Dacite - An igneous rock of volcanic origin containing plagioclase feldspar, biotite, hornblende, quartz, and pyroxene minerals. This rock has intermediate composition between andesite and rhyolite.

Daily Maximum Temperature - The highest surface air temperature recorded at a meteorological station during the course of one day (midnight to midnight).

Daily Mean Temperature - The average surface air temperature as recorded at a meteorological station for a location for a particular day. Usually, it is calculated by adding the warmest (daily maximum temperature) and coldest (daily minimum temperature) temperatures recorded on a given day, then dividing the result by 2.  

Daily Minimum Temperature - The lowest surface air temperature recorded at a meteorological station during the course of one day (midnight to midnight).

Daily Temperature Range - The difference between the warmest (daily maximum temperature) and coldest (daily minimum temperature) temperatures recorded at a meteorological station during a particular day (midnight to midnight).

Dalmation Coast - A coastline that has a string of islands located parallel to the mainland with deep bays and steep coastal cliffs. These islands represent the tops of mountains in a mountain range that has subsided because of tectonic forces.

Dam - A natural or human-constructed barrier that impounds water behind it. The creation of artificial reservoirs by dams allows humans to use this water for irrigation and power generation. However, several environmental problems have been documented by the creation of dams, including changes in stream hydrology, seismic activity, sedimentation in the reservoir, biodiversity loss and migration, and changes to riparian habitats.

Dambo - Term used to describe shallow wetlands sometimes found at the headwaters of streams in central, southern, and eastern Africa. These wetland ecosystems are dominated by grasses, sedges, and rushes and are surrounded by vegetation that generally belongs to the tropical and subtropical grasslands, savannas, and shrublands biome.

Darcy (unit) - A non-SI unit used to measure the permeability of a porous solid substance (usually soil, sediment, or rock). 1 Darcy allows a flow of 1cm³ per second (1cm³/s) of a liquid with viscosity roughly equal to water at 20°C under a pressure gradient of 1 standard atmosphere per centimeter (1 atm/cm) acting across an area of 1 cm².

Darcy's Law - Is a hydrological equation that models the flow of a fluid through a porous medium. This law was first formulated by Henri Darcy in 1856, but has been subsequently modified. The law states:

Q = kiA

where Q is the flow rate, k is the hydraulic conductivity, i is the hydraulic gradient, and A is the flow cross-section area.

Darwinism - Is a theory of biological evolution first developed by Charles Darwin (1859) and subsequently modified by others, that suggests all species of organisms originate and develop through the process of natural selection, which acts on inherited adaptations that influence an individual's capacity to compete, survive, and reproduce. Also called Darwinian Theory.

Datum - (1) A single piece of data. (2) The standard from which measurements of altitude, elevation, or spatial location are calculated.

Day Length - The period of time for a location on the Earth when insolation from the Sun can be potentially received during a day.

Daylight Saving Time - The setting of time so it is one hour ahead starting in the spring and one hour back beginning in the fall in the Northern Hemisphere. In Canada and the United States, the dates for these events are the first Sunday in April (spring ahead) and the last Sunday in October (fall back).

Dead Glacier - A glacier that no longer has the property of internal ice flow. One possible outcome of a glacier in this situation is that it becomes a permanent ice field or snowfield. Also see passive glacier and active glacier.

Debacle - The annual breakup of ice on streams in the spring.

Debris Avalanche - A type of rapid mass movement that involves the disordered movement of water, ice, soil, sediment, and rock debris down a slope. Also called a landslide.

Debris Fan - Large fan-shaped terrestrial deposit of coarse sediment found in mountainous locations with steep slopes. Debris fans form when the stream load carried by a fast-moving stream is deposited because of a sudden reduction in the velocity of the stream flow.  Similar to an alluvial fan, but with deposits of much coarser texture.

Debris Flow - A type of mass movement where there is a rapid downslope flow of a saturated mass of soil, sediment, and rock debris down a slope.

Decalcification - The permanent removal or translocation of calcium ions from a soil by leaching.

December Solstice - Date during the year when the declination of the Sun is at 23.5° South of the equator. During the December Solstice, locations in the Northern Hemisphere experience their shortest day. The December Solstice is also the first day of winter in the Northern Hemisphere. Locations in the Southern Hemisphere have their longest day on the December Solstice. This date also marks the first day of summer in the Southern Hemisphere.

Deciduous Forest - A type of forest that sheds its leaves during winter or dry seasons. Compare this with a coniferous forest.

Deciduous Vegetation - Plants that drop their leaves during winter or dry seasons. 

Declination - Location (latitude) on the Earth where the position of the Sun on a particular day is directly overhead (90° from the horizon) at solar noon. This location lies between 23.5° North and 23.5° South, depending on the time of year.

Declination Diagram - A graphic found on a topographic map that shows the angular difference between Grid North, True North, and Magnetic North, or Grid South, True South, and Magnetic South.

Décollement - Is a geological term that describes the deformation of an upper layer of rock by way of folding and faulting, with the lower layer being unaffected by these processes. This usually occurs when the stress associated with deformation causes the upper layer to slide over the lower layer of rock. 

