To visualize this process, imagine a west wind flowing against a north-south-oriented mountain range — the wind is perpendicular to the mountains. As the wind crosses over the mountains, it is lifted upward, causing the air to cool.
Cool air is unable to hold as much moisture as warm air, and any moisture that is in the air condenses to form clouds that shift to the east. When this air descends, it compresses and warms in a process called adiabatic heating. Because of the constant barrage of warm, dry winds and a lack of precipitation, these areas, known as rain shadows, become extremely dry.
Desert conditions in the Sahara are caused by weather patterns, rather than by mountain ranges. The Sahara region is dominated by a large, semi-permanent, sub-tropical high pressure system. High pressure systems are typically characterized by dry and stable conditions. Additionally, air within a high pressure zone is heavier than air in a low pressure zone, so it tends to sink.
Air needs to move up to cool, form clouds and create rain. If the high pressure system is strong and unmoving, it will prevent rain in the long-term. This is the same thing that happens in the U. Deserts exhibit extreme temperatures because of the lack of moisture in the atmosphere , including low humidity and scarce cloud cover.
Mountainous areas in front of the prevailing winds create a rain shadow. In this image from the ISS, the Sierra Nevada Mountains are perpendicular to prevailing westerly winds, creating a rain shadow to the east down in the image. Note the dramatic decrease in snow on the Inyo Mountains. Another type of desert is found in the rain shadow created from prevailing winds blowing over mountain ranges.
As the wind drives air up and over mountains, atmospheric moisture is released as snow or rain. Atmospheric pressure is lower at higher elevations, causing the moisture-laden air to cool. Cool air holds less moisture than hot air, and precipitation occurs as the wind rises up the mountain.
After releasing its moisture on the windward side of the mountains, the dry air descends on the leeward or downwind side of the mountains to create an arid region with little precipitation called a rain shadow. Finally, polar deserts , such as vast areas of the Antarctic and Arctic, are created from sinking cold air that is too cold to hold much moisture. Although they are covered with ice and snow, these deserts have very low average annual precipitation.
Solar energy converted to heat is the engine that drives the circulation of air in the atmosphere and water in the oceans. In other words, the Earth is heated unevenly depending on latitude and angle of incidence. Latitude is a line circling the Earth parallel to the equator and is measured in degrees. Generalized atmospheric circulation The figure shows the generalized air circulation within the atmosphere. Three cells of circulating air span the space between the equator and poles in both hemispheres, the Hadley Cell , the Ferrel or Midlatitude Cell, and the Polar Cell.
In the Hadley Cell located over the tropics and closest to the equatorial belt, the sun heats the air and causes it to rise. The rising air cools and releases its contained moisture as tropical rain.
The rising dried air spreads away from the equator and toward the north and south poles, where it collides with dry air in the Ferrel Cell. Arid zones between 15 o and 30 o north and south of the equator thus exist within which desert conditions predominate. The descending air flowing north and south in the Hadley and Ferrel cells also creates prevailing winds called trade winds near the equator, and westerlies in the temperate zone.
Note the arrows indicating general directions of winds in these zones. Other deserts, like the Great Basin Desert that covers parts of Utah and Nevada, owe at least part of their origin to other atmospheric phenomena. The Great Basin Desert , while somewhat affected by sinking air effects from global circulation, is a rain-shadow desert. As westerly moist air from the Pacific rises over the Sierra Nevada and other mountains, it cools and loses moisture as condensation and precipitation on the upwind or rainy side of the mountains.
Map of the Atacama desert yellow and surrounding related climate areas orange. One of the driest places on Earth is the Atacama Desert of northern Chile. The desert lies west of the Andes Mountains, in the rain shadow created by prevailing trade winds blowing west. As this warm moist air crossing the Amazon basin meets the eastern edge of the mountains, it rises, cools, and precipitates much of its water out as rain. Once over the mountains, the cool, dry air descends onto the Atacama desert.
