Page 728 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

G-9 Glossary
the lower-energy inner portion receiving sedi- ment deposits. (See Graded stream.)
Mean sea level (MSL) (16) The average of tidal levels recorded hourly at a given site over a long period, which must be at least a full lunar tidal cycle.
Medial moraine (17) Debris transported by a glacier that accumulates down the middle of the glacier, resulting from two glaciers merging their lateral moraines; forms a depo- sitional feature following glacial retreat.
Mediterranean shrubland (20) A major biome dominated by the Mediterranean (dry summer) climate and characterised by sclero- phyllous scrub and short, stunted, tough for- ests. (See Chaparral.)
Mercator projection (1) A true-shape pro- jection, with meridians appearing as equally spaced straight lines and parallels appearing as straight lines that are spaced closer togeth- er near the equator. The poles are infinitely stretched, with the 84th north parallel and 84th south parallel fixed at the same length as that of the equator. It presents false notions of the size (area) of midlatitude and poleward landmasses, but presents true compass direc- tion. (See Rhumb line.)
Mercury barometer (6) A device that mea- sures air pressure using a column of mercury in a tube; one end of the tube is sealed, and the other end is inserted in an open vessel of mercury. (See Air pressure.)
Meridian (1) A line designating an angle of longitude. (See Longitude.)
Mesocyclone (8) A large, rotating atmo- spheric circulation, initiated within a parent cumulonimbus cloud at midtroposphere el- evation; generally produces heavy rain, large hail, blustery winds, and lightning; may lead to tornado activity.
Mesosphere (3) The upper region of the homosphere from 50 to 80 km above the ground; designated by temperature criteria; atmosphere extremely rarified.
Metamorphic rock (12) One of three basic rock types, it is existing igneous and sedi- mentary rock that has undergone profound physical and chemical changes under in- creased pressure and temperature. Constitu- ent mineral structures may exhibit foliated or nonfoliated textures. (Compare Igneous rock, Sedimentary rock.)
Meteorology (8) The scientific study of the atmosphere, including its physical character- istics and motions; related chemical, physi- cal, and geological processes; the complex linkages of atmospheric systems; and weather forecasting.
Microbursts (8) Severe turbulence from a thunderstorm, smaller in size and speed than a downburst. (See downburst.)
Microclimatology (4) The study of local climates at or near Earth’s surface or up to that height above the Earth’s surface where the effects of the surface are no longer deter- minative.
Midlatitude broadleaf and mixed forest (20)
A biome in moist continental climates in ar- eas of warm-to-hot summers and cool-to-cold winters; relatively lush stands of broadleaf forests trend northward into needleleaf ever- green stands.
Midlatitude cyclone (8) An organized area of low pressure, with converging and as- cending airflow producing an interaction of air masses; migrates along storm tracks. Such lows or depressions form the dominant weather pattern in the middle and higher lati- tudes of both hemispheres.
Midlatitude grassland (20) The major biome most modified by human activity; so named because of the predominance of grasslike plants, although deciduous broadleafs appear along streams and other limited sites; loca- tion of the world’s breadbaskets of grain and livestock production.
Mid-ocean ridge (12) A submarine moun- tain range that extends more than 65000 km worldwide and averages more than 1000 km in width; centered along sea-floor spreading centers. (See Sea floor spreading.)
Milankovitch cycles (11) The consistent orbital cycles—based on the irregularities in Earth’s orbit around the Sun, its rotation on its axis, and its axial tilt—that relate to cli- matic patterns and may be an important cause of glacials and interglacials. Milutin Milanko- vitch (1879–1958), a Serbian astronomer, was the first to correlate these cycles to changes in insolation that affected temperatures on Earth.
Milky Way Galaxy (2) A flattened, disk- shaped mass in space estimated to contain up to 400 billion stars; a barred-spiral galaxy; includes our Solar System.
Miller cylindrical projection (Appendix A)
A compromise map projection that avoids the severe distortion of the Mercator projection. (See Map projection.)
Mineral (12) An element or combination of elements that forms an inorganic natural compound; described by a specific formula and crystal structure.
Mirage (4) A refraction effect when an im- age appears near the horizon where light waves are refracted by layers of air at differ- ent temperatures (and consequently of differ- ent densities).
Model (1) A simplified version of a system, representing an idealized part of the real world.
