Orography (from the Greek όρος, hill, γραφία, to write) is the study of the formation and relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as oreography, orology or oreology) falls within the broader discipline of geomorphology.
Orography has a major impact on global climate, for instance the orography of East Africa substantially determines the strength of the Indian monsoon. In geoscientific models, such as general circulation models, orography defines the lower boundary of the model over land.
When a river's tributaries or settlements by the river are listed in 'orographic sequence', they are in order from the highest (nearest the source of the river) to the lowest or mainstem (nearest the mouth). This method of listing tributaries is similar to the Strahler Stream Order, where the headwater tributaries are listed as category = 1.
Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind). This lifting can be caused by two mechanisms:
- The upward deflection of large scale horizontal flow by the orography.
- The anabatic or upward vertical propagation of moist air up an orographic slope caused by daytime heating of the mountain barrier surface.
Upon ascent, the air that is being lifted will expand and cool. This adiabatic cooling of a rising moist air parcel may lower its temperature to its dew point, thus allowing for condensation of the water vapor contained within it, and hence the formation of a cloud. If enough water vapor condenses into cloud droplets, these droplets may become large enough to fall to the ground as precipitation. In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation. Drier air (see katabatic wind) is left on the descending, generally warming, leeward side where a rain shadow is formed.
Terrain induced precipitation is a major factor for meteorologists as they forecast the local weather. Orography can play a major role in the type, amount, intensity and duration of precipitation events. Researchers have discovered that barrier width, slope steepness and updraft speed are major contributors for the optimal amount and intensity of orographic precipitation. Computer model simulations for these factors showed that narrow barriers and steeper slopes produced stronger updraft speeds which, in turn, enhanced orographic precipitation.
Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands or New Zealand, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 20 to 30 inches (510 to 760 mm) per year, and interior uplands receiving over 100 inches (2,500 mm) per year. Leeward coastal areas are especially dry—less than 20 in (510 mm) per year at Waikiki—and the tops of moderately high uplands are especially wet—about 475 in (12,100 mm) per year at Wai'ale'ale on Kaua'i.
Another well known area for orographic precipitation is the Pennines in the north of England where the west side of the Pennines receives more rain than the east because the clouds (generally arriving from the west) are forced up and over the hills and cause the rain to fall preferentially on the western slopes. This is particularly noticeable between Manchester (West) and Leeds (East) where Leeds receives less rain due to a rain shadow of 12 miles from the Pennines.
- Map of the Orography of Europe from Euratlas.com
Wikimedia Foundation. 2010.
Look at other dictionaries:
Orography — O*rog ra*phy, n. [Gr. o ros, a mountain + graphy.] That branch of science which treats of mountains and mountain systems; orology; as, the orography of Western Europe. [1913 Webster] … The Collaborative International Dictionary of English
orography — [ō räg′rə fē] n. [ ORO + GRAPHY] the branch of physical geography dealing with mountains: cf. OROLOGY orographic [ôr΄ō graf′ik] adj. orographical … English World dictionary
orography — noun Etymology: International Scientific Vocabulary Date: circa 1846 a branch of physical geography that deals with mountains … New Collegiate Dictionary
orography — orographic /awr euh graf ik, or euh /, orographical, adj. orographically, adv. /aw rog reuh fee, oh rog /, n. the branch of physical geography dealing with mountains. Also called orology /aw rol euh jee, oh rol /. [1840 50; ORO 1 + GRAPHY] * * * … Universalium
orography — noun a) The scientific study, or a physical description of mountains b) The orographic features of a region … Wiktionary
orography — n. orology, study or science of mountains; description of mountains … English contemporary dictionary
orography — orog·ra·phy … English syllables
orography — o•rog•ra•phy [[t]ɔˈrɒg rə fi, oʊˈrɒg [/t]] n. geo the branch of physical geography dealing with mountains. Also called o•rol•o•gy [[t]ɔˈrɒl ə dʒi, oʊˈrɒl [/t]] • Etymology: 1840–50 or•o•graph•ic ˌɔr əˈgræf ɪk, ˌɒr ə or o•graph′i•cal, adj. or… … From formal English to slang
orography — /ɒˈrɒgrəfi/ (say o rogruhfee) noun that branch of physical geography which deals with mountains. Also, oreography. –orographic /ɒroʊˈgræfɪk/ (say oroh grafik), orographical /ɒroʊˈgræfɪkəl/ (say oroh grafikuhl), adjective … Australian English dictionary
orography — n. (also oreography) the branch of physical geography dealing with mountains. Derivatives: orographic adj. orographical adj … Useful english dictionary