Orographic Precipitation

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What is Orography?

Orography is defined as the study of the topographic relief of mountains, and more broadly includes hills and any part of an elevated terrain region. Orography is also called oreography. Oreology or orology falls within the broader discipline of geomorphology.

What is Orographic Precipitation?

Orographic precipitation is defined as snow, rain, or other precipitation, which is formed when moist air is lifted as it moves over a range of mountains. As the air rises and cools, the orographic clouds form and serve as the precipitation source, where most falls upwind of the mountain ridge. Also falls at a shorter distance downwind of the ridge are called spillover, sometimes. On the lee side of the similar mountain range, rainfall usually becomes low, and the area is in a rain shadow. Very heavy precipitation typically takes place upwind of a prominent mountain range, which is oriented across a prevailing wind from a warm ocean.

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Generating the Precipitation

Orographic precipitation, which is also called relief precipitation, is the precipitation that is generated by a forced upward movement of air upon a physiographic upland encounter. This lifting is caused by two mechanisms:

  • The upward deflection of the large scale horizontal flow by the orography.

  • The upward vertical or anabatic propagation of moist air up an orographic slope, which is caused by the daytime heating of the mountain barrier surface.

Upon the ascent, air, which is being lifted, will cool and expand adiabatically. This adiabatic cooling of moist air parcel rise can lower its temperature to its dew point, hence allowing for water vapour condensation contained within it, and thus the cloud formation. If enough water vapour condenses into the droplets of cloud, these particular droplets can become large enough to fall to the ground as precipitation.

Terrain-induced precipitation is one of the primary factors for meteorologists because they forecast the local weather. Also, orography can play a primary major role in the amount, type, duration and intensity of precipitation events. Researchers have already discovered that slope steepness, updraft speed, and barrier width are the major contributors to the intensity and optimal amount of orographic precipitation. Computer model simulations for these factors represented that narrow barriers and the steeper slopes produced greater updraft speeds which, in turn, enhanced orographic precipitation.

Cause of Orographic Rainfall

Let us know the cause of orographic rainfall briefly, as discussed below.

Two things are directly responsible for the cause of orographic rainfall throughout the world and the rainfall can be expected or forecast by the same. Those two things are given as 

  • Humid atmospheric conditions, in which the clouds are approaching full saturation that would lead observers to believe that it will form measurable precipitation.

  • An incline, for example, which is exactly would be found going up a mountain.

Orographic rainfall is caused exactly by these two things, but in simple terms, it means “rainfall associated with or which is induced by the presence of mountains”. Nothing is more specific here except the description of rainfall, which is caused by “the presence of mountains”.

Examples of Orographic Precipitation

Examples of the precipitation, which are caused by mountains include rainfall from orographic stratus formed by forced lifting and precipitation from orographic cumuli caused by daytime heating of mountain slopes. Several classic examples of locations that have excessive annual precipitation are located on the mountain’s windward slopes facing a steady wind from a warm ocean.

Another example is the wintertime orographic stratus (which are cap clouds) often produce the primary water supply for the regions of populated semiarid like the mountainous western United States, and resultantly, these cloud systems have been a precipitation enhancement target, cloud-seeding projects intended to form snowpack augmentation.

Also, the orographic precipitation is always not limited to the ascending ground but can extend for some distance windward of the barrier base (in the upwind effect), and for a less distance to the barrier’s lee (spillover). The lee side with respect to the prevailing moist flow is often characterized because of the dry rain shadow.

Uses of Orographic Precipitation

Let us look at the important uses of orographic precipitation, which are described below.

Elevated land masses and mountain ranges have a primary impact on the global climate. For example, the elevated areas of East Africa substantially determine the strength of the Indian monsoon. Also, in scientific models, like general circulation models, orography determines the lower boundary of the model over land.

When tributaries of the river or settlements by the river are listed in the 'orographic sequence', they are present in the order from the highest (which is nearest the river source) to the lowest or mainstem (which is nearest the mouth). This listing tributaries method is the same as the Strahler Stream Order, in which, the headwater tributaries are listed as category 1.

FAQ (Frequently Asked Questions)

1. List the types of rainfall?

Rainfall is a type of Precipitation. The term "rainfall" can be used to define precipitation in the form of water drops with sizes larger than 0.5 mm. The other forms of rainfall are given as drizzle, snow, sleet, glaze and hail.

Snow will form ice crystals which are usually combined to produce flakes. When the new snow holds an initial density ranging from 0.06 to 0.15 g/cc, it is usual to assume an average density of 0.1 g/cc.

Drizzle is defined as the fine sprinkle of a number of water droplets with a size less than 0.5 mm and intensity at less than 1 mm/h is called drizzle. In this, the drops are very small, where they appear to float in the air.

2. What factors affect rainfall?

Landforms, such as plateaus, mountains affect rainfall. Orographic condensation takes place as moist air is pushed up the landform’s rain side. On the other side, the arid conditions prevailing due to moisture are squeezed out on the side of the rain. Also, the high and low-pressure zones affect rainfall.

3. Explain if heavy rainfall causes a draught elsewhere?

Yes, it can be given that there is an accompanying drought on the next side of the fault line. For instance, there was a severe drought in California on the mountain’s west side that ran both North and South with heavy rainfall on the eastern side.

4. Give the conditions of orographic rainfall?

Orographic precipitation, snow, rain, or the other forms of precipitation can be produced when the moist air is lifted as it moves over a range of mountains.