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P-Wave -  A seismic wave that moves material in push-pull fashion in the direction of its travel. This type of seismic wave can travel through solids, liquids, and gases. Also called a primary wave.
Pangaea -  Hypothetical super continent that existed in the geological past. Its break-up created the current configuration of landmasses found on the Earth.
Parallel -  A line parallel to the equator and connecting all places of the same latitude.
Parent Material -  The mineral material from which a soil forms.
Passive Remote Sensing -  Form of remote sensing where the sensor passively captures electromagnetic radiation reflected or emitted by an object.
Paternoster Lakes -  A linear series of mountain valley lakes that are formed from glacial erosion. They form behind glacial moraines or in glacially carved out rock basins. The name of this feature is related to the series of lakes looking like a string of beads.
Patterned Ground -  Term used to describe a number of surface features found in periglacial environments. These features can resemble circles, polygons, nets, steps, and stripes. The development of some of these shapes is thought to be the result of freeze-thaw action.
Peak Annual Flow -  The largest discharge produced by a stream during a one year period.
Peat -  Partially decomposed remains of plants that once flourished in a waterlogged environment.
Pebble -  A rounded piece of rock that is larger than gravel.
Pediment -  A gradually sloping bedrock surface located at the base of fluvial-eroded mountain range. Found in arid locations and normally covered by fluvial deposits.
Pediplain -  An arid landscape of little relief that is occasionally interrupted by the presence of scattered inselbergs. Formed by the coalescence of several pediments.
Perched Water Table -  Water table that is positioned above the normal water table for an area because of the presence of a impermeable rock layer.
Percolation -  Vertical movement or infiltration of water from the Earth's surface to its subsurface. Movement usually stops when the flowing water reaches the water table.
Peridotite -  Coarse grained ultramafic igneous rock composed mainly of olivine and pyroxene. The mantle is though to be composed primarily of this rock type.
Periglacial -  Landforms created by processes associated with intense freeze-thaw action in an area high latitude areas or near an alpine or continental glacier.
Perihelion -  It is the point in the Earth's orbit when it is closest to the Sun (147.5 million km). Perihelion occurs on the 3rd or 4th of January.
Period -  Geologic time unit that is shorter than an era but longer than a epoch.
Permafrost -  Zone of permanently frozen water found in high latitude soils and sediments. Five types of permafrost have been recognized: continuous permafrost, discontinuous permafrost, sporadic permafrost, alpine permafrost, and subsea permafrost.
Permeability -  A measure of the ability of soil, sediments, and rock to transport water horizontally and vertically. Permeability is dependent on the porosity of the medium the water is flowing through. Some rocks like granite have very poor permeability, while rocks like shale are actually quite pervious. As for soils, sand is the most pervious, while clay has the lowest permeability. Silt usually is somewhere in the middle.
Permian -  Last geologic period in the Paleozoic era. Occurred from 286 to 245 million years ago. This period saw the mass extinction of many corals, brachiopods, and trilobites. It also saw the diversification and growing dominance of the reptiles.
PGF -  Force due to spatial differences in atmospheric pressure. Usually expressed in millibars or kilopascals per unit distance (meters or kilometers). This force is primarily responsible for the formation of wind.
pH -  Scale used to measure the alkalinity or acidity of a substance through the determination of the concentration of hydrogen ions in solution. A pH of 7.0 is neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, to a maximum of 14.0, indicate increasing alkalinity.
Phanerozoic -  Geologic eon that occurs from 2500 million years ago to today. During this time period, life becomes more diversified and complex.
Phase Change -  Reorganization of a substance at the atomic or molecular level resulting in a change of the physical state of matter. For example, a change from solid to liquid to a gas.
Photochemical Smog -  Photochemical smog is a condition that develops when primary pollutants (oxides of nitrogen and volatile organic compounds created from fossil fuel combustion) interact under the influence of sunlight to produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants. Also see industrial smog.
Photogrammetry -  The science of using aerial photographs and other remote sensing imagery to obtain measurements of natural and human-made features on the Earth.
Photoperiod -  The duration of the daylight period.
Photosphere -  Visible surface of Sun from which radiant energy is release.
