The "mechanism" of the formation and development of deserts is subject primarily to the uneven distribution of heat and moisture on Earth, the zonality of the geographic envelope of our planet. The zonal distribution of temperatures and atmospheric pressure determines the specifics of the winds and the general circulation of the atmosphere. Above the equator, where the greatest heating of the land and water surface occurs, ascending air movements dominate.

An area of ​​calm and weak variable winds is formed here. The warm air that has risen above the equator cools somewhat and loses a large amount of moisture, which falls in the form of tropical showers. Then, in the upper atmosphere, the air flows north and south, towards the tropics. These air currents are called antitrade winds. Under the influence of the rotation of the earth in the northern hemisphere, the antitrade winds deviate to the right, in the southern hemisphere - to the left.

Approximately over latitudes of 30-40 ° C (near the subtropics), the angle of their deviation is about 90 ° C, and they begin to move along the parallels. At these latitudes, air masses descend to the heated surface, where they heat up even more, and move away from the critical saturation point. Due to the fact that in the tropics the atmospheric pressure is high all year round, and at the equator, on the contrary, it is low, near the earth's surface there is a constant movement of air masses (trade winds) from the subtropics to the equator. Under the influence of the same deflecting influence of the Earth in the northern hemisphere, the trade winds move from the northeast to the southwest, in the southern hemisphere - from the southeast to the northwest.

The trade winds capture only the lower thickness of the troposphere - 1.5-2.5 km. The trade winds prevailing in the equatorial-tropical latitudes determine the stable stratification of the atmosphere, prevent vertical movements and the development of clouds associated with them, and precipitation. Therefore, the cloudiness in these belts is not very significant, and the influx of solar radiation is the largest. As a result, there is extreme dryness of the air (relative humidity in the summer months averages about 30%) and exceptionally high summer temperatures. The average air temperature on the continents in the tropical zone in summer exceeds 30-35°C; here is the highest the globe air temperature - plus 58 ° C. The average annual amplitude of air temperature is about 20 ° C, and the daily temperature can reach up to 50 ° C, the soil surface sometimes exceeds 80 ° C.

Precipitation is very rare, in the form of showers. In subtropical latitudes (between 30 and 45° N of northern and southern latitudes), the total radiation decreases, and cyclonic activity contributes to humidification and precipitation, mainly associated with the cold season. However, sedentary depressions of thermal origin develop on the continents, causing severe aridity. Here, the average temperature of the summer months is 30 ° C or more, while the maximum temperature can reach 50 ° C. In subtropical latitudes, intermountain depressions are the most dry, where the annual precipitation does not exceed 100-200 mm.

In the temperate zone, conditions for the formation of deserts occur in inland regions such as Central Asia, where rainfall is no more than 200 mm. Due to the fact that Central Asia is fenced off from cyclones and monsoons by mountain rises, baric depression forms here in summer. The air is very dry, high temperature (up to 40 ° C or more) and very dusty. Air masses rarely penetrating here with cyclones from the oceans and from the Arctic quickly warm up and dry up.

Thus, the nature of the general circulation of the atmosphere, due to planetary features, and local geographical conditions create a peculiar climatic situation, forming to the north and south of the equator, between 15 and 45 ° C latitude, a desert zone. To this is added the influence of cold currents of tropical latitudes (Peruvian, Bengal, Western Australian, Canary and California). By creating a temperature inversion, cool, moisture-laden maritime air masses, easterly constant winds of baric maxima lead to the formation of coastal cool and foggy deserts with even less precipitation in the form of rain.

If the land covered the entire surface of the planet and there were no oceans and high mountain rises, the desert belt would be continuous and its boundaries would exactly coincide with a certain parallel. But since land occupies less than 1/3 of the globe, the distribution of deserts and their size depend on the configuration, size and structure of the surface of the continents. So, for example, the Asian deserts spread far to the north - up to 48 ° N.L. In the southern hemisphere, due to the vast water spaces of the oceans, the total area of ​​the deserts of the continents is very limited, and their distribution is more localized. Thus, the emergence, development and geographical distribution of deserts on the globe are determined by the following factors: high values ​​of radiation and radiation, a small amount of precipitation or their complete absence. The latter, in turn, is determined by the latitude of the area, the conditions of the general circulation of the atmosphere, the features of the orographic structure of the land, and the continental or oceanic position of the area.

