Taiga - See boreal forest.

Taku - Name for a katabatic type of cold wind that occurs in Alaska.

Talik - An unfrozen section of ground found above, below, or within a layer of discontinuous permafrost. These layers can also be found beneath water bodies in a layer of continuous permafrost. Several different types of talik have been distinguished: closed talik, open talik, and through talik.

Talus - An accumulation of angular rock debris from rockfalls.

Talus Slope - A slope that is composed of talus.

Tarn - A small mountain lake that occurs inside a cirque basin.

Taxon - A classification category for a group of organisms related to each other because of evolution.

Taxonomic Classification - A systematic classification of organisms based on structural and physiological connections between other species.

Tectonic Plate - An extensive layer of lithosphere that moves as a discrete unit on the surface of the Earth's asthenosphere.

Tectonic Uplift - The lifting of the terrestrial surface in elevation because of geologic processes associated with plate tectonics. For example, mountain formation occurs when two plates collide.

Tectonics - See plate tectonics.

Teleconnection - A causal linking of different atmospheric events and patterns that are geographically separate from each other by great distances. For example, the occurrence of El Niño is often associated with mild winters in the Pacific Northwest of North America, heavy winter rains in California and Oregon, droughts in Australia, New Zealand, Indonesia, and India, flooding in southern Brazil and northwest Peru, stronger than usual hurricane season in the eastern North Pacific, and wet and cool winters in the southeastern USA.

Telescope - A scientific instrument used to better image objects that are a great distance away from the viewer. The first telescopes invented used lenses and mirrors to view remote objects by collecting, magnifying, and focusing electromagnetic radiation in the visible light band. During the 20th century, scientists invented additional types of telescopes that could detect other forms of electromagnetic radiation, including gamma rays, X-rays, ultraviolet light, infrared light, radio waves, and microwaves.

Temperate Climate Zone - A region on our planet found roughly between 30 and 60° North and South latitude. This region is dominated by B, C, and D type climates according to the Köppen Climate Classification system.

Temperate Deciduous Forest - A type of forested biome found in the mid-latitudes that is dominated by deciduous trees and other vegetation. The area of temperate deciduous forests has been greatly reduced over the last 300 years. Most of this loss was caused by deforestation and the subsequent use of the land for crop production or human settlement.

Temperate Glacier - A glacier in which the ice found below 10 to 20 meters (33 to 66 feet) from its surface is at the pressure melting point. One of the three types of glaciers: cold glacier, temperate glacier, and subpolar glacier.

Temperate Rainforest - An ecosystem that is dominated by large and very tall evergreen species of trees. One location of a temperate rainforest is along the Pacific Northwest coast of North America, where annual precipitation is high, and temperatures are mild.

Temperature - Temperature is defined as the measure of the average speed of atoms and molecules. The higher the temperature, the faster they move.

Temperature Humidity Index - See discomfort index.

Temperature Inversion - A situation where a layer of warmer air exists above the Earth's surface in a typical atmosphere where air temperature decreases with altitude. In the warmer layer of air, temperature increases with altitude.

Tensiometer - A scientific instrument used to measure the quantity of water available in a soil. Tensiometers are commonly used by farmers to schedule irrigation times. 

Tephigram - Is a thermodynamic chart used for meteorological weather analysis and forecasting, usually with data obtained from a radiosonde. Variables plotted on a tephigram with altitude include air pressure, air temperature, humidity, and dew point temperature. Meteorologists use tephigrams to forecast moisture levels, cloud formation and cloud heights, atmospheric stability, maximum and minimum air temperatures, and temperature inversion layers. 

Tephra - The fragmented rock material ejected by a volcanic eruption. Also called pyroclastic material.

Tephrochronology - A geochronological technique that uses volcanic ash layers as temporal markers for relative dating in paleoenvironmental and archaeological studies. This technique relies on the fact that volcanic layers are chemically unique and are often found over extensive geographic areas.

Terminal Fall Velocity - Velocity at which a particle being transported, by wind or water, falls out of the moving medium. This velocity depends on the particle's size.

