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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.
Salinity -  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 -  Pedogenic process that concentrates salts at or near the soil surface because evapotranspiration greatly exceeds water inputs from precipitation.
SALR -  The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is less than the dry adiabatic lapse rate (9.8° Celsius per 1000 meters) because of the heat energy added to the ascending air parcel from condensation and deposition processes.
Salt -  (1) The mineral sodium chloride. (2) Compounds that are produced as the result of a metal atom replacing a hydrogen atom in an acid.
Salt Marsh -  Coastal wetland ecosystem that is inundated for some period of time by seawater. Plants that exist in this community have special adaptation to survive in the presence of high salinities in their immediate environment. Generally, found poleward of 30° North and South latitude.
Saltation -  Transport of sediment initiated by moving air or water where particles move from a resting surface to the transport medium in quick continuous repeated cycles.
Saltwater Intrusion -  The invasion of saltwater into freshwater aquifers in coastal and inland areas. This condition can be cause when groundwater, which charges the aquifer, is withdrawn faster than it is recharged by precipitation.
Sample -  A sample is a subset group of data selected from a larger population group. Most samples are drawn at random to guaranty equal representation in the data.
Sand -  Mineral particle with a size between 0.06 and 2.0 millimeters in diameter. Also see clay and silt.
Sand Dune -  A hill or ridge of aeolian sand deposits with a minimum height of less than one meter and a maximum height of about 50 meters. Found in hot deserts and along sandy coastlines.
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 -  Deposit of sometimes stratified less well sorted sand that almost resemble dunes. Common in northern Europe. Believed to form when windblown materials settle on areas of patchy snow.
Sand Wedge -  A form of ice wedge that contains accumulations of wind blown sand in long vertical layers. A form of periglacial ground ice.
Sandstone -  A type of sedimentary rock that contains a large quantity of weathered quartz grains.
Santa Ana -  A warm, dry chinook like wind that occurs in southern California. Originates from the east off an elevated desert plateau.
Santa Ana Wind -  A warm, dry chinook like wind that occurs in southern California. Originates from the east off an elevated desert plateau.
Saturated Adiabatic Lapse Rate -  The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is less than the dry adiabatic lapse rate (9.8° Celsius per 1000 meters) because of the heat energy added to the ascending air parcel from condensation and deposition processes.
Saturation -  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 caused by the cooling of the atmosphere.
Saturation Mixing Ratio -  Mass of water vapor that a kilogram of dry air can hold at saturation. Measured in grams.
Savanna -  A tropical or sub-tropical plant community characterized by trees and shrubs scattered among a cover of grasses, herbs and forbs. The climate of a savanna is tropical with a dry season occurring in the low Sun period of the year.
Scale -  A specific relative or proportional size or extent of a phenomena as measured through space and/or time.
Scarification -  Extensive movements of soil, sediment, and rock material caused by humans.
Scattering -  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 does, however, reduce the amount of incoming radiation reaching the Earth's surface. A significant proportion of scattered shortwave solar radiation is redirected back to space. The amount of scattering that takes place is dependent on two factors: wavelength of the incoming radiation and the size of the scattering particle or gas molecule. In the Earth's atmosphere, the presence of a large number of particles with a size of about 0.5 µm 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.
Schist -  A medium to coarse grained metamorphic rock with well developed bedding planes derived from the foliated recrystrallization of platy like 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 and the ability to make predictions. The overall goal of science is to better understand nature and our Universe.
Scientific Method -  The approach science uses to gain knowledge. This method tries to be unbias and neutral. Involves inductive and deductive reasoning, hypothesis testing and falsification, and predictive model testing.
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 waxy thick leaves with closing stomata which resist water loss.
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 -  An 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 a ocean that is in close proximity 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 -  Local thermal circulation pattern found at the interface between land and water. In this circulation system, surface winds blow from water to land during the daytime.
Sea Stack -  A steep pillar of rock located in the ocean a short distance from the coastline. These landforms are created when waves erode through a thin headland from both sides.
Sea-Floor Spreading -  The process of oceanic crust creation and sea-floor movement that occurs at the mid-oceanic ridge.
