Physical Geography Basics and Principals 2025 PDF

Zagazig University Faculty of Arts Department of Geography Physical Geography Basics and Principals Zagazig Dr. Reda M. Selim 2025  -1- Zagazig University Fa...

Zagazig University Faculty of Arts Department of Geography Physical Geography Basics and Principals Zagazig Dr. Reda M. Selim 2025  -1- Zagazig University Faculty of Arts Department of Geography Physical Geography Basics and Principals First Year Collected by Dr. Reda M. Selim Zagazig 2025  -2- Contents Topic Page I - Scope of Physical Geography ……………… 3 2- The Earth Inside and Out …………………... 10 3- Bodies of Water and Landforms …………… 15 4- Internal Forces Shaping the Earth …………... 22 5- External Forces Shaping the Earth ………… 32 6- Seasons and Weather ……………………….. 41 7- Climate ………………………………………... 50 8- Soils and Vegetation …………………………. 67 9- Air Masses …….……………………………... 71 10- Examples of Oceans and Seas ……….……. 80 11- Egypt’s Natural Potentials ……..…….……. 117 12- Geographical Terms ……………………….. 147 Selected References ………………………………. 167  -3- -1- Scope of Physical Geography Physical Geography examines and investigates natural phenomena spatially. We will identify some of the key elements studied by Physical Geographers. Combining these two items, we can now suggest that Physical Geography studies the spatial patterns of weather and climate, soils, vegetation, animals, water in all its forms, and landforms. Physical Geography also examines the interrelationships of these phenomena to human activities. This sub-field of Geography is academically known as the Human-Land Tradition. This area of Geography has seen very keen interest and growth in the last few decades because of the acceleration of human induced environmental degradation. Thus, Physical Geography's scope is much broader than the simple spatial study of nature. It also involves the investigation of how humans are influencing nature. Academics studying Physical Geography and other related Earth Sciences are rarely generalists. Most are in fact highly  -4- specialized in their fields of knowledge and tend to focus themselves in one of the following well defined areas of understanding in Physical Geography: - Geomorphology - studies the various landforms on the Earth's surface. - Pedology - is concerned with the study of soils. - Biogeography - is the science that investigates the spatial relationships of plants and animals. - Hydrology - is interested in the study of water in all its forms. - Meteorology - studies the circulation of the atmosphere over short time spans. - Climatology - studies the effects of weather on life and examines the circulation of the atmosphere over longer time spans. The above fields of knowledge generally have a primary role in introductory textbooks dealing with Physical Geography. Introductory Physical Geography textbooks can also contain information from other related disciplines including:  -5- - Geology - studies the form of the Earth's surface and subsurface, and the processes that create and modify it. - Ecology - the scientific study of the interactions between organisms and their environment. - Oceanography - the science that examines the biology, chemistry, physics, and geology of oceans. - Cartography - the technique of making maps. - Astronomy - the science that examines celestial bodies and the cosmos. Geography as an Environmental Science Webster's 9th Collegiate Dictionary defines Environment "... as the complex of physical, chemical, and biotic factors (such as climate, soil, and living things) that act upon an organism or an ecological community and ultimately determines its form and survival". Both Human and Physical Geography provide an important intellectual background for studying the environment. Many environmental studies/science programs offered by Universities and Colleges around the world rely on the  -6- information found in various Geography courses to help educate their students about the state of the environment. History of Physical Geography The nature of understanding in Physical Geography has changed over time. When investigating this change it becomes apparent that certain universal ideas or forces had very important ramifications to the academic study of Physical Geography. During the period from 1850 to 1950, there seems to be four main ideas that had a strong influenced on the discipline: 1- Uniformitarianism - this theory rejected the idea that catastrophic forces were responsible for the current conditions on the Earth. It suggested instead that continuing uniformity of existing processes were responsible for the present and past conditions of this planet. 2- Evolution - Charles Darwin's Origin of Species (1859) suggested that natural selection determined which individuals would pass on their genetic traits to future generations. As a result of this theory, evolutionary  -7- explanations for a variety of natural phenomena were postulated by scientists. The theories of uniformitarianism and evolution arose from a fundamental change in the way humans explained the universe and nature. During the 16th, 17th, and 18th centuries scholars began refuting belief or myth based exp lanations of the cosmos, and instead used science to help explain the mysteries of nature. Belief based explanations of the cosmos are made consistent with a larger framework of knowledge that focuses on some myth. However, theories based on science questioned the accuracy of these beliefs. 3- Exploration and Survey- much of the world had not been explored before 1900. Thus, during this period all of the fields of Physical Geography were actively involved with basic data collection. This data collection included activities like determining the elevation of land surfaces, classification and description of landforms, the measurement of the volume of flow of rivers, measurement of various phenomena associated to  -8- weather and climate, and the classification of soils, organisms, biological communities and ecosystems. 4- Conservation - beginning in the 1850s a concern for the environment began to develop as a result of the human development of once natural areas in the United States and Europe. One of the earliest statements of these ideas came from George Perkins Marsh (1864) in his book "Man in Nature" or "Physical Geography as Modified by Human Action". This book is often cited by scholars as the first significant academic contribution to conservation and environmentalism. After 1950, the following two forces largely determined the nature of Physical Geography: 1- The Quantitative Revolution - measurement became the central focus of research in Physical Geography. It was used primarily for hypothesis testing. With measurement came mapping, models, statistics, mathematics, and hypothesis testing. The quantitative revolution was also associated with a change in the way in which physical geographers studied the Earth and its  -9- phenomena. Researchers now began investigating process rather than mere description of the environment. 2- The study of Human/Land Relationships - the influence of human activity on the environment was becoming very apparent after 1950. As a result, many researchers in Physical Geography began studying the influence of humans on the environment. Some of the dominant themes in these studies included: environmental degradation and resource use; natural hazards and impact assessment; and the effect of urbanization and land-use change on natural environments. Future of Physical Geography The following list describes some of the important future trends in Physical Geography research: 1- Continued development of Applied Physical Geography for the analysis and correction of human- induced environmental problems. A student of Applied Physical Geography uses theoretical information from  - 10 - the field of Physical Geography to manage and solve problems related to natural phenomena found in the real world. 2- Remote Sensing - Advances in technology have caused the development of many new instruments for the monitoring of the Earth's resources and environment from airborne and space platforms (see three- imensional image of hurricane Andrew, Landsat image of San Francisco Bay, Landsat image of Vancouver, British Columbia, and a space radar image of Victoria, British Columbia). The most familiar use of remote sensing technology is to monitor the Earth's weather for forecasting. 3- Geographic Information Systems - A geographic information system (GIS) merges information in a computer database with spatial coordinates on a digital map. Geographic Information Systems are becoming increasingly more important for the management of resources.  - 11 - -2- The Earth Inside and Out The Solar System The “home address” of the earth is the third planet in the solar system of the sun, which is a medium-sized star on the edge of the Milky Way galaxy. Its distance from the sun is 93 million miles. The solar system consists of the sun and nine known planets, as well as other celestial bodies that orbit the sun.  - 12 - The solar system also contains comets, spheres covered with ice and dust that leave trails of vapor as they race through space. Asteroids—large chunks of rocky material— are found in space as well. As you can see in the diagram, our solar system has an asteroid belt between the orbits of Jupiter and Mars. The Structure of the Earth The earth is about 24,900 miles in circumference and about 7,900 miles in diameter. Although the earth seems like a solid ball, it is really more like a series of shells that surround one another. INSIDE THE EARTH The core is the center of the earth and is made up of iron and nickel. The outer core is liquid, but the inner core is solid. Surrounding the core is the mantle, which has several layers. The mantle contains most of the earth’s mass. Magma, which is molten rock, can form in the mantle and rise through the crust, the thin layer of rock at the earth’s surface. Study the diagram below to learn more about the earth’s interior.  - 13 - ON AND ABOVE THE EARTH Surrounding the earth is a layer of gases called the atmosphere. It contains the oxygen we breathe, protects the earth from radiation and space debris, and provides the medium for weather and climate. The solid rock portion of the earth’s surface is the lithosphere, which includes the crust and uppermost mantle. Under the ocean, the lithosphere forms the seafloor. The huge landmasses above water are called continents. There are seven continents: North America, South America, Europe,  - 14 - Asia, Africa, Australia, and Antarctica. The hydrosphere is made up of the water elements on the earth, which include oceans, seas, rivers, lakes, and water in the atmosphere. Together, the atmosphere, the lithosphere, and the hydrosphere form the biosphere, the part of the earth where plants and animals live. CONTINENTAL DRIFT In 1912, Alfred Wegener of Germany presented a new idea about continents—the continental drift hypothesis. It maintained that the earth was once a supercontinent that divided and slowly drifted apart over millions of years. Wegener called the supercontinent Pangaea (from a Greek word meaning “all earth”). An ocean called Panthalassa surrounded it. The supercontinent split into many plates that drifted, crashed into each other, and split apart several times before they came to their current positions. This process occurred over millions of years. In the 1960s, scientists studying the sea floor discovered that the youngest rocks were in the middle of the ocean, at long cracks in the crust. This suggested that the new  - 15 - sea floor was being added, pushing the continents apart. Later in this chapter, you will learn how the rocks of Earth’s surface are broken into giant plates that move and continue to shape the earth.  - 16 - -3- Bodies of Water and Landforms Bodies of Water Without both freshwater and saltwater, life on this planet would be impossible. Water not only supports plants and animals, it helps distribute heat on the earth. OCEANS AND SEAS The Ocean is an interconnected body of salt water that covers about 71 percent of our planet. It covers a little more than 60 percent of the Northern Hemisphere and about 81 percent of the Southern Hemisphere. Even though it is one ocean, geographers divide it into four main parts: the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean, which is sometimes considered part of the Atlantic. The largest of the oceans is the Pacific. The waters near Antarctica are sometimes called the Southern Ocean. OCEAN MOTION The salty water of the ocean circulates through three basic motions: currents, waves, and tides. Currents act like rivers flowing through the ocean.  - 17 - Waves are swells or ridges produced by winds. Tides are the regular rises and falls of the ocean created by the gravitational pull of the moon or the sun. The motion of the ocean helps distribute heat on the planet. Winds blow over the ocean and are either heated or cooled by the water. When the winds eventually blow over the land, they moderate the temperature of the air over the land. HYDROLOGIC CYCLE The hydrologic cycle is the continuous circulation of water between the atmosphere, the oceans, and the earth. As you can see in the diagram above, water evaporates into the atmosphere from the surface of the oceans, other bodies of water, and from plants. The water exists in the atmosphere as vapor. Eventually, the vapor cools, condenses, and falls to earth as precipitation—rain or snow. The water soaks into the ground, evaporates to the atmosphere, or flows into rivers to be recycled. LAKES, RIVERS, AND STREAMS Lakes hold more than 95 percent of all the earth’s fresh water supply. The largest freshwater lake is Lake Baikal in Russia. Its volume of water equals 18 percent of all freshwater on earth. Freshwater lakes like the Great Lakes of North America are  - 18 - the result of glacial action thousands of years ago. Saltwater lakes result from changes in the earth’s surface that cut off outlets to the sea. Saltwater lakes are created when creeks and rivers carry salts into a lake, and there is no outlet to carry the salt away. The Great Salt Lake in Utah is the remnant of a large freshwater lake—Lake Bonneville. Its water outflows were cut off, causing the remaining water to become more salty as the water evaporated. The largest saltwater lake is the Caspian Sea in Western Asia.  - 19 - Rivers and streams flow through channels and move water to or from larger bodies of water. Rivers and streams connect into drainage systems that work like the branches of a tree, with smaller branches, called tributaries, feeding into larger and larger ones. Geographers call an area drained by a major river and its tributaries a drainage basin. GROUND WATER Some water on the surface of the earth is held by the soil, and some flows into the pores of the rock below the soil. The water held in the pores of rock is called ground water. The level at which the rock is saturated marks the rim of the water table. The water table can rise or fall depending on the amount of precipitation in the region and on the amount of water pumped out of the ground. Landforms Landforms are naturally formed features on the surface of the earth. The diagram on next page shows the different kinds of landforms. OCEANIC LANDFORMS The sea floor has landforms similar to those above water. The earth’s surface from the edge of a continent to the deep part of the ocean is called the - 20 -  - 21 - continental shelf. The floor of the ocean has ridges, valleys, canyons, and plains. Ridges mark places where new crust is being formed on the edges of the tectonic plates. Mountain chains similar to those on the continents themselves cover parts of the ocean floor. The longest continuous range is the Mid-Atlantic Ridge, which extends for thousands of miles north to south through the middle of the Atlantic Ocean. Islands dot the ocean surface. Islands can be formed by volcanic action, deposits of sand, or deposits of coral skeletons. CONTINENTAL LANDFORMS To understand the types of landforms, study the illustration on previous page. The major geographic feature that separates one type of landform from another is relief. Relief is the difference in elevation of a landform from its lowest point to its highest point. There are four categories of relief: mountains, hills, plains, and plateaus. A mountain, for instance, has great relief compared with a plain, which displays very little difference between its high and low points.  - 22 - Topography is the combination of the surface shape and composition of the landforms and their distribution in a region. A topographic map shows the landforms with their vertical dimensions and their relationship to other landforms. In the next section, you will learn how internal forces of the earth help to build and change the landforms on the earth— and how those forces affect humans.  - 23 - -4- Internal Forces Shaping the Earth Plate Tectonics The internal forces that shape the earth’s surface begin beneath the lithosphere. Rock in the asthenosphere is hot enough to flow slowly. Heated rock rises, moves up toward the lithosphere, cools, and circulates downward. Riding above this circulation system are the tectonic plates, enormous moving pieces of the earth’s lithosphere. You can see the position of the tectonic plates in the map below.  - 24 - Geographers study the movement of the plates and the changes they cause in order to understand how the earth is continually being reshaped—and how earthquakes and volcanoes occur. PLATE MOVEMENT Tectonic plates move in one of four ways: 1) spreading, or moving apart; 2) subduction, or diving under another plate; 3) collision, or crashing into one another; 4) sliding past each other in a shearing motion. The diagrams below show details about plate movement. When tectonic plates come into contact, changes on the earth’s surface occur. Three types of boundaries mark plate movements: Divergent boundary—Plates move apart, spreading horizontally. Convergent boundary—Plates collide, causing either one plate to dive under the other or the edges of both plates to crumple. Transform boundary—Plates slide past one another.  - 25 - An example of a divergent boundary is the one between Saudi Arabia and Egypt. The two plates on which those countries sit are spreading apart, making the Red Sea even wider. The Red Sea is actually a part of the Great Rift Valley in Africa. If you look at the map of Africa, you will see a string of lakes along the eastern side of Africa, including Lake Tanganyika and Lake Nyasa. These lakes, along with the Red Sea, were formed in the spreading boundary.  - 26 - An example of a convergent boundary can be found in South Asia. The plate where India is located is crashing into the Asian continent and building up the Himalayas. One of the most famous examples of a transform boundary is in North America—the San Andreas Fault in California. Study the diagrams below to understand the movement of the plates and their effect on the surface of the earth. FOLDS AND FAULTS When two plates meet each other, they can cause folding and cracking of the rock. The transformation of the crust by folding or cracking occurs very slowly, often only a few centimeters or inches in a year. Because the movement is slow, the rocks, which are under great pressure, become more flexible and bend or fold, creating changes in the crust. However, sometimes the rock is not flexible and will crack under the pressures exerted by the plate movement. This fracture in the earth’s crust is called a fault. It is at the fault line that the plates move past each other.  - 27 - Earthquakes As the plates grind or slip past each other at a fault, the earth shakes or trembles. This sometimes violent movement of the earth is an earthquake. Thousands of earthquakes occur every year, but most are so slight that people cannot feel them. Only a special device called a seismograph can  - 28 - detect them. A seismograph measures the size of the waves created by an earthquake. EARTHQUAKE LOCATIONS The location in the earth where an earthquake begins is called the focus. The point directly above the focus on the earth’s surface is the epicenter. The map on page 36 outlines the major plate boundaries. Nearly 95 percent of all recorded earthquakes occur around those boundaries. Plate movement along the Pacific Rim and from southern Asia westward to southern Europe makes this region especially vulnerable to quakes. EARTHQUAKE DAMAGE Earthquakes result in squeezing, stretching, and shearing motions of the earth’s crust that damage land and structures. The changes are most noticeable in places where people live. Landslides, displacement of land, fires (from broken gas lines), and collapsed buildings are major outcomes of the ground motion. Aftershocks, or smaller-magnitude quakes, may occur after an initial shock and can sometimes continue for days afterward. An earthquake is the sudden release of energy in the form of motion. C.F. Richter developed a scale  - 29 - to measure the amount of energy released. The Richter Scale uses information collected by seismographs to determine the relative strength of an earthquake. The scale has no absolute upper limit. Most people would not notice a quake that measured 2 on the scale. A 4.5 quake will probably be reported in the news. A major quake has a measurement of 7 or more. The largest quake ever measured was 8.9 in the Kermadec Islands of the South Pacific in 1986. TSUNAMI Sometimes an earthquake causes a tsunami, a giant wave in the ocean. A tsunami can travel from the epicenter of a quake at speeds of up to 450 miles per hour, producing waves of 50 to 100 feet or higher. Tsunamis may travel across wide stretches of the ocean and do damage on distant shores. For example, in 1960 a quake near Chile created a tsunami that caused damage in Japan, almost half a world away. In December 2004, a tsunami from a quake in the Indian Ocean struck areas of Southeast Asia, South Asia, and East Africa. An estimated 225,000 people were immediately killed, and another 1.2 million were forced to leave their homes.  - 30 - Volcanoes Volcanoes are among the most spectacular of natural events. Magma, gases, and water from the lower part of the crust or the mantle collect in underground chambers. Eventually the materials pour out of a crack in the earth’s surface called a volcano. Most volcanoes are found along the tectonic plate boundaries. VOLCANIC ACTION When the magma flows out onto the land slowly, it may spread across an area and cool. Magma that has reached the earth’s surface is called lava. The most dramatic volcanic action is an eruption, in which hot lava, gases, ash, dust, and rocks explode out of vents in the earth’s crust. Often a hill or a mountain is created by lava. The landform may also be called a volcano. Volcanoes do not erupt on a predictable schedule; they may be active over many years and then stop. Sometimes they remain inactive for long periods of time—as long as hundreds of years— before becoming active again.  - 31 - RING OF FIRE The Ring of Fire, a zone around the rim of the Pacific Ocean, is the location of the vast majority of active volcanoes. You can see the zone on the map on page 37. Eight major tectonic plates meet in this zone. Volcanic action and earthquakes occur frequently there. Other volcanoes are located far from the margins of tectonic plates. They appear over “hot spots” where magma from deep in the mantle rises and melts through the lithosphere, as in volcanoes in the Hawaiian Islands. Hot springs and geysers are indicators of high temperatures in the earth’s crust. Hot springs occur when ground water circulates near a magma chamber. The water heats up and rises to the surface. The hot springs and pools of Yellowstone Park are examples of this type of activity. A geyser is a hot spring that occasionally erupts with steam jets and boiling water. Old Faithful, a geyser in Yellowstone, erupts regularly, but most geysers are irregular in their eruptions. Countries with hot springs and geysers include the United States, Iceland, and Japan.  - 32 - Not all volcanic action is bad. Volcanic ash produces fertile soil. In some parts of the world, the hot springs, steam, and heat generated by the magma are tapped for energy. In Iceland, for example, volcanic heat and steam are used for heating and hot water in the city of Reykjavik. Internal forces have a major role in shaping the earth. In the next section, you will learn how external forces also change the landscape.  - 33 - -5- External Forces Shaping the Earth Weathering In the last section, you learned about forces within the earth that changed the land. External forces, such as weathering and erosion, also alter landscapes and in some instances create the soil that is needed for plant life. Weathering refers to physical and chemical processes that change the characteristics of rock on or near the earth’s surface. Weathering occurs slowly over many years and even centuries. Weathering processes create smaller and smaller pieces of rock called sediment. Sediment is mostly identifiable as either mud, sand, or silt, which is very fine particles of rock. MECHANICAL WEATHERING Processes that break rock into smaller pieces are referred to as mechanical weathering. Mechanical weathering does not change the composition of the rock—only its size. For example, when ice crystals build up in the crack of a rock, they can actually  - 34 - create enough pressure to fracture the rock into smaller pieces. All sorts of agents can break apart rocks. Frost and even plant roots dig into crevices in the rock, splitting it. Human activities, like road construction or drilling and blasting in mining, are also mechanical weathering forces. Eventually, the smaller broken material will be combined with organic material to become soil. CHEMICAL WEATHERING Chemical weathering occurs when rock is changed into a new substance as a result of interaction between elements in the air or water and the minerals in the rock. Decomposition, or breakup, can happen in several ways. Some minerals react to oxygen in the air and begin to crumble. That is what happens when iron rusts, for example. Other minerals break down when combined with water or carbon dioxide, which form weak acids within the rock. When sulfur and nitrogen oxides mix with water, acid rain is formed. The increase of acid rain in the 20th century is believed to be speeding up some decomposition. The location and the climate in which the rocks are located have a great  - 35 - deal to do with how rocks decompose. Climates that are warm and moist will produce more chemical weathering than do cool dry areas. Rocks in cold dry and hot dry areas generally experience more mechanical weathering than chemical weathering. Erosion Erosion occurs when weathered material is moved by the action of wind, water, ice, or gravity. For erosion to occur, a transporting agent, such as water, must be present. Glaciers, waves, stream flow, or blowing winds cause erosion by grinding rock into smaller pieces. Material moved from one location to another results in the lowering of some locations and increased elevation in others. For example, water might carry topsoil from a hill into a river and gradually cause the river to become more narrow. Erosion in its many forms reshapes landforms and coastal regions, as well as riverbeds and riverbanks. WATER EROSION One form of water erosion occurs as water flows in a stream or river. The motion picks up loose material and moves it downstream. The greater the force of  - 36 - water, the greater the ability of the water to transport tiny rock particles, or sediment. Another form of erosion is abrasion, the grinding away of rock by transported particles. The heavier the load of sediment, the greater the abrasion on the banks and riverbed. A third eroding action of water occurs when the water dissolves chemical elements in the rock. The composition of the rock changes as a result. Most streams erode both vertically and horizontally— that is, the valley cut by a stream gets deeper and wider, forming a V-shaped valley. As the water slows, it drops the sediment it is carrying. When a river enters the ocean, the sediment is deposited in a fan-like landform called a delta. Wave action along coastlines also changes the land. Waves can reduce or increase beaches. Sediment deposited by wave action may build up sandbars or islands. Wave action is so powerful that in some locations, it erodes about three feet of beach per year. For some unfortunate people, a beach house with an ocean view may end up in the ocean as a result of wave action erosion.  - 37 - WIND EROSION In many ways, wind erosion is similar to water erosion because the wind transports and deposits sediment in other locations. Wind speeds must reach 11 miles per hour before fine sediment can be moved. The greater the speed of the wind, the larger the particles moved. Dust storms are capable of carrying as much as 6,000 tons of sediment per cubic mile of air. As the wind slows, the sediment is dropped. Depending on the type of wind-borne sediment, new landforms— such as sand dunes miles from seashores and rocks sculpted into fantastic forms—may be produced. Deposits of loess, windblown silt and clay sediment that produce very fertile soil, are found across the world. In northern China, for example, the deposits are several hundred feet deep. Extensive areas of loess are found in the Mississippi Valley in the United States and in the grasslands of Argentina. GLACIAL EROSION A glacier is a large, long-lasting mass of ice that moves because of gravity. Glaciers form in mountainous areas and in regions that are routinely covered  - 38 - with heavy snowfall and ice. In mountain regions, glaciers move downslope as a result of gravity. Glaciers such as ice caps and ice sheets move from the highest point on land toward the lowest point. Glaciation is the changing of landforms by slowly moving glaciers. As a glacier moves, several types of erosion occur. Rocks caught underneath the glacier are ground into finer and finer particles. Some particles are so small that they are called rock flour, which is one component of soil. Massive glaciers also cut U-shaped valleys into the land. On top of or within the ice are other rocks carried by the glacier. When the glacier melts, these rocks are left behind. Rocks left behind by a glacier may form a ridge or a hill called a moraine. Moraines can be found on the sides, down the center, or at the leading edge of a glacier. Inside or under the glacier may be tunnels formed by running water. These tunnels fill up with sediment dropped by the water. When the ice melts, it leaves a long snakelike ridge called an esker. Sometimes blocks of ice are trapped in the sediment. They melt slowly and leave behind a dent or a  - 39 - depression in the ground. These depressions are called kettles. The kettles may be filled with water forming a small lake. Building Soil Weathering and erosion are a part of the process of forming soil. Soil is the loose mixture of weathered rock, organic matter, air, and water that supports plant growth. Organic matter in the soil helps to support the growth of plants by providing needed plant food. Water and air share tiny pore-like spaces in the soil. When it rains, the pores are filled with water. As the water evaporates, drains away, or is used by the plants, the pores are filled with air. The texture of the soil, the amount of organic material called humus, and the amount of air and water in the soil all contribute to the soil’s fertility—its ability to nurture plants. SOIL FACTORS When geographers study soil, they look at five factors: Parent material The chemical composition of the original rock, or parent rock, before it decomposes affects its fertility.  - 40 - Relief Steeper slopes, such as mountainsides, are eroded easily and do not produce soil quickly. Organisms Organisms include plants, small animals like worms, ants, and bacteria that decompose material. They help to loosen soil and supply nutrients for plants. Climate Hot climates produce a soil different from that produced by cold climates. Wet climates and dry climates produce soils that are different from each other as well. Time The amount of time to produce soil varies, but a very rough average is about 2.5 cubic centimeters per century. The variety of soils—and the climates in which they are found— determine the types of vegetation that can grow in a location. Agricultural activities, such as farming, ranching, and herding, depend on this complex relationship. In the next chapter, you will learn about the climate and vegetation on the earth and how it affects human life.  - 41 - -6- Seasons and Weather Seasons Hurricanes occur frequently in the southern and eastern United States during summer and fall. During these seasons, storm systems with strong winds form over warm ocean water. EARTH’S TILT Seasons have an enormous impact on us, affecting the conditions in the atmosphere and on the earth that create our weather. As the earth revolves around the sun, it is tilted at a 23.5° angle in relation to the sun. Because of the earth’s revolution and its tilt, different parts of the earth receive the direct rays of the sun for more hours of the day at certain times in the year. This causes the changing seasons on the earth. Notice in the diagram to the right that the northern half of the earth tilts toward the sun in summer and away from the sun in winter. Two lines of latitude—the tropic of Cancer and the tropic of Capricorn—mark the points farthest north and south that the sun’s rays shine directly overhead at noon. The day  - 42 - on which this occurs is called a solstice. In the Northern Hemisphere, the summer solstice, or the beginning of summer, is the longest day of the year. Winter solstice, the beginning of winter, is the shortest.  - 43 - Another signal of seasonal change is the equinoxes. Twice a year on the equinox, the days and nights all over the world are equal in length. The equinoxes mark the beginning of spring and autumn. Weather Weather and climate are often confused. Weather is the condition of the atmosphere at a particular location and time. Climate is the term for weather conditions at a particular location over a long period of time. Northern Russia, for example, has a cold climate. WHAT CAUSES THE WEATHER? Daily weather is the complex result of several conditions. For example, the amount of solar energy received by a location varies according to the earth’s position in relation to the sun. Large masses of air absorb and distribute this solar energy, which in turn affects the weather. Other factors include: water vapor This determines whether there will be precipitation— falling water droplets in the form of rain, sleet, snow, or hail.  - 44 - cloud cover Clouds may hold water vapor. landforms and bodies of water Water heats slowly but also loses heat slowly. Land heats rapidly but loses heat quickly as well. elevation As elevation above sea level increases, the air becomes thinner and loses its ability to hold moisture. air movement Winds move the air and the solar energy and moisture that it holds. As a result, weather can change very rapidly. PRECIPITATION depends on the amount of water vapor in the air and the movement of that air. As warm air rises, it cools and loses its ability to hold water vapor. The water vapor condenses, and the water droplets form into clouds. When the amount of water in a cloud is too heavy for the air to hold, rain or snow falls from the cloud. Geographers classify precipitation as convectional, orographic, or frontal, as illustrated in the diagram below. Convectional precipitation occurs in hot, moist climates where the sun quickly heats the air. The heated air rises, and by afternoon clouds form and rain falls. Orographic  - 45 - precipitation falls on the windward side of hills or mountains that block moist air and force it upward. The air cools and rain or snow falls. The land on the leeward side is called a rain shadow because it gets little rain from the descending dry air.  - 46 - Frontal movement causes most precipitation in the middle latitudes. A front is the boundary between two air masses of different temperatures or density. Rain or snow occurs when lighter, warm air is pushed upward by the colder, denser air. The rising air cools, water vapor condenses, and precipitation falls. Weather Extremes As air masses warm and cool and move across the earth’s surface, they create weather. Sometimes the clashes between air masses cause storms, which can be severe. They disrupt the usual patterns of life and often cause major property damage and loss of human life. Hurricanes, tornadoes, blizzards, droughts, and floods are examples of extreme weather. HURRICANES Storms that form over warm, tropical ocean waters are called hurricanes—also known as typhoons in Asia. These storms are called different names around the globe: tropical cyclones, willy-willies (Australia), baguios (Philippines), and chubascos (Mexico). Hurricanes are one way heat from the tropics is moved out of the region. Air  - 47 - flowing over an ocean with a water temperature of 80°F or higher picks up huge amounts of moisture and heat energy. As these water-laden winds flow into a low-pressure core, they tighten to form an “eye.” The eye is usually 10 to 20 miles across and has clear, calm skies. But the winds moving around the eye may be as strong as 200 miles per hour. The clouds and winds stretch over a vast area, sometimes as wide as 500 miles. Upper air currents blowing from the east steer the hurricanes in a westerly direction. As the hurricane hits land, it pounds the area with howling winds and very heavy rains. It may also cause a storm surge along coastal regions. This wall of seawater, pushed ashore by the winds, may rise to 16 feet or more. The low-lying coastal regions of Bangladesh in South Asia are especially vulnerable to storm surges from tropical cyclones. TORNADOES Unlike hurricanes, which take days to develop, tornadoes form quickly and sometimes without warning. A tornado, or twister, is a powerful funnel-shaped column of spiraling air.  - 48 - Born from strong thunderstorms, tornadoes are capable of immense damage. In a tornado, winds swirl counter- clockwise around a low-pressure center. These winds may reach speeds of 300 miles per hour, blasting apart buildings and lifting objects as large as cars and mobile homes. Generally, tornadoes have small diameters (about 300 feet), travel about a mile, and last only a few minutes. However, the largest and most forceful can reach a mile across and stay on the ground for hours, hopscotching from one location to another. The largest outbreak of tornadoes in the United States occurred during a 16-hour period, April 3 and 4, 1974. A total of 148 tornadoes ripped through the Ohio and Tennessee valleys, killing 330 people. The largest share of tornadoes, about 3 of every 4, hit in the United States. On average, the U.S. National Weather Service counts 700 tornadoes each year. BLIZZARDS A blizzard is a heavy snowstorm with winds of more than 35 miles per hour and reduced visibility. These weather conditions snarl traffic, endanger livestock, and trap travelers. The greatest snowfall for a 24-hour period was 76 inches (6 feet 4 inches) in Silver Lake, Colorado, in  - 49 - 1921. A snowstorm that lasted from February 13 to 19, 1959, dumped 189 inches (almost 16 feet) of snow on Mt. Shasta, California. Because of their location, some areas of the country are frequently hit with snowstorms that produce huge amounts of snow. For example, the eastern and southern shores of the Great Lakes are snow belts that experience days and days of heavy snow resulting in enormous snow depths. Around the Lake Erie and Lake Ontario areas, the annual snowfall can be as much as 450 inches (37.5 feet). DROUGHTS A drought is a long period of time without rain or with very minimal rainfall. This lack of rain results in crop failures and drastically reduced levels in water storage facilities. In the early 1930s, a drought hit the Great Plains in the United States. Dust storms damaged farms across a 150,000- square-mile region that became known as the “Dust Bowl.” Suffering the effects of a harsh climate, thousands of families were forced to leave their land to find work elsewhere. (See the Dust Bowl Disaster feature on pages 150-151.) In 2000, a large portion of the southern  - 50 - United States was struck with a long drought. Northern Texas was particularly hard hit, with 84 straight days of no rain and extremely high temperatures. FLOODS When water spreads over land not normally covered with water, it is called a flood. Melting snow or rainwater fills streams or rivers until they reach flood stage, the point at which the banks can no longer contain the water. The water then flows into the surrounding area, called a floodplain. Floods take lives every year, especially in low, flat places like Bangladesh, where millions of people live on the flood plains and the delta. In 1993, flooding along the Mississippi and Missouri rivers claimed 50 lives and caused about $15 billion in damage. Nearly 150 rivers and their tributaries were involved. It was the largest flood ever to hit the United States.  - 51 - -7- Climate Factors Affecting Climate Four major factors influence the climate of a region: wind and ocean currents, latitude, elevation, and topography. WIND CURRENTS Wind and ocean currents help distribute the sun’s heat from one part of the world to another through convection, the transfer of heat in the atmosphere by upward motion of the air. As sunlight heats the atmosphere, the air expands, creating a zone of low air pressure. Cooler dense air in a nearby high-pressure zone rushes into the low- pressure area, causing wind. Global wind patterns are caused by the same kind of circulation on a larger scale. The hot air flows toward the poles, and the cold air moves toward the equator. The winds would blow in straight lines, but since the earth rotates they are turned at an angle. In the Northern Hemisphere, they turn to the right. In the Southern Hemisphere, they turn to the left. This bending of the winds is called the Coriolis effect.  - 52 - The map to the right shows that the wind patterns are mirror images of each other in the Northern and Southern Hemispheres. Winds are identified by the direction from which they blow; a north wind blows from the north to the south.  - 53 - OCEAN CURRENTS Ocean currents are like rivers flowing in the ocean. Moving in large circular systems, warm waters flow away from the equator toward the poles, and cold water flows back toward the equator. Winds blowing over the ocean currents affect the climate of the lands that the winds cross. For example, the warmth of the Gulf Stream and the North Atlantic Drift help keep the temperature of Europe moderate. Even though much of Europe is as far north as Canada, it enjoys a much milder climate than Canada.  - 54 - Ocean currents affect not only the temperature of an area, but also the amount of precipitation received. Cold ocean currents flowing along a coastal region chill the air and sometimes prevent warm air and the moisture it holds from falling to earth. The Atacama Desert in South America and the Namib Desert in Africa, for example, were formed partly because of cold ocean currents nearby. ZONES OF LATITUDE Geographers divide the earth into three general zones of latitude: low or tropical, middle or temperate, and high or polar. Tropical zones are found on either side of the equator. They extend to the tropic of Cancer in the Northern Hemisphere and the tropic of Capricorn in the Southern Hemisphere. Lands in tropical zones are hot all year long. In some areas, a shift in wind patterns causes variations in the seasons. For example, Tanzania experiences both a rainy season and a dry season as Indian Ocean winds blow in or away from the land. The high-latitude polar zones, which encircle the North Pole and South Pole, are cold all year. Summer temperatures in the Polar Regions may reach a high of only 50°F. The  - 55 - earth’s two temperate zones lie at the middle latitudes, between the tropics and the Polar Regions. Within the temperate zones, climates can vary greatly, ranging from relatively hot to relatively cold. These variations occur because solar heating is greater in the summer than in the winter. So summers are much warmer.  - 56 - ELEVATION Another factor in determining the climate of a region is elevation, or distance above sea level. You would think that the closer you get to the sun, the hotter it would become. But as altitude increases, the air temperature drops about 3.5°F for every 1,000 feet. Therefore, the climate gets colder as you climb a mountain or other elevated location. Climates above 12,000 feet become like those in Arctic areas—with snow and ice. For example, Mt. Kilimanjaro in east Africa is capped by snow all year long. The diagram above will help you see how latitude and elevation are related. TOPOGRAPHY Landforms also affect the climate. This is especially true of mountain areas. Remember that moisture-laden winds cool as they move up the side of a mountain, eventually releasing rain or snow. By the time the winds reach the other side of the mountain, they are dry and become warmer as they flow down the mountain. Changes in Climate Climates change over time. Scientists studying ice-core samples from thousands of years ago have noted a variety of  - 57 - changes in temperature and precipitation. Some of the changes in climate appear to be natural while others are the result of human activities. EL NIÑO The warming of the waters off the west coast of South America—known as El Niño—is a natural change in the climate. About every two to seven years, prevailing easterly winds that blow over the central Pacific Ocean slow or reverse direction, changing the ocean temperature and affecting the weather worldwide. Normally, these easterlies bring seasonal rains and push warm ocean water toward Asia and Australia. In El Niño years, however, the winds push warm water and heavy rains toward the Americas. This can cause floods and mudslides there, while Australia and Asia experience drought conditions. When the reverse occurs—that is, when the winds blow the warmer water to the lands on the western Pacific Rim— the event is called La Niña. La Niña causes increases in precipitation in places such as India and increased dryness along the Pacific coasts of the Americas.  - 58 - GLOBAL WARMING Although controversy exists over the causes of global warming, scientists agree that air temperatures are increasing. Since the late 1800s, the temperature of the earth has increased by one degree. However, estimates for the next century suggest that the increase will be almost 3.5 degrees. Some scientists believe that this warming is part of the earth’s natural warming and cooling cycles. For example, 18,000 to 20,000 years ago, the earth was in the last of several ice ages, when vast glaciers advanced over huge portions of the land mass. Other scientists argue that global temperature increases are caused by the greenhouse effect. The layer of gases released by the burning of coal and petroleum traps some solar energy, causing higher temperatures in the same way that a greenhouse traps solar energy. As more and more nations become industrialized, the amount of greenhouse gases will also increase. Scientists predict that, if global warming continues, ice caps will melt, flooding some coastal areas, covering islands, and changing the global climate. In the next section, you will learn about world climate regions.  - 59 - World Climate Regions Defining a Climate Region  - 60 - Climate regions act like a code that tells geographers much about an area without giving many local details. To define a climate region, geographers must make generalizations about what the typical weather conditions are like over many years in a location. The two most significant factors in defining different climates are temperature and precipitation. A place’s location on a continent, its topography, and its elevation may also have an impact on the climate. Geographers use a variety of methods to describe climate patterns. The most common method uses latitude to help define the climate. There are five general climate regions: tropical (low latitude), dry, mid- latitude, high latitude, and highland. Dry and highland climates occur at several different latitudes. Within the five regions, there are variations that geographers divide into smaller zones. You can see the varied climate regions on the map on pages 73–74. Although the map shows a distinct line between each of the climate regions, in reality there are transition zones between the regions. As you read about climate regions, refer  - 61 - to the climate map. You should see the latitude-related patterns that emerge in world climate regions. Types of Climates World climates are generally divided into five large regions: tropical, dry, mid-latitude, high latitude, and highland. The regions are divided into smaller sub-regions that are described below.  - 62 - TROPICAL WET This sub-region has little variation in temperature over the year—it is always hot, with an average temperature of 80°F. The days begin sunny but by afternoon have clouded up, and rain falls almost daily. The average amount of rain in a year is more than 80 inches. Tropical wet climates are found in Central and South America as well as Africa and Southwest Asia.  - 63 - TROPICAL WET AND DRY This climate is called “tropical wet and dry” because the subregion has a rainy season in summer and a dry season in winter. Temperatures are cooler in the dry season and warmer in the wet season. Rainfall is less than in the tropical wet climate subregion and occurs mostly in the wet season. Tropical wet and dry climates are found next to tropical wet climates in Africa, South and Central America, and parts of Asia. SEMI-ARID This climate subregion does receive precipitation, just not very much: about 16 inches per year. Summers are hot. Winters are mild to cold, and some semiarid locations can produce snow. The climate is found in the interior of continents, or in a zone around deserts. The region contains some of the most productive agricultural lands in the world. DESERT Some people think a desert is nothing but sand dunes. However, deserts are categorized according to the amount of rainfall, rather than by landforms, and can be hot or cool/cold. Deserts receive less than ten inches of rain per year. Hot deserts, like the Sahara and the Arabian Desert, regularly have low humidity and high temperatures during  - 64 - the day. At night, temperatures drop because the dry air cannot hold heat well. Cool/cold deserts are found in the mid-latitudes mostly in the Northern Hemisphere, often in the rain shadow of nearby mountain ranges. Summer temperatures are warm to hot, and winter temperatures range from quite cool to below freezing. MEDITERRANEAN This climate sub-region is named for the land around the Mediterranean Sea where it is located. It also exists elsewhere, such as the west coast of the United States and parts of Australia. Its summers are dry and hot, and its winters cool and rainy. This climate region supports a dense population and rich agricultural activity. MARINE WEST COAST This climate sub-region, which is located close to the ocean, is frequently cloudy, foggy, and damp. The winds over the warm ocean moderate the temperatures and keep them relatively constant. Parts of the west coast of the United States and Canada and most of Western Europe experience this climate. Precipitation in marine west coast climate regions is evenly distributed  - 65 - throughout the year. Industrial regions with marine west coast climate may have smog (a mixture of smoke and fog). HUMID SUBTROPICAL Long periods of summer heat and humidity characterize the humid subtropics. These areas are found on the east coast of continents and are often subject to hurricanes in late summer and autumn. The southeastern part of the United States and large areas of China are examples. Winters are mild to cool, depending on latitude. The climate is very suitable for raising crops, especially rice. HUMID CONTINENTAL A great variety in temperature and precipitation characterizes this climate, which is found in the mid-latitude interiors of Northern Hemisphere continents. For example, Winnipeg, Manitoba, in Canada is located deep in the North American continent. It has a humid continental climate. Air masses chilled by Arctic ice and snow flow south over these areas and frequently collide with tropical air masses, causing changing weather conditions. These areas experience four seasons. However,  - 66 - the length of each season is determined by the region’s latitude. SUB-ARCTIC Evergreen forests called taiga cover the lands in the subarctic sub-region, especially in Canada and Russia. Huge temperature variations occur in this sub-region between summer and winter. Although the summers are short and cool, the winters are always very cold. Temperatures at freezing or below freezing last five to eight months of the year. TUNDRA The flat, treeless lands forming a ring around the Arctic Ocean are called tundra. The climate subregion is also called tundra. It is almost exclusively located in the Northern Hemisphere. Very little precipitation falls here, usually less than 15 inches per year. The land has permafrost— that is, the subsoil is constantly frozen. In the summer, which lasts for only a few weeks, the temperature may reach slightly above 40°F. ICE CAP Snow, ice, and permanently freezing temperatures characterize the region, which is so cold that it rarely snows. These subregions are sometimes called polar deserts since they receive less than ten inches of precipitation  - 67 - a year. The coldest temperature ever recorded, 128.6°F below zero, was on the ice cap at Vostok, Antarctica. HIGHLANDS The highlands climate varies with latitude, elevation, other topography, and continental location. In rugged mountain areas such as the Andes of South America, climates can vary based on such factors as whether a slope faces north or south and whether it is exposed to winds carrying moisture. Understanding climate helps you understand about the general weather conditions in an area. In the next section, you will learn about the variety of soils and vegetation on the earth.  - 68 - -8- Soils and Vegetation Soil Regions Soil is a thin layer of weathered rock, humus, air, and water. It shapes human existence in many ways. The world’s food supply depends greatly on the top six inches of soil (sometimes called topsoil). Such factors as depth, texture, and humus content of the soil determine the type of vegetation that can be supported in a region. That, in turn, helps to influence which human activities may take place there. As you study the chart below,  - 69 - notice the relationship of climate to the characteristics of the soil. Soil characteristics and climate are major influences in vegetation regions. Vegetation Regions Vegetation regions are natural environments that provide the stage for human activities such as farming, raising livestock, and producing timber. Soil, temperature, and moisture influence the type of vegetation that thrives naturally in a region. Vegetation patterns are identified on the basis of the ecosystems they support. An ecosystem is an interdependent community of plants and animals. The ecosystem of a region is referred to as a biome. Biomes are further divided into forest, grassland, desert, and tundra. FORESTLANDS Forest regions are categorized by the types of trees they support—broadleaf or needle-leaf. Broadleaf trees, such as maple, oak, birch, and cottonwood, are also called deciduous trees. The rain forest is located in the tropical zone and is covered with a heavy concentration of broadleaf trees. In the tropical rain forest region, some broadleaf trees stay green all year. In the deciduous region, trees shed their leaves at least once during the year. This  - 70 - region is located almost exclusively in the Northern Hemisphere. Sometimes deciduous trees are mixed with needle-leaf trees, such as pine, fir, and cedar, to form a mixed forest region. Needle-leaf trees are also called coniferous trees because they are cone bearing. They are found in huge stands in northern regions of North America, Asia, and Europe.  - 71 - GRASSLANDS Grasslands, mostly flat regions dotted with a few trees, are called by different terms. In the tropical grassland region, the flat, grassy, mostly treeless plains are called savanna. In the Northern Hemisphere, the terms steppe or prairie are used to identify temperate grasslands. Vast areas of Eurasia are covered with steppe. In the Southern Hemisphere, the temperate grasslands may be referred to as pampas. DESERT AND TUNDRA The plants that live in these extreme climates are specially adapted to tolerate the dry or cold conditions. In the tundra, plants that hug the ground, such as mosses and lichen, are best adapted to survive the cold dry climate. In the desert, plants that can conserve water and withstand heat, such as cacti, sagebrush, or other shrubs, dot the landscape.  - 72 - -9- Air Masses An air mass is a huge, identifiable body of air possessing a relative homogeneity of temperature and moisture characteristics within compared to the air surrounding it. Most air masses have dimensions of hundreds of thousands of square kilometers and are sometimes pushed thousands of kilometers from their source regions. Boundaries between air masses are known as fronts and the various types of fronts are described elsewhere. Although air is a continuous fluid, its properties are such that it frequently organizes into air masses having differences substantial enough to cause significant weather changes as they pass. When air of the lower troposphere passes over Earth’s surface, it exchanges energy and moisture characteristics with the surface. Air slowly moving large horizontal distances starts to take on the characteristics of the surface, be it Tropical Ocean or polar tundra. Some areas are well known as air mass source regions. Source regions occur in all latitudes and are characterized by non-mountainous  - 73 - terrain and frequent dominance of high pressure. High pressure is associated with light winds and this allows air to take on the surface temperature and moisture characteristics over the course of several days. As air masses move out of their source regions, they are modified. It is not rare for Arctic air to leave Siberia and travel over central North America to the Gulf Coast. While its original temperature might be −40°C, it might moderate to temperatures slightly below freezing. This is a cold shock to the Gulf Coast but a testimony to air mass modification. One well-appreciated instance of air mass modification is lake effect snow. In early winter the Great Lakes of the United States and Canada are not yet frozen over. Polar and Arctic air masses stream over the lakes. These cold air masses do not contain much moisture but have high relative humidities. As the air passes over a few tens of kilometers of lake surface, the air mass gains water vapor and saturates. Downwind of the lakes, the arrival of the air over land initiates a small bit of lifting that cools the air lower than its dewpoint temperature. Condensation and the precipitation  - 74 - processes begin and make copious amounts of snow without the presence of a winter storm. A simple air mass classification scheme considers the surface and latitude over which air passes. For instance, consider the differences in air types that can be generated over the great Antarctic ice sheets versus the tropical reaches of the Indian Ocean. Air masses having their origins in these places will provide vastly different weather as they progress over a location. Maritime Tropical air (mT) is a product of tropical and subtropical oceans. It is warm at any time of the year and associated with a high amount of humidity and latent heat. Its air is a prolific bringer of precipitation into the middle latitudes as it is transported away from its tropical origins. This air mass is usually unstable and it has numerous summer thunderstorms. Ironically, it is this air mass that provides most of the moisture falling as snow in the middle latitudes. The maritime Tropical air is drawn into middle latitude cyclones and lifted and mixed with polar air.  - 75 - Continental tropical air (cT) is a hot, dry air mass associated with the subtropical deserts of the world. It produces the hottest temperatures on the planet while not containing enough water vapor to produce significant precipitation. Record summertime temperatures are associated with the circulation of this air into the middle latitudes. Although this air mass is unstable in the first few hundred meters above the surface because of extreme heating of the surface, thought of as a whole, continental Tropical air has great stability because of the subsidence of air into the subtropical high. Continental Polar air (cP) has source regions on the continents of the upper middle latitudes and, with the maritime Polar air mass, continental Polar is behind most of the cold fronts passing through the middle latitudes. This air is cold, dry, and usually stable. Because of these characteristics, continental Polar air is not associated with significant precipitation. Maritime polar air (mP) has its source regions over the oceans of the upper middle latitudes. It is moister and milder  - 76 - than continental Polar air. Landfalling middle latitude cyclones fueled by maritime Polar air can bring prolific precipitation to mountainous coasts of the upper middle latitudes. Arctic air (A) is the coldest air of the Northern Hemisphere. It is very cold and stable in the winter and not associated with significant precipitation. Because of the land/sea configuration around the Arctic Ocean, Arctic air originates in northern Canada, Alaska, and Siberia in the winter. Antarctic air (AA) is the coldest air mass on Earth. In fact, it is much colder than the Arctic air of the Northern Hemisphere. It is generated on the Antarctic continent, which is covered by ice sheets having surfaces frequently in excess of two kilometers above sea level. Besides being cold, the air is very stable so that organized low pressure systems and their consequent precipitation are rare. Most middle latitude locations have considerably greater snowfall totals than Antarctica because of the inability of Antarctic air masses to hold much moisture.  - 77 - Equatorial air (E) is an extremely humid air mass found in the lowest latitudes. Maritime and continental varieties are not usually designated because both types of sources produce similar temperatures and humidities. The temperatures tend not to be as hot as continental Tropical and maritime Tropical air masses because clouds and high humidities inhibit surface heating. Equatorial air masses are present year round near the Equator with incursions of other air masses being extremely rare. Equatorial air becomes unstable by the middle of the afternoon and produces daily air mass thunderstorms for which the deep tropics are so well known. The classification scheme can also include a third letter, a lower case k or w. A k in the air mass designation indicates the air mass is colder than the surface over which it passes while a w denotes the air mass is warmer than the surface. Examples of this would be cPk air as it passes over North America in the summer. The continental surface is warm, the air is cool, and this sets up instability in the lower atmosphere. Conversely, a winter mPw air mass can be warmer than snow-covered North America and tend to be stable.  - 78 - Climatologically, air masses have preferred areas of occurrence that enlarge, contract, and shift latitudes with seasons. Seasonally, the zones of dominance shift. For instance, the maritime Tropical source region strengthens, enlarges, and edges into the lower middle latitudes during the  - 79 - summer season. Also, there are geographic variations associated with the positioning of continents. Continental Tropical air vanishes from North America in winter because of the small size of the continental surface at those latitudes. In North Africa this air mass persists year round. Some locations are associated with monotony of weather caused by the firm entrenchment of air masses. If one alights in Bele´m, Brazil in the heart of tropical Amazonia, she/he is struck with the sameness of weather day after day as dictated by the presence of Equatorial air. In the middle latitudes in particular, there are places that are neither air mass origin zones nor dominated by one air mass through the year. In these places, there is a hearty daily variation of the weather, especially in winter as the polar front jet stream forces the clash of tropical and polar air masses.  - 80 - -10- Examples of Oceans and Seas Oceans 1- Atlantic Ocean THE NAME OF THE Atlantic Ocean is derived from the Greek god Atlas and means “Sea of Atlas.” Its area is approximately 41 million square mi (106 million square km) including its adjacent seas; volume is approximately 85 million cubic mi (354 million cubic km), including adjacent areas. The average depth of the ocean with adjacent seas is 10,932 ft (3,332 m). The greatest depth is in the North Atlantic at 28,232 ft (8,605 m) in the Milwaukee Deep of the Puerto Rico Trench just north of PUERTO RICO; in the South Atlantic, the greatest depth is 27,651 ft (8,428 m) at the South Sandwich Trench east of the Falkland Islands. The width of the Atlantic varies from 1,769 mi (2,848 km) between Brazil and Liberia to about 3,000 mi (4,830 km) between the United States and northern Africa.  - 81 - The ocean has a coastline of 69,357 mi (111,866 km). Adjacent areas include the CARIBBEAN SEA, Gulf of Mexico, Gulf of St. Lawrence, Hudson Bay, Baffin Bay, MEDITERRANEAN SEA, BLACK SEA, North Sea, Baltic Sea, Barents Sea, Norwegian-Greenland Sea, and Weddell Sea. The Atlantic Ocean is the second-largest of the Earth’s oceans. Covering approximately 20 percent of globe’s surface, the Atlantic is second only to the PACIFIC OCEAN in size. Because the continents bordering its waters in the north are offset to the west of those in the south, the Atlantic appears as an elongated northsouth S-shaped channel. In the north, the Atlantic is bounded by North America on the west and Europe on the east, while in the south it is bounded by South America on the west and Africa on the east. It is also linked to the Pacific Ocean by the ARCTIC OCEAN in the north and by the Drake Passage in the south. The dividing line between the Atlantic and the INDIAN OCEAN to the east has been arbitrarily set at the 20 degrees E meridian, while the dividing line with the Pacific Ocean on  - 82 - the west follows the line of shallowest depth between Cape Horn and the Antarctic Peninsula. In the north, the boundary between the North Atlantic and the Arctic Ocean lies along a system of submarine ridges that extend between Baffin Island, GREENLAND, and Scotland. There is also a boundary between the Atlantic’s northern and southern zones, formed by the equatorial counter currents that circulate just north of the equator (8 degrees north latitude) in an area known as the Intertropical Convergence Zone (ITCZ). The Atlantic Ocean appears to be the youngest of the world’s oceans. It began to form during the Jurassic period, about 150 million years ago, when a rift opened up in the supercontinent of Gondwanaland, resulting in the separation of South America and Africa. The separation continues today at the rate of several centimeters a year along the Mid-Atlantic Ridge, a great submarine mountain range that extends from ICELAND in the north to approximately 60 degrees south latitude, dividing the Atlantic into a series of somewhat equal basins (also  - 83 - known as ABYSSAL PLAINS). Roughly 930 mi (1500 km) wide, the ridge has a more rugged topography than any mountain range on land and ranges from about 0.6 to 2 mi (about 1 to 3 km) above the ocean bottom. The ridge is a continuous feature of the Atlantic floor with one exception, the Romanche Furrow near the equator where the crest of the ridge drops significantly (15,000 ft or 4,573m) below the surface, allowing deep waters to flow freely between the Atlantic’s eastern and western basins. Other transverse ridges running between the continents and the Mid-Atlantic Ridge divide the ocean floor into numerous other sub-basins including the Guiana, North American, Cape Verde, and Canaries basins in the North Atlantic and the Angola, Cape, Argentina, and Brazil basins in the South Atlantic. The large Atlantic-Antarctic Basin lies between the southernmost extension of the Mid-Atlantic Ridge and the Antarctic continent. Although all of its abyssal basins are deeper than 16,400 ft (5,000 m), with many beyond 19,680 ft (6,000 m), the average depth of the Atlantic Ocean is 2 mi (3,300 m), less than the mean depths of both the Pacific and  - 84 - Indian oceans. Unlike the other oceans, the Atlantic has a high percentage (13 percent) of shelf seas (areas where continental crust is covered by water), which is two to three times the percentage found in the other oceans. The Atlantic has a relatively small number of islands in comparison to the Pacific, with the greatest concentration found in the Caribbean region. Most of the islands are structurally part of the continents. The major islands of the Atlantic include Svalbard, Greenland, Iceland, Great Britain, IRELAND, Fernando de Noronha, the AZORES, the Madeira Islands, the CANARY