R Horizon - Soil horizon found beneath the C horizon. Consists of consolidated rock showing little sign of weathering or pedogenesis.
r- and K-Selection - A theory in ecology introduced by ecologists Robert MacArthur and E.O. Wilson in 1967 that suggests the stability of habitats influences the evolution of general traits found in organisms. In unstable habitats, where disturbance is common, successful species generally have the following traits: Small body size, short lifespan, quick maturation, reproductive effort is delayed until the end of the organism's life, production of many small offspring, and a large portion of the organism's production of biomass is used to produce offspring. Species in frequently disturbed habitats have little time for survival and growth. Organisms that show these characteristics are called r-selected species. On the other end of the spectrum are stable or predictable habitats. In these habitats, optimal conditions for survival and growth are present almost all the time. Dominant species in this environment tend to have the following characteristics: Large body size, long lifespan, slow maturation, reproductive effort occurs over most of the organism's life, only a few relatively large offspring are produced with each reproduction event, and most of the organism's production of biomass goes to promoting its own growth. Species that show these characteristics are known as K-selected species.
r-Selected Species (Malthusian Strategy) - A species that shows the following characteristics: short lifespan, early reproduction, low biomass, and the potential to produce large numbers of usually small offspring in a short period of time. Compare with K-selected species. Also see r- and K-Selection.
Radarsat - A satellite program established by the Canadian Space Agency for the purpose of remotely sensing the Earth's resources. Radarsat uses an active remote sensing system that transmits microwaves. For more information, see the Radarsat-2 webpage at:
https://www.asc-csa.gc.ca/eng/satellites/radarsat2/.
Radiant Energy - Energy in the form of electromagnetic radiation. Some uses of this term specifically refer to the radiation emitted from the Sun.
Radiation - The emission of energy from an object in the form of electromagnetic radiation.
Radiation Fog - A type of fog that is also called ground fog. Radiation fog is generated by near-surface cooling of the ground by longwave radiation loss during the evening hours. For the fog to develop, the overnight cooling must cause saturation to occur. This type of fog is usually quite shallow and close to the ground surface.
Radiative Cooling - Process where an object cools in temperature and experiences a reduction in internal heat energy because of the net loss of electromagnetic radiation. This net loss of radiation occurs because outgoing radiation exceeds incoming radiation for the object.
Radiative Forcing - A change in the amount of electromagnetic radiation available for use and/or transfer in some component of the Earth's climate system. This change can subsequently modify weather and climate patterns on our planet through variables like temperature, precipitation, wind, humidity, atmospheric pressure, etc.
Radiative Heating - Process where an object warms in temperature and experiences an increase in internal heat energy because of a net gain of electromagnetic radiation. This net gain of radiation occurs because incoming radiation exceeds outgoing radiation for the object.
Radio Waves - A form of electromagnetic radiation with a wavelength between 1.0 millimeters (0.039 inches) and 100 kilometers (62 miles).
Radioactive - An object that is releasing ionizing radiation or particles because it is experiencing radioactive decay.
Radioactive Decay - Natural decay of the nucleus of an atom where alpha or beta particles and/or gamma radiation are released at a fixed rate.
Radioactive Waste - Waste that contains a substance releasing ionizing radiation or particles because it is experiencing radioactive decay.
Radiocarbon Dating - A dating method used to determine the age of something containing organic carbon through the natural decay of the carbon-14 isotope. When a plant or animal is alive, it incorporates both carbon-14 (14C) and carbon-12 (12C) into its biomass at ratios that mirror those in the living biosphere. This reserve is assumed to be fixed and maintained by the creation of 14C by cosmic radiation. However, when the organism dies, it no longer adds 14C to its biomass. Instead, this isotope begins to decay, while the quantity of 12C remains fixed. Because this decay has a constant rate, we can use the measured ratio of 14C to 12C to determine how long has elapsed. This technique is used to date objects no older than 50,000 years.
Radioisotope or Radioactive Isotope - An unstable isotope of an element. This material decays spontaneously, releasing subatomic particles and electromagnetic radiation.
Radiometer - A general name for an instrument used to measure electromagnetic radiation over a specific wavelength range.