Sea-Level -  The average surface elevation of the world's oceans.
Sea-Level Pressure -  Average atmospheric pressure at sea-level. This value is 1013.2 millibars.
Seamount -  A volcanic mountain found on an ocean basin that has an origin not related to a mid-oceanic ridge or a tectonic subduction zone.
Seasons -  Time periods generally based on the changes in the intensity and duration of sunlight as received in the middle and high latitudes. Four seasons are normally recognized: Spring; Summer; Fall; and Winter. The astronomical definition is more precise and suggests the following time periods 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 -  Positioned or located away from land but towards an ocean or sea.
Seawater -  The mixture of water and various dissolved salts found in the world's oceans and seas.
Second Law of Thermodynamics -  This law states that heat can never pass spontaneously from a colder to a hotter body. As a result of this fact, natural processes that involve energy transfer must have one direction, and all natural processes are irreversible. This law also predicts that the entropy of an isolated system always increases with time.
Secondary Pollutant -  Atmospheric pollutants that are created chemically in the atmosphere when primary pollutants and other components of the air react. Also see primary pollutant.
Secondary 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 S-wave or a shear wave.
Sediment -  Solid material that has been or is being eroded, transported, and deposited. Transport can be due to fluvial, marine, glacial or aeolian agents.
Sediment Rating Curve -  Numerical expression or graphical curve that describes the quantitative relationship between stream discharge and the sediment transported by a particular stream.
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.
Seepage -  (1) The gradual movement of water into the soil layer. (2) Slow movement of sub-surface water to the surface. This flow is not great enough to call it 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 because of the active migration of water from around the feature. These features are found just below the active layer.
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 -  Shaking displacement usually caused by an earthquake.
Seismic Wave -  Successive wave-type displacement of rock usually caused by an earthquake.
Seismograph -  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.
Self-Regulation -  The ability of some systems to maintain a steady state equilibrium through positive and negative feedbacks.
Semi-Diurnal Tide -  Tides that have two high and two low waters per tidal period.
Sensible Heat -  Heat that can be measured by a thermometer and thus sensed by humans.
Sensible Heat Flux -  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 then can move horizontally advection (atmospheric circulation).
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.
Shale -  Fine grained sedimentary rock composed of lithified clay particles.
Shear Stress -  Stress caused by forces operating parallel to each other but in opposite directions.
Shear Wave -  A seismic wave that creates wave-like motion perpendicular to the direction of seismic energy propagation. Also called S-wave or Secondary Wave.
Sheet Flow -  The topographic movement of a thin film of water from precipitation to lower elevations. With time, this water will begin to organizing its flow into small channels called rills. The rills converge to form progressively larger channels until stream channels are formed. Occurs when the infiltration capacity of an area's soil has been exceeded. Also called overland flow or runoff.
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. Process of erosion.
Shield -  A large stable area of exposed very old (more than 600 million years) igneous and metamorphic rock found on continents. This rock forms the nucleus of the continents.
Shield Volcano -  Volcano created from alternate layers of lava flows. Shield volcanoes are slightly sloping having a gradient between 6 and 12°. Their height can be as high as 9000 meters. The chemistry of the magma of these volcanoes is basaltic.
Shore -  The land area bordering a relatively large water body like a lake or ocean.
Shoreline -  The line that separates a land surface from a water body. 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.
Sial Layer -  The part of the crust that forms the continents and is composed of relatively light, granitic rocks.
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 mean solar day.
Silcretes -  Sedimentary rock created by the chemical precipitation of silica.
Silica -  Mineral that is composed of silicon dioxide, SiO2.
Silicate -  Group of minerals that have crystal structures based on a silica tetrahedron (SiO4).
Silicate Magma -  Magma that is felsic in composition.
Sill -  Horizontal planes of igneous rock that run parallel to the grain of the original rock deposits.They form when magma enters and cools in bedding planes found within the crust. Also see intrusive igneous rock.
Silt -  Mineral particle with a size between 0.004 and 0.06 millimeters in diameter. Also see clay and sand.