In geographic science, deserts are areas that receive less than 200 millimeters of precipitation during the year, where the air is dry and on average contains less than one-third of the amount needed to saturate it, or is said to have less than 35 percent relative humidity. Deserts are areas where the average air temperature in the shade during the hottest months exceeds 25 ° C, but fluctuates sharply during the day, where many times more evaporates from the open surface of the water per year than it falls in the form of precipitation, where the vegetation is adapted to a permanent or seasonal significant lack of moisture and is usually extremely rarefied, and often completely absent in large spaces, where most animals are also well adapted to both a lack of water and high temperatures. Deserts are areas of insufficient moisture in which agriculture is usually only possible with artificial irrigation, but due to the fertility of most unleached soils and the abundance of sunlight, yields on irrigated lands can be especially high, and plants have high content sugars, proteins, oils, alkaloids and other most valuable substances.

The existence of deserts depends on the distribution of heat and moisture on the earth. The distribution of moisture is associated not so much with local winds as with common system movement throughout air shell land in general. Above the hottest belt of the earth, the equatorial, heated and therefore lighter air constantly rises and, cooling somewhat at the same time, loses a large amount of moisture that falls in the form of tropical showers. Having risen to a great height, this warm, but already dehydrated air spreads both to the north and to the south of the equator. But due to the rotation of the earth, it begins to deviate more and more to the side, pile up, condense and cool. Due to this, in the subtropical belt, the air sinks and generates surface winds that constantly blow in the opposite direction, towards the equator. These winds are called trade winds. Going down to the heated surface of the earth, the air masses warm up again, and as they move towards the equator, they continue to heat up more and more. The warmer the air, the large quantity moisture is needed to saturate it. Therefore, it turns out that the trade winds moving south have very dry air. The sky in the areas of their distribution is constantly cloudless, and the sun heats up the soil. Thus, due to the general conditions of movement of the air envelope about h km, as they say, due to the law of general circulation of the atmosphere, north and south of the equator, in areas adjacent to the tropics, namely between 15 ° and 35 ° north and south latitude, are formed belts, or zones, of subtropical deserts (see diagram).

Where should you look for deserts? If the land covered the entire surface of the earth, then the northern and southern belts of deserts would be continuous. But since the continents occupy only 29 percent of the earth's area, the distribution of deserts depends entirely on the size and shape of the continents. The influence of large continents on the climate is so great that in their central parts, several thousand kilometers away from the oceans or fenced off from them by high ridges, deserts also appear outside the zone of their general location. Such are the extratropical or intracontinental deserts, partly of America, and mainly of the greatest continent of the earth - Asia. Asian deserts reach up to 48°, and in some places even almost 50° north latitude and are associated with their origin with winds similar to the trade winds and blowing mainly south from the inland high pressure area.

southern belt. The southern hemisphere is poor in continents, and therefore the deserts in it occupy much smaller areas than in the northern one. The southern belt of deserts is divided into three separate links corresponding to three continents.

desert Central Asia and Kazakhstan. 1. Caspian lowland. 2. Ustyurt. 3. Mangyshlak. 4. Aral Kara-Kum. 5 Central Kazakhstan small hills. 6. Betpak-Dala, 7. Muyun-Kum. 8. Balkhash sands. 9. Kyzyl-Kum. 10. Kara-Kum (Turkmen). 11. Hungry steppe. 12. Fergana. desert overseas Asia . 13. Dzungarian deserts. 14, Takla Makan. 15. Tsaidam. 16. Gobi. 17. Ala Shan. 18. Ordos. 19. Salt desert Deshte-Kevir. 20. Desert Deshte-Lut. 21. Desert of Thar. Deserts of the Arabian Peninsula. 22. Syrian. 33. Big Nefud. 21. Rubel Khali. Deserts of North Africa (Sahara). 22. Big East Erg. 26. Big Western Erg. 27. Go ahead. 28. El-Jof. 29. Massif Tuareg. 30. Red desert. 31. Libyan desert. Deserts of South Africa. 32. Namib. 33. Kalahari. Deserts of Australia. 34. Great Sandy Desert. 35. Great Desert of Victoria. Deserts of North America. 36. Deserts of the Great Basin. 37. Colorado. 38. desert South America . 39. Atacama. 40. Patagonia.

In South America, despite the insignificant width of the mainland narrowing to the south, empty stretch far beyond the subtropics, reaching 50 ° south latitude, and belong to one of the driest regions of the world. For example, in the northern part of Chile, on the ocean coast, in Iquique, according to long-term data, on average, only one millimeter of precipitation falls per year.

The deserts here occupy mainly the eastern half of the southern tip of the mainland - Patagonia - and the interior of Argentina and extend northward from 50 ° south latitude to 30 °, where they end in the central regions of the mainland. To replace the Argentine deserts from 30 ° south latitude, along the western coast of South America, a kind of coastal desert begins, stretching in a narrow pod for several tens of kilometers along the coast. Pacific Ocean up to 6° south latitude.

