocean wave that travels some distance from its area of generation.

Symbiosis - Is any biological interaction between two species that involves a close living arrangement and is required for the survival of one or both of the engaged species. This interaction may negatively, positively, or have no effect on the physiological and reproductive fitness of the species involved. Biologists recognize the following types of symbiotic relationships: mutualism, commensalism, amensalism, and parasitism.

Symbiotic - A type of mutual relationship between two organisms that is necessary for either to survive.

Symbiotic Mutualism - A mutualistic biotic interaction where the species interact physically, and their relationship is biologically essential for survival. Compare with a non-symbiotic mutualism.

Sympatric Speciation - The evolution of two or more new species that occupy the same geographical area from a single ancestral species. Compare with allopatric speciation.

Sympatry - Two or more things that occur in the same geographical area. This term is often used by biologists to refer to species or populations. Compare with allopatry.

Syncline - Is a fold in rock layers that forms a trough-like bend. Compare with an anticline.

Synoptic Climatology - A subfield of climatology that examines the climate of locations and regions in relation to large-scale spatial analysis of atmospheric properties and motions. 

Synoptic Meteorology - A subfield of meteorology that examines the weather of locations and regions in relationship to large-scale spatial analysis of atmospheric properties and motions.

Synoptic Scale - Scale of geographic coverage used on daily weather maps to describe large-scale atmospheric phenomena (for example, mid-latitude cyclones, air masses, fronts, and hurricanes).

System - A system is a set of interrelated components that work through cause and effect to achieve a process or outcome.

System Attribute - The perceived character or value of a system element. For example, number, size, color, volume, and temperature are characteristics of clouds in the atmospheric system.

System Boundary - The outer edge of a system. The zone between one system and another system.

System Element - System elements are the kinds of things or substances found within a system. These system elements may be atoms or molecules or larger bodies of matter - sand grains, raindrops, the numerous plants in an ecosystem, or the various planets and other celestial bodies found in our Solar System.

System Relationship - This is the cause-and-effect association between a system's elements and/or attributes.

System State - The current value of a system's elements, attributes, and/or relationships.

Systems Theory - A field of academic study that considers phenomena (or systems) as being a set of interrelated components working together towards some kind of process.