PLACES, REGIONS, AND PHYSICAL SYSTEMS

Air, Weather, and Climate

The air surrounding Earth is called the atmosphere. It is a mixture of oxygen, nitrogen, and other gases. Compared with the enormous size of Earth, the atmosphere appears to be a thin layer of air. It is, nevertheless, deeper than any ocean.

The atmosphere surrounding a certain place may change at times. Sometimes it may be cold, at other times warm. Sometimes rain or snow may fall. The term weather refers to the condition of the atmosphere at a particular time or day in a particular place. The weather includes such factors as temperature, wind, rain, snow, fog, and hail.

Climate refers to the weather conditions over a long period of time. Weather changes daily and with the seasons. Scientists have found that climate also can change, but over much longer periods of time. Over the centuries, dramatic changes in climate have taken place in many areas of the world. Eighteen thousand years ago, for instance, the area around what is now Chicago, Illinois, was buried under an ice sheet about 3,000 feet thick.

Today Earth has a number of general climate regions. Very warm, tropical climates occur nearest the equator. Subtropical climates occur farther north and south of the equator. Some tropical and subtropical climates are humid, or moist and rainy. Others are dry. Polar climates are found near the North and South poles. Temperatures are cool to cold all year round. Regions roughly halfway between either pole and the equator in the Northern and Southern Hemispheres often have more moderate climates. Most places have four markedly different seasons each year. In mountain and highland areas, temperatures are always cool or cold, even in tropical or subtropical regions.

Land and Water

Seven large landmasses, called continents, make up most of Earth’s land surface. The continents are North America, South America, Europe, Asia, Africa, Australia, and Antarctica.

Land surfaces of Earth show many changes in altitude. These changes help define different kinds of landforms: mountains, hills, plains, and plateaus. If you consider the total amount of land area, Earth’s surface is relatively smooth. The highest mountain in the world, Mount Everest in Asia, has an altitude of 29,028 feet, or 5.5 miles. This is an insignificant height compared with the distance from Earth’s surface to its center, almost 4,000 miles.

Forces of wind and water continuously reshape the land through the process of erosion. The wind can slowly wear down mountains by blowing away small particles of sand. Rivers move millions of tons of earth from one place to another every year. The action of waves and the bits of rock they carry wear away seacoasts. The tremendous weight of glaciers grind up rocks as the thick ice sheets move slowly over the land. The glaciers may fill valleys with bits of rock and decaying plant and animal matter they have dragged along.

During the ice ages thousands of years ago, glacial erosion disintegrated many rocks and smoothed the surface of huge land areas. The disintegrated rock mixed with decaying plant and animal matter to make up Earth’s soil.

Earth is home to humans and to many kinds of plants and animals. None of these life forms could exist without water. Water is one of the most important and abundant substances found on Earth’s surface. If the planet were named for the main substance on its surface, it would be called water, not Earth. The water in oceans, rivers, and lakes, together with that frozen in the polar regions, covers three-fourths of the planet’s surface.

The Atlantic, Pacific, Indian, and Arctic oceans all help to separate the continents. The water in the oceans, however, all comes together. This means that there is actually only one, worldwide ocean.

Earth has only a certain amount of water, but this water is used over and over again. Water in nature is not destroyed: it constantly moves from the clouds to the ground and then back again to the clouds. This movement is called the water cycle.

During the water cycle, water falls from the clouds as rain, snow, or hail. Most of the water falls on the ocean or other bodies of water; some of it remains on the land; some of it is absorbed and then given off again by living things. The heat of the Sun causes water to evaporate from Earth’s surface. Then the water vapor in the air forms clouds. Eventually, the water falls from the clouds to the land and the cycle begins again.

Plants and Animals

You have already learned that parts of Earth have different climate regions depending on their distance from the equator and the poles. Climate greatly influences the natural vegetation, or wild plants that grow in a region. In polar regions known as tundra, tiny flowering plants and mosses are the only vegetation. They grow during the short summer season and may feed herds of elk or caribou. Many different animals live in the huge evergreen forests that edge the tundra in the Northern Hemisphere.

Moderate climate regions of both the Northern and Southern Hemispheres have many different zones of natural vegetation. Where rainfall is plentiful, large forestlands are possible. In these forests, the trees drop their leaves every year during the fall and winter months. Where rainfall is less plentiful, grasses will grow in the wild. Where rainfall is scarce, arid deserts with hardly any vegetation take over. Both large and small animals—deer, bison, rabbits, prairie dogs—inhabit the forests and grasslands. Mostly small animals live in the deserts.

Warm temperatures encourage heavy natural vegetation in tropical and subtropical regions. Where rainfall is heavy, dense rain forests cover large areas. These forests are filled with birds, snakes, and various small- to medium-sized animals. Where annual rainfall occurs only during certain months—the rainy season—tall grasses and grazing animals make up the landscape. During the dry season, the grasses wither and turn brown, and the animals move on to other feeding grounds.

The entire continent of Antarctica is a frozen ice cap the year round. It has no vegetation. Along some coastal areas, penguins, sea lions, and other animals fish the ocean waters for food.

Throughout human existence, people have used plants and animals to meet their needs and wants. Plants still provide the bulk of the world’s food, clothing, and shelter. Animals meet the same needs but to a lesser extent. In many parts of the world, animals are vital as beasts of burden. They may pull farming implements or carry goods for traveling merchants.

Human activity over the centuries has changed animal life and natural vegetation regions forever. As agriculture developed, humans cleared miles and miles of natural forests in moderate climate regions. In parts of Europe and North America forest areas have virtually disappeared. Grasslands, too, are gone. In their place are fields of wheat, corn, and other plants that people have domesticated, or improved from the wild.

Some animals have benefited from human activity, while others have suffered. Animals domesticated and cared for by humans, like chickens, sheep, cattle, and dogs, have increased greatly in numbers. At the same time, other kinds of animals are in danger of extinction. Some governments have restricted hunting of endangered animals to preserve wildlife.