Onshore winds from the Pacific are cooled by the Peru Humboldt ocean current. This super-cooled air holds almost no moisture and, with these three factors, some locations in the Atacama Desert have received no measured precipitation for several years. This desert is the driest, non-polar location on Earth. The polar vortex of mid-November, This cold, descending air shown in purple is characteristic of polar circulation.
Notice in the figure that the polar regions are also areas of predominantly high pressure created by descending cold dry air, the Polar Cells. As with the other cells, cold air, which holds much less moisture than warm air, descends to create polar deserts. This is why historically, land near the north and south poles has always been so dry.
In the inertial frame of reference of the top picture, the ball moves in a straight line. The observer, represented as a red dot, standing in the rotating frame of reference sees the ball following a curved path. This perceived curvature is due to the Coriolis Effect and centrifugal forces. The Earth rotates toward the east where the sun rises. Think of spinning a weight on a string around your head. The speed of the weight depends on the length of the string. When a fluid like air or water moves from a lower latitude to a higher latitude , the fluid maintains its momentum from moving at a higher speed, so it will travel relatively faster eastward than the Earth beneath at the higher latitudes.
Since each hemisphere has three atmospheric cells moving respectively north and south relative to the Earth beneath them, the Coriolis effect deflects these moving air masses to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect also deflects moving masses of water in the ocean currents. These are deflected to the right or west by the Coriolis effect.
This deflected air generates the prevailing trade winds that European sailors used to cross the Atlantic Ocean and reach South America and the Caribbean Islands in their tall ships. This air movement is mirrored in the Hadley Cell in the southern hemisphere; the lower altitude air current flowing equatorward is deflected to the left, creating trade winds that blow to the northwest.
In the southern hemisphere Mid- Latitude or Ferrel Cell, the poleward flowing surface air is deflected to the left and flows southeast creating the Southern Hemisphere westerlies. Another Coriolis-generated deflection produces the Polar Cells. At 60 o north and south latitude , relatively warmer rising air flows poleward cooling and converging at the poles where it sinks in the polar high.
This sinking dry air creates the polar deserts , the driest deserts on Earth. Persistence of ice and snow is a result of cold temperatures at these dry locations. Artillerymen must take the Coriolis effect into account on ballistic trajectories when making long-distance targeting calculations. Geologists note how its effect on air and oceanic currents creates deserts in designated zones around the Earth as well as the surface currents in the ocean. The Coriolis effect causes the ocean gyre Large circular ocean currents formed by global atmospheric circulation patters.
It also causes low pressure systems and intense tropical storms to rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Explanation of Coriolis Effect.
Use this quiz to check your comprehension of this section. Click directly on the answer button, not on the answer bar. Because of the Coriolis Effect in the Southern Hemisphere, low pressure systems like hurricanes rotate in which direction? Think about it! The Coriolis Effect causes moving masses to veer to the left in the Southern Hemisphere, thus low pressure storms rotate clockwise; hurricanes rotate counterclockwise in the Northern Hemisphere. All but sinking polar air are influenced by the Coriolis Effect.
Sinking polar air is caused by the lack of solar energy at such high latitudes, and would happen even if the Earth did not spin. Which latitudes have sinking air with dry conditions due to atmospheric circulation? What makes the Great Basin unique in North America?
It is called a basin because all water in the area stays or evaporates, as water does not drain to the oceans. Restart quiz. Weathering takes place in desert climates by the same means as other climates, only at a slower rate. While higher temperatures typically spur faster chemical weathering , water is the main agent of weathering , and lack of water slows both mechanical and chemical weathering. Low precipitation levels also mean less runoff as well as ice wedging.
W hen precipitation does occur in the desert, it is often heavy and may result in flash floods in which a lot of material may be dislodged and moved quickly. Newspaper rock, near Canyonlands National Park, has many petroglyphs carved into desert varnish. One unique weathering product in deserts is desert varnish.