Radiosonde - Is a battery-powered meteorological device attached to a weather balloon to remotely measure weather variables with altitude. Upper air meteorological measurements are an important input for numerical weather forecasting models. Variables measured by this device include altitude, geographic location, air temperature, atmospheric pressure, wind speed, wind direction, and relative humidity. Data from these measurements are transmitted to ground receivers using a radio frequency of 403 or 1680 MHz. More than 800 radiosonde launch sites exist worldwide. Radiosondes are launched at these sites every 12 hours at about 45 minutes before 0:00 and 12:00 Coordinated Universal Time (UTC).
Radon Gas - A colorless, odorless, naturally occurring radioactive gas of the chemical element radon (radon-222). This gas is produced from the radioactive decay of uranium or thorium. Uranium or thorium are the two most abundant radioactive elements on our planet. Radon gas has been found to accumulate in some houses because it seeps in from the ground.
Rain - A form of precipitation. Rain is a liquid deposit that falls from clouds in the atmosphere to the ground surface. Rain has a diameter between 0.5 and 5.0 millimeters.
Rain Factor - A measure used to determine the aridity of a location or area. Calculated by the following formula:
Rainforest - A rainforest is a dense forest ecosystem with high biodiversity, characterized by high rainfall, lush vegetation, and a closed canopy that blocks much of the sunlight from reaching the forest floor. Two types of rainforest are commonly recognized: Tropical Rainforest and Temperate Rainforest.
Rain Gauge - An instrument that measures the amount of rain that falls at a location over a period of time. See standard rain gauge and tipping bucket rain gauge.
Rainbow - A common optical phenomenon observed in the atmosphere that usually resembles a curve consisting of the various colors of the spectrum of light. Rainbows are created by the reflection and refraction of sunlight by water droplets. Rainbows are always found in the opposite direction to the Sun.
Raindrop Impact - A force produced by a falling raindrop on a rock, sediment, or soil surface.
Rainfall - The occurrence of rain falling to the Earth's surface.
Rainfall Intensity - Is the rate at which rain is being received at the ground surface. Meteorologists often use a tipping bucket rain gauge to measure this quantity.
Rainshadow Effect - The reduction of precipitation commonly found on the leeward side of a mountain. This reduction in precipitation is the result of compression warming of descending air.
Rainsplash - Soil erosion caused by the impact of raindrops.
Rainwash - The erosion of soil by overland flow (runoff). Typically occurs together with rainsplash, which plays an important role in dislodging soil particles for transport.
Raised Beach - A former beach that is now located some distance inland from the shoreline. Raised beaches can result from a fall in sea level because of tectonic uplift, isostatic rebound, or climate change. Also called the marine terrace and the perched coastline.
Randkluft - A gap that exists between the ice of a glacier and the bedrock it rests against or flows over. Randkluft often forms at the back of smaller cirque-type glaciers where warmer rock melts the ice next to it. These gaps can be many meters (feet) deep.
Random - A process or event that occurs by chance.
Range - A statistical measure of the dispersion of observation values in a data set. The range is determined by the difference between the largest and smallest observed values.
Rangeland - A land-use type that supplies vegetation for consumption by grazing and browsing animals. This land-use type is normally not intensively managed.
Ranker - A shallow, immature soil with limited surface organic matter accumulation that has developed on non-calcareous bedrock.
Rapids - A section of stream channel where steep topography creates a sudden increase in flow velocity and turbulence. Rapids often have rocks protruding above the water surface.
Rating Curve - (1) The line on a graph that describes the cause and effect relationship that exists between stream discharge and water level (stage) in a stream. (2) The line on a graph that describes the cause-and-effect relationship that exists between stream discharge, suspended sediment, and solute transport.
Rational Formula - A formula used to estimate run-off in cubic meters of peak stream discharge (Q) from measured precipitation intensity - mm/hr (I), drainage basin area (A), and a coefficient that varies with the nature of the surface of the basin area (C).
Q = CAI
Raunkiaer's Life Forms - A system that classifies plants according to the location of their growth bud. This system was developed by Christen C. Raunkiaer.
