A desert is defined as an area deficient in moisture, typically as a result of receiving, on average, less than ten inches or two hundred and fifty millimeters of rainfall in a year. As a result of this lack of rainfall, deserts are very dry and have sparse vegetation and animal life. But how do deserts actually form?
Deserts form when atmospheric air is too cold to hold moisture. Despite the cold atmospheric air, desert surface air is hot because there is no water vapor to deflect the sun’s heat. Deserts form because of Hadley Cells, cold oceanic upwellings, mountain rain shadows, deep inland locations, and extreme cold.
Deserts are one of the Earth’s five main biomes, and they cover approximately one-third of the Earth’s surface area. This article looks at how deserts form—what causes the lack of precipitation and how this affects the formation of desert features. Deserts can develop as a result of a combination of two or more of the different formation models.
What Are The Different Types Of Desert?
Deserts can fall under 3 different types:
- extremely arid
Additionally, Antarctica is considered to be a fourth type of desert.
Extremely arid deserts are regions where there is no precipitation throughout the year. These types of deserts cover approximately four percent of the Earth’s surface.
Arid deserts are regions where there is some precipitation, but this is countered by extremely high rates of evapotranspiration. Arid deserts account for about fifteen percent of the Earth’s surface area.
Semi-arid deserts receive more precipitation than arid deserts, but the rate of precipitation is still exceeded by the rate of evapotranspiration. Semi-arid deserts cover approximately fourteen percent of the Earth’s surface.
Desert Formation: Hadley Cells
Most of the Earth’s deserts are located on either side of the equator, specifically in the bands extending fifteen to thirty degrees North and South of the equator. This is because of the presence of Hadley Cells.
Hadley Cells are atmospheric circulations that start by carrying air up from the equator. This air then hits the stratosphere, which acts as a roof containment causing the air to move outward until it is approximately thirty degrees North or South of the equator. Once the air reaches these points, it moves down and along the Earth’s surface back towards the equator again. And so the cycle continues.
Hadley Cells are driven primarily by atmospheric temperatures. As you know, the Earth’s atmosphere is hottest at the equator and cools down towards the poles, where it is the coldest. Why is this?
Well, at the equator, the rays of the sun are hitting the Earth’s surface straight on. So, the heat is concentrated on a smaller surface area of the Earth.
Towards the poles, however, the curvature of the Earth means that the sun’s rays are hitting the surface at an angle that disperses the heat over a larger surface area. The Hong Kong Observatory gives a great illustration of this using torches. You can see their illustration HERE.
Because the air at the equator has a higher temperature, it is less dense. This means two things: first, that the air is capable of holding more moisture, and two, it will rise. As it rises, it cools. Cold air is incapable of holding as much water vapor as hot air, and so precipitation occurs. This is why the climate around the equator is so humid.
By the time the air reaches the thirty-degree mark North and South of the equator, it is very dry and very cold. This is why there is no precipitation at these locations, and deserts form. Cold air is denser, and so it sinks. This air then moves from areas of high density to areas of low density, i.e., the equator, heating up, and the cycle begins again.
But, if the atmospheric air over the desert areas is cold, why are deserts so hot?
When water vapor is in the air, some of the sun’s thermal energy is absorbed by the moisture, some is reflected back to the atmosphere by clouds, while the rest heats up the surface air and the ground.
Over desert regions, there is no moisture or clouds, so all of the sun’s heat is absorbed by the air above the ground and the ground itself, making a desert hot—at least during the day.
At night, the temperatures drop dramatically in a desert. This is because water vapor also acts as an insulator for heat. Thus, the lack of water vapor in the air above deserts means that the heat is lost quickly back into the atmosphere.
Furthermore, the heat absorbed by the ground rapidly escapes into the colder surface air (heat travels from high temperature areas to low temperature areas) and is also lost to the atmosphere.
Desert Formation: Cold Water Upwelling
Did you know that there are coastal deserts? The idea of a coastal desert may seem strange, but they form because of the cold ocean currents that travel along the western edges of the continents at these locations.
At these locations, the prevailing winds travel parallel to the coastline. However, they do not create oceanic currents that travel parallel to the coast. Instead, the rotation of the Earth causes the winds to drive the currents away from the coast.
