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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. A number of 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.
Tectonic Plate -  An extensive layer of lithosphere that moves as a discrete unit on the surface of the Earth's asthenosphere.
Temperate Deciduous Forest -  Forested biome found in the mid-latitudes and dominated by deciduous vegetation.
Temperate Glacier -  Glacier in which the ice found below 10 to 20 meters from its surface is at the pressure melting point. One of the three types of glaciers: cold glacier; temperate glacier; and subpolar glacier.
Temperate Rain Forest -  An ecosystem that is dominated by large and very tall evergreen trees. This biome occurs 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 Inversion -  Situation where a layer of warmer air exists above the Earth's surface in a normal atmosphere where air temperature decreases with altitude. In the warmer layer of air, temperature increases with altitude.
Tephra -  Fragmented rock material ejected by a volcanic explosion. Also called pyroclastic material.
Terminal Fall Velocity -  Velocity at which a particle being transported by wind or water falls out of the moving medium. This velocity is dependent on the size of the particle.
Terminal Moraine -  Moraine that marks the maximum advance of a glacier.
Terminal Velocity -  Maximum speed that can be achieve by a body falling through a fluid like water or air.
Terminus -  End or snout of a glacier.
Terrace -  An elevated surface above the existing level of a floodplain or shore that is created by stream or ocean wave erosion.
Tertiary -  Geologic period that occurred roughly 1.6 to 65 million years ago. During this period, mammals become a dominant species on the planet.
Tertiary Consumer -  Organisms that occupy the fourth trophic level in the grazing food chain. These organisms are carnivores. Also known as a secondary carnivore.
Tetrahedron -  Silicon atom joined by four oxygen atoms (SiO4). The atomic properties of this molecule cause it to develop a unique three dimensional crystal lattice that is pyramid shaped.
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 -  Line of deepest water in a stream channel as seen from above. Normally associated with the zone of greatest velocity in the stream.
Thematic Map -  Map that displays the geographical distribution of one phenomenon or the spatial associations that occur between a few phenomena. Compare with reference map.
Thematic Mapper -  Remote sensing device found on Landsat satellites that scans images in seven spectral bands from visible to thermal infrared.
Theory -  Proposed explanation for the causal mechanisms responsible for a phenomenon or a set of facts. Also see hypothesis.
Thermal Circulation -  Atmospheric circulation caused by the heating and cooling of air.
Thermal Equator -  Continuous area on the globe that has the highest surface temperatures because of the presence of the Intertropical Convergence Zone.
Thermal High -  Area of high pressure in the atmosphere caused by the area having warmer temperatures relative to the air around it.
Thermal Infrared Radiation -  Form of electromagnetic radiation with a wavelength between 3 to 14 micrometers (µm).
Thermal Low -  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.
Thermocline -  Boundary in a body of water where the greatest vertical change in temperature occurs. This boundary is usually the transition zone between the layer of warm water near the surface that is mixed and the cold deep water layer.
Thermodynamic Equilibrium -  This type of equilibrium describes a condition in a system where the distribution of mass and energy moves towards maximum entropy.
Thermodynamic Laws -  Laws that describe the physical processes, relationships, and phenomena associated with heat.
Thermokarst -  Landscape dominated by depressions, pits, and caves that is created by the thawing of ground ice in high latitude locations. Resembles karst landscape but is not created by chemical weathering.
Thermometer -  Device used to measure temperature.
Thermosphere -  Atmospheric layer above the mesosphere (above 80 kilometers) characterized by air temperatures rising rapidly with height. The thermosphere is the hottest layer in the atmosphere. In the thermosphere, gamma, X-ray, and specific wavelengths of ultraviolet radiation are absorbed by certain gases in the atmosphere. The absorbed radiation is then converted into heat energy. Temperatures in this layer can be greater than 1200° Celsius.
Third Law of Thermodynamics -  This law states if all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would result and all energy would be randomly distributed.
Threatened Species -  Species that is still plentiful in its natural range but is likely to become endangered because of declining population numbers.
Threshold -  The level of magnitude of a system process at which sudden or rapid change occurs.
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 factors like particle cohesion. Also see critical entrainment velocity.
Through Talik -  Is a form of localized unfrozen ground (talik) in an area of permafrost. It is open to the ground surface and to an area of unfrozen ground beneath it. Permafrost encases it along the sides.
Throughfall -  Describes the process of precipitation passing through the plant canopy. This process is controlled by factors like: plant leaf and stem density, type of the precipitation, intensity of the precipitation and duration of the precipitation event. The amount of precipitation passing through varies greatly with vegetation type.
Throughflow -  The roughly horizontal flow of water through soil or regolith.
Thrust Fault -  A geologic fault where the hanging wall is forced over the foot wall.
Thunder -  Sound created when lightning causes the rapid expansion of atmospheric gases along its strike path.
Thunderstorm -  A storm several kilometers in diameter created by the rapid lifting of moist warm air which creates a cumulonimbus cloud. Thunderstorms can have the following severe weather associated with them: strong winds; hail; lightning; tornadoes; thunder; and heavy rain.