Rayleigh Scattering - One of three types of atmospheric scattering. In this type, the particles responsible for the scattering are smaller than the wavelengths of the radiation striking them. Particles responsible for Rayleigh scattering are primarily gas molecules. This type of scattering occurs in the atmosphere at an altitude of about 10 km (6.2 mi). Rayleigh scattering does not affect the various wavelengths of solar radiation uniformly and is most effective at ultraviolet and shorter wavelengths in the visible spectrum. Rayleigh scattering is responsible for causing the sky to appear blue. Also see mie scattering and non-selective scattering.
Reach - Often describes a stretch of stream channel between two points along its course. May also refer to a place where a stream channel widens significantly.
Realized Niche - Describes the part of the fundamental niche that a species actually occupies. It includes the effects of biotic interactions such as interspecific competition and predation on a species’ geographic distribution.
Recessional Moraine - A moraine that is created during a glacial retreat pause. Also called a stadial moraine.
Recharge - The process of refilling a store of water found beneath the ground surface. This term usually refers to an aquifer, but can also describe groundwater or water below the water table.
Recharge Area - The area on the Earth's surface that receives water for below-ground storage into a particular aquifer.
Rectangular Coordinate System - A system that measures the location of points on the Earth on a two-dimensional coordinate plane. See the Universal Transverse Mercator (UTM) Grid System.
Recumbent Fold - A fold in rock layers where the axial plane (fold center) is almost horizontal. Compare with anticline, syncline, monocline, and overturned fold.
Recurrence Interval - The average time period that separates natural events of a specific magnitude. For example, floods at a specific stream discharge level occur on average every X number of years.
Red Beds - A generic term used to describe sedimentary rocks (sandstones, siltstones, and shales) that are red in color because of the presence of ferric oxides coating sediment grains.
Red List - Is an inventory produced by the International Union for Conservation of Nature (IUCN) that categorizes the extinction risk of over 60,000 evaluated plant and animal species. The Red List has nine evaluation categories: extinct, extinct in the wild, critically endangered, endangered, vulnerable, near threatened, least concerned, data deficient, and not evaluated. This inventory can be found at the following website: https://www.iucnredlist.org.
Redox Potential (Reduction Potential) - A chemical measure of the gain (reduction) or loss (oxidation) of electrons in a chemical reaction. Instruments that measure redox potential do so in volts (V) or millivolts (mV). A measure of 0.00 volts/millivolts indicates a balance is occurring between chemical reduction and oxidation. Positive redox values indicate that the substance is gaining oxygen, while negative values indicate oxygen is being lost. Redox potential is mainly used to determine if oxidizing or reducing conditions are common in water or soil. This measurement can also be used to gauge water quality, with oxidizing conditions usually indicating good water quality.
Reduction - (1) A chemical process that involves the removal of oxygen from a compound. (2) A form of chemical weathering.
Reductionist - The study and understanding of a phenomenon isolated from other connected levels of cause and effect.
Reef - A ridge of rocks found in the tidal zone along a coastline. One common type of reef is the coral reef.
Reef Limestone - A type of limestone sedimentary rock that forms because of the lithification of mainly coral reefs.
Re-Entrants - A prominent surface indentation in an escarpment, ridge, or shoreline.
Reference Map - A map that shows natural and human-made objects from the geographical environment with an emphasis on location. Compare with the thematic map.
Reflected Infrared Radiation - A form of electromagnetic radiation with a wavelength between 0.7 and 3.0 micrometers (µm).
Reflected Wave - A water wave that reflects off the shore or another obstacle and is redirected back towards the sea or lake.
Reflection - The process of returning sound or light waves back to their source.
Reflection (Atmospheric) - A process where insolation is redirected by 180° after striking a particle. This redirection causes 100% loss. Most of the reflection in the Earth's atmosphere occurs in clouds, where light is intercepted by particles of liquid and frozen water. The reflectivity of a cloud can range from 40 to 90%.
Reflective Beach - A type of beach that is characterized by being relatively narrow and having a steep profile gradient. This particular morphology is the result of the prevalence of destructive waves. Opposite of a dissipative beach. Also called a swell beach.
