The great american desert

The land that is known as the "Great American Desert" is an area containing 900 000 square miles of land in western United States. It is an area divided into three parts: on the east are the Great Plains, in the center the Rocky Mountains, and farther west the part that is known as the Great Basin. It is difficult to determine exactly where the desert begins and where it ends because it establishes its own borders. How far the desert extends varies on the eastern side according to the amount of rain that falls. On this side one sees the neverending struggle between the desert plants and the grasses that grow on the plains, each tries to occupy as much land as possible. Where there is little rain the desert may extend as far as states of Texas, Kansas North and South Dakota.

Many people think that a desert consists of nothing but sand. But most desert land also contains rocky places, basins surrounded by mountains, and canyons. All of these kinds of land are found within the borders of the Great American Desert.

One fifth of the world's surface is desert, which supports no more than four percent of the total population. The world's deserts vary in appearance, but they all have features that are similar. They are very dry, hot during the day and cold at night. The wind blows and few animals and plants can live there.

In 1824 school books in the United States indicated that the Great American Desert extended from the Mississippi Valley to the eastern side of the Rocky Mountains. As more travelers went west, the maps changed and included more desert land. But when settlers learned that animals and crops could be grown on this land, they changed their ideas. The land first known as the Great American Desert became known as the Great Plains. By 1870 the idea that all the land west of the Mississippi River was a vast desert had disappeared entirely from maps and school books.

Few books agree on the borders of the modern American Desert. Perhaps this lack of agreement results from the fact that there are mountains in the center of this region. However, mountains are the principal cause of deserts. They prevent the passage of the cold winds over them to let the rain fall on the desert land. This is true in most dry regions of the world where cold ocean currents force the clouds to lose their moisture on the sea instead of on the land.

Many scientists think that land receiving only ten inches of rain a year should be known as a desert. However, land on the American Desert receives an average of about 12 inches a year. Some places receive as little as 3 inches a year and others as much as 16 inches a year.

Other scientists believe that deserts cannot be determined by the amount of rain that falls on them because so many things combine to determine what effect the rain has on plant and animal life. Often on the high plains and in the Great Basin the rains are sudden and heavy and do the land little good. Great amounts of water run off the dry hard land not sinking into it. Floods are caused which do considerable harm.

The rate of evaporation is as important as the amount of rain. This process is very rapid on the Great American Desert because of the small amount of moisture in the air and the strong winds that blow.

The air above the warm parts of the ocean can easily pick up large quantities of water. When this warm wet air moves over the land, heavy rains result. This does not happen along the coast of the southern part of the state of California where the desert meets the ocean. Here the ocean currents are cold and the winds along the coast prevent the passage of moisture inland.

In northern California the winds blow off warmer waters. But as they continue east, a series of mountain ranges causes them to lose much of their moisture. As a result cities on the western side of the mountains receive large amounts of rain.

Most of the world's deserts possess a supply of underground water. But it often remains so deep under the surface that it can be taken out only with powerful machines. And in a few places there are wells where the water comes out of the earth by itself. A few such wells exist in the state of New Mexico.

On almost all of the world's deserts there are rivers that have their source in the rainy highlands and flow across dry land: the Nile River in Egypt, the Indus in Pakistan and the Colorado in the United States. The Colorado River has its source in the state of Wyoming and flows south into the Gulf of California. It is the fifth largest river in the United States. Like other desert rivers the Colorado lacks branch rivers and, as a result, the amount of water becomes smaller as it flows downstream.

The Great American Desert is a land of great differences. The lowest land is 270 below sea level – Death Valley in California. The highest mountains are 14 000 feet above sea level. Parts of the desert in summer may be as hot as 130 degrees. And in the northern part of the desert the temperature may be 10 degrees below zero in the winter.

In many places rain water that flows from the highlands passes through the rock. Sometimes lakes are created that are not very deep. Rapid evaporation removes the water from them leaving great areas of shiny white salt and minerals. The Great Salt Lake in Utah is one of these lakes, but it receives enough rain to allow a high level of water to exist all the time.