Siltstone -  Fine grained sedimentary rock composed of lithified silt particles.
Silurian -  Geologic period that occurred roughly 408 to 438 million years ago. During this period, the first plant and insect species appear.
Sima Layer -  The part of the crust that forms the ocean basins and lower layers in the crust and is composed of relatively heavy, basaltic rocks.
Sink -  (1) Site of the storage of some material. (2) Another name for sinkhole.
Sinkhole -  A pit like hole in found in areas of karst. These features are caused by the weathering of limestone or dolomite by subsurface drainage. Also called a sink or doline.
Sinusoidal Equal-Area Projection -  Map projection that represents areas in their true form on a two-dimensional map. Distances are only correct along parallels and central meridian. Shapes become more distorted away from the central meridian and close to the poles.
Slate -  A fine grained metamorphic rock with well developed bedding planes derived from the slight recrystrallization of shale.
Sleet -  A type of precipitation. Ice pellets or sleet are transparent or translucent spheres of frozen water that fall from clouds. Ice pellets have a diameter less than 5 millimeters. To form, this type of precipitation requires an environment where raindrops develop in an atmosphere where air temperature is above freezing. These raindrops then fall into a lower layer of air with freezing temperatures. In this lower layer of cold air, the raindrops freeze into small ice pellets. Like freezing rain, an air temperature inversion is required for development of ice pellets. Also see ice pellets.
Sling Psychrometer -  Psychrometer that uses a rotating handle and a whirling motion to ventilate its wet-bulb thermometer.
Slip-Face -  The lee side of a dune where material accumulates and slides or rolls downslope.
Slope Aspect -  Main compass direction (North, North East, East, South East, South, South West, West, and North West) that a slope faces.
Slope Failure -  The downslope movement of soil and sediment by processes of mass movement.
Slump -  Form of mass movement where material moves suddenly along a curvilinear plane. Also called a 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 -  Generic term used to describe mixtures of pollutants in the atmosphere. Also see industrial smog and photochemical smog.
Snout -  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 nuclei. Snowflakes begin their life as very tiny crystals developing on a six-sided hexagonal deposition nuclei. The developing snowflak, then grows fastest at the six points of the nuclei as these surfaces are more exposed to atmosphere's water vapor. Snowfall is most common with the frontal lifting associated with mid-latitude cyclones during fall, winter, and spring months when air temperatures are below freezing.
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 Melt -  Conversion of snow into runoff and groundwater flow with the onset of warmer temperatures.
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. Snow pellets develop when supercooled droplets freeze on snowflakes. Snow pellets often fall for a brief time 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 of permanent snow accumulation. Usually at high altitudes or latitudes.
Soil -  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 Creep -  Slow mass movement of soil downslope. Occurs where the stresses on the slope material are too small to create a rapid failure.
Soil Erosion -  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 enhance this natural process.
Soil Fertility -  The ability of a soil to provide nutrients for plant growth.
Soil Horizon -  Layer within a soil profile that differs physically, biologically or chemically from layers above and/or below it.
Soil Moisture Recharge -  The process of water filling the pore space found in a soil (storage).
Soil Organic Matter -  Organic constituents of soil.
Soil Permeability -  The rate at which water and air move vertically through a soil.
Soil Porosity -  The volume of water that can be held in a soil. Also refers to the ratio of the volume of voids to the total volume of the soil.
Soil Profile -  Vertical arrangement of layers or horizons in a soil.
Soil Science -  The study of soils from an interdisciplinary perspective.
Soil Solution -  Aqueous liquid found within a soil. This liquid normally contains ions released from mineral particles, organic matter or plant roots and leaves.
Soil Structure -  General term that describes how mineral and particles organic matter of are organized and clumped together in a soil.
Soil Taxonomy -  The classification of a soil in a hierarchical system based on its various properties. Grouping 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, from, or through the soil.
Solar Altitude -  Height of the Sun above the horizon from either True North or True South.
Solar Constant -  A term used to describe the average quantity of solar insolation received by a horizontal surface at the edge of the Earth's atmosphere. This value is approximately 1370 Watts per square meter.