Decomposition - (1) To chemically or physically break down a mass of matter into smaller parts or chemical elements. (2) The breakdown of organic matter into smaller parts or inorganic constituents by decomposing organisms.

Decomposer - A type of detritivore. Decomposers play an important role in recycling organic matter back into inorganic nutrients in ecosystems. This recycling is done by decomposing complex organic matter, then converting the resulting simpler organic products into inorganic compounds and atoms. Most of the recycled inorganic nutrients are then consumed by producers. Bacteria and fungi are the most common decomposers found in many ecosystems. Also see detritus feeders.

Deduction - A form of logical speculation where the explanation of some cause and effect process involves first proposing a hypothesis, and then trying to find empirical facts that support or refute this idea. Compare this form of logic with induction.

Deep Ocean - Bottom most layer in an ocean or a sea with great depth. Normally found at a distance beyond 1800 meters (5900 feet) from the seawater surface. This layer receives no sunlight, and the many living organisms that live here mainly consume organic matter received from shallower depths. Also called deep sea.

Deep Ocean Water - See bottom water.

Deep Sea - See deep ocean.

Deep Weathering - A term that refers to the very active weathering that occurs in warm and moist tropical climates. Often, rock outcrop surfaces have a weathered layer up to 30 meters (100 feet) deep. This deep weathering occurs because the combination of warm temperatures, abundant moisture, and humic acid from vegetation maximizes the effectiveness of chemical weathering.

Deepening - Refers to a forecasted lowering in atmospheric pressure for a cyclonic low pressure system. Such a weather condition will often initiate higher wind speeds, an increase in cloud development, and an increase in precipitation associated with the low pressure system.

Deflation - A process where wind erosion creates blowout depressions or deflation hollows by removing and transporting sediment and soil.

Deflation Hollow - A ground surface depression or hollow commonly found in arid and semiarid regions caused by wind erosion. Also see the related blowout depressions.

Deflocculate - The physical or chemical dispersion of the mineral and organic particles making up soil aggregates. In natural conditions, deflocculation of soil aggregates can be induced by the addition of sodium.

Deforestation - The removal of many trees from a habitat dominated by forest. Over the last three centuries, a significant proportion of Earth's forest cover has been lost to this process, much of it converted into agricultural fields, grazing land for livestock, and urban land-use. Currently, deforestation is occurring at high rates in the tropical and subtropical regions of our planet. In some parts of the world, like the USA, forest cover is increasing. 

Deformation - (1) A change in the volume or shape of solid matter. (2) A change in the nature of a mass of rock because of applied force.

Deglaciation - The loss of glaciers or ice sheets from the Earth's surface because of melting and sublimation due to warmer temperatures, a decline in snowfall, or both.

Degradation - The readjustment of the stream's long profile where the stream channel is lowered by the erosion of the stream bed. Usually associated with an increase in stream discharge.

Degree - (1) An angular unit of measurement equal to 1/360 of the circumference of a circle. (2) A unit used to measure temperature using Celsius, Kelvin, or Fahrenheit scales.

Degree Day - A measurement that determines the difference between an actual temperature for a particular day relative to a base temperature (Actual Temperature - Base Temperature). Often, degree days are calculated over a period longer than one day.  Also see heating degree day, cooling degree day, and growing degree day.

Delta - A large deposit of alluvial sediment located at the mouth of a stream where it enters a body of standing water, like a lake or an ocean.

Demersal Zone - The part of the ocean  that is near and affected by the seabed. It sits just above the benthic zone. This zone can occur at various depths. Many commercial fisheries target demersal fish species. Also see (in order of depth in a water body) pelagic zone, photic zone, aphotic zone, mesopelagic zone, bathyal zone, abyssal zone, hadal zone, and benthic zone.

Demographer - Someone who studies the numerical and statistical characteristics of a human population (population dynamics), at the local, regional, or global scale.

Demography - The science that studies the numerical and statistical characteristics of a human population (population dynamics), at the local, regional, or global scale.

Demoiselle - A column of unconsolidated sediment with a boulder at its top, which protects it from erosion.

Dendritic - Term used to describe the stream channel pattern that is completely random. This pattern resembles the branching pattern of blood vessels or tree branches.

Dendrochronology - The science that analyzes the annual growth rings of some species of woody plants. This analysis investigates factors in the plant's surrounding environment that influence its growth. This technique has been successfully used to reconstruct historical patterns of change in climate variables such as temperature and precipitation.

Dendroecology - Is the scientific field of knowledge that applies tree-ring analysis to understanding phenomena in Ecology. 

Denitrification - The conversion of nitrates into gaseous nitrogen (N₂) and nitrous oxide (N₂O). 

Densitometer - (1) An instrument used to measure the density of a substance. (2) An instrument used to measure the optical density of photographic film.

Density (of Matter) - Refers to the quantity of mass per unit volume. For gases, density is the number of atoms or molecules per unit volume.