In South Africa, the deserts are confined to the most southwestern outskirts of the mainland. Here along the coast Atlantic Ocean from 18 ° to 31 ° south latitude stretches the Namib coastal desert, moving away from the ocean to the southeast along the Orange River valley. And to the east of the Namib, in the central regions of the southern part of the mainland, there is a peculiar, mostly stony, semi-desert; there were, as they say now, the Kalahari desert steppe.

Australia has the largest deserts in the southern hemisphere. Deserts here cover the entire interior of the mainland and occupy more than half of its area, forming sandy massifs.

northern belt. In the northern hemisphere, deserts occupy relatively small areas in the New World, but in the Old World they form an almost continuous belt, starting at the western coast of Africa and ending only in East China.

In North America, the deserts are located partly on the shores of the Pacific Ocean, but mainly extend north from Baja California into the Lower Colorado region and into the Great Salt Lake basin. They are also found in the central regions of Mexico.

In Africa, deserts occupy almost the entire northern half of the mainland, stretching from 12-15 ° north latitude in places to the shores of the Mediterranean Sea. North Africa is. but in essence, the single and largest desert in the world, covering the Sahara, Algeria (except for the mountainous and coastal parts), Libya, Egypt and most of the Sudan. Here is the most extensive hot region of the world, where the air temperature in the shade often exceeds 50 ° C, and 58 ° was observed.

In Asia, deserts are located on an even larger area, but are divided into separate arrays. On the Arabian Peninsula, they cover almost the entire territory, except for the mountainous parts. Through Mesopotamia, they go east to Iran, Balochistan, Afghanistan and India. A huge area of ​​deserts is located in the more northern, extratropical regions of Central, Central and East Asia.

A small investigation into the recent appearance of deserts on our planet, with photographs of abandoned ancient Russian cities in North America.

I remember once in the army, about twenty years ago, one of my colleagues, somehow became emotional and spoke out to me, apparently mistaking him for his own. "We're still distorting history!" I hatched at him, “What story? Who are we distorting? But for some reason he suddenly stopped and fell silent, as if a fish had eaten sand. Then I didn’t enter at all, absolutely, what did he blurt out about, whom was he going to distort there?
Why distort if I don't know what? And no one will help, no one will tell. So now you have to dig yourself in order to know what to distort.

Well, for example, I have always wondered why there are deserts on earth? After all, this is absolutely not a normal state of nature. Where did they come from? As a child, like everyone else, I was probably interested in dinosaurs. And therefore I knew, based on paleontological data, that there were no deserts before, everywhere there are traces of ancient life, a tropical climate.
But nowhere is there any information about the origin of deserts. Only once, somewhere in the journal "Science and Life", there was a version, using the Sahara desert as an example. The fact that there were tropics on the site of the Sahara is no secret to anyone. The same Sphinx is corroded by tropical downpours from above, and not from the side by winds. So it was said there that for reasons and mechanisms that are still incomprehensible to us, every 10,000 years there is a change from a tropical climate to a desert one and vice versa. Now it is supposedly the turn of the desert, and after some time the desert will recede again, and its place will be taken by a violent life that will grow from small and rare oases, in which the original forms of life are preserved and live.
Yes, but something does not add up, however. Why exactly in the Sahara, and in other places, what kind of reservation is this, nothing changes? The selectivity is enviable, of course. Well, let's not discuss this nonsense, like others, let's do a better personal investigation. So let's see with our own eyes.
Firstly, it is striking that the deserts on earth are strictly local, but there are no bindings, as if scattered randomly. Around normal nature and climate, and suddenly bam, a spot of the desert, like spilled, some kind of acid, by accident, anywhere.
But as we now know, not entirely by chance and not entirely randomly. It is a strange coincidence, but just in the place of modern deserts there were centers of the former civilization. This can be seen even in old maps, of which there are many. Here, for example, one for 1575. It has no deserts at all. And on the site of modern deserts, a rather flourishing climate and high density signs of civilization, with cities and metropolitan areas. I wish Columbus had such a map in his time! And then he grabbed the poor fellow, it is not clear what slums, to the wild Papuans. And so I would immediately sail to civilized cities. Although he would most likely have been mistaken for a beggar there, and perhaps they would not have been allowed into the city.