Reforestation - The reestablishment of forest cover by natural means or by humans planting seeds or young trees. Compare with afforestation.
Refraction - The process by which the direction of travel of insolation (solar radiation) is redirected after entering another medium.
Refugia (singular Refugium) - A term in Biology and Ecology that describes habitats that contain an isolated population of a relic species. These relic organisms were once more widespread and abundant in the past. Then, some factor reduced the geographic range and abundance of this species. Some mechanisms that could cause a species to become restricted to refugia include climate change, human activities like hunting or deforestation, and competition or predation by other species.
Reg - A rocky desert landscape. See desert pavement.
Regime - In the academic fields of Climatology and Hydrology, this term describes the average variation of a variable (system attribute) of some system over a specified time period. For example, in a stream, we may be interested in the typical fluctuations (regime) in stream discharge over a calendar year. During this period, the stream is characterized by low discharge in winter, high discharge in spring when snowmelt occurs, and moderate discharge in summer and fall due to generally constant rainfall during these seasons.
Regime Theory - The engineering science associated with building stable channel shapes for human-made irrigation ditches, canals, and stream channels. This design is done to efficiently transport water and sediment while reducing erosion.
Region - A term used in geography that describes an area of the Earth where some natural or human-made phenomena display similar traits.
Regional Metamorphism - A large-scale metamorphic modification of existing rock through the heat and pressure of plutons created at tectonic zones of subduction.
Regolith - A loose layer of rocky material overlying bedrock.
Regosol Soil - Soil order (type) of the Canadian System of Soil Classification. A regosol is generally any young, underdeveloped soil that lacks identifying soil horizons. For more information on this soil type, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - https://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/index.html.
Regulating Services - One of four benefits humans receive from ecosystems, as described in the United Nations’ Millennium Ecosystem Assessment. This service suggests that humans can benefit from ecosystems because they regulate other natural phenomena important to human well-being. For example, the role wetlands play in limiting floods, the fact that forests and other vegetation modify and moderate climate patterns, and the function birds have in controlling the population size of pest insects. Compare with cultural services, provisioning services, and supporting services.
Rejuvenation - A term used in Geomorphology that describes the cyclical renewal of some landform or geomorphic process to a youthful stage because of factors like tectonic uplift, volcanism, mountain building, etc.
Relative Age - The age of a geologic event, deposit, rock, or fossil measured relative to some other phenomenon (younger or older). Relative age does not determine how old something is in terms of time. Compare with absolute age.
Relative Humidity - The ratio between the actual amount of water vapor held in the atmosphere compared to the amount required for saturation. Relative humidity is influenced by temperature and atmospheric pressure.
Relaxation Time - The time required by some system to re-equilibrate itself after a change in energy input.
Relief - The range of topographic elevation within a specified region of our planet or some other celestial body with a solid surface.
Remanié - A glacier that receives the input component of its mass balance from snow and ice avalanches and not from a connected snowfield.
Remote Sensing - The gathering of information from an object or surface without direct contact. The methods used for remote sensing have changed over time. Before 1940, this was commonly done using balloons and aircraft, with information captured in photographs. After 1960, the use of satellites for remote sensing became more common, and the information was collected from these platforms using a variety of non-photographic instruments. An excellent example of satellite-based remote sensing is NASA's Landsat program.
Remote Sensor - A mechanical device used to remotely sense an object or phenomenon.
Rendzina - A shallow, poorly developed soil that forms from rocks like limestone, dolomite, chalk, and gypsum that easily decompose by weathering. These soils have a rich humus layer and a grayish-brown color.
Renewable Resource - Is a substance or thing that is consumed as a resource and that can replenish itself under the right conditions.
Repetti Discontinuity - A boundary located in the Earth's mantle at a depth of 900 to 1000 kilometers below the surface that causes seismic wave velocities to rise in speed quickly. Also called Birch Discontinuity.