Density Current - A current of gas or liquid that is kept in motion because of gravity and the fact that the current has a greater density than the gas or liquid around it. Density contrasts between the current and the surrounding material can occur due to differences in temperature, salinity, or suspended sediment concentration. The thermohaline circulation, an ocean current system, is an example of a density current. In specific areas of the North Atlantic Ocean and Southern Ocean, high rates of evaporation cause seawater to cool in temperature and increase in salinity which in turn increases its density. This higher-density seawater then sinks, initiating deep-ocean currents. 

Density Dependent - Factors that vary in their ability to control the population size of organisms. These factors are most effective when population densities are high. Such factors include resource availability (food and shelter), biotic competition, predation, and pathogens. Compare with density independent factors.

Density Independent - Factors that can control the population size of organisms regardless of density. Such factors include weather, floods, fire, and asteroid impact. Compare with density dependent factors.

Denudation - (1) The erosion or wearing down of a landmass. (2) Removal of the vegetative cover from an area.

Denudation Chronology - The temporal reconstruction of how erosion has influenced a landscape's development and form. Such reconstructions use available geological and geomorphological evidence.

Denudation Rates - The rate of land surface reduction due to erosional processes. Usually expressed in millimeters per 1000 years.

Denudation Slope - A hill slope that experiences a net loss of material from its surface because weathering and erosion cause a net export of soil and sediment. This loss of material results in a progressive lowering of the slope's surface over time.

Deoxyribonucleic Acid (DNA) - A form of nucleic acid that is organized into a double-helix molecule. DNA is used by most organisms to chemically encode their genetic information and to direct the development and functioning of cells. This direction requires RNA, which represents a copy of a portion of DNA. Found in the nucleus of cells.

Dependent Variable - A variable in a statistical test whose observation's values are thought to be controlled through cause and effect by another independent variable modeled in the test.

Depleted Soil - A soil that has suffered significant nutrient loss because of poor cropping practices or excessive leaching.

Depletion Curve - A graph that describes the discharge of water within a drainage basin from the various components of storage (throughflow and groundwater flow) as measured at a particular location on a stream. This graph can be used to predict the gradual reduction in base flow over time.  Also called the base flow recession curve.

Deposit - (1) An accumulation of something within or on top of a larger mass of some substance. (2) A layer of sediment laid down after being transported by wind, water, or ice.

Deposition - (1) The process involved in depositing something. (2) The change in the state of matter from gas to solid that occurs with cooling. This term is typically used in meteorology to describe the formation of ice from water vapor. This process releases latent heat energy to the environment. (3) The laying down of sediment transported by wind, water, or ice.

Deposition Nuclei - Six-sided microscopic particle that allows for the deposition of water as ice crystals in the atmosphere. This is the required starting point for snowflake formation. Deposition normally occurs on these particles when the relative humidity becomes 100%.

Depositional Landform - Is a landform formed from the deposition of weathered and eroded surface materials. On occasion, these deposits can be compressed and altered by pressure, heat, and chemical processes into sedimentary rocks. This includes landforms with some of the following geomorphic features: beaches, deltas, floodplains, and glacial moraines.

Depression - (1) A concave hollow found on the Earth's surface. (2) A term used to describe a cyclone or an atmospheric low pressure system.

Depression Storage - This refers to the capture of water in depressions found on the ground surface during a precipitation event. When enough water accumulates and the depressions overflow, overland flow begins. Over time, the water in the hollows will eventually infiltrate into the ground and evaporate back to the atmosphere. 

Depth Hoar - An accumulation of large (up to 10 mm), faceted cup-shaped ice crystals at the base of a snowfield or snowpack. Depth hoar forms when rising water vapor deposits around existing ice crystals. Depth hoar crystals form in isolation from each other, creating a layer that is prone to sliding, allowing accumulated snow above it to slide. This process can produce a snow avalanche. Also called sugar snow.

Deranged Drainage - A landscape drainage pattern that is highly irregular. Areas that have experienced continental glaciation may have this type of drainage pattern.

Desalinization (or Desalination) - Is the conversion of seawater or any other saline brine into freshwater. Several processes are now used to remove salts and minerals from saline water. The freshwater produced from this process is often used for human consumption, irrigation, and other agricultural uses.

Desert - (1) A biome that has plants and animals adapted to survive severe drought conditions. In this habitat, evaporation exceeds precipitation, and the average annual precipitation is less than 25 centimeters (10 inches). (2) An area that receives low precipitation and has relatively high evapotranspiration. 

Desert Pavement - A veneer of coarse particles left on the ground after the erosion of finer particles by wind.

Desert Varnish - Is a thin, hard, sometimes glassy coating found on rock surfaces in arid environments that are not exposed to much precipitation or wind. This very thin coating can be colored yellow, orange, red, brown, or black. Black colored desert varnish contains a high concentration of the metal manganese. The process that creates this coating involves thin clay accumulations on the rock chemically reacting with other deposited substances in the presence of moisture and high temperatures. Also called rock varnish, rock rust, and desert patina.

Desertification - Conversion of marginal rangeland or cropland to a more desert-like type of land. Desertification can be caused by overgrazing, soil erosion, prolonged drought, or climate change.

Desiccation - (1) A state of extreme dryness. (2) The process of drying causes something to lose its water. (3) A climatic trend resulting in less precipitation and/or greater evaporation for a particular location or region.  

