Orographic Precipitation

Dhristi JEE 2022-24

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. Orography, in the case of a general circulation model, can also define the lower boundary or the model over land.


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. orographic precipitation is also known by the term 'relief precipitation'. 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.


Therefore, the orthographic effect on rainfall is the reason for the planet's sharpest climatic changes.


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 vapor condensation contained within it, and thus the cloud formation. If enough water vapor 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.


What is Orographic Lift?

Orographic lift is the process in which the moving air mass meets a mountain or any other geographical feature, and rises up over it. This happens due to the interruption in airflow. During orographic lift, if the air is moist then it forms clouds. 


Orographic lift can also raise the humidity to 100% and in favorable conditions, cause precipitation. Moreover, cooling results in rain or snow.


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. Also, when rainfall evaporates before reaching the earth (because it passes through dry air) it is known as virage.

  • 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.


Drawbacks of Orographic Precipitation

  • During orographic precipitation, cloud seeding causes floods, hurricanes which can cause harm. To many lives.

  • Once started, this precipitation is uncontrollable.

  • Windward side experiences a huge amount of rainfall and the leeward side experiences dry conditions.


Orographic Cloud

When the flow is humid enough, clouds form on the windward side of the mountain and are called orographic clouds. orographic clouds are formed only during orographic effect.


Rain Shadow

When the air goes to the leeward side, it loses moisture due to rainfall. this descending air gets warmer as it comes in contact with the heat of the troposphere. This absorbs the remaining moisture from it and creates a dry region called a rain shadow. As a result of this, areas around the leeward side are effectively drier because the orographic precipitation removes all the moisture from the rain shadow. 

 

Effect of Precipitation  on Air Pollution

The atmospheric reactions and dynamic processes are strongly linked to air quality and climate conditions. for example, due to orographic rainfall, humidity increases in that particular area. this leads to the increase in moisture content in air. We know that the increase in moisture level of the air can cause air pollution. along with this, it also causes many respiratory diseases to the humans surviving in that area. These conditions also affect the air quality and hence making it unfit for exchange of gasses in the environment. therefore, precipitation affects the air and can become a cause of air pollution.


Meteorology Impact

The impact of topography on precipitation is an important component used by meteorologists to forecast local weather. The type, intensity, and duration of precipitation are all influenced by orography, or high topography. According to studies, the volume and severity of orographic precipitation is determined by the breadth of the land barrier, the steepness of the slope, and the speed at which the air flows upward. According to computer models, narrow sides and steeper slopes create greater updrafts, which result in faster vertical speeds, which increases the quantity of precipitation.


Notable Occurrence Locations

Orographic precipitation is well-known in several parts of the world. On oceanic islands like hawaii and new zealand, mountain rainfall is prevalent. Much of the rain falls on the windward sides of these sites, while the opposite parts remain generally dry, occasionally desert-like. As a result of this occurrence, there are significant differences in rainfall. The interior uplands, which get over 100 inches (2,500 mm) per year, receive much less rain (20 to 30 inches or 510 to 760 mm) than coastal locations. The shores of the leeward islands are unusually arid. In hawaii, for example, waikiki receives less than 20 inches (510 millimetres) of yearly rainfall, but high places such as waikiki receive 475 inches (12,100 millimetres). The pennine mountain range in northern england is another area known for orographic precipitation.


Conclusion

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. orography can also define the lower boundary or the model over land for a general circulation model. Orographic precipitation is the precipitation that is generated by a forced upward movement of air upon a physiographic upland encounter.

Book your Free Demo session
Get a flavour of LIVE classes here at Vedantu
FAQs (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.


Moreover, Rainfall Has Mainly 3 Types :

  • Relief rainfall - when warm and moist air rises up over mountains, it causes relief rainfall.

  • Convectional rainfall - this rainfall usually happens during the summers, when the sun heats the land. 

  • Frontal rainfall - when a warm front meets a cold front then frontal rainfall takes place. in this, the warm air rises up and cools. this cooled down air condenses to form clouds and then rains heavily. 

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. Moreover, factors such as land slope, topography, wind velocity etc also affect the effective rain. To make the rainfall pattern effective, we must take the first step to adopt an eco-friendly lifestyle because 'pollution' also affects effective rainfall. 

3. Explain if heavy rainfall causes a drought elsewhere?

Yes, it can be given that there is an accompanying drought on the next side of the fault line. A drought can be defined as the shortage of water. It is possible for a region with high rainfall to suffer drought because it all depends on the water cycle. 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. In india, orographic rainfall occurs at the step slope of the western ghats side. It can also be experienced in other sloppy regions of the west coast of india such as pondicherry and the area around it. In orographic rainfall, the windward slopes get more rainfall. 

5. What is the difference between convectional rainfall and orographic rainfall?

Below are some points that shows the difference between convectional rainfall and orographic rainfall :

  • Convectional rainfall occurs when the moist air (over the heated ground) becomes warmer than the surrounding whereas the orographic rainfall takes place when moist wind is forced to rise over a mountain or upward heading slope. 

  • The conventional rainfall generally occurs in the tropical region (where temperature and evaporation is high) whereas orographic rainfall occurs in the upwind side of the mountain region. 

  • In conventional rainfall, we can experience heavy rainfall with thunder and lightning. but, in orographic rainfall, leeside of the mountain gets less rainfall due to the lack of moisture in the clouds. 

Comment