People often use the adjectives “hot,” “dry,” and “empty” to describe deserts, but these words do not tell the whole story. Although some deserts are very hot, with daytime temperatures as high as 5. And most deserts, far from being empty and lifeless, are home to a variety of plants, animals, and othwer organisms. People have adapted to life in the desert for thousands of years. The red fox is considered a more specialised form of Vulpes than the Afghan, corsac and Bengal foxes in the direction of size and adaptation to carnivory; the skull. Western California; Southern Nevada; Far southwestern and western Arizona; Far southwestern tip of Utah; Sonora and Sinaloa, Mexico Including Tibur One thing all deserts have in common is that they are arid, or dry. Most experts agree that a desert is an area of land that receives no more than 2. The amount of evaporation in a desert often greatly exceeds the annual rainfall. In all deserts, there is little water available for plants and other organisms. Deserts are found on every continent and cover about one- fifth of Earth’s land area. They are home to around 1 billion people—one- sixth of the Earth’s population. Although the word “desert” may bring to mind a sea of shifting sand, dunes cover only about 1. Some deserts are mountainous. Others are dry expanses of rock, sand, or salt flats. Kinds of Deserts. The world’s deserts can be divided into five types—subtropical, coastal, rain shadow, interior, and polar. Deserts are divided into these types according to the causes of their dryness. Subtropical Deserts. Subtropical deserts are caused by the circulation patterns of air masses. They are found along the Tropic of Cancer, between 1. Equator, or along the Tropic of Capricorn, between 1. Equator. Hot, moist air rises into the atmosphere near the Equator. As the air rises, it cools and drops its moisture as heavy tropical rains. The resulting cooler, drier air mass moves away from the Equator. As it approaches the tropics, the air descends and warms up again. The descending air hinders the formation of clouds, so very little rain falls on the land below. The world’s largest hot desert, the Sahara, is a subtropical desert in northern Africa. The Sahara Desert is almost the size of the entire continental United States. Other subtropical deserts include the Kalahari Desert in southern Africa and the Tanami Desert in northern Australia. Coastal Deserts. Cold ocean currents contribute to the formation of coastal deserts. Air blowing toward shore, chilled by contact with cold water, produces a layer of fog. This heavy fog drifts onto land. Although humidity is high, the atmospheric changes that normally cause rainfall are not present. A coastal desert may be almost totally rainless, yet damp with fog. The Atacama Desert, on the Pacific shores of Chile, is a coastal desert. Some areas of the Atacama are often covered by fog. But the region can go decades without rainfall.
In fact, the Atacama Desert is the driest place on Earth. Some weather stations in the Atacama have never recorded a drop of rain. Rain Shadow Deserts. Rain shadow deserts exist near the leeward slopes of some mountain ranges. Leeward slopes face away from prevailing winds. When moisture- laden air hits a mountain range, it is forced to rise. The air then cools and forms clouds that drop moisture on the windward (wind- facing) slopes. When the air moves over the mountaintop and begins to descend the leeward slopes, there is little moisture left. The descending air warms up, making it difficult for clouds to form. Death Valley, in the U. S. Death Valley, the lowest and driest place in North America, is in the rain shadow of the Sierra Nevada mountains. Interior Deserts. Interior deserts, which are found in the heart of continents, exist because no moisture- laden winds reach them. By the time air masses from coastal areas reach the interior, they have lost all their moisture. Interior deserts are sometimes called inland deserts. The Gobi Desert, in China and Mongolia, lies hundreds of kilometers from the ocean. Winds that reach the Gobi have long since lost their moisture. The Gobi is also in the rain shadow of the Himalaya mountains to the south. Polar Deserts. Parts of the Arctic and the Antarctic are classified as deserts. These polar deserts contain great quantities of water, but most of it is locked in glaciers and ice sheets year- round. So, despite the presence of millions of liters of water, there is actually little available for plants and animals. The largest desert in the world is also the coldest. Almost the entire continent of Antarctica is a polar desert, experiencing little precipitation. Few organisms can withstand the freezing, dry climate of Antarctica. Changing Deserts. The regions that are deserts today were not always so dry. Between 8. 00. 