بحث , تقرير إنجليزي جاهز / DESERTS للصف العاشر

When people think of deserts, the following image comes to their minds : hot and arid land, vast expanses of sand, soil of reddish brown color, a sky of brilliant blue, no or very few plants, cacti, spiny leaves and camel is the only animal that they can think of.

The truth is not like that. It is not very common to find sand in a desert, but small rocks, pebbles and loose gravel on the surface layer instead. Only 15% of the world’ s desert surface is pure sand. (Parts of the Sahara and Arabia desert)

Desert occupies about one fifth to one third of the earth’ s surface. The rainfall pattern is not a seasonal one. Instead, rain usually falls in the form of sudden, violent thunderstorms. There may be several storms in a year or none for several years. The " average rainfall each year" is not calculated based on one year’ s rainfall, but on the total rainfall in a long period of time.

" How about the polar regions with sheets of floating ice?" You may ask. Large amount of water is locked up in the icecap. The average rainfall in the cold deserts located in polar regions is within 25cm.

The rate of evaporation ( known as " Moisture Index" ) can be used to classify deserts as there are extremely high temperatures, low humidities and very little cloud cover.

How extreme are the temperatures ? The daytime air temperature can reach 58oC (136oF) in the Sahara. Soil temperature may reach 80oC (176oF). For half a year, the average temperature of the Mongolian desert is below the freezing point. In Antarctica, the winter mean temperature is -30oC (41oF).

First of all lets talk about the weather in deserts .We all know that rain rarely falls in desert and when it comes, it comes in the form of thunderstorm.

In sandy desert, the rain usually drains away promptly and only change the landscapecomparatively slightly. In contrast, the torrential downpour in rocky deserts drains into wadis (rocky watercourses that is dry except after heavy rain). This deepen the dry valleys. Heavy downpour can build up into flash flood, carrying sand, gravel and then large rocks and boulders. Thus, at the end of most wadis, there is an enormous bank of sand and stone( known as "alluvial fan" ). The surplus sediment from the flash flood forms muddy lakes of different size and duration
Between wadis, there are flat plateaux in different extent called mesas. The mesas are isolated by the continuously widened wadi. The isolated mesas then become flat-topped, step-sided island in the desert, know as a buttle.

These lakes are particular seen in Australian desert. They lasts long enough to breed creatures like shrimps, frogs and wildfowl. Some of the lakes formed have high salt ************************************************** *************, which is thought to be derived from salt in the atmosphere, brought from oceanic spray.

Shallow, low-bottom-gradient lakes can be moved by wind stress over many square kilometers. When they dry up, an area of clay, silt, or sand encrusted with salt is found, known as playa.

Action of wind
Wind dehydrates soils and living things. Sand and dust particles are moved by desert winds. Desert winds also remove organic debris that makes the soil fertile. Since plants are scarce in deserts, wind erosion occurs more easily. Take Prairie States of North America as an example, a productive area was reduced in the 1930s to desert by desert wind (devastating tornado winds from desert), over-cropping and over-cultivating. The fine dust can be carried to kilometers away and thousands of meters up. Large amount of the fine duct rest in more temperate or moister regions and from the basis of loess (a fertile soil).

Impact of changes in temperature
Heat and cold produces the least observable effect in sandy desert. In contrast, their impact is much greater in rocky deserts.

In cold desert, rainfall is frozen at night in winter. When water goes into the tiny cervices and expands, the rocks are forced to split up.

In hot deserts, the fragmenting force of temperature is slower. Rock surfaces reach 70oC or 80oC at midday and cool down to freezing point at midnight. Expansion under the sun and contraction at night weaken the surface layers and cause flaking.

Secondly , lets talk about the different inhabitats that live in such places and how they adapt themselves in order to survive in such harsh weathers .

Desert plants have developed various ways to get as much water as possible and to reduce water loss.

Some plants have long roots to get water deep in the soil, or branched roots to get water over a wide area. With thick waxy layer on the stems and leaves, water can be retained and the tissues are protected from strong sunshine. Some plants have pin-like leaves to reduce water loss. Barrel Cactus

Many desert plants are succulents and they store water in their swollen stems or leaves.

Some desert plants are the " drought evaders" . They exist as seeds before the rain comes, and grow when it rains. They flower quickly to produce seeds and then die. There are " drought resisters" — perennial plants that possess the abilities of storing water, locating underground water or minimizing the use of water by various measures.

Very often, when people think of mammals in desert, they usually think of camels only. In fact, almost all of the mammalian orders have their representatives in the deserts.

To avoid the heat, some small mammals live in shelters underground, where the temperature fluctuations are not as much as those on the surface. Most important of all, the maximum temperature in burrows is much lower than that on the surface. They usually come out at night to search for food and store the food in burrows.
The Kangaroo Rat

Large mammals, like gazelles, onyx, asses and camels, find their shelters in the shade of a rock or tree. As the excessive heat may kill them, some of them lose heat by evaporation, which can only increase the staying power by 2-3oC. Of course, water must be available for sweating. They get water from waterholes and by eating plants and preys. The Mountain Lion

The small mammals have greater surface area to weight ratio. They gain and lose heat quickly and thus face greater problems in maintaining an acceptable body temperature range. Any animal living in desert has to have a gut that is able to excrete almost dry faecal material and a kidney that draws minimum water from blood. There are "summer hibernates" that minimize their ****bolic rates in summer to avoid water lost through respiration.

