Glossary of Terms: D

Day Length

Period of time for a location on the Earth when insolation from the Sun is being received.

 

Daylight Savings 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 is the first Sunday in April (spring ahead) and the last Sunday in October (fall back).

 

Debris Flow

A type of mass movement where there is a downslope flow of a saturated mass of soil, sediment, and rock debris.

 

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 June solstice. This date also marks the first day of summer in the Southern Hemisphere.

 

Declination

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

 

Deciduous Vegetation

Type of vegetation that sheds its leaves during winter or dry seasons. Compare with coniferous vegetation.

 

Decomposition

(1) To chemically or physically breakdown a mass of matter into smaller parts or chemical elements.

(2) 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 and then coverting the less complex organic products into inorganic compounds and atoms. Much of the recycled inorganic nutrients are then consumed by producers. Bacteria and fungi are the most common decomposers found in most ecosystems. Also see detritus feeders.

 

Deduction

Inference in which the conclusion about particulars follows necessarily from general theory. In a science like Physical Geography, deductive reasoning would involve stating a theory first and then trying to find facts that reject this idea.

 

Deflation

Process where wind erosion creates blowout depressions or deflation hollows by removing and transporting sediment and soil.

 

Deflation Hollow

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

 

Deforestation

Removal of trees from a habitat dominated by forest.

 

Degradation

Readjustment of the stream profile where the stream channel is lowered by the erosion of the stream bed. Usually associated with high discharges.

 

Delta

Large deposit of alluvial sediment located at the mouth of a stream where it enters a body of standing water.

 

Dendritic

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

 

Denitrification

Conversion of nitrates into gaseous nitrogen and nitrous oxide.

 

Denudation

(1) The erosion or wearing down of a landmass.

(2) Removal of the vegetative cover from an area.

 

Density (of Matter)

Refers to the quantity of mass per unit volume. For gases, density involves the number of atoms and molecules per unit volume.

 

Deoxyribonucleic Acid (DNA)

Form of nucleic acid that is organized into a double-helix molecule. DNA is used by most organisms to chemically code their genetics 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

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.

 

Deposition

(1) The change in state of matter from gas to solid that occurs with cooling. Usually used in meteorology when discussing the formation of ice from water vapor. This process releases latent heat energy to the environment.

(2) Laying down of sediment transported by wind, water, or ice.

 

Depositional Landform

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

 

Deposition Nuclei

Six-sided microscopic particle that allows for deposition of water as ice crystals in the atmosphere. Nucleus for the formation of snowflakes. Deposition normally occurs on these particles when relative humidity becomes 100%.

 

Depression

(1) Concave hollow found on the Earth's surface.

(2) Term used to describe a cyclone or an atmospheric low pressure system.

 

Deranged Drainage

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

 

Desert

(1) Biome that has plants and animals adapted to survive severe drought conditions. In this habitat, evaporation exceeds precipitation and the average amount of precipitation is less than 25 centimeters a year.

(2) Area that receives low precipitation. Also see cold desert and warm desert.

 

Desertification

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

 

Desert Pavement

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

 

Detachment

One of three distinct processes involved in erosion. This process involves the disengagement of a particle from its surroundings.

 

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.

Detritus

Shed tissues, dead body parts, and waste products of organisms. In most ecosystems, detritus accumulates at the soil surface and other types of 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

Model describing the conversion of organic energy in a community or ecosystem into inorganic elements and compounds through decomposition. The organisms involved in this conversion are called detritivores.

 

Detritivore

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.

 

Deuterium

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

 

Devonian

Geologic period that occurred roughly 360 to 408 million years ago. During this period, the first amphibians and trees appear.

 

Dew

Condensation of water on the Earth's surface because of atmospheric cooling.

 

Dew Point

Dew point is the temperature at which water vapor saturates from an air mass into liquid or solid usually forming rain, snow, frost or dew. Dew point normally occurs when a mass of air has a relative humidity of 100%. If the dew point is below freezing, it is referred to as the frost point.

