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.
Uses
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 the opposite of the
Strahler Stream Order, where the headwater tributaries are listed as category = 1.
Precipitation
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:
1) The upward deflection of large scale horizontal flow by the Orography.
2) 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 per year, and interior uplands receiving over per year. Leeward coastal areas are especially dry—less than per year at
Waikiki—and the tops of moderately high uplands are especially wet—about 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.
See also