The water that moves seaward has to be replaced, and this is achieved through upwelling. As the upper regions of the ocean are blown seaward, cold water from the lower regions rises up.
Air currents traveling over these cold ocean waters are cooled down. As they cool down, they lose their capacity to hold water, and precipitation occurs before they reach more inland locations along the coast.
As there is no water vapor in the air reaching these locations, deserts form. Once again, the air above these deserts is heated, unhindered, by the sun, resulting in the high daytime temperatures. Heat then rapidly escapes into the atmosphere at night, accounting for the low nighttime temperatures.
What can occur are advection fogs, which form when the cold ocean air encounters warm land surfaces.
Desert Formation: Mountain Rain Shadows
Some deserts are located right next to snowy mountain ranges. This is because, in these areas, the mountains create what is known as rain shadows.
Sub-tropical trade winds, which originate from the North-East (northern hemisphere) or South-East (southern hemisphere), are blocked by any mountain ranges in their path. The air is pushed up over the mountains, but as the air rises, its temperature decreases.
The air holds less moisture at these lower temperatures, and so precipitation occurs over the mountain. By the time the air arrives on the other side of the mountain (always the western side because of where the trade winds originate), it is dry, resulting in the formation of deserts in these rain shadows.
Desert Formation: Deep Inland Locations
Deserts can form in regions that are so far from the coast that no moisture remains in the air when it reaches these locations.
Desert Formation: Extreme Cold
Antarctica is considered to be a desert, even though it is covered in snow and not sand. This is because these areas still experience extremely low levels of precipitation.
As mentioned previously, the sun’s thermal energy reaching the Earth at the poles is spread over a very large surface area, accounting for the low temperatures. Because the air here is so cold, it holds very little moisture.
Any moisture it does hold will be precipitated again as snow. The low temperatures also prevent the fallen snow from melting and being evaporated in any significant quantity, so the snow just builds up.
Desert Formation: Desertification
Desertification is the process whereby non-desert land is transformed into deserts. The cause of desertification is a combination of three factors:
- Climate change
- More animals on the land
- Increased human populations
These three factors cause a loss of protective vegetation covering the land, exposing the soil to the effects of wind and rain, increasing the rate of evaporation from the grounds, and increasing the risk of weathering (soil erosion).
How Climate Change Contributes To Desertification
As our climate changes, it is showing a downward trend in the amount and reliability of rainfall. This means that the occurrence and intensity of periods of drought increase. Bodies of water, from rivers to waterholes, dry up during these rainless periods.
Loss of surface water and precipitation means that vegetation cover dies, leading to desertification.
Furthermore, global warming causes a global rise in temperatures. As air temperatures rise, the rate of evaporation increases, but the rate of condensation decreases, and less rain falls. Once again, this contributes to the death of vegetation and the loss of protective ground covering.
More Animals On The Land Contributes To Desertification
The land can only support a certain number of grazing animals. This number depends on the amount of vegetation, the rate at which the vegetation regrows, the time the land is allowed to recover, the number of animals per area, etc.
As people increase their livestock numbers, there are more individual animals grazing on smaller portions of land. Furthermore, the increase means that there is no chance to rotate the grazing areas, and the land is not allowed time to recover.
This is known as overgrazing. It not only removes the vegetation cover but is also strips the soils of its nutrients, which means that vegetation cannot regrow, even if given a chance to do so, leading to desertification.
Increased Human Population Contributes To Desertification
As there are more mouths to feed, farmers have to expand their farmlands, which involves pulling up natural vegetation to plant crops.
Additionally, farmers have to increase the frequency at which each pasture is used, which means the land has less time to recover and it becomes infertile.
More people also require more livestock, so it contributes to more animals being on the land.
Yet another result of increased human populations is the increased need for wood, which leads to deforestation, which also contributes to desertification.
Examples Of Deserts & How They Formed
Here, we’ll take a look at some of the deserts around the world and how they were formed…
The Sahara Desert
The Sahara Desert is always going to be one of the first deserts that people think of; this is because it holds the title of the largest desert in the world.
It spans the entire east-to-west width and covers almost all of Northern Africa, a staggering surface area of three million, three hundred, and twenty thousand square miles (eight million, six hundred thousand square kilometers).