Representative Concentration Pathways (RCP) - Four emission scenarios used to model future climate change on Earth in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. These four scenarios are called Representative Concentration Pathway 2.6 (RCP 2.6), 4.5 (RCP 4.5), 6.0 (RCP 6.0), and 8.5 (RCP 8.5) and are based on four assumptions about how future human population growth and socioeconomic development will unfold from now until 2100. The numbers used in the description name of these scenarios refer to the quantity of radiative forcing that will occur in the year 2100 relative to pre-industrial times (+2.6, +4.5, +6.0, and +8.5 W/m2). Replaced with Shared Socioeconomic Pathways (SSPs).
Representative Fraction - The expression of map scale as a mathematical ratio.
Reproductive Isolation - A process involved in the evolution of new species where a group of similar interbreeding individuals of the same species is divided into two or more sub-populations. Because of a barrier to gene flow, these subpopulations are no longer able to exchange genes. Further, within each sub-population, mutations occur, producing new adaptations, and the gene frequency of sub-populations begins to change because they each exist in different habitats. Within each habitat, natural selection determines which new adaptations become successful. If habitats differ greatly, given enough time, the process of reproductive isolation can cause each subpopulation to become a new species.
Reptile - A group of terrestrial vertebrate animals that includes turtles, tortoises, snakes, lizards, crocodiles, and alligators. These organisms have four limbs and have a sprawling posture, or are descended from ancestors that had four limbs. Most reptiles lay eggs as part of their reproduction. They are also covered by scales and are ectothermic (cold-blooded). See the Encyclopedia of Life for more information on this group of organisms. https://eol.org/pages/1703/overview.
Resequent Stream - A stream whose course follows the slope of the land topography, but at an elevation that is lower than the original land surface. These streams flow in a direction that is determined by the configuration of underlying rock layers.
Residence Time - A widely used term in science that refers to the average length of time a particle of matter is in some system. For example, the residence time of a typical water molecule in the atmosphere component of the hydrologic cycle is about nine days.
Residual Strength - A level of deformation strength for a soil that is just below its shear strength. Engineers and soil scientists measure residual strength with a device called a shear box. Also called residual shear strength.
Resource - Anything obtained from the environment to meet the needs of an organism. Many different types of resources have been defined, including: potential resource, actual resource, non-renewable resource, renewable resource, natural resource, economic resource, and biological resource.
Resource Partitioning - The evolutionary process of species living in the same ecosystem dividing up resources so that each species develops different resource requirements to avoid competition and possible extinction. Also see ecological niche, fundamental niche, and realized niche.
Respiration - Also called cellular respiration.
Resultant Wind - The average direction of wind for a particular location given as a single vector for a specified length of time.
Retention Curve - See soil moisture retention curve.
Retention Forces - The forces found in soil, sediment, and rock pores that hold water to the surface of these materials against the downward force of gravity. Three different forces are involved in this process. The strongest force is the molecular force between elements and compounds at the surfaces of soil, sediment, and rock. The water retained by this force is found within 0.0002 millimeters of the material's surface. Matric force holds soil water from 0.0002 to 0.06 millimeters from the surface of soil, sediment, and rock. This force is due to two processes: surface molecular attraction (adhesion and absorption) to water, and the cohesion that water molecules have to each other. This force declines with distance from the surface holding the water and becomes negligible beyond 0.06 millimeters. Beyond 0.06 millimeters from the material, water moves freely under the effect of gravity.
Retrogradation - The natural retreat of a shoreline caused by an increase in beach erosion. Compare with progradation.
Retrogressive Succession - A plant succession where the plant community becomes simplistic and contains fewer species and less biomass over time.
Return Period - An estimate of the time it will take for an event to occur again. This estimate is usually made statistically from historical data. Return periods are routinely calculated for stream discharge thresholds, earthquakes, volcanic eruptions, and floods. Also called the recurrence interval.
Reverse Fault - Is a type of vertical geologic fault where the hanging wall is forced over the foot wall. Reverse faults are usually caused by tectonic compressional forces. One important consequence of this type of fault is that older strata can be pushed over younger strata. Also called a thrust fault.
Reversing Dune - A sand dune that is intermediate between a star and transverse dunes. The dune ridge is asymmetrical and has two slip-faces.
Revolution - See earth revolution.
