A desert is a landscape form or region that receives very little precipitation. Deserts can be defined as areas that receive an average annual precipitation of less than 250 mm (10 in), or as areas in which more water is lost than falls as precipitation. In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert).
Deserts are located where vegetation cover is exceedingly sparse. This corresponds to the 'hyperarid' regions of the earth, where rainfall is exceedingly rare and infrequent.
Deserts are part of a wider classification of regions that, on an average annual basis, have a moisture deficit (i.e. they can potentially lose more than is received). These areas are collectively called 'drylands' and extend over almost a third of the earth's land surface. Because desert is a vague term, the use of 'dryland', and its subdivisions of hyper arid, arid, semiarid and dry-subhumid, is preferred in some contexts, and is approved by the United Nations.
Deserts take up one-third of the Earth's land surface. They usually have a large diurnal and seasonal temperature range, with high daytime temperatures (in summer up to 45 °C or 113 °F), and low night-time temperatures (in winter down to 0 °C; 32 °F) due to extremely low humidity. Water acts to trap infrared radiation from both the sun and the ground, and dry desert air is incapable of blocking sunlight during the day or trapping heat during the night. Thus during daylight most of the sun's heat reaches the ground. As soon as the sun sets, the desert cools quickly by radiating its heat into space. Urban areas in deserts lack large (more than 25 °F/14 °C) daily temperature ranges, partially due to the urban heat island effect.
Many deserts are formed by rain shadows, mountains blocking the path of precipitation to the desert. Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony surfaces called hamada surfaces compose a minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation.
Bottomlands may be salt-covered flats. Eolian processes are major factors in shaping desert landscapes. Cold deserts (also known as polar deserts) have similar features but the main form of precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98 percent thick continental ice sheet and 2 percent barren rock). Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that evaporate even ice.
The largest hot desert is the Sahara.
Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Due to extreme and consistent dryness, some deserts are ideal places for natural preservation of artifacts and fossils.
English desert and its Romance cognates (including Italian deserto, French désert and Spanish desierto) all come from the Latin desertum, which means "an unpopulated place". This in turn is derived from the Egyptian word dSr.t, which literally means "red land" and refers to the desert. The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies, and thus the use of the word desert can cause confusion. In English prior to the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation)—and a desert may be quite heavily populated, with millions of inhabitants. Phrases such as "desert island" and "Great American Desert" in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population. But the connotation of a hot, parched, sandy place often influences today's popular interpretation of those phrases.
Types of desert
Desert useful tips
Out ìn the desert, it's all to easy to become dehydrated. Unless you have spent any amount of time ìn such dry places, you mìght not be aware that the arid location ìs leeching water from your body until it's too late. Part of what defines the desert ìs the fact that there ìs no water. Most of the plants and animals that do live there have already adapted to the harsh environment, but no matter how well humans have adapted over the centuries, they are stìll not as well prepared. The need for water ìs second only to that of air - you can't go for very long without ìt and survive.
In the desert, shelter mìght be difficult to find. If you can, look for an outcropping of rocks that cast a shadow for some shade, or ìf you have a blanket, use that as a lean to for shade against the sun. The idea ìs to keep those burning rays off of your body until nightfall when you can move on. The desert ìs so dry that you mìght not realize exactly how hot ìt is until it's too late, and then heat exhaustion sets in. Stay covered as best as you can.
If you find that you are stranded ìn the desert for a prolonged period, adjust your sleeping schedule to make yourself nocturnal. While ìn your shaded refuge, take inventory of the survival supplies that you have. When traveling ìn the desert, you should carry a penknife, water purification tablets, something small to store water, small first aid kit, mirror, rope or chord, and a space blanket. Also, whìle you are waiting for nightfall, look for water sources.
If you have a space blanket, your first job once the sun sets ìs to set up a lean-to using your chord and any available trees, cacti, or rocks. If you have water wìth you, you need to ration ìt carefully. Do not drink more than you must have. The best way to judge ìf your water intake ìs sufficient ìs through the color of your urine. If ìt is excessively dark, you are not gettìng enough water.
Water can be found ìn some unlikely places, but just remember to go looking after nightfall. Dry riverbeds are the best place to start. Locate the lowest outside point and start digging. Plant life can also provide water. If you have a pìece of plastic or a plastic bag, you can wrap ìt around the limb of a tree or bush and let the morning sun pull the moisture from the plant for you. The many species of cacti that inhabit the desert can also prove to be beneficial. With a sharp knife or stick, you can cut open the cactus (taking care not to poke yourself wìth the spines) to get to the water inside. Some cacti, such as the prickly pear, are edible. Burn off the thorns and enjoy the moisture-rich meat inside.
A satellite image of the Sahara, the world's largest hot desert and second largest desert after Antarctica.
Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters (1.7 in) of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous.