Also known as desert patina or rock rust, this is thin dark brown layers of clay minerals and iron and manganese oxides that form on very stable surfaces within arid environments. The exact way this material forms is still unknown, though cosmogenic and biologic mechanisms have been proposed.
While water is still the dominant agent of erosion in most desert environments, wind is a notable agent of weathering and erosion in many deserts. This includes suspended sediment traveling in haboobs , or large dust storms, that frequent deserts.
Deposits of windblown dust are called loess. Loess deposits cover wide areas of the midwestern United States, much of it from rock flour that melted out of the ice sheets during the last ice age. Loess was also blown from desert regions in the West. Possessing lower energy than water, wind transport nevertheless moves sand, silt, and dust. As noted in chapter 11 , the load carried by a fluid air is a fluid like water is distributed among bedload and suspended load. As with water, in wind these components depend on wind velocity.
Since saltating sand grains are constantly impacting other sand grains, wind blown sand grains are commonly quite well rounded with frosted surfaces.
Saltation is a cascading effect of sand movement creating a zone of wind blown sand up to a meter or so above the ground. This zone of saltating sand is a powerful erosive agent in which bedrock features are effectively sandblasted. The fine-grained suspended load is effectively sorted from the sand near the surface carrying the silt and dust into haboobs. When wind velocity is high enough to slide or roll materials along the surface, the process is called creep A slow and steady movement.
Used with faults, mass wasting in soils, and grain movement. One extreme version of sediment movement was shrouded in mystery for years: Sliding stones. Also called sailing stones and sliding rocks , these are large moving boulders along flat surfaces in deserts, leaving trails.
For years, scientists and enthusiasts attempted to explain their movement, with little definitive results. In recent years, several experimental and observational studies have confirmed that the stones, imbedded in thin layers of ice, are propelled by friction from high winds.
These studies include measurements of actual movement, as well as re-creations of the conditions, with resulting movement in the lab. A bedrock outcrop which has such a sandblasted shape is called a yardang. Rocks and boulders lying on the surface may be blasted and polished by saltating sand.
When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such wind abraded desert rocks are called ventifacts.
In places with sand and silt accumulations, clumps of vegetation often anchor sediment on the desert surface. Yet, winds may be sufficient to remove materials not anchored by vegetation. The bowl-shaped depression remaining on the surface is called a blowout. What does saltation do to the surface of saltating sand grains? Saltation causes lots of impacts among grains that produces rounding and frosting of the grain surfaces. Why is the rate of weathering and erosion slower in desert regions? The lack of water slows both chemical and mechanical weathering.
Drilling into aquifers provides water for drinking, agriculture , industry , and hygiene. However, it comes at a cost to the environment. Aquifers take a long time to refill. If desert communities use groundwater faster than it is replenished, water shortages can occur.
The Mojave Desert, in southern California and Nevada, for instance, is sinking due to aquifer depletion. The water level in the aquifer has sunk as much as 30 meters feet since the s, while the land above the aquifer has sunk as much as 10 centimeters 4 inches.
River s sometimes provide water in a desert. People often modify rivers to help distribute and store water in a desert. The Nile River ecosystem dominate s the eastern part of the Sahara Desert, for instance. The Nile provides the most reliable, plentiful source of freshwater in the region. The Aswan Dam harnesses the power of the Nile for hydroelectricity used in industry.
Construction of the Aswan High Dam was a huge engineering project. Local desert communities can divert rivers on a smaller scale. Throughout the Middle East, communities have dug artificial wadis, where freshwater can flow during rainy seasons. In countries like Yemen, artificial wadis can carry enough water for whitewater rafting trips during certain times of the year.
When deserts and water supplies cross state and national borders, people often fight over water rights. Rapidly expanding populations in California, Nevada, and Arizona have compounded the problem.
Agreements that were made in the early 20th century failed to account for Native American water rights. Mexican access to the Colorado, which has its delta in the Mexican state of Baja California, was ignored. Desert agriculture, including cotton production, demanded a large portion of the Colorado.