As for the same Sahara, again, for some reason no one observes that it does not end with the African continent. And smoothly and without delay extends further to the east. Arabia, Persia, Pakistan, Central Asia and the mountainous desert of the Himalayas. And the Himalayas, as you know, were the haven of a prosperous Shambhala. But now she is not there, and they drove her either into fairy tales or into another dimension. Now the Himalayas is a wild and lifeless desert. There are people there in small numbers, but they live very hard, barely survive. For one wife, for two husbands they have a provision rate. And who is smart enough to say that someone was smart enough to come there from more favorable places? And they live there not because they stayed from good times, accidentally survived and continue to exist there.

But we will consider the Sahara-Himalayan anomaly and make a detailed alignment later. In the meantime, let's go from the edges. So to speak from distant cordons. For example, let's look at our native America. It immediately catches the eye that the American desert has a center. A clear center from which it diverges at an equal distance along the radius. Moreover, the epicenter is sharply highlighted in red. As if it was the result of an explosion of enormous power. It was as if someone had used some kind of super-powerful weapon of mass destruction. Like a quasi-electronic bomb of multiplier action.

And now let's take and look into the very center of the desert. Who was so mercilessly bombed there? Let's land short. And immediately we run into this.

It looks like some kind of huge temple complex.

For some reason, we see more recent chips, from the main surface, and not uniform weathering.

One gets the impression that either man cooperated with nature, or nature is absolutely reasonable.

Well, here cooperation with the mind is absolute.

Oh, there is a lot of this, the hand gets tired of looking. I specifically give from one patch only. But there are dozens of kilometers between these urban-type settlements. But whose are they? The Anglo-Saxons have only been living there for two hundred years. Buildings clearly more years. Yes, and they were built using polygonal masonry, which neither the Anglo-Saxons owned then, and no one in our time can reproduce it. With all the seemingly simplicity. But how, without a binder solution, stones of completely different sizes are folded into absolutely even, smooth and durable walls?? Don't believe? Try to do it yourself. That's what they were, these stones planed, a blaster or a tomahawk?

By the way, an attempt to reproduce polygonal masonry,

the pebbles, however, are loosely folded, which is understandable, the bricks are all crooked.

It even breaks Van Dam, or a second-grader kid. But this is the present, even Chuck Norissa cannot be broken.

And here is the Russian stove. In America. In Arizona.

So who built these buildings here? The Anglo-Saxons have nothing to do with it. What they are capable of, we know well.

And half-naked chungachgooks with tomahawks on mustangs cannot be cut down either. Moreover, their range is much to the south, in Central America. Then who broke this? Similar to ancient Greece. Turkey is full of it. There, however, similar structures date back two to three millennia. What do Americans say about this? Nothing, perhaps?

This is generally something incomprehensible, but something vaguely reminiscent. So they actually say that the Phoenicians have been to America? Well, that means the Phoenicians, and we will attribute, sowing architecture. True, they were mostly sailors, and these places are far inland. And the civilization here was quite significant and too thorough for mere travelers.

By the way, as you can see, many buildings are destroyed. And monotonously, as with a powerful blast wave. Just like the buildings at the nuclear test site, the masonry has been demolished from above. Well, who did it? As my youngest daughter says when she messes up something. "It's semo!" Some stones are melted and red from exposure to ultra-high temperature and black, oily streaks. There are swimming pools in the yards, but, of course, without water, everything has dried up or evaporated.

Well, these are wall paintings from various nearby places, petroglyphs, sort of.

At the bottom of the astronomical, something similar, such as Capricorn, Aries. And the zigzag line is probably some kind of number. Since we know that one of the ways to write numbers was, the number of angles. By the way, our figures are based precisely on this principle.

Well, now it seems to be in order, we at least roughly know what needs to be distorted.

And this is dear Africa!

Let's take a look at the South African Kalahari Desert. Right in her very geographic center. And immediately we get to some ruins, some buildings.

Do you want to freak out? Have you ever seen river dams in the desert? See.

And this is a giant brick. Grown into the ground. With even edges, size 6 x 35 meters. Like Baalbek, only many thousands of kilometers away opposite side mainland.

I do not know if this 2.5 km diameter crater has anything to do with the formation of the desert. But it is right at the epicenter of the Kalahari.

And these are fried pebbles. Evidence of exposure to ultra-high temperatures. Which instantly evaporated huge lakes and hundreds of rivers.

Former river. Former lake.

Now ostriches cut through local saigas there. They run fast and they are not hot. And from the planet Plyuk brought scorpions. It’s just not clear how people still live there, and it’s completely incomprehensible what they do there? After all, for hundreds of kilometers you can not see any fields, orchards and orchards. Some bare roads.

In short, see for yourself what was and what has become.