Some mammals, like hares, have large ears densely populated with blood vessels to remove heat.
Desert Carnivores obtain water from their preys. The dog and cat families lack sweat glands. They remove heat by panting which in turn lose much water so they are found mostly at the desert margins near a permanent water source.

With feathers, birds can keep out of external cold, heat loss and as well as external heat gained, for feathers can trap a layer of insulating air. The feathers are under muscular control and can be held erect. When there is a breeze, the upright feathers direct the air movement down to cool the skin.

Birds cannot sweat. They pant or flutter to remove heat.

Some flesh-eating birds don’ t need to drink as they can get enough moisture from their preys. The seed-eating birds need to drink daily and thus they are restricted in a number of places
where they can nest. Roadrunner
Birds find shelters underground, in the shade of rocks or bushes. Flying at high attitudes enables them to avoid heat. Birds can tolerate 3oC body temperature rise above normal level. No birds can survive at above 45oC.
Reptiles possess scaly skin to conserve moisture. They can control their body temperature by gaining or losing heat from surroundings. By using external heat, they do not need as much energy as the birds and mammals. Hence they need less food and breathe at a lower rate, which help them to conserve moisture.

Black-collared Lizard
The skins of the amphibians are soft and moist. This property allows the skin to respire and absorb oxygen from air and water. It seems unlikely that they can be found in deserts. Though, some frogs and toads manage to survive in the arid area. They must complete their life cycles before the wetlands evaporate after the rain, for their eggs have to develop in lakes or pools.

Most of the small animals like insects and arachnids (eg. Spiders, scorpions) have waterproof cuticles or shell mainly composed of a substance called chitin, which is hard, strong and impervious to many gases and liquids. It prevents them from drying up by the heat. The different stages of the invertebrates have various ways to prevent drying up. The egg is surrounded by a tough shell that is impervious to drought. It can last for months or even years, waiting for the rain to come. The larval stage often develops underground to avoid the heat. In the pupal stage, the cocoon or chrysalis protect the moth. The adult invertebrates, as protected by the chitinous exoskeleton, may live a long and productive life. It may lead a short life in the extreme, depending on the fat stored in its body. The short life is long enough for it to mate and lay eggs.
Tarantulas

Have you ever thought of the problems that face people who live in deserts ??Well there are quite a few .

If on the morning of a hot summer day, a healthy adult is placed in the middle of a desert without water for an hour, he will lose 0.9l of water by perspiring and he will be very thirsty. At the mid afternoon, he will lose about 5 to 8 kg of his weight. At night, if the daytime temperature is higher than 48.9oC, he may be dead. 3.8l of water a day is still not enough to save a man’s life from the heat for a week.

Loss of salt and clothing are also major problems .Through continuous sweating and intake of fresh water, the salt concentration in the body falls and causes severe cramping of muscles and headaches. Thus not only water , a source of salt is also essential.To avoid the heat, people in deserts have to dress up carefully :The long, flowing robes shield the skin from the sun and allow air to reach the body. The clothes are loose-fitting to prevent immediate sweat evaporation so that the body does not dehydrate so quickly in the very dry air. Headgear shields the head from the sun. Veils protect the face and keep the sand out of the mouth. The clothes also keep the people warm at night and in winter.

As for the homes ,some people live in permanent homes such as the mud houses of some Native Americans. The nomads carry with them portable homes as they move their livestock from place to place. All desert homes need to be well insulated from extreme heat and cold. Tents made of densely woven goat’s hair or thick-walled homes are suitable. Since rain is torrential, they need to be water proof.

Peolpe living in the deserts are called nomads who can be classified into three main groups : hunter-gatherers, pastoralists and traders.

Hunter gatherers move as small, independent bands within the area in which they know where are the waterholes, important trees, patches of smaller food plants and potential food animals. They move daily, monthly or semi-annually.

Postoralists move from place to place depending on the needs of their domesticated stock like sheep, goats or camels. Their targets are places with prolonged and predictable grass supply. Some of them may also hunt while others may cultivate an area near where they settle and grow cereal crops. They may also trade when they get a chance.

Traders are mobile merchants with trains of pack animals like camels, asses, mules or yaks with them. They supply goods produced on one side of the desert to the town people.

To conclude , you can see that there are lots of information that most people don’t know about deserts . Most of us think there is only one type of deserts , hot deserts , and even a bigger number of people don’t know the variuos plants and animals that live in deserts and how they cope themselves to the weather conditions . This report is just a summary of the great wonders of deserts as there is lots more that scientists are trying to discover nowadays .