Terminal Moraine - A moraine that marks the maximum advance of a glacier.

Terminal Velocity - The maximum speed that can be achieved by a body falling through a fluid like water or air.

Terminus - The end or snout of a glacier.

Terrace - An elevated terrestrial area above the existing level of a floodplain or shoreline created by stream or ocean wave erosion.

Terrane - (1) The various dominant rock formations found in an area. Usually, an identified terrane shows no resemblance to the spatially adjacent rock formations along its boundary. (2) A group of related rock formations. 

Terrestrial Magnetism - (1) The fact that Earth has a magnetic field and magnetic properties. (2) A subfield of Geophysics that studies the Earth's magnetic field.

Territory (Animal) - A spatial region that an animal of a particular species protects from other species or other members of its species. Some animals use scent marking to establish the location of their territory. The purpose of a territory is related to defending resources required to maximize fitness.

Tertiary - A former term used to describe the geologic period roughly 2 to 65 million years ago. The tertiary has been replaced by the Paleogene (65.5 to 23.0 million years ago) and Neogene Periods (23.0 to 2.6 million years ago).  See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Tertiary Consumer - Organisms that occupy the fourth trophic level in the grazing food chain. These organisms are carnivores. Also known as a secondary carnivore.

Tethys Ocean - The tropical ocean basin that existed between the continental masses of Gondwana and Laurasia from 250 to 65 million years ago.

Tetrahedron - Silicon atom joined by four oxygen atoms (SiO₄). The atomic properties of this molecule lead to a unique pyramid-shaped three-dimensional crystal lattice.

Texture - The relative quantities of the different types and sizes of mineral particles in a deposit of sediment. Also see the related soil texture.

Thalweg - An imaginary line of the deepest water in a stream channel as seen from above, normally associated with the zone of greatest velocity in a stream.

Thaw Lake - A lake that forms from the melting of ground ice in areas where permafrost is common.  Thaw lakes are common in northern Canada and Siberia.

Thematic Map - A map that displays the geographical distribution of one phenomenon or the spatial associations that occur between several phenomena. Compare with a reference map.

Thematic Mapper - A remote sensing device found on Landsat satellites that scans images in seven spectral bands from visible light to infrared radiation. 

Theory - A logically proposed explanation for the causal mechanisms responsible for an observed phenomenon or a set of facts. Also see hypothesis.

Theory of Island Biogeography - A theory published in 1967 by R.H. MacArthur and E.O. Wilson that proposed a mathematical model to explain the colonization, establishment, and extinction of species on islands. This theory has been modified and applied to problems in Conservation Biology.

Thermal - A rising mass of warmer and less dense air in the lower atmosphere. Thermals commonly form by the localized solar heating of the ground surface. A form of atmospheric convection.

Thermal Circulation - A type of air circulation caused by the heating and cooling of air.

Thermal Conductivity - The ability of a substance to transfer heat energy through its mass by conduction. Some substances with high thermal conductivity include diamond, silver, copper, gold, aluminum, and iron. Conversely, some substances with low thermal conductivity include gases such as carbon dioxide, argon, water vapor, nitrogen, oxygen, snow, foam, and fiberglass.

Thermal Depression - A temporary region of low atmospheric pressure created by intense solar heating at the Earth's surface.

Thermal Efficiency Index - A relative measure of the effectiveness of temperature in producing growth in plants. Calculated by adding for all months of the year (T -  32)/4, where T is the mean monthly temperature in °F.  Thornthwaite first used the term in 1931. Abbreviated T-E index.

Thermal Equator - A narrow zone with the highest mean annual surface air temperatures that circles our planet near the equator. On a daily basis, the location of this zone migrates with changes in the Sun's inclination. Generally located at the same latitude as the Intertropical Convergence Zone. Also called the heat equator.

Thermal High - An area of high pressure in the atmosphere caused by the area having warmer temperatures relative to the air around it.

Thermal Infrared Radiation - A form of electromagnetic radiation with a wavelength between 3 and 14 micrometers (µm).