The environmental impact of dams was not considered when the structures were built. States of the Colorado River Basin continue to negotiate today to prepare for population growth, agricultural development , and the possibility of future droughts. Life in the Desert Plants and animals adapt to desert habitat s in many ways.
Desert plants grow far apart, allowing them to obtain as much water around them as possible. This spacing gives some desert regions a desolate appearance.
In some deserts, plants have unique leaves to capture sunlight for photosynthesis , the process plants use to make food. Small pore s in the leaves, called stomata , take in carbon dioxide. When they open, they also release water vapor.
In the desert, all these stomata would quickly dry out a plant. So desert plants typically have tiny, waxy leaves. Cactus es have no leaves at all. They produce food in their green stems.
Some desert plants, such as cactuses, have shallow, wide-spreading root system s. The plants soak up water quickly and store it in their cell s. Saguaro cactuses, which live in the Sonoran Desert of Arizona and northern Mexico, expand like accordions to store water in the cells of their trunks and branches.
A large saguaro is a living storage tower that can hold hundreds of liters of water. Other desert plants have very deep roots. The roots of a mesquite tree, for example, can reach water more than 30 meters feet underground. Mesquites, saguaros, and many other desert plants also have thorns to protect them from grazing animal s.
Many desert plants are annual s, which means they only live for one season. Their seeds may lie dormant for years during long dry spells. When rain finally comes, the seeds sprout rapidly. Plants grow, bloom, produce new seeds, and die, often in a short span of time. A soaking rain can change a desert into a wonderland of flowers almost overnight.
Animals that have adapted to a desert environment are called xerocole s. Xerocoles include species of insects, reptiles, birds, and mammals. Some xerocoles avoid the sun by resting in scarce shade. Many escape the heat in cool burrow s they dig in the ground.
The fennec fox, for example, is native to the Sahara Desert. Fennec fox communities work together to dig large burrows, some as large as 93 square meters 1, square feet. Dew can collect in these burrows, providing the foxes with fresh water. However, fennec foxes have adapted so they do not have to drink water at all: Their kidney s retain enough water from the food they eat.
Most xerocoles are nocturnal. They sleep through the hot days and do their hunting and foraging at night. Deserts that seem desolate during the day are very active in the cool nighttime air. Foxes, coyotes, rats, and rabbits are all nocturnal desert mammals. Snakes and lizards are familiar desert reptiles. Insects such as moths and flies are abundant in the desert. Most desert birds are restricted to areas near water, such as river banks.
However, some birds, such as the roadrunner, have adapted to life in the desert. The roadrunner, native to the deserts of North America, obtains water from its food. Some xerocoles have bodies that help them handle the heat.
Some desert vultures urinate on their own legs, cooling them by evaporation. Many desert animals have developed ingenious ways of getting the water they need. The thorny devil, a lizard that lives in the Australian Outback , has a system of tiny grooves and channels on its body that lead to its mouth. The lizard catches rain and dew in these grooves and sucks them into its mouth by gulping.
Camels are very efficient water users. The animals do not store water in their humps, as people once believed. The humps store fat.
Hydrogen molecule s in the fat combine with inhaled oxygen to form water. During a shortage of food or water, camels draw upon this fat for nutrition and moisture. Dromedary camels, native to the Arabian and Sahara deserts, can lose up to 30 percent of their body weight without harm. People and the Desert About 1 billion people live in deserts. Many of these people rely on centuries-old customs to make their lives as comfortable as possible Civilization s throughout the Middle East and Maghreb have adapted their clothing to the hot, dry conditions of the Sahara and Arabian deserts.
Clothing is versatile and based on robes made of rectangles of fabric. Long-sleeved, full-length, and often white, these robes shield all but the head and hands from the wind, sand, heat, and cold. White reflects sunlight, and the loose fit allows cooling air to flow across the skin.