Tidal Current -  Regional scale ocean current that is created the tidal rise and fall of the ocean surface.
Tidal Period -  Time it takes for one tidal cycle.
Tidal Zone -  Area along the coastline that is influence by the rise and fall of tides.
Tide -  Cyclical rise and fall of the surface of the oceans. Caused by the gravitational attraction of the Sun and moon on the Earth.
Till -  Heterogeneous sediment deposited directly by a glacier. The particles within this deposit have not been size sorted by the action of wind or water.
Till Plain -  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 carried.
Time -  Measurable period in which cause and effect occurs and systems function.
TIROS (Television and Infrared Observation Satellite) -  Series of meteorological satellites launched by the United States starting in 1960. The main purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting. 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.
Tombolo -  A coastal feature that forms when a belt 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 -  Map that displays topography through the use of elevation contour lines. Base elevation on these maps is usually sea-level.
Topographic Profile -  A two-dimensional diagram that describes the landscape in vertical cross-section
Topography -  The relief exhibited by a surface.
Topset Bed -  Horizontal deltaic deposit composed of coarse alluvial sediment. Represents current or past surface of the delta.
Tornado -  A vortex of rapidly moving air associated with some severe thunderstorms. Winds within the tornado funnel may exceed 500 kilometers per hour.
Tornado Alley -  Region in North America which receives a extraordinary high number of tornadoes. This region stretches from central Texas to Illinois and Indiana.
Tornado Warning -  A warning issued to the public that a tornado has been observed by an individual in a specified region. This warning can also be issued if meteorological information indicates a high probability that a tornado will develop in a specified region.
Tornado Watch -  A forecast issued to the public that a tornado may occur in a specified region.
Total Column Ozone -  A measurement of ozone concentration in the atmosphere.
Traction -  Erosional movement of particles by rolling, sliding and shuffling along the eroded surface. Occurs in all erosional mediums (air, water, and ice).
Trade Winds -  Surface winds that generally dominate air flow in the tropics. These winds blow from about 30° North and South latitude (subtropical high pressure zone) to the equator (intertropical convergence zone). Trade winds in the Northern Hemisphere have northeast to southwest direction and are referred to as the Northeast Trades. Southern Hemisphere trade winds have southeast to northwest direction but are called the Southeast Trades.
Transform Fault -  Massive strike-slip fault continental in size. Examples of such faults occur along tectonic plate boundaries and at the mid-oceanic ridge.
Transparency -  The ability of a medium to allow light to pass through it.
Transpiration -  Transpiration is 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 do have the ability to open and close their stomata. Transpiration is a passive process largely controlled by the humidity of the atmospheric and the moisture content of the soil. Of the transpired water passing through a plant only 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.
Triassic -  Geologic period that occurred roughly 208 to 245 million years ago. During this period, the first dinosaurs appeared.
Tributary -  A smaller branching stream channel that flows into a main stream channel. Opposite of distributary.
Trophic Level -  Level of organization in the grazing food chain.
Trophic Pyramid -  A graphic model describing the distribution of energy, biomass, or some other measurable quantity between the different trophic levels found in an ecosystem.
Tropic of Cancer -  Latitude of 23.5° North. Northern limit of the Sun's declination.
Tropic of Capricorn -  Latitude of 23.5° South. Southern limit of the Sun's declination.
Tropical Cyclone -  An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour. Another name for hurricane.
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. Can develop into a hurricane.
Tropical Disturbance -  An organized group of thunderstorms often found over a tropical ocean that generates a slight cyclonic flow of less than 37 kilometers per hour. Can develop into a hurricane.
Tropical Rainforest -  Forested biome found near the equator and dominated by evergreen vegetation.
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. 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 above the Earth's surface.
Troposphere -  Layer in the atmosphere found from the surface to a height of between 8 to 16 kilometers of altitude (average height 11 kilometers). The troposphere is thinnest at poles and gradually increases in thickness as one approaches the equator. This atmospheric layer contains about 80% of the total mass of the atmosphere. 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° Celsius per 1000 meters (commonly called the Environmental Lapse Rate), until an average temperature of -56.5° Celsius is reached at the top of the troposphere.
Trough -  An elongated area of low pressure in the atmosphere.
True North -  Direction of the North Pole from an observer on the Earth.
True South -  Direction of the South Pole from an observer on the Earth.
Tsunami -  Large ocean wave created from an earthquake or volcanic eruption. Open ocean wave height may be as high as 1 meter. When entering shallow coastal waters, land configuration can amplify waves to heights of over 15 meters.
Tundra -  High latitude biome dominated by a few species of dwarf shrubs, a few grasses, sedges, lichens, and mosses. Productivity is low in this biome because of the extremes of climate.
Turbulent Flow -  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 which one can reject the null hypothesis are located either side of the center of the probability distribution.
Typhoon -  An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour. Another name for hurricane.