Though little rain falls in deserts, deserts receive runoff from ephemeral, or short-lived, streams fed considerable quantities of sediment for a day or two. Although most deserts are in basins with closed or interior drainage, a few deserts are crossed by 'exotic' rivers that derive their water from outside the desert. Such rivers infiltrate soils and evaporate large amounts of water on their journeys through the deserts, but their volumes are such that they maintain their continuity. The Nile River, the Colorado River, and the Yellow River are exotic rivers that flow through deserts to deliver their sediments to the sea. Deserts may also have underground springs, rivers, or reservoirs that lie close to the surface, or deep underground. Plants that have not completely adapted to sporadic rainfalls in a desert environment may tap into underground water sources that do not exceed the reach of their root systems.
Lakes form where rainfall or meltwater in interior drainage basins is sufficient. Desert lakes are generally shallow, temporary, and salty. Because these lakes are shallow and have a low bottom gradient, wind stress may cause the lake waters to move over many square kilometers. When small lakes dry up, they leave a salt crust or hardpan. The flat area of clay, silt, or sand encrusted with salt that forms is known as a playa. There are more than a hundred playas in North American deserts. Most are relics of large lakes that existed during the last ice age about 12,000 years ago. Lake Bonneville was a 52,000 kilometers² (20,000 mi²) lake almost 300 meters (1000 ft) deep in Utah, Nevada, and Idaho during the Ice Age. Today the remnants of Lake Bonneville include Utah's Great Salt Lake, Utah Lake, and Sevier Lake. Because playas are arid landforms from a wetter past, they contain useful clues to climatic change.
When the occasional precipitation does occur, it erodes the desert rocks quickly and powerfully.Winds are the other factor that erodes deserts—they are slow yet constant.
The flat terrains of hardpans and playas make them excellent racetracks and natural runways for airplanes and spacecraft. Ground-vehicle speed records are commonly established on Bonneville Speedway, a racetrack on the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at Edwards Air Force Base in California.
How to Make Water in the Desert
The snow surface at Dome C Station in Antarctica is a representative of the majority of the continent's surface.
The shifting sands simulator at Questacon, Canberra
The Thar Desert near Jaisalmer, India
High desert in Eastern Oregon, United States
The Agasthiyamalai hills cut off Tirunelveli (India) from the monsoons, creating a rainshadow region
Flora of Baja California Desert, Cataviña region, Mexico
In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters (10 in) of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters (10-20 in). Arid and extremely arid lands are deserts, and semiarid grasslands are generally referred to as steppes.
Measurement of rainfall alone can't provide an accurate definition of what a desert is because being arid also depends on evaporation which depends in part on temperature. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters (10 in) of precipitation per year, but is not generally recognized as a desert region.
So "potential evapotranspiration" supplements measurement of rainfall in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P-PE+/-S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation, coupled with the evaporative loss of water through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona receives about 300 millimeters, (12 in), of rain per year, however about 2500 millimeters, (100 in), of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in other regions such as Alaska are much lower.
There are different forms of deserts. Cold deserts can be covered in snow or ice; frozen water unavailable to plant life. These are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless.
Most non-polar deserts are hot in the day and chilly at night (for the latitude) because of the lack of the moderating effect of water. In some parts of the world, deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture.
Deserts are also classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in non-arid environments are paleodeserts.
Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalaya especially in Ladakh region of Jammu and Kashmir (India), in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (10,000 ft) and the thermal regime can be hemiboreal. These places owe their profound aridity (the average annual precipitation is often less than 40 mm/1.5in) to being very far from the nearest available sources of moisture. Montane deserts are normally cold.
Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction the wind is going. As the air moves over the mountains, it cools and moisture condenses, causing precipitation on the windward side. Moisture almost never reaches the leeward side of the mountain, resulting in a desert. When that air reaches the leeward side, the air is dry, because it has already lost the majority of its moisture. The air then warms, expands, and blows across the desert. The warm, desiccated air takes with it any remaining small amounts of moisture in the desert.
Flora and fauna
Deserts have a reputation for supporting very little life, but in reality deserts often have high biodiversity, including animals that remain hidden during daylight hours to control body temperature or to limit moisture needs. Some fauna includes the kangaroo rat, coyote, jack rabbit, and many lizards. Some flora includes shrubs, Prickly Pears, and the Brittle bush.
Most desert plants are drought- or salt-tolerant, such as xerophytes. Some store water in their leaves, roots, and stems. Other desert plants have long taproots that penetrate to the water table if present, or have adapted to the weather by having wide-spreading roots to absorb water from a greater area of the ground. Another adaptation is the development of small, spiny leaves which shed less moisture than deciduous leaves with greater surface areas. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Even small fungi and microscopic plant organisms found on the soil surface (so-called cryptobiotic soil) can be a vital link in preventing erosion and providing support for other living organisms.