Desiccation Crack - Fracture in the surface of fine-textured soils subject to intense solar heating.

Desiccation Polygons - Polygon-shaped fractures in the surface of fine-textured soils subject to intense solar heating.

Desilication - (1) The removal of silica from a layer in a soil profile because of leaching. Common soil processes in locations with high precipitation. (2) The removal of silica from minerals and rocks because of chemical weathering.  

Desquammation - See exfoliation.

Destructive Wave - Are deep ocean waves with a short wavelength that wash up on the shoreline. Because destructive waves are tall, they produce a very strong downward force on the beach. The large amount of energy generated by the downward force actively erodes beach deposits and produces coastal cliffs. It also creates a strong backwash and a narrow beach profile. Destructive waves have ahigh frequency, producing only about 13 to 15 waves per minute. Compare with a constructive wave.

Detachment - One of the three distinct processes involved in erosion. This process involves a particle disengaging from its surroundings. Also see entrainment and transport.

Detrital Rock - Sedimentary rock that is composed of particles transported to their place of deposition by erosional processes. Examples of such rock include sandstone and shale.

Detritivore - A heterotrophic organism that feeds on detritus. Examples of such organisms include earthworms, termites, slugs, snails, bacteria, and fungi. Two types of detritivores are generally recognized: decomposers and detritus feeders.

Detritus - The shed tissues, dead body parts, and waste products of organisms. In most ecosystems, detritus accumulates at the soil surface and in other surface sediments.

Detritus Feeder - A type of detritivore. Detritus feeders acquire the nutrients they need from partially decomposed organic matter found in shed animal tissues, plant litter, dead bodies of plants and animals, and animal waste products. Some examples of detritus feeders include various species of beetles, various species of ants, earthworms, and termites. Also see decomposer.

Detritus Food Chain - A model describing the conversion of organic matter in a community or ecosystem by organisms into inorganic elements and compounds through decomposition. The organisms involved in this conversion are called detritivores.

Deuterium - Isotope of hydrogen, with a nucleus containing one proton and one neutron, and an atomic mass number of 2.

Devonian - One of six geologic periods that occurred from 416 to 359 million years ago during the Paleozoic Era.  During this time, terrestrial arthropods become common, primitive sharks are numerous in the oceans, forests cover a large proportion of the land surface, and the first amphibians appear. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Dew - Condensation of water on the surface of an object because of atmospheric cooling. Occurs when a mass of moist air is cooled to its dew point.

Dew Point - Is the temperature at which water vapor saturates from an air mass into liquid, forming rain or dew. Dew point normally occurs when a mass of air has a relative humidity of 100%, and temperatures are above 0°C. If the dew point is below freezing, it is referred to as the frost point.

Diabase - See dolerite.

Diachronous - This is a bed or stratum of sedimentary rock that can be dated to two or more points in geologic time.

Diaclinal Stream - A stream that flows in a direction that is perpendicular (90°) to the strike of below-ground bedrock strata.

Diagenesis - Refers to the changes that have occurred to a sediment after it has been deposited. This can include processes like cementation and compaction.

Diapir - A type of rock intrusion where the movement of mobile salt deposits or gas-rich muds can cause the overlying rocks to fold upward, producing an anticline in the strata. Diapirs can cause the ground surface to the dome.

Diastrophism - The tectonic alteration of the shape of the Earth's surface through faulting, folding, and orogenies.

Diatom - A specific type of unicellular algae that lives in freshwater or seawater. Diatoms differ from other algae because their cell walls are composed of hydrated silicon dioxide. There are about 200 genera and 100,000 species of diatoms. Diatoms are useful organisms for science because they can be used to monitor and reconstruct past environmental conditions. 

Diatomaceous Earth - A naturally occurring, crumbly, and soft sedimentary deposit made of the fossilized remains of diatoms. Diatomaceous earth has many uses by humans, including a filtration medium, a polish, an abrasive insecticide, a filler in plastics and rubber, a component in dynamite, and a thermal insulator.  

Diatom Ooze - A deep ocean deposit composed of at least 30% siliceous diatoms. This unconsolidated soft ocean floor deposit accumulates in abyssal zones where cold nutrient-rich waters occur, and seawater is saturated with dissolved silica. Covers about 15% of the ocean floor. Also called siliceous ooze. Compare with calcareous ooze.

Diatreme - A general term describing volcanic pipes and vents that cut through overlying layers of sedimentary rock.

Dieback - A term describing a significant decline in health or death of a single plant or many plants due to physical damage, pathogens, or the toxic effects of pollutants. An example of dieback is the reduced vigor and mortality of forests observed in Germany and the northeast United States in the 1980s. Multiple factors were implicated in this dieback, including acid precipitation, drought, parasites, and pathogens.

Differential Ablation - An uneven pattern of ablation on the top of a glacier caused by spatial variations of surface albedo and solar radiation absorption.

Differential Erosion - An uneven pattern of erosion on some spatial scale caused by differences in the factors responsible for erosion.

Differential Weathering - An uneven pattern of weathering caused by spatial differences in the factors responsible for weathering.