But this is the natural state of nature. Which has always been everywhere. And now somewhere, still left. But what was not destroyed at once, is being achieved now.

But living in the desert is much better. You can ride motorcycles, from refueling to refueling.

Definition and geographical features of deserts

Desert - a zonal type of landscape that has developed under conditions of moisture deficiency (arid desert) or heat (cold desert) and is characterized by very sparse and depleted phytocenoses (a set of plants). Deserts are common in the temperate zone of the Northern Hemisphere, subtropical and tropical zones of the Northern and Southern Hemispheres.

A desert is a certain geographical phenomenon, a landscape that lives its own special life, has its own laws, has its own features, forms of change during development or degradation. Deserted lands (even restored ones) retain their differences from grazing, irrigated and industrially developed territories. Within each of them, too, there are differences.

Arid deserts are distinguished by high summer seasonal (or even annual) air temperatures, low annual precipitation (usually from 100 to 200 mm), lack of surface runoff, salinity of groundwater, and uneven rainfall. It is in the desert that the following are observed: the absolute minimum of annual precipitation (from 0 to 10-15 mm.); absolute maximum air temperature (59°С), heating of the soil surface up to 80°С, decrease in relative air humidity up to 5-10%.

In the desert, the features of aridity (aridity) are most pronounced and reach the extreme, beyond which the complete destruction of the biological life of the landscape and the loss of economic potential begin, the transition to an extra-arid state (where the probability of permanent droughts is 75-100%).

Cold desert - A type of desert in which the scarcity of vegetation is due mainly to low temperatures. At the same time, ice and alpine deserts are distinguished. Separately, snowy deserts are distinguished (in Antarctica and the Arctic - the Arctic desert). The territory of deserts, as a rule, is drainless, sometimes they are crossed by transit rivers (Syr Darya, Amu Darya, Nile, Huang He, etc.); there are many lakes and rivers that dry up, often changing their shape and size (Lob Nor, Chad, Eir), periodically drying up watercourses are characteristic.

Groundwater is often mineralized. Soils are poorly developed, characterized by the predominance of water-soluble salts in the soil solution over organic matter, salt crusts are common.

Deserts have existed since ancient geological times and have their own long evolutionary history. As a result of the periodic change in the climate of the Earth, the boundaries of desert territories also changed. Most of the world's deserts were formed on ancient platforms, and occupy the oldest land areas.

Deserts in Asia, Africa and Australia are usually located at an altitude of 200 to 600 m, in Central Africa and North America - at an altitude of 1000 m above sea level, within the ancient river, delta and lake boundaries. Often they occupy areas of foothill troughs or intermountain depressions (depressions). Some deserts were formed on the site of large lakes of the Quaternary period. Surface deposits of the desert are associated with the geological structure of the territory and exogenous processes - Quaternary and modern (stony and gravel eluvium on Paleogene-Neogene and Cretaceous conglomerates; pebble, sandy or loamy-argillaceous proluvial deposits of piedmont plains; sandy ancient river valleys, eolian sands). The relief is low-mountain, small hills, plains (alluvial and proluvial), large lacustrine depressions. In other deserts, sandy massifs arose in arid conditions due to the dispersal of gneisses, quartzites and other metamorphic rocks or sedimentary rocks of the Cretaceous and Jurassic ages. In the temperate zone, conditions for the formation of deserts occur in inland areas such as Central Asia, where precipitation does not exceed 200 mm.

Desert formation

Deserts are one of the landscapes of the Earth, which arose just as naturally as all others, thanks primarily to a peculiar distribution along earth's surface heat and moisture and the related development of organic life, the formation of biogeocenotic systems. Such an understanding of the desert - as a certain landscape with features and properties of nature inherent only to it, which arose in certain latitudes of the Earth - does not cause serious disagreements in the scientific literature. If disputes arise, then about the main indicators of deserts - climatic, botanical, etc. The very concept of "desert" in genetic terms and how certain system geobiocenosis is perceived unambiguously.

The process of formation and development of deserts is based, first of all, on the uneven distribution of heat and moisture on the Earth, the zonality of the geographic envelope of our planet. The zonal distribution of temperatures and atmospheric pressure determines the specifics of the winds and the general circulation of the atmosphere. Above the equator, where the greatest heating of the land and water surface occurs, ascending air movements dominate.