Thermal Low - An area of low pressure in the atmosphere caused by the area having cooler temperatures relative to the air around it.

Thermal Metamorphism - Is the metamorphic alteration of rock because of intense heat released from processes related to plate tectonics.

Thermal Pollution - A form of pollution that involves an increase in the normal temperature of water. Normally involves a sensitive aquatic ecosystem in a water body such as a stream, pond, lake, or ocean. One common source of the heat associated with this type of pollution is factories or power plants that use water as a coolant.

Thermal Wind - Is the wind shear caused by the horizontal temperature gradient between two levels in a geostrophic wind. 

Thermistor - Is a type of electrical resistor whose resistance to electricity varies measurably with temperature. As a result, thermistors are used for temperature sensors and self-regulating heating elements.

Thermocline - A thermally defined boundary in a body of water where the greatest vertical change in temperature occurs. This boundary is usually the transition zone between the mixed, warm-water layer (epilimnion) near the surface and the cold, deep-water layer (hypolimnion).

Thermocouple - A device widely used to measure temperature. Uses the electric voltage produced at the junction where different conductors or semiconductors meet to determine temperature. Metals are often used for a conductor. Voltage differences are calibrated to the measured temperature.

Thermodynamic Equilibrium - This type of equilibrium describes a condition in a system in which the distribution of mass and energy approaches a state of maximum entropy.

Thermodynamic Laws - Laws that describe the physical processes, relationships, and phenomena associated with heat.

Thermohaline Circulation - A large-scale system of ocean circulation that is powered by changes in seawater density because of surface heating and inputs of freshwater. This ocean circulation system operates in a closed loop that takes the current from the ocean’s surface to great depths.  Formation zones of the current occurs in two locations: the North Atlantic Ocean and the Southern Ocean. At these sites, high rates of evaporation causing the seawater to become more saline and dense. This dense seawater then begins to sink vertically like an underwater waterfall until the current reaches the ocean floor. At the ocean floor, the current beings to move in a horizontal direction to complete the loop.

Thermokarst - A type of landscape dominated by depressions, pits, and caves that is created by the thawing of ground ice in high latitude locations. It resembles a karst landscape but is not created by chemical weathering.

Thermoluminescence - A type of luminescence that occurs in some crystalline minerals when they are subjected to heat. The crystalline mineral must have previously absorbed energy from electromagnetic radiation or other ionizing radiation for the luminescence to occur. This property has been used to date objects as one can quantify the relationship between the amount of light emitted and the length of time the object has been absorbing radiation.

Thermometer - A device used to measure temperature. A variety of different devices have been invented to measure temperature by converting some physical change into a numerical value. Some of the methods employed in these devices include thermal expansion of substances, pressure changes in substances, and the measurement of electromagnetic radiation. 

Thermopile - A series of multiple thermocouples designed to convert heat energy into electrical energy. Often used to measure and monitor temperature.

Thermosphere - Atmospheric layer above the mesosphere (above 80 kilometers or 50 miles) characterized by air temperatures rising rapidly with height. The thermosphere is the hottest layer in the atmosphere. In the thermosphere, gamma rays, X-rays, and specific ultraviolet wavelengths are absorbed by certain atmospheric gases. The absorbed radiation is then converted into heat energy. Temperatures in this layer can exceed 1,200°C (2,190°F).

Thiessen Polygon - A mathematical procedure for partitioning the area surrounding some points located on a two-dimensional surface (plane) into polygons. As shown on the diagram, polygons are created by drawing a line (colored black) that is equidistant and perpendicular to another line (colored red) that connects adjacent points located on the plane.

Third Law of Thermodynamics - This law states that if all the thermal motion of atoms (kinetic energy) making up a substance could be removed, a state called absolute zero would occur.

Tholoid - A volcanic cone found inside a volcanic crater or a caldera volcano.

Thornthwaite Climate Classification - A method for classifying the climates of locations on the Earth developed by C.W. Thornthwaite in 1931 and revised in 1948. This method is different from the Köppen Climate Classification system in that it considers potential evapotranspiration as a variable in categorizing a location's climate.