Diffluence - (1) A flow of air that moves away from the center becomes more spread out with distance traveled (see image). Compare with a confluence. (2) A term that refers to a valley glacier overflowing its topographic enclosure into neighboring valleys.

Diffused Solar Radiation - Solar radiation received by the Earth's atmosphere or surface that has been modified by atmospheric scattering. Also called diffused radiation.

Diffusion - (1) Molecular mixing of one substance into another substance. (2) Redirection or refraction of solar insolation in many directions. This process causes the beam of traveling radiation to become less intense.

Digital Elevation Model (DEM) - A three-dimensional representation of the surface of a planet or some other celestial body. Digital elevation models can be constructed from measurements made during a ground surface or from some type of remote sensing instrument. Elevation measurements can be represented in a DEM in raster or vector form. DEMs are often used in geographic information systems as a base layer. 

Digital Image Processing - The use of computer techniques and algorithms to simplify, modify, and enhance image information numerically at the pixel level. Sometimes image data involved in this procedure is captured in an analog format and pre-processed to get it into a numerical value for pixel-level mathematical processing. Digital image processing is commonly used in remote sensing, robotics, and medical imaging.

Digitizing - The process of transforming analog information into digital form so it can be used in a computer.

Dilation - (1) The enlargement or widening of some object. (2) The enlargement or widening of fissures in rock because of the freezing of water into ice. (3) The enlargement or widening of an opening in rock because of the igneous intrusion of magma.

Dilution Effect - Describes the reduction in the concentration of dissolved substances in a stream's flow after a storm runoff event. This is caused by the dilution effect of water added from precipitation.

Dilution Gauging - A technique used in measuring stream discharge where a set amount of tracer (like sodium chloride salt) is added to the stream's flow at some point. At some location downstream, a measurement is taken to determine the amount of dilution that has taken place, and from this value, stream discharge is approximated mathematically.

Dimethyl Sulfide (DMS) - An organosulfur compound produced through organic decay of dimethylsulfoniopropionate. Large amounts of this substance are released into the atmosphere from marine phytoplankton. In the atmosphere, dimethyl sulfide is oxidized to produce sulfur dioxide, dimethyl sulfoxide, dimethyl sulfone, methanesulfonic acid, and sulfuric acid. Sulfuric acid is important in creating condensation nuclei, which in turn influences cloud formation and precipitation. Dimethyl sulfide has the chemical formula (CH₃)₂S.

Dinosaur - A diverse group of vertebrate animals that first appeared on Earth about 230 million years ago during the Triassic period. More than a thousand different species of dinosaurs have been identified and named from the fossil record. These organisms died out at the end of the Cretaceous period about 65.5 million years ago. This mass extinction is thought to have been caused by the impact of an asteroid on Earth's surface. This is known as the Cretaceous–Paleogene extinction event.

Diorite - A coarse-grained igneous rock of intrusive origin that is darker and chemically more mafic than granite.

Dip - One of the directional properties of a geologic structure such as a fold or a fault. Dip is the inclination angle of the formation as measured at right angles to strike. The concept of dip is illustrated in the diagram where N, E, S, and W cardinal directions, the blue plane is an ideal horizontal plane, green plane is an ideal vertical plane, 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 strike.

Dip Fault - A geologic fault that has a strike that is perpendicular to the strike of surrounding rock strata. Dip faults are parallel to the true dip of the rock strata.

Dip Slope - A land surface that has a slope that is in the same direction as the dip of underlying bedrock strata.

Dip Stream - A stream that is oriented in the same direction as the dip of the underlying strata in the landscape.

Diphotic Zone - Depth below the surface of a water body where sunlight becomes too dim to support photosynthesis.

Diploid - A cell that contains two sets of chromosomes. Also see haploid.

Direct Runoff - The water from a precipitation or snowmelt event that flows over the ground surface directly into a stream or other water body. On a stream hydrograph, direct runoff begins when stream discharge rises after a precipitation event and continues until the flow declines back to being mainly composed of base flow contributions. Also called quickflow.

Direct Solar Radiation - Solar radiation received by the Earth's atmosphere or surface, which has not been modified by atmospheric scattering.

Dirt Band - A zone of discolored ice in a glacier because of the presence of sediment.

Dirt Cone - A mound of sediment on the surface of a glacier that has a cone shape. Dirt cones can reach several meters in height and contain an ice core. They form when a patch of sediment protects the ice beneath it from melting. The ice under the patch is protected from heating while the surrounding ice melts. Over time, the differential melting causes the dirt cone to form. 

Disappearing Stream - A stream that flows underground for some portion of its course. Common in karst landscapes. 

Disaster - A natural or human-made occurrence that causes significant damage and loss of life, human and other. Events caused by natural forces are called natural disasters.

Discharge - See stream discharge.