Reynolds Number - A calculated value that measures flow pattern characteristics of gases and liquids. In general, it is used to determine if the flow is laminar or turbulent. The following equation can be used to calculate the Reynolds number (Re):
Re = (ρ V L)/μ
Where ρ is the density of the fluid, V is the average flow velocity of the fluid, μ is the viscosity of the fluid, and L is the length or diameter of the fluid. When used to characterize the type of flow occurring in a stream channel, the hydraulic radius must be determined and substituted for L in the equation above. For streams, Re values below 500 indicate laminar flow, while Re values above 750 indicate turbulent flow.
Rheidity - The ability of some solid materials to deform into viscous flow under specific conditions of stress. Most types of rocks can display this behavior.
Rhizome - A modified stem of a plant that is found below the ground surface. Rhizomes are used to store carbohydrates, proteins, and nutrients. Rhizomes can produce roots and shoots.
Rhizosphere - The zone in a soil that is directly influenced by plant roots, their secretions, and associated microorganisms like bacteria and fungi.
Rhodoliths - A form of free-living crustose red algae that is branching, unattached, rigid, and spherical in shape. Rhodoliths are similar in appearance to coral. Rhodolith beds are found in our planet's oceans and seas, from the tropics to polar regions. More specifically, they can be found in reefs, shallow lagoons, tidal channels, sea-grass beds, and in waters with a depth down to about 60 meters (180 feet). Their growth form results from numerous species of red algae building cell walls that contain calcium carbonate.
Rhourd - A type of sand dune that has a pyramid shape.
Rhumb Line - A line of constant compass direction or bearing which crosses the meridians at the same angle. A part of a great circle.
Rhyolite - A fine-grained extrusive igneous rock that is rich in quartz and potassium feldspar. Derived from felsic magma.
Ria Coast - An extensively carved out coast with conspicuous headlands and deep re-entrants.
Ribbon Falls - A spectacular, narrow waterfall often found at the edge of a hanging valley.
Ribonucleic Acid (RNA) - A form of nucleic acid. Ribonucleic acid is used by most organisms to read the genetic information in DNA and to produce specific organic molecules essential to the development and functioning of cells.
Richter Scale - A logarithmic measurement scale of earthquake magnitude. This scale measures the energy released by the largest seismic wave associated with the earthquake.
Riegel - A ridge of bedrock in a glacial valley that was exposed because of glacial erosion.
Riffle - A type of bar deposit found on the stream bed. Often associated with these alluvial deposits are pools. Also see bedforms.
Rift - The zone between two diverging tectonic plates. The mid-oceanic ridge is an area where such plate divergence is occurring. Also see continental rift zone.
Rift Valley - A steep-sided valley found on the Earth's surface created by tectonic rifting.
Rill - A very small, steep-sided channel carrying water. This landscape feature is intermittent and forms only for a short period after rainfall.
Rime - A deposit of ice crystals that occurs when fog or supercooled water droplets come in contact with an object with a temperature below freezing (0°C). This deposit develops outward on the windward side of the object.
Ring of Fire - See Circum-Pacific Belt.
Rip Current - A strong, relatively narrow current of water that flows seaward against breaking waves.
Riparian - The land and habitat located next to streams, lakes, and wetlands. As habitats, riparian areas have high biodiversity and contain many species specifically adapted to living in aquatic environments.
Ripple - Stream bed alluvial depositional feature. Small-scale wave-like undulations that form in fine sand or silt from the flow of water in one direction. Spacing is usually less than 50 centimeters (1.5 feet) apart, with the vertical distance from the trough to the crest of each ripple being no more than 3 centimeters (1.2 inches). Found in slow-moving streams. Also see bedforms.
Rising Limb - Refers to the section of a hydrograph plot where there is a rapid increase in recorded stream discharge, usually caused by precipitation input.
River - A long, narrow channel of water that flows as a function of gravity and elevation across the Earth's surface. Many rivers empty into lakes, seas, or oceans. Also called a stream.
River Metamorphosis - A modification in the stream channel morphology caused by a change in stream discharge and stream load that exceeds a threshold level.