These robes of loose cloth can be adjusted folded for length, sleeves, and pockets, depending on the wearer and the climate. A thobe is a full-length, long-sleeved white robe. An abaya is a sleeveless cloak that protects the wearer from dust and heat. A djebba is a short, square pullover shirt worn by men. A kaffiyeh is a rectangular piece of cloth folded loosely around the head to protect the wearer from sun exposure, dust, and sand. It can be folded and unfolded to cover the mouth, nose, and eyes.
Kaffiyehs are secured around the head with a cord called an agal. A turban is similar to a kaffiyeh, but wrapped around the head instead of being secured with an agal. Turbans are also much longer—up to six meters 20 feet! Desert dwellers have also adapted their shelters for the unique climate. The ancient Anasazi peoples of the southwestern United States and northern Mexico constructed huge apartment complexes in the rocky cliff s of the Sonoran Desert.
These cliff dwellings, sometimes dozens of meters off the ground, were constructed with thick, earthen walls that provided insulation. Although temperatures outside varied greatly from day to night, temperatures inside did not. Tiny, high windows let in only a little light and helped keep out dust and sand. The need to find food and water has led many desert civilizations to become nomadic. Nomadic cultures are those that do not have permanent settlements. In the deserts of the Middle East and Asia, nomadic tent communities continue to flourish.
Tent walls are made of thick, sturdy cloth that can keep out sand and dust, but also allow cool breezes to blow through. Tents can be rolled up and transported on pack animal s usually horses, donkeys, or camels.
Nomad s move frequently so their flocks of sheep and goats will have water and grazing land. Besides animals like camels and goats, a variety of desert vegetation is found in oases and along the shores of rivers and lakes.
Figs, olives, and oranges thrive in desert oases and have been harvested for centuries. Some desert areas rely on resource s brought from more fertile areas—food trucked in from distant farmlands or, more frequently, water piped from wetter regions. Large areas of desert soil are irrigate d by water pumped from underground sources or brought by canal from distant rivers or lakes.
The booming Inland Empire of southeastern California is made up of deserts the Mojave and the Sonoran that rely on water for agriculture, industry, and residential development. Canal s and aqueduct s supply the Inland Empire with water from the Colorado River, to the east, and the Sierra Nevada snowmelt to the north. A variety of crops can thrive in these irrigated oases. Sugar cane is a very water-intensive crop mostly harvested in tropical regions.
However, sugar cane is also harvested in the deserts of Pakistan and Australia. Water for irrigation is transported from hundreds of kilometers away, or drilled from hundreds of meters underground. Oases in desert climates have been popular spots for tourists for centuries. The Dead Sea has had flourishing spas since the time of King David. Air transportation and the development of air conditioning have made the sunny climate of deserts even more accessible and attractive to people from colder regions.
Desert parks, such as Death Valley National Park, California, attract thousands of visitors every year. But in cities, structures like buildings, roads, and parking lots hold on to daytime heat long after the sun sets. This is called the urban heat island effect. It is less pronounced in desert cities than cities built in heavily forested areas. New York was built on wetland habitat, and Atlanta was built in a wooded area.
They may be only slightly warmer than the surrounding desert. Deserts can hold economically valuable resources that drive civilizations and economies. The most notable desert resource in the world is the massive oil reserve s in the Arabian Desert of the Middle East.
More than half of the proven oil reserves in the world lie beneath the sands of the Arabian Desert, mostly in Saudi Arabia. The oil industry draws companies, migrant workers, engineers, geologist s, and biologist s to the Middle East. Desertification Desertification is the process of productive cropland turning into non-productive, desert-like environments.
Desertification usually happens in semi-arid areas that border deserts. Human activities are a primary cause of desertification. These activities include overgrazing of livestock , deforestation , overcultivation of farmland, and poor irrigation practices. Overgrazing and deforestation remove plants that anchor the soil. As a result, wind and water erode the nutrient -rich topsoil.
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