ISLANDS, the CAPE VERDE Islands, BERMUDA, the West Indies, ASCENSION, ST. HELENA, Tristan da Cunha, the FALKLAND ISLANDS, and the SOUTH GEORGIA Islands. NEWFOUNDLAND is the principal island on the North American shelf, the British Isles the major island group of the Eurafrican shelf, the Falkland Islands the only major group on the South American shelf, and the South Sandwich Islands on the Antarctic shelf. The islands of Puerto Rico, Hispaniola, JAMAICA, and CUBA (the Antilles) are part of an oceanic arc, while the Madeiras,  - 85 - Canaries, Cape Verde, and the Sao Tome and Principe group are the peaks of submarine ridges. The Azores, Saint Paul’s Rocks, Ascension, and the Tristan da Cunha group are peaks of the Mid-Atlantic Ridge system, while the large island of Iceland is a volcanic hotspot at the northern end of the Mid- Atlantic Ridge. Bermuda rises from the floor of the North American Basin, and St. Helena from the Angola Basin. The Atlantic consists of four major water masses. The North and South Atlantic central waters constitute the surface waters. Sub-Antarctic intermediate waters extend to depths of 3,300 ft (1,000 m), while the North Atlantic deep waters reach depths of as much as 13,200 ft (4,000 m). The Antarctic bottom waters are found at depths greater than 13,200 ft (4,000 m). Waters in the North Atlantic have a clockwise circulation (due to the Coriolis force), while those in the South Atlantic circulate counterclockwise. In addition, the land area that drains into the Atlantic is four times that of either the Pacific or Indian oceans. The major river drainage basins affecting the Atlantic include waters from many of the principal rivers of the world, among them the ST. LAWRENCE, MISSISSIPPI, Orinoco, AMAZON, Paraná,  - 86 - CONGO, NIGER, and LOIRE, and the rivers emptying into the North, Baltic, and MEDITERRANEAN seas.  - 87 - The circulatory system of the Atlantic’s surface waters consists of two large gyres (or circular current systems), with one in the North Atlantic and the other in the South Atlantic. These gyres or current systems tend to be wind driven but are also influenced by the rotation of the Earth. The currents of the North Atlantic (the North Equatorial Current, the Canaries Current, and the GULF STREAM) flow in a clockwise direction from the equator to about 45 degrees north latitude, while those in the South Atlantic (the Brazil, Benguela, and South Equatorial currents) flow counterclockwise from near the equator to about 45 degrees south latitude. As you approach the polar zones, the currents are less completely defined, with one rotating counterclockwise in the Arctic regions of the North Atlantic and another in the South Atlantic rotating clockwise near Antarctica. Surface salinity values are influenced by evaporation, precipitation, river inflow, and melting of sea ice. The salinity of the surface waters in the open Atlantic range from 33 to 37 parts per thousand, depending on latitude and season. Minimum salinity values are usually found at high  - 88 - latitudes and along coasts of continents where large river flows affect concentration. Given the volume of water discharged by the Amazon River in northeastern South America, minimum salinity values for the Atlantic are found just north of the equator. The area with the highest salinity values occurs in a part of the Atlantic referred to as the SARGASSO SEA. The Sargasso is somewhat of an ocean desert, with very little rainfall. Given the regions latitude, rates of evapotranspiration (evaporation exceeding rainfall) are quite high leading to the high salinity values in the surface waters. Surface water temperatures, which are influenced by latitude, current systems, and season, range from 28 degrees F to 84 degrees F (-2 degrees C to 29 degrees C). The most active circulation is found in the uppermost layer of warm water. Below this, circulation becomes increasingly sluggish as the temperature decreases. Surface temperatures range from 32 degrees F (0 degrees C) at the Arctic and Antarctic margins, to 81 degrees F (27 degrees C) at the equator. At depths below about 6,600 ft (2,000 m), temperatures of 36 degrees F (2 degrees C) are prevalent; and in bottom waters,  - 89 - those below 13,200 ft (4,000 m), temperatures of 30 degrees F (-1degrees C) are common. The ocean has also contributed significantly to the development and economy of the countries around it. Besides its major transatlantic transportation and communication routes, the Atlantic contains some of the world’s most productive fisheries. The most productive of these include the Grand Banks off Newfoundland, the shelf area off Nova Scotia, Georges Bank off Cape Cod, the Bahama Banks, the waters around Iceland, the Irish Sea, the Dogger Bank near the North Sea, and the Falkland Banks. The major species of fish caught in these areas are cod, haddock, hake, herring, and mackerel. There are also abundant petroleum deposits in the sedimentary rocks of the continental shelves. Large amounts of petroleum are currently being extracted in the North Sea and in the Caribbean Sea and Gulf of Mexico region, with lesser amounts coming from the Gulf of Guinea near the African coast. Actively mined mineral resources include titanium, zircon, and monazite (phosphates of the cerium  - 90 - metals), off the eastern coast of Florida, and tin and iron ore, off the equatorial coast of Africa. 2- Indian Ocean MEASURING approximately 26.5 million square mi (68.5 million square km), the Indian Ocean includes the Andaman Sea, ARABIAN SEA, BAY OF BENGAL, Flores Sea, Great Australian Bight, Gulf of Aden, Gulf of Oman, Java Sea, Mozambique Channel, PERSIAN GULF, RED SEA, Savu Sea, Straits of Malacca, Timor Sea, and other tributary water bodies. The ocean is located between Africa, Southern Ocean, Asia, and AUSTRALIA. The Indian Ocean is the third-largest of the world’s five oceans (after the PACIFIC OCEAN and ATLANTIC OCEAN, but larger than the Southern Ocean and Arctic Ocean). The Southern Ocean was delineated in a spring 2000 decision by the International Hydrographic Organization, which consolidated a fifth world ocean from the southern portions of the Atlantic Ocean, Indian Ocean, and Pacific Ocean. This new ocean extends from the coast of ANTARCTICA north to 60 degrees South latitude or the  - 91 - Antarctic Treaty Limit. Five major choke points along commercial sea lanes provide access to the Indian Ocean. These waterways are the Suez Canal (EGYPT), BAB EL MANDEB (DJIBOUTI, YEMEN), Strait of Hormuz (IRAN- OMAN), and Straits of Malacca (INDONESIA- MALAYSIA), and the Lombok Strait (INDONESIA). The Indian Ocean has held the historic TRADE ROUTES from Occident to Orient since the dawn of maritime commerce in the ancient world. Spices, slaves, and marvelous though modest handcrafts moved among the many ports the Indian Ocean carried by centuries of monsoon winds and currents. This trade now bears the modern manufactured goods of industrialized nations and the petroleum required to fuel the world’s economies. Never an insular sea, the Indian Ocean now serves not only the ports of its own shores but the far reaches of the countries of Europe and the Americas. Historical accounts include the tales of Egyptian, Greek, and Roman commercial and colonial exploits. The settlement of Madagascar by peoples from Indonesia in ancient times points to the reciprocal flow of culture and  - 92 - commerce from the east. Centuries of trade and exploration were carried out by the Islamic merchants and seaman of the Arabian Peninsula, who took their goods and their faith to the lands of the African coast, India, and Southeast Asia. Before the Europeans made good a sea passage to the waters of the Indian Ocean, Admiral Zheng He of the Ming Dynasty of China led several maritime expeditions across these fated waters from 1405 to 1433. By 1497, Vasco da Gama navigated around the Cape of Good Hope and began the Portuguese fight for domination of the spice trade across the Indian Ocean. This model of bold commercial venture supported by national navies and prestige was followed by the Dutch, French, and British over the next two centuries. European colonial interests in the Indian Ocean LITTORAL continued until after World War II. The independence of regional states from their colonial masters, growth and development among the newly industrialized states, and the increased flows of manufactured goods and petroleum have made the Indian Ocean truly an international sea.  - 93 - During the Cold War, the Indian Ocean was an arena of competition between East and West. Regional navies were often overshadowed by the number and modern capabilities of the U.S. and Soviet fleets. Establishment of a permanent support and operational base on the island of Diego Garcia, strategically central in the Indian Ocean, was a clear sign of the American intent to remain active in the region. The United States maintains the most active, modern, and capable military forces in the Indian Ocean. Throughout the long history of foreign economic exploitation and domination, the Indian Ocean has continuously been a rich resource to the peoples of the region. It constantly supplied transportation routes and rich fisheries along the coasts. The Indian Ocean continues to be a major fishing ground with fleets from many nations vying for the limited resource. Fishing stocks are being depleted by a combination of overfishing and pollution along the Indian Ocean coasts. The overexploitation of fish stocks is mostly due to large fishing vessels operating illegally near the coast. The growth of regional populations, particularly in INDIA  - 94 - and Indonesia, will add pressure on the already challenged marine resources.  - 95 - This population increase and industrial development creates major pollution problems in the most critical fishing areas. Industrial effluents contain heavy metals and chemical wastes. Pesticides and organic wastes flow untreated into the coastal waters from cities and agricultural land. Oil pollution from accidents, ballast dumping, and offshore oil extraction is on the increase. The nations that share the management and use of the Indian Ocean have no comprehensive plan for conservation and management of the resources or uses of this global asset. Mineral resources and especially offshore petroleum extraction are continuing to grow as a commercial interest to nations. Large reserves of hydrocarbons are being tapped in the offshore areas of SAUDI ARABIA, IRAN, INDIA, and AUSTRALIA. An estimated 40 percent of the world’s offshore oil production comes from the Indian Ocean. Beach sands rich in heavy minerals and offshore placer deposits are actively exploited by bordering countries, particularly India, SOUTH AFRICA, Indonesia, SRI LANKA, and THAILAND. The mining of polymetallic nodules from the  - 96 - seabed remain a tempting, but technologically challenging operation. Climate and weather patterns are dominated by the annual monsoon. This weather cycle is attributed to low atmospheric pressure over Southwest Asia created by hot, rising summer air, which causes southwest-to-northeast winds and currents during the summer months. A high pressure over North Asia created by cold, falling, winter air results in northeast-to-southwest winds during the winter months. For half the year (April to October), the winds in this region are from the southwest, reversing in the other half of the year. This monsoon (season) weather pattern dominates the region on land and sea, setting the pace of life onshore and off. Tropical cyclones occur during May/June and October/November in the northern Indian Ocean and January/February in the southern Indian Ocean. Primary adjoining arms of the Indian Ocean are the Persian Gulf and the Red Sea. The deepest known point is 25,443 ft (7,758 m), off the southern coast of Indonesia in the  - 97 - Java Trench. The Indian Ocean contains numerous islands, the largest of which are MADAGASCAR and SRI LANKA. Smaller islands that constitute independent countries include the MALDIVES, the SEYCHELLES, and MAURITIUS. The major rivers that flow into the Indian Ocean include the waters of the Limpopo and Zambezi rivers from Africa, as well as the IRRAWADDY, Brahmaputra, GANGES, and INDUS from east to west along Asia. The combined waters of the Tigris and Euphrates rivers mix with the Indian Ocean via the Persian Gulf. The Indian Ocean is of geostrategic interest as it is a transit route for a major portion of the world’s oil supply. In addition the commerce flowing by ship from Asia to Europe also sails this sea. Some major ports and harbors of the Indian Ocean are Chennai (Madras; India), Colombo (Sri Lanka), Durban (SOUTH AFRICA), JAKARTA (Indonesia), KOLKATA (Calcutta; India), Melbourne (Australia), MUMBAI (Bombay; India), and Richards Bay (South Africa).  - 98 - Seas 1- Adriatic Sea THE ADRIATIC SEA (in Italian “Mar Adriatico,” in Serbian “Jadransko more”) is a northwest-to-southeast arm of the MEDITERRANEAN SEA. The sea separates the Italian peninsula from the Austro-Hungarian, Montenegrin, and Albanian littorals, and the Italian Apennine Mountains from the Balkan Dinaric Alps. The western coast is Italian and the eastern comprises slovenia, croatia, bosnia and herzegovina, serbia and montenegro, and albania. The name derives from the Italian town of Adria (Hadria), designating in early historic times the sea’s upper portion. The term was later extended geographically to the south. The Adriatic has a total surface area of about 60,000 square mi (160,000 square km), with a maximum length of about 480 mi (770 km) and a width of nearly 100 mi (160 km); however, the Strait of Otranto, connecting the Adriatic and the Ionian Sea to the south, is 45 mi (72 km) in breadth. The mean depth is 133 fathoms (240 m), but the northern portion of the Adriatic is shallowest between the southern  - 99 - promontory of Istria and Rimini (about 25 fathoms or 46 m), where the low Italian littoral merges in the northwest into marshes and lagoons along the delta of the Po River. The freshwater Po and Adige are the major rivers flowing into the saline Adriatic and account for substantial silting. The Po’s sediment extended the coastline for 2 mi or 3.2 km within the last two millennia.  - 100 - The area between Šibenik and Ortona (Croatia and ITALY) exceeds 100 fathoms (180 m) in depth, but west of Durrës (Albania) and south of Dubrovnik (Croatia); the basin exceeds 500 fathoms (900 m). The rocky east coast has many long and narrow islands with the long axes lying parallel to the mainland coast and elevations of a few hundred feet; larger islands such as Brac have elevations of 2,552 ft (778 m). There are more than 1,000 islands in the Adriatic, although only 66 are inhabited, notably near Venice (Italy), and Trieste (Italy). Due to eutrophication and minimal tidal flow, making the sea a shallow, closed system, the Adriatic has notable water and air pollution yet remains an important tourism and fishing locale. 2- Aegean Sea THE AEGEAN SEA is an arm of the MEDITERRANEAN SEA, located between the Greek peninsula to the west and TURKEY to the east. The Aegean is connected through the Dardanelles, the Sea of Marmara, and the Bosporus with the BLACK SEA, while the island of Crete is considered to be the southern boundary. In all, it is  - 101 - about 380 mi (611 km) long and 186 mi (299 km) wide. It has a total area of approximately 83,000 square mi (214,000 square km). As for the name Aegean, there are several explanations: 1) named after the town of Aegae; 2) derived from the queen of the Amazons, Aegea, who died in the sea; and 3) stemmed from Aegeus, the father of Theseus, who drowned himself in the sea when he mistakenly thought his son had died in a distant war. The formation of the sea occurred when the Tethys Sea or Seaway began to shrink over the last 120 million years as the approaching African and European plates closed off the Mediterranean and surrounding seas. During the last Ice Age (2,000,000 to 18,000 B.C.E.), the shallow and narrow Straits of Gibraltar blocked off much of the ATLANTIC OCEAN waters, which led to a saline crisis as a high rate of evaporation in the nearly arid region created a shallow, briny basin. Even today, the generally shallow (average depth of 4,921 ft or 1,500 m) Mediterranean Sea has a low exchange rate with the Atlantic and is saltier. The maximum depth of the Aegean is found east of Crete, where it reaches 3,543 m (11,627 ft). The rocks that make up  - 102 - the floor of the sea are mainly limestone, though proximity to a plate boundary has allowed volcanic activity to alter it.  - 103 - The Aegean is studded with numerous large and small islands that are the mountain peaks of Aegeis, the name given to a submerged land mass. The Aegean islands can be divided into seven groups: the Thracian Sea group, the East Aegean group, the Northern Sporades, the Cyclades, the Saronic Islands, the Dodecanese, and Crete. North winds prevail over the Aegean Sea, although periodically, the cold gale-force Bora katabatic drainage wind thunders into the sea from the Balkans. The low tides generally follow those of the eastern Mediterranean. However, the tide of Euripus, the strait between Greece and the island of Euboea, demonstrates a violent and uncertain character, leading to the term the Euripus Phenomenon. Cold water masses with fluctuating temperatures flow out of the higher BLACK SEA, impacting the deep waters of the Aegean. The low concentration of phosphates and nitrates, necessary for marine life, limits fishing to sardines and sponges. Likewise, the barren, rocky soil hinders agriculture. Thus, tourism remains the major source of income for the Aegean coastal countries.  - 104 - 3- Arabian Sea THE ARABIAN SEA covers approximately 1,491,000 square mi (3,862,000 square km) and is located between the Arabian and Indian peninsulas in the northwestern area of the INDIAN OCEAN, bounded by INDIA, PAKISTAN, IRAN, OMAN, YEMEN, and the HORN OF AFRICA. The Arabian Sea has been the historic trade route from Occident to Orient since the dawn of commerce between the cradles of civilization. Dhows (sailing vessels) full of spices and slaves have given way to petroleum tankers and container ships, yet the trade continues unabated. As early as the 8th century and onward, Arabian and Persian mariners learned to navigate this area by using prevailing winds and the surface currents generated by the summer and winter monsoons. For half the year (April– October), the winds in this region are from the southwest, reversing in the other half of the year. This monsoon (season) weather pattern dominates the region on land and sea, setting the pace of commercial activities and much of life.  - 105 - Commercial fishing is a major activity in the Arabian Sea with the leading species fished being sardines, prawns, and mackerel. India accounts for 23.6 percent of its annual haul mostly from waters 6 to 9 mi (10 to 15 km) from the coast. Fishing stocks are being depleted because of a combination of overfishing and pollution along the Indian coast. Some 65 percent of all fish taken from the Arabian Sea continues to be from local fishermen in traditional boats as their single means of livelihood. The overexploitation of fish stocks is mostly due to large fishing vessels operating illegally near the coast. The growth of regional populations, particularly India, will add pressure on the already challenged marine resources. This population increase and industrial development create major pollution problems. Industrial effluents contain heavy metals and chemical wastes. Pesticides and organic wastes flow untreated into the coastal waters from cities and agricultural land. Oil pollution from accidents and ballast dumping is on the increase. The nations that share the management and use of the Arabian Sea have no comprehensive plan for  - 106 - conservation and management of the resources or uses of this critical area of ocean. Primary branches of the Arabian Sea are the Gulf of Oman, which joins it to the Persian Gulf via the Strait of Hormuz and the Gulf of Aden, which joins it with the Red Sea via the BAB EL MANDEB. There are no islands in the middle of the Arabian Sea, where depths average in excess of 9,800 ft (3,000 m). Deep water reaches close to the bordering lands except in the northeast, off Pakistan and India. The deepest known point in the Arabian Sea is at Wheatley Deep, where depths are more than 19,000 ft (5,800 m). The principal waterway draining directly into the Arabian Sea is the INDUS RIVER. Costal islands exist around the Arabian Sea and have proven significant for political and military purposes. The sea is of geostrategic interest as it is the transit route for a major portion of the world’s oil supply. In addition, the commerce flowing by ship from Asia to Europe also sails this sea.  - 107 - 4- Aral Sea THE ARAL SEA (Aral’skoye More) is one of the world’s largest lakes or inland seas. It is located to the east of the CASPIAN SEA in the Central Asian republics of Kazakhstan and Uzbekistan. The region is arid, with the Karakum Desert lying to the west of the Aral Sea and the Kyzylkum Desert to the east. The sea is shallow with no outlet; the water level is determined by the balance between loss from evaporation, input from rivers, groundwater, and precipitation. Its main source of water is two rivers: the Syr Darya (the ancient River Jaxartes) and the AMU DARYA (the ancient River Oxus), which rise in the foothills of the northern HIMALAYAN mountains. During the last 3 million years, the lake has periodically flooded and experienced episodes of desiccation as the Earth has cooled and warmed. Recent changes are, however, the result of human activity, and the sea now has a negative water balance; that is, there is a net loss of water annually. This is mainly because the Amu Darya and Syr Darya are subject to considerable water extraction for irrigation. Consequently, the Aral Sea has shrunk in size, especially in  - 108 - the last 70 years as cotton production in Central Asia has intensified. Even in the last 40 years, the area of the Aral Sea has decreased by about 50 percent and its level has dropped by more than 56 ft (17 m). Almost 19.8 million acres (8 million hectares) of land in the region are now under irrigation, compared with only 7.4 million acres (2.9 million hectares) in 1900 and 12.4 million acres (5 million hectares) in 1960. Moreover, much irrigation is inefficient as water loss from the canal network is high through exposure of a large surface area of water that encourages evaporation. The Aral Sea is no longer one water body; its volume has decreased by two-thirds and since 2001 there have been three water bodies. The shoreline has decreased by 300 mi (480 km), isolating settlements from the water’s edge in salt desert. Water quality has also altered as salinity and mineral content have increased, and wind-blown sediment is a problem. Fish and wildlife losses have ensued and no commercial fishing is possible; vegetation cover has diminished as fewer species can tolerate the harsh  - 109 - environment. Livelihoods based on fishing and hunting have disappeared. Human health problems have also developed, partly because of poor water quality attribut