Orography is the study of thetopographic relief ofmountains,[1] and can more broadly include hills, and any part of a region's elevated terrain.[2] Orography (also known asoreography,orology, ororeology) falls within the broader discipline ofgeomorphology.[3] The term orography comes from theGreek:όρος, hill,γράφω, to write.
Mountain ranges and elevated land masses have a major impact onglobal climate. For instance, the elevated areas ofEast Africa substantially determine the strength of theIndian monsoon.[4] In scientific models, such asgeneral circulation models, orography defines the lower boundary of the model over land.[citation needed]
When a river'stributaries 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 ormainstem (nearest the mouth).[citation needed] This method of listing tributaries is similar to theStrahler Stream Order, where the headwater tributaries are listed as category 1.
 | It has been suggested that this section besplit out into a new article titledOrographic precipitation. (Discuss)(November 2020) |
Orographic precipitation, also known as relief precipitation, isprecipitation generated by a forced upward movement of air upon encountering a physiographic upland (seeanabatic wind). This lifting can be caused by:
Upon ascent, the air that is being lifted expands and cools adiabatically. Thisadiabatic cooling of a rising moist air parcel may lower its temperature to itsdew point, thus allowing for condensation of the water vapor contained within it, and hence the formation of acloud. If enough water vapor condenses into cloud droplets, these droplets may become large enough to fall to the ground as precipitation.
Terrain-induced precipitation is a major factor formeteorologists to consider when they forecast the local weather. Orography can play a major role in determining the type, amount, intensity, and duration of precipitation events. Researchers have discovered that barrier width, slope steepness, andupdraft speed are major contributors when it comes to achieving the optimal amount and intensity of orographic precipitation.Computer models simulating these factors have shown that narrow barriers and steeper slopes produce stronger updraft speeds, which in turn increase orographic precipitation.
Orographic precipitation is known to occur on oceanicislands, such as theHawaiian Islands andNew Zealand; much of the rainfall received on such islands is on the windward side, and theleeward side tends to be quite dry, almostdesert-like. This phenomenon results in substantial local gradients in the amount 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 atWaikiki—and the tops of moderately high uplands are especially wet—about 475 in (12,100 mm) per year atWaiʻaleʻale onKauaʻi.
Another area in which orographic precipitation is known to occur is thePennines in the north ofEngland: the west side of the Pennines receives more rain than the east because the clouds are forced up and over the hills and cause the rain to tend to fall on the western slopes. This is particularly noticeable betweenManchester (to the west) andLeeds (to the east); Leeds receives less rain due to a rain shadow of 12 miles (19 km) from the Pennines.
