As a child, I often wondered why do deserts exist? Later I found one more interesting question. Why deserts exist only in the western part of the continent. If this question also disturbed you, here all your doubts will be cleared.
There are three reasons for the existence of a dessert. They are (i) High Pressure (ii) Off-shore movement of air and (iii) Cold oceanic currents. The combination of all these three or two can cause the existence of a desert.
We have divided our earth into 180 imaginary lines each called a latitude. Some latitude experience Low-pressure area and some High-pressure. A low-pressure area is where the winds raise, forms clouds and hence rainfall. A High-pressure area is where the winds descend (wind come down from the sky) and no cloud formation hence no rainfall.
We have a high-pressure area around 30° and 90° latitude. Locate deserts western part of the continent around these latitudes.
Although winds can be categorised in many types on our earth. But here we are just talking about two types of winds i.e. (i) On-shore winds and (ii) Off-shore winds.
On-shore winds- When winds move from sea to the land bringing moisture to the land and forming clouds hence Orographic rainfall. The most common example of this type of rainfall and on-shore winds is the Indian Monsoon. When winds bring a lot of moisture from Arabian sea to the western coast of India. These winds counter with the mountains of the western coast and raise forming clouds hence bringing Orographic rainfall.
Off-shore winds- These winds are just opposite of the on-shore winds. When winds blow from land to the sea. It is called the off-shore winds. These winds are mostly dry and hot.
Between 30° north to 30° south, trade winds blow. Trade winds blow from east to the west direction. And between 30° north/south to 60° north/south, Westerlies blow. Westerlies blow from west to the east. And again between 60° north/south to 90° north/south easterlies blow. They blow from east to the west. Locate the region in these locations where the winds blow from land to sea. Did you find any desert?
Cold Oceanic Currents
Based on temperature, ocean currents are divided into two types i.e. (i) Warm currents and (ii) Cold currents.
Warm currents- As the name suggests, they are warm. The ocean currents that move from tropical area to sub-tropical or polar region are called ‘warm currents’.
Cold currents- The ocean currents that move from polar region to subtropical area and tropical area are called ‘cold currents’. All the cold deserts are because of cold currents.
Can you see the Sechura desert and Atacama desert on the western part of Peru and Chile respectively? They are mainly because of the cold current which came from the south pole.