Among the important features of the southern part of the Great American Desert are the great piles of sand and vast areas of land where desert plants grow. In the distance, pointed mountain tops and large formations of sharp rocks may be seen.

Not many plants and animals can live on the very dry part of the desert. The cactus is the most common of the plants that grow on the desert. These cactus plants can live unharmed by great changes of climate. They can hold water in reserve and thus continue to grow during the hottest months. They develop long roots near the surface of the ground. These roots can easily use the rain water. Cactus plants also develop long roots that extend deep into the ground to use the sources of water far below the surface. Sometimes these roots are so long that they extend thirty feet down into the ground. They may also serve to keep the plants secure in a strong wind storm. Plants which do not have long roots are usually not very tall and grow close to the ground. Thus they cannot blow away in a strong wind.

Many desert plants are able to turn leaves to avoid the direct rays of the sun. Others have hard or shiny surfaces that prevent loss of moisture.

Like the native plants certain animals also adapt to life on the desert. They depend on the few scattered streams for water, and many go out of their shelter only in the cool of the evening. Some animals are able to manufacture water within their own bodies from their dry food. Therefore they never need to drink water.

The colour of almost all desert animals is similar to the surroundings and thus it serves as a protection to them. Smaller animals dig into ground where the rays of the sun cannot touch them. During the cold winter months they remain asleep.

EUROPE

Position and size. – With an area of about 3 750 000 square miles, Europe is the smallest of the continents excepting Australia. It is characterized by the comparatively great length of the coastline, broken up everywhere by peninsulas, gulfs, bays and fringed by islands with the result that only the heart of Russia is more than 500 miles from the sea. Europe lies almost entirely in middle latitudes – the North Temperate Zone; only a small fragment in the north is within the Arctic Circle.

Climate. – In general, it may be said that a number of factors have a determining influence on the character of European weather and climate. The factors may be grouped as follows:

a) The western coasts of the continent are bathed by warm current – the North Atlantic Drift, which is a continuation of the Gulf Stream. The existence of this warm current, especially round the British Isles, has undoubtedly an important effect in ameliorating winter conditions. But the effect of the warm waters themselves is enormously enhanced by the prevalent south-westerly wind; for the warmth is communicated through winds and not by the actual warm current.

2. According to the modern concepts of air-mass meteorology, a very important difference is found between cold Polar air and warm air coming from tropical regions. The position and amount of the cold Polar air varies with the season. The margin, the Polar front, in winter may be regarded as following roughly the 32° F isotherm. The currents of warm tropical air which reach Europe is the Westerlies exert their influences on the remainder of Europe.

3. The configuration of Europe, particularly the existence of the Mediterranean Sea and its continuation in the Black Sea, as well as, to a less extent, the existence of the Baltic Sea, permits the penetration eastwards of oceanic conditions.

We may now consider climatic conditions in Europe by contrasting the winter with the summer.

Winter conditions. – Owing to the warm currents of the sea and the wind system, in the winter months the whole of Europe, with the possible exception of Iceland, lies in the belt of the westerly winds, thus enjoying the warm, moisture-laden winds from the Atlantic Ocean. The extra-tropical, high-pressure belt at this season lies to the south of Europe, over the Sahara and its continuation to the Atlantic that is, to the south of the Azores. But the eastern part of the continent at this season is very near the great land mass of Central Asia, which gets extremely cold. One may picture a great mass of cold heavy air centred over Asia and eastern Europe, giving rise to a permanent high-pressure system in the winter. The warm moisture-laden air masses from the Atlantic move up against this as against a wail, and either find their way away to the north-east past the coast of Norway, or they escape to the south along the Mediterranean. At times this great high-pressure system of eastern Europe, with its cold, out-blowing winds, extends its influence even as far as the eastern shores of the British Isles and gives rise there to spells of cold and frosty, though often sunny, weather. Indeed, it may be said that the winter weather of the whole of Europe is determined by the relative strength or importance of the great pressure system: the semi-permanent low-pressure system over Iceland, the permanent high-pressure system over Europe and the high-pressure system south of the Azores. Bearing these facts in mind, it is not difficult to understand why in winter months it gets steadily, colder as one travels eastwards in Europe, and that the isotherm of 32°, or freezing-point, divides the continent roughly into two halves. Nor is it difficult to understand why the whole of the western and Mediterranean margins receive a considerable proportion of their rain in the winter (more than half the total in the west of the British Isles, western France, and the Mediterranean), whereas as the result of the cold, out-blowing winds, precipitation of any sort is less in eastern Europe in winter than in summer.

