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R Horizon -  Soil horizon found beneath the C horizon. Consists of consolidated rock showing little sign of weathering or pedogenesis.
Radiant Energy -  Energy in the form of electromagnetic waves and photons. In some cases it refers to the radiation emitted from the Sun.
Radiation -  The emission of energy from an object in the form of electromagnetic waves and photons.
Radiation Fog -  A type of fog that is also called ground fog. Radiation fog is generated by near surface cooling by radiation loss during the evening hours. For the fog to develop, the overnight cooling must cause saturation occur. This type of fog is normally quite shallow.
Rain -  A form of precipitation. It is any liquid deposit that falls from clouds in the atmosphere to the ground surface. Rain normally has a diameter between than 0.5 and 5.0 millimetres.
Rain Gauge -  Instrument that measures the rain that falls at a location over a period of time.
Raindrop Impact -  Force exerted by a falling raindrop on a rock, sediment, or soil surface.
Rainshadow Effect -  Reduction of precipitation commonly found on the leeward side of a mountain. The reduction in precipitation is the result of compression warming of descending air.
Rainsplash -  Soil erosion caused from the impact of raindrops.
Rainwash -  The erosion of soil by overland flow. Normally occurs in concert with rainsplash.
Random -  Process or event that occurs by chance.
Range -  A statistical measure of the dispersion of observation values in a data set. Determined by taking the difference between the largest and the smallest observed value.
Rangeland -  Land-use type that supplies vegetation for consumption by grazing and browsing animals. This land-use type is normally not intensively managed.
Re-Entrants -  A prominent indentation in an escarpment, ridge or shoreline.
Reach -  An expanse of a stream channel.
Recessional Moraine -  Moraine that is created during a pause in the retreat of a glacier. Also called a stadial moraine.
Recharge Area -  The area on the Earth's surface that receives water for storage into a particular aquifer.
Rectangular Coordinate System -  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 which the axial plane is almost horizontal.
Recurrence Interval -  The average time period that separates natural events of a specific magnitude. For example, floods of a specific stream discharge level.
Reduction -  (1) Chemical process that involves the removal of oxygen from a compound. (2) A form of chemical weathering.
Reef -  A ridge of rocks found in the tidal zone along a coastline. One common type of reef is the coral reef.
Reference Map -  Map that shows natural and human-made objects from the geographical environment with an emphasis on location. Compare with thematic map.
Reflected Infrared Radiation -  Form of electromagnetic radiation with a wavelength between 0.7 to 3.0 micrometers (µm).
Reflected Wave -  A water wave that reflects off the shore or another obstacle and is redirected towards the sea or lake.
Reflection -  Process of returning sound or light waves back to their source.
Reflection (Atmospheric) -  Process where insolation is redirect by 180° after striking a particle. This redirection causes 100% loss. Most of the reflection in the Earth's atmosphere occurs in clouds because of light's interception with particles of liquid and frozen water. The reflectivity of a cloud can range from 40-90%.
Refraction -  Process where insolation is redirect to a new direction of travel after entering another medium.
Reg -  A rocky desert landscape. See desert pavement.
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 -  Large scale metamorphic modification of existing rock through the heat and pressure of plutons created at tectonic zones of subduction.
Regolith -  Loose layer of rocky material overlying bedrock.
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.
Relief -  The range of topographic elevation within a specific area.
Remote Sensing -  The gathering of information from an object or surface without direct contact.
Remote Sensor -  Mechanical devices used to remotely sense an object or phenomenon.
Representative Fraction -  The expression of map scale as a mathematical ratio.
Reptile -  Group of terrestrial vertebrate animals that includes turtles, tortoises, snakes, lizards, crocodiles, and alligators.
Resource -  Anything obtained from the environment to meet the needs of a species.
Reverse Fault -  This vertical fault develops when compressional force causes the displacement of one block of rock over another.
Revolution -  Refers to the orbit of the Earth around the Sun. This celestial motion takes 365 1/4 days to complete one cycle. Further, the Earth's orbit around the Sun is not circular, but elliptical. Also see Earth revolution.
RF -  The expression of map scale as a mathematical ratio.
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 -  Spectacular narrow waterfalls that occur at the edge of a hanging valley.
Richter Scale -  A logarithmic measurement scale of earthquake magnitude. This scale measures the energy released by the largest seismic wave associated with the earthquake.
Riffle -  Bar deposit found on the bed of streams. Associated with these deposits are pools.
Rift -  Zone between two diverging tectonic plates. The mid-oceanic ridge is an area where such plate divergence is occurring.
Rift Valley -  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 for only a short period of time after a rainfall.
Rime -  Deposit of ice crystals that occurs when fog or super cooled water droplets comes in contact with an object with a temperature below freezing (0° Celsius). This deposit develops outward on the windward side of the object.
Ring of Fire -  A zone circling the edge of the Pacific Ocean basin where tectonic subduction causes the formation of volcanoes and trenches. Also called the Circum-Pacific Belt.
Rip Current -  A strong relatively narrow current of water that flows seaward against breaking waves.
Ripple -  Stream bed deposit found streams. Ripples are only a few centimeters in height and spacing and are found in slow moving streams with fine textured beds.
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.
Robinson Projection -  Map projection system that tries to present more accurate representations of area. Distortion is mainly manifested in terms of map direction and distance.
Roche Moutonnee -  A feature of glacial erosion that resembles an asymmetrical rock mound. It is smooth and gently sloping on the side of ice advance. The lee-side of this feature is steep and jagged.
Rock -  A compact and consolidated mass of mineral matter. Three types of rock are recognized: igneous, sedimentary, and metamorphic.
Rock Cycle -  General model describing the geomorphic and geologic processes involved in the creation, modification and recycling of rocks.
Rock Flour -  Very finely ground rock fragments that form between the base of a glacier and the underlying bedrock surface.
Rock Slide -  Large scale mass movement of rock materials downslope.
Rockfall -  Type of mass movement that involves the detachment and movement of a small block of rock from a cliff face to its base. Normally occurs when the rock has well defined bedding planes that are exaggerated by freeze-thaw action or thermal expansion and contraction.
Roll Cloud -  A dense, cigar shaped cloud found above the gust front of a thunderstorm. Air within the cloud rotates around the long axis.
Rossby Wave -  A large wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with short waves. Also called long wave.
Rotation -  Is a line drawn through the Earth around the planet rotates. The point at which the polar axis intercepts the Earth's surface in the Northern Hemisphere is called the North Pole. Likewise, the point at which the polar axis intercepts the Earth's surface in the Southern Hemisphere is called the South Pole. Also see Earth rotation.
Rotational Slip -  Form of mass movement where material moves suddenly along a curvilinear plane. Also called a slump.
Runoff -  The topographic flow of water from precipitation to stream channels located at lower elevations. Occurs when the infiltration capacity of an area's soil has been exceeded. It also refers to the water leaving an area of drainage. Also called overland flow.