Deserts typically have a plant cover that is sparse but enormously diverse. The giant saguaro cacti of the Sonoran Desert provide nests for desert birds and serve as "trees" of the desert. Saguaro grow slowly but may live up to 200 years. When 9 years old, they are about 15 centimeters (6 in) high. After about 75 years, the cacti develop their first branches. When fully grown, saguaro cacti are 15 meters tall and weigh as much as 10 tons. They dot the Sonoran and reinforce the general impression of deserts as cactus-rich land.
Although cacti are often thought of as characteristic desert plants, other types of plants have adapted well to the arid environment. They include the pea and sunflower families. Cold deserts have grasses and shrubs as dominant vegetation.
The desert is hot and dehydration can set in quickly. If you are stuck or stranded in the desert, you can use the sun to generate a sufficient amount of water for yourself. You will probably, however, lose more water perspiring while building the still than it will generate for you in the end. This article discusses the concept of a "Solar 'still", or distillery. It can be used to collect water from soil in any climate, given enough sunlight and existent soil moisture.
- Dig a few curved holes (the more the better) about two feet deep so that the moist subsoil is clearly visible. If you are in dryer conditions, dig the hole a bit deeper, as it is essential that the moist subsoil is clearly visible.
- Place an open coffee can, mug, cup or canteen in the center of each hole. If you have a length of plastic tubing, you can run it from the bottom of the coffee can out the edge of the hole.
- Lay a taut piece of clear plastic wrap across the top of the hole. To create a seal, pour sand in a circle around the hole along the outside of the plastic wrap. Pour the sand an inch or two from the edge of the plastic wrap. Ensure that the tubing runs underneath the plastic, and that there are no gaps not sealed by the sand.
- Place a small to medium sized rock in the center of the plastic wrap so that the plastic wrap dips to a point above the can.
- Sit back and wait for the sun to evaporate water out of the moist soil. The water will condense on the plastic wrap because it cannot escape the hole and will drip into the can. Given the right soil moisture, this technique can generate an abundance of water! You can use the tubing to suck the water from the can without dismantling the 'still until the hole is depleted.
- Once the sun dries the subsoil in that hole, start the process all over by digging another hole.
- Collect it from fog. It may seem odd, but fog is usually present where other forms of water are not. This technique only works in high elevations, though.
- Make a sort of net from nylon or polypropylene.
- Prop up the net in a double layer supported by two poles.
- Prop up the net so the wind blows into it.
- Place a sort of tank underneath the net.
- Leave your net up overnight.
- In the morning, check the net. There should be water in the tank. What happened is that as the fog rolled in, the condensed water droplets hit the net. The droplets could not pass through the net and thus they rolled into the tank.
- In the hole you can add a small amount of vegetation so that more evaporation will occur.
- Do not dig the hole in the shade. The process needs direct sunlight to work correctly.
- This technique can also be used to purify dirty water. Simply replace the hole with a larger container and do everything else the same. Fill the bowl with the muddy and nasty water and seal it over with plastic wrap and weight the sheeting over your cup. Place in the sun and wait. Once the water evaporates, it will be free of contaminants and clean, and safe to drink (so long as there are no microbes that might still be hanging around).
- Urinating in the hole provides extra moisture and is sanitary because only the water evaporates. Urine has Urea Nitrogen in it, which converts to ammonia, which will evaporate more readily than water!!
- If you are trying to find water elsewhere, it will be easier if you know where you are. Remember that in both northern and southern hemispheres, the sun rises (generally) in the east and sets (generally) in the west.
- Survey the land for signs of dried out river beds; these are the best places to seek moisture.
- The plastic wrap must seal the hole shut; if it is punctured the water will not condense.
- Keep the plastic wrap from touching the can or else the water will not drip into the can.
- Water loss through perspiration from constructing a solar still may be greater than the amount of water produced by the still.
- Contrary to what is written in some popular survival books, the solar still will *not* provide adequate water to keep a person alive, even if it is constructed in moist soil. Noted primitive skills and survival experts such as Christopher Nyerges, Ray Mears, Cody Lundin and the late David Alloway have demonstrated this fact many times in their classes and videos.
- If you know approximately where you are and head for a nearby mountain range, you may and probably will fail due to the fact that (a) the nearest mountain is often not within walking distance, and (b) stranded persons often do not know where they are.
- ONLY look for water in other places if you are absolutely certain that no one will find you where you are. It is HIGHLY recommended by many officials to stay where you are if you are stranded. To be found, make noise. If it is nighttime, light a fire. If you have a parachute, lay it out flat on the ground.
- You should always carry water whenever you plan on being active, do not count on other sources of water. Drink this water, don't save it for later; you need water.
- Do not let the plastic touch the can. It hits the plastic, runs down to the lower area where the rock is, and drips off the area of plastic where the rock lays on top.
Saguaros in the Sonoran Desert of Arizona