Discomfort Index - This calculation combines the effects of temperature and humidity to measure the degree of discomfort an individual would experience in warm weather. In the United States, depending on data availability, three different equations are commonly used to determine the discomfort index:

Discomfort Index = 0.4 x (Td + Tw) + 15
Discomfort Index = (0.55 x Td) + (0.2 x Tdp) + 17.5
Discomfort Index = Td - (0.55 - 0.55RH) x (Td - 58)

where Td is the dry-bulb temperature in °F, Tw is the wet-bulb temperature in °F, Tdp is the dew point temperature in °F, and RH is relative humidity (%) expressed as a decimal. Approximately 50% of the population will be uncomfortable when the discomfort index reaches 75, and almost everyone will be uncomfortable when the discomfort index reaches 79. Also called the temperature-humidity index (THI). In Canada, this type of measurement involves a different calculation and is called the humidex.

Disconformity - Is a type of unconformity found in strata lying between horizontal layers of sedimentary rocks. Disconformities are identified by the presence of surface erosional features. 

Discontinuity - (1) A change with depth beneath the ground surface in the physical characteristics of the interior of the Earth as determined by seismic information. (2) An abrupt change in meteorological measurements of air temperature, atmospheric pressure, precipitation, humidity, wind speed and direction suggesting a frontal boundary between different air masses.

Discontinuous Permafrost - A form of permafrost that contains numerous scattered patches of unfrozen ground.

Discordance - A situation where adjacent rock strata in a bed are not aligned with each other.

Discordant - (1) Something that is not ordered as it should be. (2) An igneous intrusion that has penetrated the layers of country rock. (3) A landscape with topographic features that do not reflect the underlying structural geology.

Discordant Coast - A situation where the shape of a coastline is quite different from the geologic structure and local topography of the land adjacent to it. Compare with a concordant coast.

Discordant Drainage -  A pattern of stream drainage that is not being influenced by the underlying geologic structure of the landscape. Compare with accordant drainage.

Discordant Junction - The joining of a tributary to a main stream channel that is not at the same elevation and as a result produces a waterfall.

Disintegration - The process where weathering and erosion cause the breakdown of rock into smaller fragments.

Disjunct Distribution - A pattern of geographical distribution where two or more populations of a particular species or genus are very isolated from each other because of distance or some other landscape feature. 

Dislocation - The spatial displacement of once-connected rock formations because of tectonic movement on either side of a fault.

Dispersal - The act of an organism leaving its place of birth or usual activity for another location for a permanent establishment.

Dispersion - The pattern of things distributed spatially.

Dispersion Diagram - A graph used to display the numerical distribution of a set of data.

Dissection - The modification of a previously flat landscape into one with many incised valleys with steep slopes because of stream erosion.

Dissipative Beach - A type of beach that is characterized by being relatively wide and having a gentle profile gradient. This particular morphology results from the prevalence of constructive waves. On these beaches, breaking waves normally lose much of their energy before reaching the beach face. Also, these beaches typically have an abundant supply of medium- to fine-grained sand. Compare with a reflective beach.

Dissociation - A chemical process where a compound or molecule breaks up into simpler constituents.

Dissolution - The process of a substance dissolving and dispersing within a liquid.

Dissolved Load  - The portion of the stream load that is in solution in the flowing water. Also see bed load.

Dissolved Oxygen (DO) - A measurement made to determine the quantity of dissolved oxygen in water. The quantity of dissolved oxygen in aquatic ecosystems is extremely important to the survival of fish and other aquatic life. Oxygen can be removed from water through plant and animal cellular respiration. The standard unit of measurement is milligrams per liter (mg/l) or parts per million (ppm). Dissolved oxygen levels above 5 milligrams per liter are considered ideal. Many fish cannot survive for long at levels less than 3 milligrams per liter. Water becomes hypoxic when DO is below 1 milligram per liter.

Dissolved Solids - See total dissolved solids.

Distance Ratio - Method for measuring the gradient of a slope. Simply involves dividing the vertical change in distance (rise) by the horizontal change in distance (run) or rise/run. The measurement is usually presented as a percentage or relative to a unit distance traveled horizontally.

Distributary - A smaller branching stream channel that flows away from a main stream channel. These features are common on deltas. Opposite of tributary.

Distributional Limit - The spatial boundary that defines the edge of a species geographical range.

Disturbance - (1) The partial or complete alteration of a community or an ecosystem by a biotic or abiotic factor. (2) A cyclonic low pressure system. (3) An external event that causes a temporary change in the usual cause-and-effect processes associated with a system.

Diurnal Range - The daily range between maximum and minimum values of some meteorological variables, such as surface air temperature, relative humidity, or atmospheric pressure.

Diurnal Tide - A tide that has one high and one low water per tidal period.

Divergence - Horizontal outflow of wind from an area. In a surface divergence, outflow originates from the upper atmosphere.

Divergent Evolution - Creation of two or more unique species from one ancestral species through the differential evolution of isolated populations.

Diversity - See species diversity.

Diversivore - A species that has a varied diet that varies from plants to animals.

Divide - The topographic ridge that separates drainage basins.

Dobson Unit (DU) - A unit used to measure the density of ozone in the Earth's atmosphere. One Dobson Unit equals a layer of pure ozone gas 10 microns (µm) thick at standard temperature and pressure. 

Doldrums - An area of low atmospheric pressure and calm prevailing winds located at the equator. Similar to the Intertropical Convergence Zone.