Threatened Species - A term used to describe the possible extinction risk of a species. Normally, a threatened species is still plentiful in its natural range but is likely to become endangered due to declining population numbers. Compare with endangered species.  

Threshold - A term used in systems theory that describes a situation where exceeding a specific level of some phenomena or substance within a system causes a sudden or rapid non-linear change in the way the system works.

Threshold Velocity - Velocity required to cause entrainment in the erosional agents of wind, water, or ice. Threshold velocity is usually higher than the velocity required for transport because of factors like particle cohesion. Also see critical entrainment velocity.

Throughfall - Describes the process of precipitation passing through the plant canopy. This process is controlled by factors like plant leaf and stem density, precipitation type, precipitation intensity, and the duration of the precipitation event. The amount of precipitation passing through varies greatly with vegetation type.

Throughflow - The roughly downward horizontal flow of water through soil, sediment, or regolith. This type of subsurface water flow can have two outcomes: (1) it can become part of the groundwater, or (2) it can return to the surface and enter a stream, lake, or other water body. Throughflow is often considered a component of interflow.

Through Talik - Is a form of localized unfrozen ground (talik) in an area of permafrost. It is open to the ground surface and the area of unfrozen ground beneath it. However, permafrost is found along the sides of a through talik.

Thrust Fault - See reverse fault. 

Thunder - The loud sound produced when lightning causes the rapid expansion of the gases in the atmosphere along its path.

Thunderstorm - A storm that typically ranges in size from several kilometers (several miles) to over 50 kilometers (30 miles) in diameter, created by the quick lifting of moist warm air, which creates a cumulonimbus cloud. Thunderstorms can occur in isolation as a single-celled storm or as a multi-celled storm arranged in a cluster or as a line of adjacent cells (called a squall line) found along or ahead of a mid-latitude cyclone cold front. Thunderstorms can be accompanied by severe weather, including strong winds, hail, lightning, tornadoes, thunder, and heavy rain.

Tidal Current - A regional or local-scale ocean current that is created by the rise and fall of a tide on the ocean surface. Also see ebb current and flood current.

Tidal Cycle - The cyclic change in the height of sea level or the surface of other bodies of water caused by the gravitational attraction of the Moon and Sun with the Earth's surface. One tidal cycle takes approximately 24 hours and 52 minutes.

Tidal Flats - A flat area of clay, silt, and sand deposits devoid of vegetation found in an estuary that is repeatedly submerged and exposed to the atmosphere by varying tidal levels. This area is found adjacent to the tidal marsh at a lower elevation. Also called a mudflat.

Tidal Marsh - A type of wetland found in areas influenced by tidal seawater inundation. Most tidal marshes are restricted to middle and high latitudes. These marshes can vary in salinity because of the addition of freshwater from an associated stream. Plant species that dominate these ecosystems include sedges and grasses. Shown is a tidal marsh near the mouth of the Fraser River, British Columbia, Canada.

Tidal Period - The time it takes for one tidal cycle.

Tidal Prism - (1) Refers to the volume of seawater between mean high tide and mean low tide for an estuary, inlet, or other semi-enclosed area of ocean. (2) Volume of seawater leaving an estuary, inlet, or other semi-enclosed area of ocean as the result of an ebb tide.

Tidal Range - The vertical distance in sea level height between high tide and the coming low tide.

Tidal Zone - Area along the coastline that is influenced by the rise and fall of tides.

Tide - The cyclical rise and fall of the surface of the oceans. Tides are caused by the gravitational attraction of the Sun and Moon on the Earth. Very large lakes can also experience tides, with water levels varying by several centimeters (inches). Also see low tide and high tide.

Till - The sediment deposited directly by a glacier. The particles within this deposit are quite varied in terms of size and have not been size-sorted by the action of wind or water.

Till Fabric - Refers to the arrangement, orientation, and textural characteristics of particles found in the till. A quick analysis of particle orientation can approximate the direction the glacier ice traveled.

Till Plain - An extensive flat plain of till that forms when a sheet of ice becomes detached from the main body of the glacier and melts in place, depositing the sediments it once carried.