In western and southern Europe most rain falls where there are mountain ranges which intercept the winds.

Summer Conditions. – At this season the wind system of the world have moved to the north so that only the northern part of Europe is under the influence of the Westerlies. The southern part of Europe, namely, the countries surrounding the Mediterranean Sea – lies within the influence of the high-pressure belt which surrounds the globe just outside the Tropics. The high pressure reigning in the summer months over the Mediterranean prevents the penetration of the cooling or rain-bearing winds from the Atlantic Ocean; consequently the Mediterranean lands suffer from considerable heat, and comparative or even complete rainlessness – the typical Mediterranean climate. In the Atlantic the high pressure center of the Azores which forms pert of this belt of high pressure, is north of its winter position, and frequently extends its influence as far as the British Isles. On the other hand, the Polar front is farther to the north, and the belt of cyclones which is associated with it tends to lie to the north of Iceland, and to affect such regions as the British Isles far less than in the winter.

In eastern Europe the conditions of winter are reversed. The great continental land mass becomes greatly heated and a large low-pressure area is the result. There is a tendency for the low pressure to be particularly marked over southern Russia, and towards this area the rain-bearing winds from the Atlantic blow, and result in the light spring rain of the steppe lands of south-eastern Europe. Most of central and eastern Europe thus have the greater part of their rain, that is to say, more than half their annual amount, in the summer half of the year rather than in the winter.

The south-westerly winds do not blow steadily but travel across the continent rather as a succession of cyclones or depressions with intervening wedges of high pressure which characterize the whole of the westerly wind belt. Thus the rainfall of Europe brought by these winds is partly orographical, partly cyclonic in its origin; with the result that although the rainfall on the mountains is nearly always heavier than on the lowlands even as far from the Atlantic as south-eastern Europe, the rainfall is sufficient for agriculture, only in the south of the Soviet Union surrounding the Caspian does it drop below the minimum amount for agriculture.

Climatic Regions. – The continent of Europe includes five of the great climatic regions of the world:

1. The Mediterranean.

2. The Cool Temperate Oceanic.

3. The mid-Latitude Continental or mid-Latitude Grasslands.

4. The Cold Temperate or Coniferous Forest:

5. The Tundra.

Natural Vegetation and Soils. – When we study the natural vegetation of Europe we must bear in mind that so much of the continent is densely populated and has been populated for many generations, that but little of the natural vegetation remains. It is possible, however to distinguish the following broad major divisions,

1) The Mediterranean Region. – Mediterranean climate is characterized essentially by its cool moist winters, and its hot, dry summers. In such a climate, where the hot seasons and wet seasons do not coincide, soil forms but slowly, and it is only in local patches of alluvium that one finds rich soil in Mediterranean lands. Moreover, the young fold mountains which play such a prominent part in the formation of the relief of Mediterranean lands are composed to a very considerable extent of hard limestones, which tend to give only poor, thin, red soil. The natural vegetation of Mediterranean lands therefore has to overcome not only the difficulties of the climate when the moisture is available in the winter and spring but not in the summer, but also has to overcome the difficulties of thin, poor soil. The resulting vegetation is essentially woody rather than herbaceous; and may vary from the low scrubland through Mediterranean woodland, in which the trees are characterized by protective devices against the loss of moisture, to, in the wetter and more favoured parts, forests of such trees as cork oak and pines.