Dolerite - A medium-grained, dark colored, mafic igneous rock containing the minerals plagioclase feldspar, pyroxene, and olivine. This rock is often found in dykes and sills. Also called diabase.

Doline - A funnel-shaped surface depression in an area with limestone bedrock. This landscape feature forms because of solution weathering and the underground collapse of sedimentary layers.

Dolocrete - A magnesium carbonate-rich type of calcrete soil layer. Often forms near the water table as a precipitate from brackish groundwater.

Dolomite - (1) Sedimentary rock formed from CaMg(CO₃)₂. (2) Mineral with the chemical formula CaMg(CO₃)₂.

Domain - The highest level of organization in the taxonomic system currently employed to classify a species. Currently, scientists recognize three domains: Bacteria, Archaea, and Eukaryota.

Dome - (1) A topographically rounded mountain or hilltop. (2) A roughly symmetrical anticline fold that dips equally in all directions. (3) A rounded accumulation of snow on the top of a mountain. (4) A topographically rounded hill composed of granite. (5) A topographically rounded igneous intrusion. (6) A natural below-ground dome-shaped hollow containing salt, oil, or natural gas.

Dome Dune - A mound of sand that is circular or elliptical in shape and has no slip-faces. This type of sand dune is formed by the modification of a stationary barchan dune.

Domestication - The modification of genetic characteristics of plants and animals to produce strains that are better suited and more productive for human use and consumption. Often, domesticated organisms no longer have the adaptations required for successful survival in the wild.

Dominant Organism - A species that is extremely important to the abiotic and/or biotic functioning of a habitat, community, or ecosystem.

Dominant Wind -  The wind direction that produces the highest wind speeds for a location or region over a specific period. Compare with the prevailing wind.

Doppler Radar - A specialized type of radar used in weather forecasting to image areas where precipitation is falling. Precipitation patterns in thunderstorm clouds can help identify where a storm may generate tornadoes.

Dormant Volcano - A volcano that is not extinct and should show activity sometime in the future.

Downcutting - The lowering of a stream's base-level in the landscape because of erosion at its stream bed.

Downdraft - The downward movement of a mass of air in the broader atmosphere. Compare with updraft.

Downthrow - The downward displacement of rock strata along one side of a vertical geological fault.

Downwasting - The reduction in the thickness of a continental glacier because of increased ablation in a warming climate.

Downwelling - The accumulation and sinking of seawater at some location in an ocean.

Downwelling Current - An ocean current that travels downward into the ocean because of the convergence of opposing horizontal currents or because of an accumulation of seawater.

Draa - An area of aeolian deposits, including various types of sand dunes, that is smaller than a sand sea. 

Drain - A human-constructed channel for the unrestricted flow of water from an area. 

Drainage - (1) Can refer to a natural flow of surface water via overland flow and stream flow from an area. (2) The downward movement of water received at the ground surface through a soil because of percolation.

Drainage Basin - The land surface region drained by a length of stream channel. Also called the catchment area. In Canada and the USA, the term watershed is synonymous with drainage basin.

Drainage Density - Is the measure of the length of the stream channel per unit area of the drainage basin. Mathematically, it is expressed as: 

Drainage Density (Dd) = Stream Length / Basin Area

Drainage Divide - The topographic border between adjacent drainage basins. 

Drainage Network - A system of interconnected stream channels found in a drainage basin.

Drainage Pattern - The geometric pattern that a stream's channels take in the landscape. These patterns are controlled by factors such as slope, climate, vegetation, and bedrock resistance to erosion.

Drainage Wind - A wind common to mountainous regions that involves heavy cold air flowing along the ground from high to low elevations because of gravity. Also see katabatic wind.

Drawdown - The degree to which removing groundwater from a well lowers the underground elevation of the water table.

Drift - (1) Any solid material deposited by a glacier. (2) The movement of continents across the Earth's surface is due to tectonic forces. (3) The frictional movement of near-surface ocean seawater because of large-scale surface wind patterns.

Drift Ice - Are pieces of broken sea ice that have freely moved away from the ice-shelf where they formed. 

Drift Potential - A vector measurement of the wind's ability to move sand in an aeolian environment over some period, often a year. These measurements are expressed proportionally on a sand rose over 360° of compass direction.

Dripstone - Deposits, like stalactites and stalagmites, that form from a salt-rich solution on the roof, walls, and floor of a cave. 

Drizzle - A form of precipitation where the water droplets are smaller than rain. Drizzle has a diameter between 0.2 and 0.5 mm (0.008 to 0.02 in). Often falls from stratus and stratocumulus clouds.

Dropstone - A rock fragment once encased in floating ice that becomes free and falls into soft sediments. 

Drought - A climatic condition where water loss due to evapotranspiration is greater than water inputs through precipitation.

Drowned Valley - A valley that has become submerged by seawater because of a rise in sea level.

Drumlin - A hill-shaped deposit of till. The shape of these features resembles an elongated teaspoon lying bowl down. The tapered end of the drumlin points to the direction of glacier advance. Drumlins come in assorted sizes. Lengths can range from 100 to 5,000 meters (330 to 16,400 feet), and heights can be as great as 200 meters (650 feet). 

