Orographic Effect

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Orographic Effect

Not many people are familiar with the impact of orographic uplift on climatic conditions; knowing what orographic effect is all about will perhaps help in understanding the same.

Many times, you must have noticed that the mountainous regions of the world receive more rainfall as compared to the coastal areas. Similarly, you must have also come across mountains with lush green vegetation on one side and barren land on the other. These occurrences are attributed to several factors; orographic effect is one of them.

Going by its simplest definition, orographic effect is an atmospheric condition which is triggered as a result of forced upward movement of air upon encountering a mountain or highland. This phenomenon eventually results in precipitation on the windward side of the mountain and rain shadow on its leeward side.

Orography: An Overview

Even though the term orography technically refers to the study of the formation and relief of mountains, in a broad sense it is also used to refer to the formation and relief of other elevated terrain. Basically, it is a discipline of geomorphology, which in itself is a branch of geology, that revolves around the study of rocks and landforms. The study of formation and relief of mountains is very important, as it plays a crucial role in determining the climate of various regions of the world.

What is the Orographic Effect?

In geography, the term ‘orographic effect’ refers to the weather condition triggered by upward movement of air mass on coming in contact with a mountain or some other elevated terrain. It has a crucial role to play in elevated mountainous regions of the world receiving more rainfall as opposed to the plains. Simply put, it is all about how mountains and other elevated terrain alter the weather conditions in a particular region.

When the air mass moves over a rising terrain, it is automatically forced to move upwards. This upward movement of air mass triggered by rising terrain is referred to as the ‘orographic lift’. As the mass of air continues to rise, it is subjected to adiabatic cooling―cooling of a body of air without the addition or subtraction of heat or thermal energy. This phenomenon, wherein temperature decreases as altitude increases, eventually results in condensation of air. 

When relative humidity is 100 percent, it triggers cloud formation and results in precipitation, which is referred to as ‘orographic precipitation’. Some of the most prominent examples of regions receiving orographic rainfall include the northwestern United States, Appalachian Mountains in West Virginia, the Western Ghats and lesser Himalayas in India, eastern seaboard of Australia, etc.

While the windward side of a mountain experiences heavy rainfall, all the moisture in the clouds is drained, owing to which the air which descends on the leeward side is dry and warm. This, in turn, results in a dry spell, which is known as ‘rain shadow effect’ on the leeward side of the mountain.

While the regions in the windward side of the mountain receive rainfall between 80 – 100 inches and even more at times, those regions on the leeward side, which are not farther than 10 – 20 miles, hardly receive 10 inches of rainfall. Such is the drastic difference in the windward and leeward sides of a mountain experiencing orographic effect. Some examples of such regions are California’s Central Valley, Kauai island of Hawaii, areas east of the Cascade Range, etc.

This phenomenon has a crucial role to play in determining the climatic conditions of various mountainous regions across the world. It is because of this phenomenon that windward sides of mountains receive rainfall in plenty and become lush green with vegetation cover, while the leeward sides experience dry spell and become barren. In quite a few cases, the regions which are subjected to the rain shadow effect for long periods fall prey to desertification―a phenomenon which is observed in the Death Valley in California.

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