 

Diffused Solar Radiation

Solar radiation received by the Earth's atmosphere or surface that has been modified by atmospheric scattering.

 

Diffusion

(1) Molecular mixing of one substance into another substance.

(2) Redirection or refraction of solar insolation in many directions. Process cause the beam of traveling radiation to become less intense.

 

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.

 

Diploid

Cell that contains two sets of chromosomes. Also see haploid.

 

Direct Solar Radiation

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

 

Discharge

See stream discharge.

 

Discontinuous Permafrost

Form of permafrost that contains numerous scattered pockets of unfrozen ground.

 

Dispersal

An organism leaving its place or birth or activity for another location.

 

Dissociation

Chemical process where a compound or molecule breaks up into simpler constituents.

 

Dissolution

The process of a substance dissolving and dispersing into a liquid.

 

Dissolved Load

Portion of the stream load that is in solution in the flowing water.

 

Distance Ratio

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

 

Distributary

A smaller branching stream channel that flows away from a main stream channel. Common on deltas. Opposite of tributary.

 

Distributional Limit

Spatial boundary that defines the edge of a species geographical range.

 

Disturbance

(1) Partial or complete alteration of a community or an ecosystem by a biotic or abiotic factor.

(2) Cyclonic low pressure system.

 

Diurnal Tide

Tides that have 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.

 

Divide

The topographic ridge that separates drainage basins.

 

Doldrums

Area of low atmospheric pressure and calm westerly winds located at the equator. Similar to Intertropical Convergence Zone.

 

Dolomite

(1) Sedimentary rock formed from CaMg(CO₃)₂.

(2) Mineral with the chemical formula CaMg(CO₃)₂.

 

Downdraft

Downward movement of air in the atmosphere.

 

Downwelling Current

Ocean current that travels downward into the ocean because of the convergence of opposing horizontal currents or because of an accumulation of seawater.

 

Dune

(1) Stream bed deposit found streams whose channel is composed mainly of sand and silt. Dunes are about 10 or more centimeters in height and are spaced a meter or more apart and are common in streams with high velocities.

(2) 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.

 

Dust Dome

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

 

Drainage Basin

Land surface region drained by a length of stream channel.

 

Drainage Density

Is the measure of the length of stream channel per unit area of drainage basin. Mathematically its is expressed as:

Drainage Density (Dd) = Stream Length / Basin Area

 

Drainage Divide

Topographic border between adjacent drainage basins or watersheds.

 

Drainage Network

System of interconnected stream channels found in a drainage basin.

 

Drainage Pattern

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.

 

Drift

Any material deposited by a glacier.

 

Drought

Climatic condition where water loss due to evapotranspiration is greater than water inputs through precipitation.

 

Drumlin

A hill shaped deposit of till. The shape of these features resembles an elongated teaspoon laying 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 and heights can be as great as 200 meters.

 

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° Celsius per 1000 meters because of adiabatic cooling.

 

Dry-Bulb Thermometer

Thermometer on a psychrometer used to determine current air temperature. This measurement and the reading from a wet-bulb thermometer are then used for the determination of relative humidity or dew point from a psychrometric table.

 

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. 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.

 

Dyke

Thin vertical veins of igneous rock that form when magma enters and cools in fractures found within the crust. Also see intrusive igneous rock.

 

Dynamic Equilibrium

A dynamic equilibrium occurs when a system displays unrepeated average states through time.

 

Dynamic Metamorphism

Form of metamorphism that causes only the structural alteration of rock through pressure. The minerals in the altered rocks do not change chemically. The extreme pressures associated with mountain building can cause this type of metamorphism.

 

Dyne

A unit of force that creates an acceleration on a mass of 1 gram equal to 1 centimeter per second. 10⁵ dynes equals one newton.

 

 

Citation: Pidwirny, M. (2006). "Glossary of Terms: D". Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/physgeoglos/d.html