Druse - A coating of mineral crystals on a rock fracture surface. 

Dry Adiabatic Lapse Rate (DALR) - The rate of decline in the temperature of a rising parcel of air before it has reached saturation. This rate of temperature decline is 9.8°C per 1,000 meters (5.4°F per 1,000 feet) because of adiabatic cooling (see graphic). Compare with the saturated adiabatic lapse rate.

Dry Deposition - The transport of gases and minute liquid and solid particles from the atmosphere to the ground surface without the aid of precipitation or fog. This commonly happens for the nutrients nitrogen, sulfur, chloride, calcium, and sodium. Compare with wet deposition. 

Dry Line - A boundary that separates dry and moist air in the warm sector of a mid-latitude cyclone wave. Found ahead of the cold front. 

Dry Snow - A powdery type of snow that forms in very cold atmospheric conditions. Individual dry snow crystals do not stick together. Dry snow has a density of less than 0.1 kilograms per cubic meter.

Dry Spell - A prolonged period without precipitation. In the United States, it is defined as 14 consecutive days with precipitation, while in Britain, it is defined as 15 consecutive days with no more than 1 mm (0.04 inches) of precipitation on any one day.

Dry Valley - A valley that rarely has a stream channel with water flowing in it. There are many explanations as to why these features occur.

Dry-Bulb Thermometer - One of two thermometers on a sling psychrometer used to measure current air temperature. This measurement and the reading from a wet-bulb thermometer are then used to determine relative humidity or dew point from a psychrometric table.

Dumpy Level - An optically based surveying instrument used to confirm and establish points along the same horizontal plane (levelling).

Dune - (1) Stream bed alluvial deposit found in streams whose channel is composed mainly of sand and silt. Dunes are about 10 or more centimeters (4 or more inches) in height and are spaced a meter or more (3 or more feet) apart and are common in streams with high velocities. Also see bedforms. (2) A terrestrial deposit of sand that resembles a mound or ridge that was formed from aeolian processes. Also see sand dune.

Dune Field - An extensive region covered by numerous sand dunes.

Duricrust - A hard crust that forms at or near the ground surface because of chemical processes associated with soil formation and rock weathering. Common in desert and tropical regions of our planet. Types of duricrusts include alcrete, calcrete, ferricrete, and silcrete. 

Duripan - A hardpan soil layer created when soil particles are cemented together by silica deposited by illuviation.

Dust - Very small particles or organic and inorganic matter that can remain suspended in the atmosphere for some time. Humans have increased atmospheric dust levels through activities such as livestock grazing, industrial emissions, vehicle use, and soil plowing.

Dust Bowl - A period during the 1930s when wind erosion from major dust storms caused significant loss of topsoil in the United States and Canadian prairies. This environmental problem was due to the combined effects of prolonged drought and poor agricultural soil management. Also called the Dirty Thirties.

Dust Devil - A vortex of rapidly moving air and loose soil created by a strong localized convection current in the atmosphere. Dust devils range in size from 1 to 10 meters  (3 to 30 feet) wide to more than 1000 meters (3000 feet) in height. 

Dust Dome - A dome of air that surrounds a city, created from the urban heat island effect that traps air pollutants like particulate matter. 

Dust Storm - A wind storm that carries a considerable load of loose soil because of high wind speeds. Dust storms are often associated with thunderstorm gust fronts that occur in arid and semi-arid regions with limited vegetation to protect the topsoil from wind erosion.

Dust Veil Index (DVI) - A numerical estimate of the impact that volcanic dust and ash can have on short term cooling of the Earth's climate. This index was developed by climatologist Hubert Lamb in 1970. The eruption of Krakatoa in 1883 had a dust veil index of 1000.

Dwarf Planet - (1) According to the International Astronomical Union (IAU), this is a celestial body orbiting the Sun that is massive enough to be spherical because of its gravity. However, this celestial body has not cleared its orbital path through space of objects. Pluto is considered a dwarf planet. (2) A similar body orbiting another star.

Dyke - Thin vertical veins of igneous rock that form when magma enters and cools in fissures and faults found within the crust. Also see intrusive igneous rock. Compare with batholith, sill, laccolith, lopolith, and phacolith.

Dyke Spring - A spring of water that is associated with a dyke that has produced a below-ground channel that moves groundwater to the surface.

Dyke Swarm - An extensive area of numerous dykes that have all formed because of the same single igneous intrusion.

Dynamic Equilibrium - A dynamic equilibrium occurs when the measured state of a system shows many time-averaged values that repeat.

Dynamic Metamorphism - A form of metamorphism that causes only the structural alteration of rock through pressure. The minerals in the altered rocks do not undergo chemical changes. The extreme pressures associated with mountain building can cause this type of metamorphism.

Dyne - A unit of force acting on a mass of 1 gram, creating a rate of acceleration equal to 1 centimeter per second for every second the force is applied in the same direction. 100,000 dynes equals one newton.

Dystrophic Lake - A lake with high concentrations of humic acid and substances derived from organic matter decomposition in its waters. The water of this type of lake is tea-colored, nutrient-poor, has a low pH, and low plant productivity.