2) The Cool Temperate Oceanic Climatic Region, including the sub­division of north-western and central Europe, and central USSR. With the cooler, and by no means rainless, summer, when vegetation processes can go on, the principal vegetation of this region is the Deciduous Forest: forest, because the rain throughout the year maintains a deep-seated water supply well fitted to trees; deciduous because the cold of the winter provides a good resting season. Wherever increasing severity of conditions is apparent, the deciduous forest tends to be replaced by coniferous forest; hence the large patches of coniferous forest associated with all the major mountain areas and with such areas of poor soil as the sandy tracts of the Lands in France, or of the heathlands of southern England. It should be noted that deciduous forest tracts are associated with fine rich brown soil, the colour of which is due to the humus, or decayed vegetable matter resulting from the leaf fall taking place every autumn. The actual formation of soil from the weathering of rocks is considerably more rapid than in Mediterranean climates and is hastened by the never-ceasing work of the earth-worm, which is particularly active in these humus-rich soils of temperate lands. Where cleared, the deciduous forest tends naturally to be replaced by the rich green grassy meadows so characteristic of the British Isles. The climate is sufficiently moist throughout the year to permit the grass to remain green. It must be remembered, however, that large tracts of the Deciduous Forest belt in Europe have probably never been forested e. g. some sandy tracts which evidently were covered with heathland and moorland.

3) The Northern Coniferous Forest Belt. – This region stretches as a broad belt across the north of Europe, through Norway and Sweden, nearly the whole of Finland, and across the north of Russia. In these northern latitudes soil forms slowly, and, in addition, the complex of ancient rocks which underlies much of this tract does not furnish material readily available for soil formation; the whole area was swept bare of loose deposits and soil by the great ice sheets – and these three factors combined have resulted in large areas of poor sterile soil. Nor is it enriched by leaf mould in the same way as it is in the deciduous forests, so that even where mineral debris has accumulated the light siliceous soils of the Northern Coniferous Forest form a well-marked type known as podsols. Except in favoured hollows where glacial debris forms better soils, there is comparatively little agriculture possible. Moreover the climatic conditions do not favour arable farming, and in particular, very little cereal farming is possible. These northern regions, therefore remain comparatively thinly populated and forests cover very large areas. In these forests two trees are predominant – the Norway spruce and the Scot pine. The latter occurs especially in the sandier and lighter soils, the former especially in damper regions. As we go northwards into Polar regions, so the growth of forest trees is slower. It takes about 50 or 60 years for a forest that has been cut over to grow again to trees large enough for lumber in the southern tracts; towards the northern parts of this belt, the growth is so much slower that it takes about 200 years for a tree to grow sufficiently to be cut for lumber.

4. The Tundra Region. – In the extreme north of Russia and of Norway one comes into those regions which are beyond the Polar limit of tree growth. Similar conditions are reached in the higher parts of the mountain divide between Norway and Sweden. A low scrub or woodland usually occurs beyond the limit of the Northern Coniferous Forest but in the Tundra proper the ground is simply covered by a thick growth of lichens, of which the so-called "reindeer moss" is the most important constituent together with a large proportion of mosses, and usually a number of low bushes six inches to a foot high, of dwarf birch. In places however, the Tundra regions are occupied by surprisingly rich meadow lands.

5) The Steppe lands of South-eastern Russia. In the south-east of European part of the Soviet Union, with its late spring and early summer rainfall and its cold winters, the conditions are ideal for the growth of grass, but inimical to the growth of trees; so here existed the low, treeless, rolling grassy plains known as the steppes now replaced by ploughlands. They occupy the whole of the southern portion of Russia in Europe, with the exception of the Mediterranean fringe of the Crimea, the desert surrounding the Caspian and the mountainous belt of the Caucasus. They stretch some distance to the south-west into the plains of Rumania, whilst the Great Hungarian Plain may be regarded as an outlier of the main belt. The wide, open, rolling plains of the Anatolian plateau in Asia Minor are intermediate in character between the Russian sleppelands and the Mediterranean vegetation. They are for the most part treeless except near the water-courses, but instead of a luxuriant growth of grass one finds a number of the smaller plants and flowers (especially in the spring) that one associates more particularly with Mediterranean regions.

6) The Mid-Latitude Desert. – Very, very dry stepplands fading into desert country occur in Russia round the northern fringe of the Caspian Sea. The desert is a partly climatic, with a low rainfall, partly edaphic (or, due to soil) owing to the large proportion of salt in the soil where one approaches the great salt lake of the Caspian Sea itself.