Solar Day -  Time required for the Earth to complete one rotation relative to the Sun.
Solar Energy -  Direct or diffused shortwave solar radiation that is received in the Earth's atmosphere or at its surface. Also see insolation.
Solar Noon -  Point of time during the day when the Sun is aligned with True North and True South.
Solar Radiation -  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 the Sun.
Solar Wind -  Mass of ionized gas emitted to space by the Sun. 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.
Solid -  A state of matter where molecules where the mass of the substance does not have the property of flow.
Solifluction -  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.
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 -  Part of the soil that is capable of supporting life.
Solution -  (1) Form of chemical weathering where rocks and minerals are dissolved by water. Materials entering the mixture can alter the chemical nature of the solution and can 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.
Source Region -  Area where air masses originate and come to possess their moisture and temperature characteristics.
South Magnetic Pole -  Location in the Southern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual 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.
Southern Oscillation -  Reversal of atmospheric circulation in tropical Pacific Ocean that triggers the development of an El Niño.
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.
Spatial Analysis -  The examination of the spatial pattern of natural and human-made phenomena using numerical analysis and statistics.
Spatial Tradition -  Academic tradition in modern Geography that investigates geographic phenomena from a strictly spatial perspective.
Specific Heat -  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 -  Measurement of atmospheric humidity. Specific humidity is the mass of water vapor in a given mass of air. Normally expressed in grams of water vapor per kilogram of air at a specific temperature.
Spectrum -  Is a graph that describes the quantity of radiation that is emitted from a body at particular wavelengths.
Speed of Light -  Velocity of light in a vacuum. This velocity is approximately 3 x 108 meters per second. It takes light from the Sun 8 minutes and 20 seconds to reach the Earth.
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 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 the result of the deposition of sediments by longshore drift.
Sporadic Permafrost -  Form of permafrost that exists as small islands of frozen ground in otherwise unfrozen soil and sediments.
SPOT (Centre National d'Etudes Spatiales) -  Series of satellites developed by the French Space Agency, with the cooperation with Belgium and Sweden for the purpose of remotely monitoring resources on the Earth. The first SPOT satellite was launched in 1986. See the following website for more information - SPOT Image.
Spring -  (1) Season between winter and summer. Astronomically it is the period from the vernal equinox to the summer solstice in the Northern Hemisphere. (2) A natural flow of water from the sub-surface to the surface. Usually occurs when the water table intersects the Earth's surface.
Spring Tide -  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 Sun are complementary to each other. Contrasts with neap tide.
Squall Line -  A band of thunderstorm development found ahead of a cold front.
Stability -  The capability of a system to tolerate or recover from disturbance or an environmental stress.
Stable Atmosphere -  Condition in the atmosphere where isolated air parcels have a tendency to sink. The parcels of air tend to be cooler than the air that surrounds them.
Stable Equilibrium -  In a stable equilibrium the system displays tendencies to return to the same equilibrium after disturbance.
Stadial Moraine -  Moraine that is created during a pause in the retreat of a glacier. Also called a recessional moraine.
Stage -  The elevation of the water surface in a stream channel.
Standard Atmospheric Pressure -  A pressure of 101.32 kilopascals or 1013.2 millibars.
Standard Deviation -  A statistical measure of the dispersion of observation values in a data set. Calculated by determining the square root of the variance.
Star -  A large and very massive, self-luminous celestial body of gas that illuminates via the radiation derived from its internal source of energy.
State of Matter -  Form of matter. Matter can exist in three different forms gas, liquid, and solid.
Static Equilibrium -  Static 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 State Equilibrium -  In this type of equilibrium the average condition of the system remains unchanged over time.
Steam Fog -  A type of fog produced from the advection of cold air over warm water or warm or moist land. This type of fog is sometimes called evaporation fog or sea smoke.