Tillite - A lithified sedimentary rock composed of an amalgamation of non-weathered rock and glacial till.

Timberline - See tree line.

Time - A measurable period in which the processes of cause and effect occur, resulting in the function of a system. With time, we can order events from past to present and predict future events.

Tipping Bucket Rain Gauge - This meteorological instrument consists of a funnel that directs falling rain into a small double-sided container that moves up and down like a seesaw. After a set amount of precipitation collects, the seesaw tips, emptying the collected water onto one side of the container and sending a signal to a remote device that makes a recording. This device allows one to accurately determine how much rain falls over time.

TIROS (Television and Infrared Observation Satellite) - Series of meteorological satellites launched by the United States starting in 1960. The primary purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting. The TIROS program was very successful, providing the first accurate weather forecasts based on data gathered from space. TIROS began continuous monitoring of the Earth's weather in 1962.

Tissue - A group of similar cells in a multicellular organism organized into a structure that performs a specific purpose for its survival.

Tolerance Model of Succession - This model of plant succession posits that changes in plant species dominance over time are driven by competition for resources. Later colonizing species can tolerate lower resource levels due to competition and can grow to maturity in the presence of early species, eventually outcompeting them.

Tolerance Range - The limits of tolerance a species has to an abiotic factor or condition in the environment.

Tombolo - A coastal feature that forms when a line of sand and/or gravel is deposited between an island and the mainland. This feature is above sea level for most of the time. 

Topographic Map - A map that displays topography and elevation through the use of contour lines. The base elevation on topographic maps is usually sea level.

Topographic Profile - A two-dimensional diagram that describes the landscape in a vertical cross-section.

Topography - (1) The vertical relief shown by a horizontal surface. (2) A subfield of Earth Science and Space Science that studies the shape of the surface of the Earth and other celestial objects.

Toposequence - A succession of different soil types that occurs along a topographic gradient. Differences between dissimilar soils are caused by their distinct topographic positions.  

Topset Bed - A horizontal deltaic deposit composed of coarse alluvial sediment. Represents the current or past surface of the delta.

Topsoil - The uppermost layer of a soil, usually the top 5 to 20 cm (2 to 8 inches).

Tornado - A vortex of rapidly moving air associated with some severe thunderstorms.  Within the tornado, funnel winds can exceed 500 kilometers per hour (310 miles per hour).

Tornado Alley - A popular term used to describe a region in North America that receives an extraordinarily high number of tornadoes. This term has not been explicitly defined in mainstream meteorology or climatology. One definition of this term suggests the region extends from north-central Texas northwards to South Dakota and the southern edge of Minnesota.

Tornado Warning - A warning issued to the public that an individual has observed a tornado in a specified region. This warning can also be issued if meteorological information indicates a high probability of a tornado developing in a specified region.

Tornado Watch - A weather forecast issued to the public that a tornado may occur in a specified region.

Torrent - A very fast-moving and chaotic flow of water or volcanic lava.

Total Column Ozone - A measurement of ozone concentration in the Earth's atmosphere, usually in Dobson Units (DU). Total column ozone measurements are used to monitor seasonal ozone holes that form over Antarctica and to a lesser extent over the Arctic. 

Total Dissolved Solids (TDS) - A measurement made to determine the quantity of dissolved inorganic and organic substances in a sample of water. Two techniques are used to measure total dissolved solids: gravimetry and conductivity. Gravimetry involves evaporating the water and then measuring the mass of the remaining solids. Conductivity measures the ionic content of a solution, typically by measuring electrical resistance.

Tower Karst - A spectacular karst landscape dominated by very tall column-shaped hills with rounded tops.

Toxin - A potentially poisonous substance created within living cells or organisms.

Tracer - A substance added by scientists to the moving water found in some component of the surface or subsurface hydrologic system and used to determine the direction or rate of water transport. 

Traction -  The erosional movement of particles by rolling, sliding, and shuffling along the eroded surface. Occurs in all erosional mechanisms involved in sediment transport (air, water, and ice).