0 and 3. BCE, for example, the Sahara had a much milder, moister climate. Climatologists identify this period as the “Green Sahara.”Archaeological evidence of past settlements is abundant in the middle of what are arid, unproductive areas of the Sahara today. This evidence includes rock paintings, graves, and tools. Fossils and artifacts show that lime and olive trees, oaks, and oleanders once bloomed in the Sahara. Elephants, gazelles, rhinos, giraffes, and people used stream- fed pools and lakes. There were three or four other moist periods in the Sahara. Similar lush conditions existed as recently as 2. Between the moist periods came periods of dryness much like today’s. The Sahara is not the only desert to have dramatic climate change. The Ghaggar River, in what is now India and Pakistan, was a major water source for Mohenjo- daro, an urban area of the ancient. Indus Valley Civilization. Over time, the Ghaggar changed course and now only flows during the rainy monsoon season. Mohenjo- daro is now a part of the vast Thar and Cholistan deserts. Most of Earth’s deserts will continue to undergo periods of climate change. Desert Characteristics. Humidity—water vapor in the air—is near zero in most deserts. Light rains often evaporate in the dry air, never reaching the ground. Rainstorms sometimes come as violent cloudbursts. A cloudburst may bring as much as 2. Desert humidity is usually so low that not enough water vapor exists to form clouds. The sun’s rays beat down through cloudless skies and bake the land. The ground heats the air so much that air rises in waves you can actually see. These shimmering waves confuse the eye, causing travelers to see distorted images called mirages. Temperature extremes are a characteristic of most deserts. In some deserts, temperatures rise so high that people are at risk of dehydration and even death. At night, these areas cool quickly because they lack the insulation provided by humidity and clouds. Temperatures can drop to 4. In the Chihuahuan Desert, in the United States and Mexico, temperatures can vary by dozens of degrees in one day. Daytime temperatures in the Chihuahua can climb beyond 3. With little vegetation to block it, the wind can carry sand and dust across entire continents and even oceans. Windstorms in the Sahara hurl so much material into the air that African dust sometimes crosses the Atlantic Ocean. Sunsets on the Atlantic coast of the U. S. These features differ from those of wetter regions, which are often gently rounded by regular rainfall and softened by lush vegetation. Water helps carve desert lands. During a sudden storm, water scours the dry, hard- baked land, gathering sand, rocks, and other loose material as it flows. As the muddy water roars downhill, it cuts deep channels, called arroyos or wadis. A thunderstorm can send a fast- moving torrent of water—a flash flood—down a dry arroyo. A flash flood like this can sweep away anything and anyone in its path. Many desert regions discourage visitors from hiking or camping in arroyos for this reason. Even urban areas in deserts can be vulnerable to flash floods. The city of Jeddah, Saudi Arabia, sits in the Arabian Desert. In 2. 01. 1, Jeddah was struck by a sudden thunderstorm and flash flood. Roads and buildings were washed away, and more than 1. Even in a desert, water and wind eventually wear away softer rock. Sometimes, rock is carved into tablelike formations such as mesas and buttes. At the foot of these formations, water drops its burden of gravel, sand, and other sediment, forming deposits called alluvial fans. Many deserts have no drainage to a river, lake, or ocean. Rainwater, including water from flash floods, collects in large depressions called basins. The shallow lakes that form in basins eventually evaporate, leaving playas, or salt- surfaced lake beds. Playas, also called sinks, pans, or salt flats, can be hundreds of kilometers wide. The Black Rock Desert in the U. S. The hard, flat surface of desert salt flats are often ideal for car racing. In 1. 99. 7, British pilot Andy Green set the land speed record in Black Rock Desert—1,2. Green’s vehicle, the Thrust. SSC, was the first car to break the sound barrier. Wind is the primary sculptor of a desert’s hills of sand, called dunes. Wind builds dunes that rise as high as 1. Dunes migrate constantly with the wind. They usually shift a few meters a year, but a particularly violent sandstorm can move a dune 2. Sandstorms may bury everything in their path—rocks, fields, and even towns. One legend holds that the Persian Emperor Cambyses II sent an army of 5. Siwa Oasis in western Egypt around 5. BCE. Halfway there, an enormous sandstorm swallowed the entire group. Archaeologists in the Sahara have been unsuccessfully looking for the “Lost Army of Cambyses” ever since. Water in the Desert. Rain is usually the main source of water in a desert, but it falls very rarely. Many desert dwellers rely on groundwater, stored in aquifers below the surface. Groundwater comes from rain or other precipitation, like snow or hail. It seeps into the ground, where it can remain for thousands of years. Underground water sometimes rises to the surface, forming springs or seeps. A fertile green area called an oasis, or cienega, may exist near such a water source. About 9. 0 major, inhabited oases dot the Sahara. These oases are supported by some of the world’s largest supplies of underground water. People, animals, and plants all surround these oases, which provide stable access to water, food, and shelter.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
January 2017
Categories |