Stefan-Boltzmann Law -  This radiation law suggests the amount of radiation given off by a body is proportional to the 4th power of its temperature as measured in Kelvin units. This law can be expressed by the following simple equation: E* = sT 4 where E* is the amount of radiation emitted by the body in Watts per square meter, s 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. The redirection of water by this process causes the ground area around the plant's stem to receive additional moisture. The amount of stemflow is determined by leaf shape and stem and branch architecture. In general, deciduous trees have more stemflow than coniferous vegetation.
Storm Surge -  Relatively rapid rise in the height of the ocean along a coastline. Often caused by the storm winds pushing water towards land.
Storm Track -  The path taken by a storm (thunderstorm, mid-latitude cyclone or hurricane) or the average path taken by storms.
Stoss -  Side of a slope that faces the direction of flow of ice, wind, or water. Opposite of lee.
Strata -  The layers or beds found in sedimentary rock.
Stratified Drift -  A type of glacial drift that has been partially sorted by glaciofluvial meltwater.
Stratigraphy -  Subdiscipline of geology that studies sequence, spacing, composition, and spatial distribution of sedimentary deposits and rocks.
Stratocumulus Clouds -  Low altitude gray colored cloud composed of water droplets that has a patchy appearance. Each cloud patch consists of a rounded mass. This cloud has a somewhat uniform base and normally covers the entire sky. Between the patches blue sky can be seen. Found in an altitude range from the surface to 3,000 meters.
Stratopause -  The stratopause is a relatively thin atmospheric transition layer found between the stratosphere and the mesosphere. The height of this layer is about 50 kilometers above the Earth's surface.
Stratosphere -  Atmospheric layer found at an average altitude of 11 to 50 kilometers 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 -  Volcano created from alternate layers of flows and exploded rock. Their height ranges from 100 to 3,500 meters tall. The chemistry of the magma of these volcanoes is quite variable ranging from basalt to granite. Also see composite volcano.
Stratus Clouds -  Low altitude gray colored cloud composed of water droplets. This cloud has a uniform base and normally covers the entire sky. It is also quite thick and can obscure the Sun. Light precipitation is often found falling from it. Found in an altitude range from the surface to 3,000 meters.
Stream -  A long narrow channel of water that flows as a function of gravity and elevation across the Earth's surface. Many streams empty into lakes, seas or oceans.
Stream Bank -  Sides of the stream channel.
Stream Bed -  Bottom of the stream channel.
Stream Channel -  Long trough-like depression that is normally occupied by the water in a stream.
Stream Discharge -  A river or stream's rate of flow over a particular period of time. 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 -  The flow of water in a river 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. In normally consists of three components: bed load (pebbles and sand which move along the stream bed without being permanently suspend in the flowing water), suspended load (silts and clays in suspension) and dissolved load (material in solution).
Stream Long Profile -  Vertical and horizontal profile of the stream. Most streams have a profile that is concave shaped.
Stream Order -  The relative position, or rank, of a stream channel segment in a drainage network.
Striations -  Grooves of scratches found in surface rock that are the result of glacial abrasion.
Strike -  One of the directional properties of a geologic structure such as a fold or a fault. Strike is the horizontal directional taken by an imaginary line drawn on the plane of the formation. Also see dip.
Strike-Slip Fault -  Fault that primarily displays horizontal displacement.
Structural Landform -  Is a landform created by massive Earth movements due to plate tectonics. This includes landforms with some of the following geomorphic features: fold mountains, rift valleys, and volcanoes.
Subduction -  Process of plate tectonics where one lithospheric plate is pushed below another into the asthenosphere.
Subduction Zone -  Linear area where tectonic subduction takes place.
Sublimation -  Process where ice changes into water vapor without first becoming liquid. This process requires approximately 680 calories of heat energy for each gram of water converted.
Submarine Canyon -  V-shaped canyons cut into the continental slope to a deep of up to 1200 meters. These features are normally associated with major rivers
Subpolar Glacier -  Glacier in which the ice found from the its surface to base has a temperature as cold as -30° Celsius throughout the year. This 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 atmospheric low 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.
Subsea Permafrost -  Form of permafrost that exists beneath the sea in ocean sediments.
Subsidence -  Lowering or sinking of the Earth's surface.
Subsolar Point -  The location on the Earth where the Sun is directly overhead. Also see declination.