Trade Winds - Surface winds that generally dominate air flow in the tropics. These winds blow from about 30° North latitude (Subtropical High Pressure Zone) to the equator (Intertropical Convergence Zone) and 30° South latitude (Subtropical High Pressure Zone) to the equator (Intertropical Convergence Zone). Trade winds in the Northern Hemisphere flow from northeast to southwest and are referred to as the Northeast Trade Winds. Southern Hemisphere trade winds flow from southeast to northwest and are called the Southeast Trade Winds.

Transform Fault - A massive strike-slip fault, continental in size. Examples of transform faults are found along tectonic plate boundaries and at the mid-oceanic ridge.

Transgression - Occurs when a rise in sea level causes the progressive inundation of land areas by seawater, resulting in the deposition of marine sediments over terrestrial deposits. Transgressions have a significant role in the Earth's geological history, altering sedimentary sequences and the paleoenvironment.

Translational Slide - A type of mass movement where material moves suddenly downslope along a flat plane. This type of mass movement may move material a considerable distance because of the lack of rotational movement at the slide's rupture surface. Compare with a rotational slide.

Transmission (Atmospheric) - The process of solar radiation passing through the atmosphere without being absorbed, reflected, or backscattered to space. The amount of transmission occurring at any place and/or time is primarily controlled by the presence of clouds and the density of particulate matter in the atmosphere. 

Transparency - The ability of a substance to transmit light through itself.

Transpiration - The process of water loss from plants through stomata. Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues. Some dry environment plants can open and close their stomata. Transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Of the transpired water passing through a plant, only about 1% is used in the growth process. Transpiration also transports nutrients from the soil into the roots and carries them to the various cells of the plant.

Transport - One of three distinct processes involved in erosion. It is the movement of eroded material in the medium of air, water, or ice. Also see entrainment and detachment.

Transverse Crevasse - A crevasse (large and deep fracture in the ice) on the brittle surface of a glacier that runs perpendicular to the direction of ice flow. Form when the glacier flows over a steeper elevational gradient.

Transverse Dune - A long and irregular sand dune that forms at right angles to the wind direction. These dunes form when there is an abundant supply of sand and relatively weak winds. These dunes have a single long slip-face. 

Transverse Mercator Map Projection - A widely used mapping projection system used in national and international mapping systems worldwide to depict areas with east-west extent no more than a few degrees. This map projection system is an adaptation of the Mercator Map Projection.

Transverse Rib - A stream bedform feature that consists of a ridge of gravel and pebbles that runs across the channel and perpendicular to water flow. Often, transverse ribs occur in a repetitive sequence and are evenly spaced. Found in high-energy stream systems with shallow water flow. 

Tree - A large woody plant with a trunk that extends some distance above the ground surface, supporting branches and leaves.

Tree Line - A transition zone between habitats that can support the growth of trees and an adjacent area where tree growth is not supported by the environment. Also called timber line or timberline.

Triassic - A geologic period that occurred roughly 208 to 245 million years ago. The Triassic begins after the Permian-Triassic Extinction Event and, as a result, starts with a depleted biosphere. During this period, the first dinosaurs appeared. One of three periods during the Mesozoic. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Tributary - A smaller branching stream channel that flows into the main stream channel. The opposite of distributary.

Trophic Level - A level of organization found in the grazing food chain.

Trophic Pyramid - A graphic model that describes the distribution of energy, biomass, or some other measurable quantity between the different trophic levels found in an ecosystem.

Tropic of Cancer - The latitude of 23.5° North. The northern limit of the Sun's declination.

Tropic of Capricorn - The latitude of 23.5° South. The southern limit of the Sun's declination.

Tropical Broadleaf Forest - A form of tropical rainforest where the trees have dark green broad leaves. These forests receive high rainfall year-round and do not shed their leaves. Compare with a tropical deciduous forest.

Tropical Climate Zone - A region on our planet found roughly between 0 and 30° North and South latitude. This region is dominated by A and B type climates according to the Köppen Climate Classification system.

Tropical Cyclone - Another name for a hurricane.