The warm air that has risen above the equator cools somewhat and loses a large amount of moisture, which falls in the form of tropical showers. Then, in the upper atmosphere, the air flows north and south, towards the tropics. These air currents are called antitrade winds. Under the influence of the rotation of the earth in the northern hemisphere, the antitrade winds deviate to the right, in the southern hemisphere - to the left. Approximately over latitudes of 30-40°С (near the subtropics), their angle of deviation is about 90°С, and they begin to move along the parallels. At these latitudes, air masses descend to the heated surface, where they heat up even more, and move away from the critical saturation point. Due to the fact that in the tropics the atmospheric pressure is high all year round, and at the equator, on the contrary, it is lower, near the earth's surface there is a constant movement of air masses (trade winds) from the subtropics to the equator.

The formation of the relief of deserts occurs under the influence of wind and water erosion. Deserts are characterized by some of the same type of natural processes that are prerequisites for morphogenesis: erosion, water accumulation, blowing and eolian accumulations of sand masses. For a desert of the same geomorphological type and degree of aridity, the same relief forms are characteristic.

In the most common sandy deserts, these are bare mobile and fixed immobile deflationary-accumulative eolian forms (dunes, ridges, mounds, dunes, etc.), often combined with solonchaks; in denudation and mountainous deserts - steep cliffs (chinks), remnants, dry streams, drainless basins, salt lakes, etc. Desert territories are located either near young high mountain systems (Karakum and Kyzylkum, deserts Central Asia- Alashan and Ordos, South American deserts), or - with ancient mountains (Northern Sahara).

In any landscape, one can find the interaction of natural elements, a chain of connections that create the necessary balance in geobiocenotic systems. In this case, the quantitative ratio of natural components can be different. The ability of geobiocenoses to maintain their functions on the verge of a critical, depressed state and then restore the disturbed balance is an important property of the geographical environment, meets the vital interests organic world and ultimately people.

The desert is equally characterized by natural connections, and balance, and the ability to maintain productivity with a changing ratio of natural elements. But the connections themselves and the critical point in a depressed state natural forces the desert is thinner, more sensitive, more reactive. Excessive tension or disruption of communications quickly turns into an extreme state, causing a natural disaster. Dangerous for desert vegetation are droughts, excessive moisture deficiency in the air and soil, sandstorms, a sharp drop in the level of groundwater, drying up of wells, changes in the chemical composition of well waters (their mineralization, an increased presence of hydrogen sulfide) and other equally serious changes.

In desert conditions, the opportunity to awaken the extreme forces of nature is easier than in other landscapes. They are dangerous due to their irreversibility or very slow self-healing processes. Therefore, the ways and means of development natural resources deserts, ways of adaptation of living organisms, including people, their resettlement, the resulting relationships with local nature are not at all similar to those observed in other natural areas.

Desertification - back side and an inevitable concomitant of the extensive agricultural and industrial exploitation of the natural resources of the desert.

Construction of roads, mining facilities, laying of pipelines, etc. make significant changes to the fragile natural ecosystems of the desert.

However, with the immediate implementation of appropriate reclamation work, the landscape is usually restored; if environmental measures are ignored, the imbalance of desert ecosystems reaches such a degree that desertification processes become irreversible. According to UNO specialists, 19% of the land is on the verge of desertification (a combination of physical, geographical and anthropogenic processes leading to the destruction of ecosystems in arid regions, the degradation of all forms of organic life and, as a result, to a decrease in the natural and economic potential of these territories).

The "mechanism" of the formation and development of deserts is primarily subject to the uneven distribution of heat and moisture on Earth, the zonality of the geographic envelope of our planet. The zonal distribution of temperatures and atmospheric pressure determines the specifics of the winds and the general circulation of the atmosphere. Above the equator, where the greatest heating of the land and water surface occurs, ascending air movements dominate. An area of ​​calm and weak variable winds is formed here. The warm air that has risen above the equator cools somewhat and loses a large amount of moisture, which falls in the form of tropical showers. Then, in the upper atmosphere, the air flows north and south, towards the tropics. These air currents are called antitrade winds. Under the influence of the rotation of the earth in the northern hemisphere, the antitrade winds deviate to the right, and in the southern hemisphere - to the left. Approximately over latitudes of 30-40 ° C (near the subtropics), the angle of their deviation is about 90 ° C, and they begin to move along the parallels. At these latitudes, air masses descend to the heated surface, where they heat up even more, and move away from the critical saturation point. Due to the fact that in the tropics the atmospheric pressure is high all year round, and at the equator, on the contrary, it is low, near the earth's surface there is a constant movement of air masses (trade winds) from the subtropics to the equator. Under the influence of the same deflecting influence of the Earth in the northern hemisphere, the trade winds move from the northeast to the southwest, in the southern hemisphere - from the southeast to the northwest. The trade winds capture only the lower thickness of the troposphere - 1.5-2.5 km. The trade winds prevailing in the equatorial-tropical latitudes determine the stable stratification of the atmosphere, prevent vertical movements and the development of clouds associated with them, and precipitation. Therefore, the cloudiness in these belts is not very significant, and the influx of solar radiation is the largest. As a result, there is extreme dryness of the air (relative humidity in the summer months averages about 30%) and exceptionally high summer temperatures. The average air temperature on the continents in the tropical zone in summer exceeds 30-35°C; here is the highest air temperature on the globe - plus 58 ° С. the form of showers. In subtropical latitudes (between 30 and 45° N of northern and southern latitudes), the total radiation decreases, and cyclonic activity contributes to humidification and precipitation, mainly associated with the cold season. However, sedentary depressions of thermal origin develop on the continents, causing severe aridity. Here, the average temperature of the summer months is 30 ° C or more, while the maximum temperature can reach 50 ° C. In subtropical latitudes, intermountain depressions are the most dry, where the annual precipitation does not exceed 100-200 mm.