River Valley - See stream valley.
Riverscape - Refers to the landform features that can be found associated with a river (stream). Some of the features common to a riverscape include alluvial fan, oxbow lake, delta, levee, floodplain, terrace, waterfall, meander, gorge, and stream valley.
Robinson Map Projection - A map projection system that tries to present more accurate representations of area. Distortion is mainly manifested in terms of map direction and distance.
Roche Moutonnée - A feature of glacial erosion that resembles an asymmetrical rock mound. It is smooth and gently sloping on the side of the ice advance. The lee side of this feature is steep and jagged, suggesting that it was formed through plucking rather than abrasion. These landform features can vary greatly in size. Lengths range from several meters (feet) to a couple of kilometers (miles), and heights range from tens of centimeters to hundreds of meters (feet). A roche moutonnée can form under valley glaciers and continental glaciers.
Rock - A compact and consolidated mass of mineral matter. Three types of rock are recognized: igneous, sedimentary, and metamorphic.
Rock Cycle - A general model describing the geomorphic and geologic processes involved in the long-term creation, modification, and recycling of rocks.
Rock Drumlin - A glacial landform feature that is almost the same as a roche moutonnée. Rock drumlins differ from a roche moutonnée in two respects: 1) their steep lee-side is not jagged but smooth, and 2) they can be covered by a thin layer of till. Lengths range from several meters (feet) to a couple of kilometers (miles), and heights range from tens of centimeters to hundreds of meters (feet). Rock drumlins can form under valley glaciers and continental glaciers.
Rock Flour - Very finely ground rock fragments that form between the base of a glacier and the underlying bedrock surface.
Rock Glacier - A lobe of mainly rock debris with some ice that is found in some alpine valleys. The appearance of a rock glacier is quite similar to that of a glacier. Rock glaciers also exhibit some downslope movement.
Rock Mass Strength - A quantitative rating system that measures the resistance of a large mass of rock to erosion.
Rock Slide - A large-scale mass movement of rock materials downslope.
Rockfall - A type of mass movement that involves the detachment and movement of a small block of rock from a cliff face to its base. Often occurs when the rock has well-defined bedding planes, which are exaggerated by freeze-thaw action or by thermal expansion and contraction (insolation weathering).
Roll Cloud - A dense, cigar-shaped cloud found associated with a cold front or above the gust front of a severe thunderstorm. The air within the roll cloud rotates around the long axis. Also called an arcus cloud.
Rossby Wave - A large wave in the polar jet stream and the upper air westerlies that extends from the middle to the upper troposphere. This atmospheric feature is often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with short waves. Also called long wave.
Rotation - See earth rotation.
Rotational Slide - A type of spoon-shaped mass movement where material moves suddenly downslope along a curvilinear rupture surface. The rotational nature of these slides tends to quickly restore a resting equilibrium to the materials moved downslope. Also called a slump and a rotational slip. Compare with the translational slide.
Rotor Streaming - A meteorological phenomenon where turbulent eddies develop in horizontal atmospheric flow on the downwind side of a mountain barrier. Rotor streaming is usually associated with the first trough of a lee wave.
Roughness - (1) The level to which a surface lacks smoothness. (2) The degree of resistance that a surface inflicts on the flow of a liquid or a gas.
Roundness - A measure applied to particles found in a sediment. Roundness determines the level of abrasion these particles were subjected to by examining the sharpness of their outer surface. In other words, have the edges and corners of the particle become rounded.
Ruderal Species - According to J.P. Grime's theory of plant strategies, these species have the highest fitness in habitats with a low intensity of stress and a high intensity of disturbance. Simply, these plants are able to outcompete other species by having adaptations that help them survive the effects of disturbance. Ruderal plant species tend to be short-lived annuals or perennials with high growth rates. During their rapid growth, they allocate much of their biomass production to making many easily dispersed seeds. Compare with competitive and stress-tolerant species.
Runoff - The topographic flow of water from precipitation to stream channels located at lower elevations. Runoff occurs when the infiltration capacity of an area's soil has been exceeded. It also refers to water leaving a drainage area. Also called overland flow.