Photosynthesis -  Is the chemical process where plants and some bacteria can capture and organically fix the energy of the Sun. This chemical reaction can be described by the following simple equation: 6CO2 + 6H2O + light energy >>> C6H12O6 + 6O2
Physical Geography -  Field of knowledge that studies natural features and phenomena on the Earth from a spatial perspective. Subdiscipline of Geography.
Physical Weathering -  Breakdown of rock and minerals into small sized particles through mechanical stress.
Phytoplankton -  Small photosynthetic organisms, mostly algae and bacteria, found inhabiting aquatic ecosystems. Also see plankton and zooplankton.
Piedmont Glacier -  A large glacier formed from the merger of several alpine glaciers.
Pingo -  A large conical mound that contains an ice core. This feature can be up to 60 to 70 meters in height. Form in regions of permafrost. Common in the Mackenzie Delta region of Canada. Also see the related palsa.
Pitted Topography -  Landscape characterized by numerous kettle holes on a glacial outwash plain.
Place -  A term used in geography that describes the factors that make the location of natural and human-made phenomena unique.
Plagioclase Feldspar -  A type of feldspar that is rich in sodium and calcium. Common rock forming mineral.
Plane of the Ecliptic -  Hypothetical two-dimensional surface in which the Earth's orbit around the Sun occurs.
Planet -  (1) Any one of the nine primary celestial bodies that orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (2) A similar body orbiting another star.
Plankton -  Minute plant (phytoplankton) and animal organisms (zooplankton) that are found in aquatic ecosystems.
Plant -  Organisms belonging to the kingdom Plantae. These organisms have the following general characteristics: lack of locomotion, lack of a nervous system, and cellulose cell walls. Most plants can photosynthesize.
Plastic Deformation -  Irreversible change in the shape of a material without fracture as the result of the force of compression or expansion.
Plate Tectonics -  Theory suggesting that the Earth's surface is composed of a number of oceanic and continental plates. Driven by convection currents in the mantle, these plates have the ability to slowly move across the Earth's plastic asthenosphere. This theory is very important to geology and geomorphology because it helps to explain the occurrence and formation of mountains, folds, faults, volcanoes, earthquakes, ocean trenches, and the mid-oceanic ridges.
Plateau Basalt -  An accumulation of horizontal flows of basaltic lava. Also called flood basalts.
Platform -  Horizontal sedimentary deposits found on top of continental shield deposits.
Playa -  A dry lake bed found in a desert.
Pleistocene Epoch -  Period of time from about 2 million years ago to 10,000 years ago. During this period areas of land at higher and middle latitudes where covered with glacial ice.
Plucking -  Erosive process of particle detachment by moving glacial ice. In this process, basal ice freezes in rock surface cracks. As the main body of the glacial ice moves material around the ice in the cracks is pulled and plucked out. Also called quarrying.
Pluton -  Any mass of intrusive igneous rock.
Point Bar -  Stream bar deposit that is normally located on the inside of a channel bend.
Polar Axis -  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.
Polar Cell -  Three-dimensional atmospheric circulation cell located at roughly 60 to 90° North and South of the equator. Vertical air flow in the Polar cell consists of rising air at the polar font and descending air at the polar vortex.
Polar Easterlies -  Winds that originate at the polar highs and blow to the subpolar lows in a east to west direction.
Polar Front -  Weather front located typically in the mid-latitudes that separates arctic and polar air masses from tropical air masses. Along the polar front we get the development of the mid-latitude cyclone. Above the polar front exists the polar jet stream.
Polar High -  Surface area of atmospheric high pressure located at about 90° North and South latitude. These high pressure systems produced by vertically descending air currents from the polar vortex.
Polar Jet Stream -  Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers. This jet stream flows from west to east at speeds between 110 to 185 kilometers per hour. Also see jet stream and subtropical jet stream.
Polar Stratospheric Clouds -  High altitude clouds found in the stratosphere where the temperature is less than -85° Celsius. Commonly found over Antarctica. Have a role in the creation of the ozone hole over Antarctica.
Polar Vortex -  High pressure system located in the upper atmosphere at the polar regions. In this system, air in the upper troposphere moves into the vortex center and then descends to the Earth's surface to create the polar highs.
Pollutant -  A substance that has a harmful effect on the health, survival, or activities of humans or other living organisms.