In the temperate zone, conditions for the formation of deserts occur in inland regions such as Central Asia, where rainfall is no more than 200 mm. Due to the fact that Central Asia is fenced off from cyclones and monsoons by mountain rises, baric depression forms here in summer. The air is very dry, high temperature (up to 40 ° C or more) and very dusty. Air masses rarely penetrating here with cyclones from the oceans and from the Arctic quickly warm up and dry up.

Thus, the nature of the general circulation of the atmosphere is determined by planetary features, and local geographical conditions create a peculiar climatic situation that forms a desert zone north and south of the equator, between 15 and 45 ° C latitude. To this is added the influence of cold currents of tropical latitudes (Peruvian, Bengal, Western Australian, California). By creating a temperature inversion, cool, moisture-laden maritime air masses, easterly constant winds of baric maxima lead to the formation of coastal cool and foggy deserts with even less precipitation in the form of rain.

If the land covered the entire surface of the planet and there were no oceans and high mountain rises, the desert belt would be continuous and its boundaries would exactly coincide with a certain parallel. But since land occupies less than 1/3 of the globe, the distribution of deserts and their size depend on the configuration, size and structure of the surface of the continents. So, for example, the Asian deserts spread far to the north - up to 48 ° N N. sh. In the southern hemisphere, due to the vast water spaces of the oceans, the total area of ​​the deserts of the continents is very limited, and their distribution is more localized. Thus, the emergence, development and geographical distribution of deserts on the globe are determined by the following factors: high values ​​of radiation and radiation, a small amount of precipitation or their complete absence. The latter, in turn, is determined by the latitude of the area, the conditions of the general circulation of the atmosphere, the features of the orographic structure of the land, and the continental or oceanic position of the area.

The desert is characterized by high summer temperatures, low annual precipitation - more often from 100 to 200 mm, lack of surface runoff, often a predominance of sandy substrate and a large role of eolian processes, groundwater salinity and migration of water-soluble salts in the soil, an uneven amount of precipitation, which determines the structure , productivity and fodder capacity of desert plants. One of the features of the distribution of deserts is the island, local nature of their geographical location. Desert lands do not form a continuous strip on any continent, like the arctic, tundra, taiga or tropical zones. This is due to the presence within the desert zone of large mountain structures with their greatest peaks and significant expanses of water. In this respect, deserts do not fully obey the law of zoning.

In the northern hemisphere, the desert territories of the African continent lie between 15 ° C and 30 ° N, where the largest desert in the world, the Sahara, is located. In the southern hemisphere, they are located between 6 and 33 ° S, covering the Kalahari, Namib and Karoo deserts, as well as the desert territories of Somalia and Ethiopia. In North America, deserts are confined to the southwestern part of the continent between 22 and 24 ° N, where the Sonoran, Mojave, Gila, and other deserts are located. Significant areas of the Great Basin and the Chihuahua desert are by nature quite close to the conditions of the arid steppe. In South America, deserts, located between 5 and 30 ° S, form an elongated strip (more than 3 thousand km) along the western, Pacific coast of the mainland. The deserts of Asia are located between 15 and 48-50 ° N and include such large deserts as Rub al-Khali, Great Nefud, on the Arabian Peninsula, Deshte-Kevir, Deshte-Lut, Dashti-Margo, Registan, Kharan in Iran and Afghanistan; Karakum in Turkmenistan, Kyzylkum in Uzbekistan, Muyunkum in Kazakhstan; Thar in India and Thal in Pakistan; Gobi in Mongolia and China; Takla-Makan, Alashan, Beishan, Caidasi in China. Deserts in Australia cover a vast area between 20 and 34 ° S. latitude. and are represented by the Great Victoria, Simpson, Gibson and Great Sandy deserts.