Tropical Deciduous Forest - A form of tropical rainforest where the trees are deciduous. These forests do experience seasonal droughts, and during this time, the trees shed their leaves to reduce water loss. Compare with a tropical broadleaf forest.

Tropical Depression - An organized group of thunderstorms often found over a tropical ocean that generates a cyclonic flow of between 37 and 63 kilometers per hour (23 to 38 miles per hour). This type of storm can develop into a hurricane.

Tropical Disturbance - An organized group of thunderstorms often found over a tropical ocean that generates a slightly cyclonic flow of less than 37 kilometers (23 miles) per hour. Can develop into a hurricane.

Tropical Rainforest - A forested biome found at and near the equator. Two major types of tropical forests are found in this biome: tropical broadleaf forest and tropical deciduous forest.

Tropical Savanna - See savanna.

Tropical Storm - An organized group of thunderstorms often found over a tropical ocean that generates a cyclonic flow of between 64 and 118 kilometers per hour (39 to 73 miles per hour). Often develops into a hurricane.

Tropopause - The tropopause is a relatively thin atmospheric transition layer found between the troposphere and the stratosphere. The height of this layer varies from 8 to 16 kilometers (5.0 to 10.0 miles) above the Earth's surface.

Troposphere - Layer in the atmosphere found from the surface to a height of between 8 and 16 kilometers (5.0 and 10.0 miles) of altitude [average height 11 kilometers (6.8 miles)]. The troposphere is thinnest at the poles and gradually thickens as one approaches the equator. This atmospheric layer accounts for about 80% of the atmosphere's total mass. It is also the layer where the majority of our planet's weather occurs. Maximum air temperature occurs near the Earth's surface in this layer. With increasing altitude, air temperature drops uniformly with increasing height at an average rate of 6.5°C per 1,000 meters (3.6°F per 1,000 feet) (commonly called the Environmental Lapse Rate), until an average temperature of -56.5°C (-70°F) is reached at the top of the troposphere.

Trough - An elongated area of low pressure in the atmosphere.

True Dip - The actual direction of inclination of a geologic feature based on dip and strike information. Compare with an apparent dip. 

True North - Direction to the North Pole from an observer on the Earth.

True South - Direction to the South Pole from an observer on the Earth.

Truncated Spur - An elevated ridge formed at the intersection of two valleys with different base elevations because of erosional truncation. In between, adjacent truncated spurs are a hanging valley and an associated waterfal.

Tsunami - A large ocean wave created from an earthquake or volcanic eruption. The wave height of a tsunami on the open ocean may be as high as 1 meter (3.3 feet). When entering shallow coastal waters, the land configuration can amplify wave heights to over 15 meters (49 feet).

Tufa - Deposits of limestone formed by the precipitation of carbonate minerals in water bodies. .

Tuff - A type of sedimentary or igneous rock composed of volcanic ash that has become compacted and individual particles cemented over time.

Tundra - An extensive high latitude biome dominated by a few species of dwarf shrubs, a few grasses, sedges, lichens, and mosses. Biological productivity is low in this biome because of the extremes of climate.

Tunnel Valley - A type of valley found in many regions of the world that were once covered by continental glaciers. These valleys form specifically from erosion caused by subglacial streams. These valleys can be quite long (up to 100 km or 60 mi), wide (up to 400 m or 1,300 ft), and deep (up to 400 m or 1,300 ft). Tunnel valleys are typically U-shaped.

Turbulence - A situation where the flow of a fluid (gas or liquid) exhibits chaotic flow characteristics.

Turbulent Flow - A movement of water within a stream that occurs as discrete eddies and vortices. Turbulent flow is caused by channel topography and friction.

Two-Tailed Statistical Test - Is an inferential statistical test where the values for the rejection of the null hypothesis are located on either side of the center of a probability distribution.

Type Species - A single species that is commonly used to describe a much larger group of related species that belong to a particular genus.

Typhoon - Another name used to describe a hurricane. This term is most often used to describe tropical storms occurring in the Indian Ocean and the western Pacific Ocean.