The outer regions of the Sahara Desert are semi-arid, changing to arid further in, with a central region of extreme aridity.
Most of the Sahara Desert is found within fifteen to thirty degrees north of the equator and was formed by the effects of Hadley Cells.
Desert formation along the western coast was also contributed to by cold water upwelling.
The effects of being deep inland are also counted as one of the formative processes for the central regions of the Sahara Desert.
Furthermore, desertification is worst in the Sahel countries (Ethiopia, Sudan, Chad, Niger, and Somalia), which are located along the southernmost edge of the Sahara Desert.
The Gobi Desert
The Gobi Desert is located in Central Asia, and it also has extremely arid, arid, and semi-arid regions. But it is only about five hundred square miles (one million, three hundred thousand square kilometers).
The Gobi Desert is located further North than the Hadley Cells thirty-degree mark, so it was not formed by Hadley Cells. It is also far from any coastline. The major formation process for the Gobi Desert is that it is located so deeply inland that no water remains in the air that reaches it.
The Great Basin Desert
The Great Basin Desert is the largest of the four divisions of the North American Desert. It is arid and semi-arid desert land, covering approximately one hundred and ninety thousand square miles (four hundred and ninety two thousand square kilometers).
The Great Basin Desert was formed primarily as a result of the rain shadow created by the Sierra Nevada mountains, located West of the desert.
How Do Desert Landscapes Form?
Some deserts have hard-packed dirt, while others have soft, loose sand. Some deserts are flat as far as the eye can see, while others are dotted with barren mountain ranges, rocky hills, and sand dunes. So, how is a desert’s landscape formed?
As with any other landscape, desert landscapes are formed through erosion, transport, and deposition. These are controlled by:
- Mechanical weathering
- The effects of wind
- The effects of water
- Climate change
Desert Landscape Formation: Mechanical Weathering
Diurnal Temperature Differences
As we have already mentioned, the daytime temperatures in a desert are extremely high, while the nighttime temperatures drop very low.
Daytime temperatures in deserts typically exceed one hundred and four degrees Fahrenheit (forty degrees Celsius). Nighttime temperatures in deserts typically fall to around thirty two degrees Fahrenheit (zero degrees Celsius).
Insolation weathering: During the day, the sun’s rays shine unhindered by vegetation onto the desert’s surface. The surface layers, therefore, heat up throughout the day and can reach temperatures of one hundred and seventy six degrees Fahrenheit (eighty degrees Celsius). This causes the rocks to heat up and expand during the day before cooling and contracting at night.
Deserts are formed from different layers of different rocks. These different rocks heat up and expand and cool down and contract at varying rates, causing mechanical stress. Over time, these rocks start to crack and break.
When entire layers of rock peel off, it is known as exfoliation, and it produces rounded landforms called exfoliation domes. When the rocks fall apart granularly, it is called granular disintegration.
Frost shattering: This occurs in colder and mountainous deserts. The rainfall that does happen here seeps into the joints and cracks of desert rocks. During the warmer days, the water is in its liquid form. Then, at night, when the temperatures drop below freezing, the water becomes ice.
As you know, ice is less dense than water, and so it takes up more space. As it expands, it presses against the surrounding rock, creating pressure on the already weak joints and cracks. During the day, this pressure is suddenly removed as the water liquefies again.
This constant increase and decrease of pressure undermine the desert rocks and eventually blocks shatter off from the main part of the rock.
Rain contains salts, which seep into the surface layers of desert rocks when rain does occur. Furthermore, salts are leached out of the rock itself and carried to the surface layers by capillary action.
During the daytime, when temperatures are high, the moisture in which these salts are found is evaporated, leaving behind crystallized salt particles, which expand within the crevasses and joints of the upper rock layers.
Over time, the pressure caused by the crystalized salt expansion results in pieces of the rock breaking off.
Desert Landscape Formation: The Effects Of Wind
The ground composition of desert landscapes is affected by what once was there, i.e., before the tectonic plates shifted.
Some desert areas are thought to have once been the lowland deposition location of rivers that flowed from the surrounding highlands. In the basins, these rivers deposited stones, rocks, and pebbles along with sand, clay, and silt.