According to the amount of precipitation on Earth, they distinguish: the area of ​​\u200b\u200barid territories, classified by the nature of the vegetation cover and the amount of precipitation, is 46,749 thousand km², that is, about 32% of the earth's land area. At the same time, about 40 million km² falls on the share of typical deserts (extraarid and arid), and only 7044 thousand km² per year fall on the share of semi-arid lands. Arid territories make up 21.4 million km², where the amount of precipitation varies from 50 to 150 mm, and semi-arid 21.0 million km² - with precipitation from 150 to 200 mm.

The areas of arid territories are presented in Table 1.

Table 1 Areas of arid territories

The American scientist Dregne (1968) believes that, according to the nature of the soil cover, arid lands occupy 31.5% of the land area, which is 46149 million km² (Table 2).

Most of the world's deserts were formed on geological platforms and occupy the oldest land areas. Deserts are one of the landscapes of the Earth, which arose just as naturally as all others, thanks to the peculiar distribution of heat and moisture over the earth's surface and the development of organic life associated with this, the formation of biogeocenotic systems. A desert is a certain geographical phenomenon, a landscape that lives its own special life, has its own laws, has its own features, forms of change during development or degradation. Speaking of the desert as a planetary and naturally occurring phenomenon, this concept should not be understood as something monotonous, of the same type. Most deserts are surrounded by mountains or, more commonly, bordered by mountains. In some places, deserts are located next to young high mountain systems, in others - with ancient, heavily destroyed mountains. The first should include the Karakum and Kyzylkum, the deserts of Central Asia - Alashan and Ordos, the South American deserts; the second should include the Northern Sahara.

Mountains for deserts are areas of formation of liquid runoff, which comes to the plain in the form of transit rivers and small, with "blind" mouths. Of great importance for deserts is also underground and under-channel runoff, which feeds their groundwater. Mountains are areas from which destruction products are carried out, for which deserts serve as a place of accumulation. The deserts of the sewage regions are distinguished by the insignificant distribution of ancient alluvial and lacustrine deposits (Sahara, etc.). On the contrary, drainless regions (the Turan lowland, the Iranian highlands, etc.) are distinguished by thick strata of deposits.

Surface deposits of deserts are peculiar. This is what they owe geological structure territory and natural processes. According to MP Petrov (1973), the surface deposits of deserts are everywhere of the same type. Deserts are characterized by some of the same type of natural processes that are prerequisites for morphogenesis: erosion, water accumulation, blowing and eolian accumulations of sand masses. It should be noted that the similarities between deserts are found in a large number of features. The features of the differences are less noticeable and limited to a few examples, up to quite sharply. The differences are most related to geographic location deserts in various thermal zones of the Earth: tropical, subtropical, temperate. The first two belts contain the deserts of North and South America, the Near and Middle East, India, and Australia. Among them are continental and oceanic deserts. In the latter, the climate is moderated by the proximity of the ocean, which is why the differences between heat and water balances, precipitation and evaporation are not similar to the corresponding values ​​that characterize continental deserts. However, for oceanic deserts great importance have ocean currents washing the continents - warm and cold. A warm current saturates the air masses coming from the ocean with moisture, and they bring precipitation to the coast. The cold current, on the contrary, intercepts the moisture of the air masses, and they enter the mainland dry, increasing the aridity of the coasts. Oceanic deserts are located off the western coasts of Africa and South America.

In the temperate zone of Asia and North America are continental deserts. They lie within the continents (the deserts of Central Asia) and are distinguished by arid and extra-arid conditions, a sharp discrepancy between the thermal regime and precipitation, high evaporation, and contrasts in summer and winter temperatures. The differences in the nature of deserts are also influenced by their altitudinal position.

Mountain deserts, as well as those located in intermountain depressions, are usually distinguished by an increased aridity of the climate. The variety of similarities and differences between deserts is primarily associated with their location in different latitudes of both hemispheres, in the hot and temperate zones of the Earth. In this regard, the Sahara may have more similarities with the Australian desert and more differences with the Karakum and Kyzylkum in Central Asia. Equally, the deserts formed in the mountains may have a number of natural anomalies among themselves, but there are even more differences with the deserts of the plains. Differences occur in average and extreme temperatures during the same season of the year, in the time of precipitation (for example, the eastern hemisphere of Central Asia receives more precipitation in summer from monsoon winds, and the deserts of Central Asia and Kazakhstan - in spring).