Mohorovicˇ ic´ discontinuity, or Moho (12)
The boundary between the crust and the rest of the lithospheric upper mantle; named for the Yugoslavian seismologist Mohorovicˇ ic´ ; a zone of sharp material and density contrasts.
Moist adiabatic rate (MAR) (7) The rate at which a saturated parcel of air cools in as- cent; a rate of 6 C° per 1000 m. This rate may vary, with moisture content and temperature, from 4 C° to 10 C° per 1000 m. (See Adiabatic; compare Dry adiabatic rate.)
Moisture droplet (7) A tiny water particle that constitutes the initial composition of clouds. Each droplet measures approximately 0.002 cm in diameter and is invisible to the unaided eye.
Mollisols (Appendix B) A soil order in the Soil Taxonomy. These have a mollic epipe- don and a humus-rich organic content high in alkalinity. Some of the world’s most signifi- cant agricultural soils are Mollisols.
Moment magnitude (M) scale (13) An earthquake magnitude scale. Considers the amount of fault slippage, the size of the area that ruptured, and the nature of the materi- als that faulted in estimating the magnitude of an earthquake—an assessment of the seis- mic moment. Replaces the Richter scale (am- plitude magnitude); especially valuable in assessing larger-magnitude events.
Monsoon (6) An annual cycle of dryness and wetness, with seasonally shifting winds produced by changing atmospheric pressure systems; affects India, Southeast Asia, Indo- nesia, northern Australia, and portions of Africa. From the Arabic word mausim, mean- ing “season.”
Montane forest (20) Needleleaf forest asso- ciated with mountain elevations. (See Needle- leaf forest.)
Moraine (17) Marginal glacial deposits (lat- eral, medial, terminal, ground) of unsorted and unstratified material.
Mountain and valley breezes (6) A light wind produced as cooler mountain air flows downslope at night and as warmer valley air flows upslope during the day.
Movement (1) A major theme in geography involving migration, communication, and the interaction of people and processes across space.
Mudflow (14) Fluid downslope flows of ma- terial containing more water than earthflows.
Natural levee (15) A long, low ridge that forms on both sides of a stream in a developed floodplain; a depositional product (coarse gravels and sand) of river flooding.
Neap tide (16) Unusually low tidal range produced during the first and third quarters of the Moon, with an offsetting pull from the Sun. (Compare Spring tide.)
Needleleaf forest (20) Consists of pine, spruce, fir, and larch and stretches from the east coast of Canada westward to Alaska and continuing from Siberia westward across the entire extent of Russia to the European Plain; called the taiga (a Russian word) or the bo- real forest; principally in the microthermal climates. Includes montane forests that may be at lower latitudes at higher elevations.
Negative feedback (1) Feedback that tends to slow or dampen responses in a system; promotes self-regulation in a system; far more common than positive feedback in living sys- tems. (See Feedback loop; compare Positive feedback.)
Net primary productivity (19) The net pho- tosynthesis (photosynthesis minus respiration) for a given community; considers all growth and all reduction factors that affect the amount of useful chemical energy (biomass) fixed in an ecosystem.
outgoing longwave energy.
Nickpoint (knickpoint) (15) The point at which the longitudinal profile of a stream is abruptly broken by a change in gradient; for example, a waterfall, rapids, or cascade.
Nimbostratus (7) Rain-producing, dark, gray- ish stratiform clouds characterised by gentle drizzle.
Net radiation (NET R) (4)
The net all-wave radiation available at Earth’s surface; the fi- nal outcome of the radiation balance process between incoming shortwave insolation and
Nitrogen dioxide (NO) (3) A noxious 2
(harmful) reddish-brown gas produced in com- bustion engines; can be damaging to human respiratory tracts and to plants; participates in photochemical reactions and acid deposition.
Noctilucent cloud (3) A rare, shining band of ice crystals that may glow at high latitudes long after sunset; formed within the meso- sphere, where cosmic and meteoric dust act as nuclei for the formation of ice crystals.
Normal fault (13) A type of geologic fault in rocks. Tension produces strain that breaks a rock, with one side moving vertically relative to the other side along an inclined fault plane. (Compare Reverse fault.)
Normal lapse rate (3) The average rate of temperature decrease with increasing altitude in the lower atmosphere; an average value of 6.4 C° per km, or 1000 m. (Compare Environ- mental lapse rate.)