Now, the pebbles, stones, and rocks are being exposed in the barren desert landscapes as wind erosion removes the smaller and light silt, sand, and clay particles in a process call deflation.
Once the surface is covered only in these pebbles, rocks, and stones, it is protected from further deflation erosion.
In other places, chemical weathering loosens hard-packed dirt, and the resultant sand is also removed by deflation, creating deflation hollows.
A second wind erosion process responsible for shaping desert landscapes is known as abrasion.
One particular wind transportation method called saltation, which will be described in the following section, causes rocks to be eroded through a sandblasting action. The areas of rocky outcroppings that are near the ground are eroded away by abrasion.
This is why you get top-heavy desert formations like rock pedestals.
There are three main wind transportation methods in a desert.
Suspension: The finest material (diameters less than 0.006 inches or 0.15 millimeters) is moved by suspension. Being small and light, these particles are easily picked up by the winds and carried high and far.
Sandstorms are caused when wind speeds are such that vast quantities of material can be suspended and moved at the same time.
Saltation: Saltation is the process whereby particles that are between 0.006 inches (0.15 millimeters) and 0.01 inches (0.25 millimeters) in diameter are moved.
Particles moved by saltation are too big to be lifted high off the ground, rarely achieving heights beyond three feet (1 meter). They are also not carried for very long before being deposited again.
Soil creep or traction: As the particles carried by saltation return to the ground, the force can dislodge larger stones and sand particles, which are rolled along the desert’s surface in the process of soil creep or traction.
Deposition of saltation and surface creep particles can result in the formation of desert dunes. Large areas of dunes are called ergs, but they are only found in the Sahara and the Arabian Deserts.
There are eight main sand dune morphologies:
- Barchan: crescent-shaped dunes at right-angles to the wind direction, and with the concave edge on the downwind side. These are formed when the wind blows in a constant direction over limited amounts of sand.
- Barchanoid ridge: rows of uneven dunes at right-angles to the wind direction, formed and moved when the wind blows in a constant direction over limited amounts of sand.
- Transverse: rows of wave-like dunes formed by fluctuating winds that travel in a constant direction over thick sand.
- Dome: these are formed by strong winds blowing over areas of plenty of coarse sand.
- Seif: long, linear dunes at parallel angles to the wind direction, formed by persistent winds that exhibit diurnal or seasonal direction changes, blowing over large amounts of sand.
- Parabolic: curved dunes at right-angles to the wind direction and with the convex edge on the downwind side. These are formed when the wind blows in a constant direction over limited amounts of sand.
- Star: star-shaped dunes formed when the wind blows in many different directions over a limited amount of sand.
- Reversing: irregular and undulating shaped dunes, formed by the flow of equal intensity but opposite direction winds over a limited amount of sand.
The morphology of a sand dune is determined by the amount of sand that is available, the direction of the wind, how much vegetation there is, and whether the ground is smooth and sandy or rocky and uneven.
Some sand dunes form around obstacles, while others form simply because of the ebb and flow of winds. Some dunes are mobile, while others do not move at all.
Desert Landscape Formation: The Effects Of Water
Now, you might think that in a desert where there is very little rainfall and which is known for being dry and barren, water would not feature at all in the formation of the landscape. However, in most desert regions, rain does fall.
It is always infrequent and irregular. Most often, it is in small quantities, but sometimes there can be high-intensity rainstorms that actually cause flash floods.
Additionally, some deserts have rivers flowing through them—just think of the Grand Canyon. The paths of these rivers cut into the landscape, shaping it over time.
Deserts are regions that experience less than ten inches (two hundred and fifty millimeters) of rainfall each year.
Deserts form when the atmospheric air is cooled to the point at which it cannot hold any moisture. The moisture is lost as precipitation in areas adjacent to desert regions, and the atmospheric air above these deserts is characteristically dry.
The loss of moisture can be the result of Hadley Cells, cold oceanic upwellings along western coastlines, the rain shadow effect of mountains (desserts always from on the western side of mountains), being located deep inland, and extremely low temperatures (Antarctica). Furthermore, desertification is how deserts are spreading across larger areas of the Earth.
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