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Desert s are areas that receive very little precipitation. And most deserts, far from being empty and lifeless, are home to a variety of plants, animals, and other organism s. People have adapt ed to life in the desert for thousands of years. One thing all deserts have in common is that they are aridor dry. Most experts agree that a desert is an area of land that receives no more than 25 centimeters 10 inches of precipitation a year.
The amount of evaporation in a desert often greatly exceed s the annual rainfall. In all deserts, there is little water available for plants and other organisms. Some deserts are mountainous. Others are dry expanses of rock, sand, or salt flat s. Deserts are divided into these types according to the causes of their dryness.
Subtropical Deserts Subtropical desert s are caused by the circulation patterns of air mass es. They are found along the Tropic of Cancerbetween 15 and 30 degrees north of the Equatoror along the Tropic of Capricornbetween 15 and 30 degrees south of the 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 hinder s the formation of cloud s, so very little rain falls on the land below. The Sahara Desert is almost the size of the entire continental United States.
Coastal Deserts Cold ocean current s contribute to the formation of coastal desert s. Air blowing toward shorechilled 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 Deserton the Pacific shores of Chile, is a coastal desert. Some areas of the Atacama are often covered by fog.
But the region can go decade s without rainfall. In fact, the Atacama Desert is the driest place on Earth. Some weather station s in the Atacama have never recorded a drop of rain. Rain Shadow Deserts Rain shadow desert s exist near the leeward slopes of some mountain range s. Leeward slopes face away from prevailing wind s. 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 Valleyin the U. Death Valley, the lowest and driest place in North America, is in the rain shadow of the Sierra Nevada mountains.
Interior Deserts Interior desert s, 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 Desertin 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 desert s contain great quantities of water, but most of it is locked in glacier s and ice sheet s 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 and BCE, for example, the Sahara had a much milder, moister climate. This evidence includes rock paintings, grave s, and tools. Fossil s and artifact s show that lime and olive trees, oaks, and oleander s 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 25, years ago. 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-daroan 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. 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 cloudburst s. A cloudburst may bring as much as 25 centimeters 10 inches of rain in a single hour—the only rain the desert gets all year.
Desert humidity is usually so low that not enough water vapor exists to form clouds. 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 mirage s. 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. In the Chihuahuan Desert, in the United States and Mexico, temperatures can vary by dozens of degrees in one day. Winds at speeds of about kilometers per hour 60 miles per hour sweep through some deserts. 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. First-time visitors to deserts are often amazed by the unusual landscape s, which may include dunes, towering bare peaks, flat-topped rock formations, and smoothly polished canyon 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 scour s 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 arroyo s or wadi s. 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. InJeddah was struck by a sudden thunderstorm and flash flood.
Ro and buildings were washed away, and more than people died. Even in a desert, water and wind eventually wear away softer rock. Sometimes, rock is carved into tablelike formations such as mesa s and butte s. At the foot of these formations, water drops its burden of gravelsand, and other sediment, forming deposits called alluvial fan s. Many deserts have no drain age to a river, lake, or ocean. Rainwater, including water from flash floods, collects in large depressions called basin s. The shallow lakes that form in basins eventually evaporate, leaving playa s, 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. The hard, flat surface of desert salt flats are often ideal for car racing. InBritish pilot Andy Green set the land speed record in Black Rock Desert—1, kilometers per hour miles per hour.
Wind builds dunes that rise as high as meters feet. Dunes migrate constantly with the wind. They usually shift a few meters a year, but a particularly violent sandstorm can move a dune 20 meters 65 feet in a single day. Sandstorms may bury everything in their path—rocks, fields, and even towns. Halfway there, an enormous sandstorm swallowed the entire group. 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 groundwaterstored in aquifer s below the surface. Groundwater Rainy cold daylets warm eachother up dominant male seeks pet 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 spring s or seeps. A fertile green area called an oasisor cienegamay exist near such a water source. About 90 major, inhabited oases dot the Sahara. People, animals, and plants all surround these oases, which provide stable access to water, food, and shelter. Many desert cities, from the American Southwest to the Middle Eastrely heavily on such aquifers to fill their water needs.
Rural Israeli communities called kibbutzim rely on aquifers to furnish water for crop s and even fish farming in the dry Negev Desert. Drilling into aquifers provides water for drinking, agricultureindustryand hygiene.Rainy cold daylets warm eachother up dominant male seeks pet
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