Earth

Earth is our home planet. Its surface is mostly water (about 70%), and it has a moderately dense nitrogen-and-oxygen atmosphere that supports life—the only known life in the Universe. Rich in iron and nickel, Earth is a dense, rocky ball orbiting the Sun with its only natural satellite, the Moon. A complete revolution of the earth around the Sun takes about one year, while a rotation on its axis takes one day. The surface of Earth is constantly changing, as the continents slowly drift about on the turbulent foundation of partially molten rock beneath them. Collisions between landmasses build mountains; erosion wears them down. Slow changes in the climate cause equally slow changes in the vegetation and animals inhabiting a place.

Land

The lands of our planet are in a constant, though slow, state of change. Landmasses move, collide, and break apart according to a process called plate tectonics. The lithosphere is not one huge shell of rock; it is composed of several large pieces called plates. These pieces are constantly in motion, because Earth's interior is dynamic, with its core still molten and with large-scale convective currents in the upper mantle. The giant furnace beneath all of us moves our land no more than a few centimeters a year, but this is enough to have profound consequences.

Consider North America. The center of the continent is the magnificent expanse of the Great Plains and the Canadian Prairies. Flat and wide is the land around Winnipeg, Topeka, and Amarillo. On the eastern edge, the rolling folds of the Appalachian Mountains grace western North Carolina, Virginia, and Pennsylvania. In the west, the jagged, crumpled Rockies thrust skyward, tall, stark, and snow-capped.

These two great ranges represent one of the two basic land-altering processes: mountain building. Two hundred million years ago, North America was moving east, driven by the restless engine beneath it. In a shattering, slow-motion collision, it rammed into what is now Europe and North Africa. The land crumpled, and the ancient Appalachians rose. At that time, they were the mightiest mountains on Earth. A hundred million years later, North America was driven back west. Now the western edge of the continent rumbled along over the Pacific plate, and about 80 million years ago, a massive spate of mountain building formed the Rockies.

During the time since the Appalachians rose, the other land-altering process, erosion, has been hard at work on them. Battered by wind and water, their once sheer flanks have been worn into the low, rolling hills of today. Eventually they will be gone—and sometime long after that, so will the Rockies.

Mountain building can be seen today in the Himalayas, which are still rising as India moves northward into the underbelly of Asia, crumpling parts of Nepal and Tibet nearly into the stratosphere. Erosion rules in Arizona's Grand Canyon, which gradually is deepening and widening as the Colorado river slices now into ancient granite two billion years old. In time, the Canyon too will be gone.

This unending cycle of mountain building (caused by movement of the crustal plates) and erosion (by wind and water) has formed every part of Earth's surface today. Where there are mountains, as in the long ranks of the Andes or the Urals, there is subterranean conflict. Where a crustal plate rides over another one, burying and melting it in the hot regions below the lithosphere, volcanoes rise, dramatically illustrated by Mt. St. Helens in Washington and the other sleeping giants that loom near Seattle and Portland. Where lands lie wide and arid, they are sculpted into long, scalloped cliffs, as one sees in the deserts of New Mexico, Arizona, and Utah. Without ever being aware of it, we humans spend our lives on the ultimate roller coaster.

Water

Earth is mostly covered with water. The mighty Pacific Ocean covers nearly half the earth; from the proper vantage point in space one would see nothing but water, dotted here and there with tiny islands, with only Australia and the coasts of Asia and the Americas rimming the edge of the globe.

The existence of oceans implies that there are large areas of the lithosphere that are lower than others. This is because the entire lithosphere rides on a pliable layer of rock in the upper mantle called the asthenosphere. Parts of the lithosphere are made of relatively light rocks, while others are made of heavier, denser rocks. Just as corks float mostly above water while ice cubes float nearly submerged, the less dense parts of the lithosphere ride higher on the asthenosphere than the more dense ones. Earth therefore has huge basins, and early in the planet's history these basins filled with water condensing and raining out of the primordial atmosphere. Additional water was brought to Earth by the impacts of comets, whose nuclei are made of water ice.

The atmosphere has large circulation patterns, and so do the oceans. Massive streams of warm and cold water flow through them. One of the most familiar is the Gulf Stream, which brings warm water up the eastern coast of the United States.

Circulation patterns in the oceans and in the atmosphere are driven by temperature differences between adjacent areas and by the rotation of Earth, which helps create circular, or rotary, flows. Oceans play a critical role in the overall energy balance and weather patterns of our planet. Storms are ultimately generated by moisture in the atmosphere, and evaporation from the oceans is the prime source of such moisture. Oceans respond less dramatically to changes in energy input than land does, so the temperature over a given patch of ocean is far more stable than one on land.

Structure of the atmosphere

Earth's atmosphere is the gaseous region above its lithosphere, composed of nitrogen (78% by number), oxygen (21%), and other gases (1%). It is only about 50 mi (80 km) from the ground to space: on a typical, 12-in (30 cm) globe the atmosphere would be less than 2 mm thick. The atmosphere has several layers. The most dense and significant of these is the troposphere; all weather occurs in this layer, and commercial jets cruise near its upper boundary, 6 mi (10 km) above Earth's surface. The stratosphere lies between 6 and 31 mi (10 and 50 km) above, and it is here that the ozone layer lies. In the mesosphere and the thermosphere one finds aurorae occurring after eruptions on the Sun; radio communications "bounce" off the ionosphere back to Earth, which is why you can sometimes pick up a Memphis AM radio station while you are driving through Montana.

The atmosphere is an insulator of almost miraculous stability. Only 50 mi (80 km) away is the cold of outer space, but the surface remains temperate. Heat is stored by the land and the atmosphere during the day, but the resulting heat radiation (infrared) from the surface is prevented from radiating away by gases in the atmosphere that trap infrared radiation. This is the well-known greenhouse effect, and it plays an important role in the atmospheric energy budget. It is well for us that Earth's climate is this stable. A global temperature decrease of two degrees could trigger the next advance of the current ice age, while an increase of three degrees could melt the polar ice caps, submerging every coastal city in the world.

Weather

Despite this overall stability, the troposphere is nevertheless a turbulent place. It is in a state of constant circulation, driven by Earth's rotation as well as the constant heating and cooling that occurs during each 24-hour period.

The largest circulation patterns in the troposphere are the Hadley cells. There are three of them in each hemisphere, with the middle, or Ferrel cell, lying over the latitudes spanned by the continental United States. Northward-flowing surface air in the Ferrel cell is deflected toward the east by the Coriolis force, with the result that winds—and weather systems—move from west to east in the middle latitudes of the northern hemisphere.

Near the top of the troposphere are the jet streams, fast-flowing currents of air that circle Earth in sinuous paths. If you have ever taken a commercial plane flight, you have experienced the jet stream: eastbound flights get where they are going much faster than westbound flights.

Circulation on a smaller scale appears in the cyclones and anticyclones, commonly called low and high pressure cells. Lows typically bring unsettled or stormy weather, while highs mean sunny skies. Weather in most areas follows a basic pattern of alternating pleasant weather and storms, as the endless progression of highs and lows, generated by Earth's rotation and temperature variation, passes by. This is a great simplification, however, and weather in any given place may be affected, or even dominated, by local features. The climate in Los Angeles is entirely different from that in Las Vegas, though the two cities are not very far apart. Here, local features—specifically, the mountains between them—are as important as the larger circulation patterns.