Pollution -  Physical, chemical, or biological change in the characteristics of some component of the atmosphere, hydrosphere, lithosphere, or biosphere that adversely influences the health, survival, or activities of humans or other living organisms.
Polycyclic Landform -  Landform that shows the repeated influence of one or more major geomorphic processes over geological time. Major geomorphic processes are: weathering, erosion, deposition, and massive Earth movements caused by plate tectonics.
Polygenetic Landform -  Landform that shows the influence of two or more major geomorphic processes. Major geomorphic processes are: weathering, erosion, deposition, and massive earth movements caused by plate tectonics.
Pool -  Scoured depression found on the bed of streams. Associated with riffles.
Population -  (1) Refers to all the individuals of a given species in a specific area or region at a certain time. Its significance is more than that of a number of individuals because not all individuals are identical. Populations contain genetic variation within themselves and between other populations. (2) A statistical population is the entire collection of people, animals, plants or things from which we may collect data from.
Population Density -  Number of individuals of a particular species found in a specified area.
Population Parameter -  A value used to represent a certain quantifiable characteristic of a population. For example, the population mean is a parameter that is often used to indicate the central value of quantity.
Pore Ice -  A form of periglacial ground ice that is found in the spaces that exist between particles of soil.
Porosity -  The void spaces found in rock, sediment, or soil. Commonly measured as the percentage of void space in a volume of substance.
Porous -  The void spaces found in rock, sediment, or soil. Commonly measured as the percentage of void space in a volume of substance.
Precambrian -  Span of geologic time that dates from 4.6 billion to 570 million years ago. Made up of three geologic eras: Hadean, Archean, and Proterozoic.
Precambrian Shield -  Another term for shield.
Precession of the Equinox -  Wobble in the Earth's polar axis. This motion influences the timing aphelion and perihelion over a cyclical period of 23,000 years.
Precipitable Water -  Amount of water potentially available in the atmosphere for precipitation. Usually measured in a vertical column that extends from the Earth's surface to the upper edge of the troposphere.
Precipitation -  (1) Is any aqueous deposit, in liquid or solid form, that develops in a saturated atmosphere (relative humidity equals 100%) and falls to the ground generally from clouds. Most clouds, however, do not produce precipitation. In many clouds, water droplets and ice crystals are too small to overcome natural updrafts found in the atmosphere. As a result, the tiny water droplets and ice crystals remain suspended in the atmosphere as clouds. (2) The state of being precipitated from a solution.
Prediction -  Forecast or extrapolation of the future state of a system from current or past states.
Pressure -  Is defined as the force acting on a surface from another mass per unit area.
Pressure Gradient Force -  Force due to spatial differences in atmospheric pressure. Usually expressed in millibars or kilopascals per unit distance (meters or kilometers). This force is primarily responsible for the formation of wind.
Pressure Melting Point -  Temperature at which minerals deep within the Earth and ice below the surface of a glacier are caused to melt because of the introduction of pressure.
Prevailing Wind -  Dominant direction that a wind blows from for a location or region.
Primary Pollutant -  Air pollutants that enter the atmosphere directly. Also see secondary pollutant.
Primary Wave -  A seismic wave that moves material in push-pull fashion in the direction of its travel. This type of seismic wave can travel through solids, liquids, and gases. Also called a primary wave.
Prime Meridian -  The location from which meridians of longitude are measured. Has the measure of 0° of longitude. The prime meridian was selected by international agreement to run through Greenwich, England.
Probability -  Statistical chance that an event will occur.
Progradation -  The natural extension of a shoreline seaward.
Proterozoic -  Geologic eon that occurred from 570 to 2500 million years ago. During this time period, the first single-celled and multi-celled eukaryotic organisms evolved and developed.
Proxy Data -  Data that measures the cause and effect relationship between two variables indirectly.
Psychrometer -  Instrument used to measure atmospheric humidity. It consists of two thermometers (wet-bulb and a dry-bulb) one of which has its bulb covered by a moistened wick. Humidity is determined by the difference in readings between the two thermometers after air has passed over both of them for a specific time period.
Psychrometric Table -  Table of values that allows for the determination of relative humidity and dew point from dry-bulb and wet-bulb temperatures recorded on a psychrometer.
Pyroclastic Material -  Pieces of volcanic rock thrown out in a volcanic explosion.