Subtropical High Pressure Zone -  Surface zone of atmospheric high pressure located at about 30° North and South latitude. These high pressure systems 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. This jet stream flows from west to east and has a speed that is somewhat slower that the polar jet stream. Also see jet stream.
Succession -  Directional cumulative change in the types plant species that occupy a given area, through time.
Succulent Vegetation -  Group of plants that have the ability to survive in deserts and other dry climates by having no leaves. Instead their branches and stems that are photosynthetic. This adaptation reduces the surface area for evaporation thus reducing the loss of scarce water.
Sulfate Aerosol -  Type of solid compound commonly found in the atmosphere. These particles play an important role in reflecting, absorbing, and scattering incoming insolation. 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 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.
Sulfuric Acid -  Acid with the chemical formula H2SO4.
Summer -  Season between spring and fall. Astronomically it is the period from the summer solstice to the autumnal equinox in the Northern Hemisphere.
Summer Solstice -  The summer solstice denotes the first day of the summer season. For the Northern Hemisphere, the date of summer solstice is either on June 21 or 22 (changes yearly). December 21 or 22 is the date of the summer solstice for the Southern Hemisphere. During the summer solstice, locations in their respective hemispheres experience the longest day of the year.
Sun -  Luminous star around which the Earth and other planets revolve around. The Sun emits 63,000,000 Watts per square meter of electromagnetic radiation. The Sun has an average distance from the Earth of about 150,000,000 kilometers. The Earth's orbit is not circular but elliptical.
Sunrise -  Moment of time when the Sun's edge first appears above the Earth's horizon.
Sunset -  Moment of 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.
Super-Saturation -  Atmospheric condition where saturation occurs at a relative humidity greater than 100% because of a shortage of deposition or condensation nuclei.
Supercooled Water -  Cooling of water below 0° Celsius without freezing. Common in clouds where there is a deficiency of condensation nuclei.
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 -  Process where heat energy is transferred into land and ocean surfaces on the Earth. Much of this transfer takes place when solar radiation absorbed at the land or ocean surface is converted into heat energy. On land surfaces, surface heat is transfered down into the ground by conduction. Heat energy is transfered to greater depths in ocean surfaces because liquids have the ability mix by convection. Heat energy stored in ocean waters can also move quickly over large horizontal distances in a poleward direction through ocean currents.
Surface Tension -  Tension of a liquid's surface. Due to the forces of attraction between molecules.
Surface Wave -  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 waves on the ocean.
Surge -  A large, destructive ocean wave caused by very low atmospheric pressure and strong winds. Hurricanes often cause a surge of the ocean surface.
Suspended Load -  Portion of the stream load that is carried almost permanently suspended in flowing water.
Suspension -  Erosional movement of sediment continually 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.
Swash -  A 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 the area of its generation.
Symbiotic -  Mutual relationship between two organisms which is necessary for either to survive.
Symbiotic Mutualism -  Mutualistic interaction where the species interact physically and their relationship is biologically essential for survival.
Syncline -  A fold in rock layers that forms a trough-like bend.
synoptic chart -  A weather chart reflecting the state of the atmosphere over a large area at a given moment.
Synoptic Scale -  Scale of geographic coverage used on daily weather maps to describe large scale atmospheric phenomenon (for example, mid-latitude cyclone, air masses, fronts, and hurricanes).
synoptic weather map -  A weather chart reflecting the state of the atmosphere over a large area at a given moment.
System -  A system is a set of interrelated components working together towards some kind of process.
System Attribute -  A system attribute is the perceived characteristic of a system element. For example, number, size, color, volume, and temperature may be some of the perceived characteristics of clouds in the atmospheric system.
System Boundary -  Outer edge of system. Zone between one system and another system.
System Element -  System elements are the kinds of things or substances composing the system. They may be atoms or molecules, or larger bodies of matter-sand grains, rain drops, plants, or cows.
System Relationship -  Is the association that exist between the elements and attributes of a system based on cause and effect.
System State -  Current value of a system's elements, attributes, and/or relationships.