We live our lives at the bottom of an ocean of air called the atmosphere. The atmosphere surrounds the solid and liquid parts of Earth—the land and water—but it is just as truly a part of Earth as they are.
But the atmosphere was not always there. Some 4 billion years ago, Earth had yet to grow the atmospheric skin that all life depends upon today. There was no blue sky to mark the day, and stars did not twinkle at night, since there was little air to catch and scatter their light. As Earth turned, the side facing away from the Sun froze while the side facing it broiled, since there was little air to hold the heat or deflect it.BIRTH OF THE ATMOSPHERE
As the planet Earth solidified, its bulk produced enough gravity to capture gases from space—but only rather heavy gases such as methane, ammonia, water vapor, and noble gases such as neon, argon, and krypton. Scientists believe that these were the elements of the planet's first atmosphere.
As early Earth stabilized into a solid planet, great volcanoes formed on its surface. The volcanoes spewed out vast quantities of carbon monoxide and carbon dioxide, which were gradually incorporated into the early atmosphere as well.
Earth's early atmosphere was too thin and weak to deflect incoming meteorites. Each day, thousands crashed to the surface. On impact, the meteorites vaporized, adding more water vapor, ammonia, and other gases to the surrounding air.
Sometime over the next billion years, life appeared on Earth. These first organisms survived on a "diet" of water vapor, carbon dioxide, and ammonia. At some point, life arose that was capable of photosynthesis; that is, it could harness the energy of the Sun to drive a reaction in which carbon dioxide and water are used to create organic molecules and oxygen.
Over hundreds of millions of years, the oxygen produced by these first photosynthetic organisms accumulated in the atmosphere. Scientists refer to this as the Great Oxidation Event (GOE). It occurred about 2.3 billion years ago. The widespread development of land plants some 400 million years ago likely raised oxygen levels to what they are today.
Almost simultaneously, the amount of carbon dioxide in the air plummeted. Much was captured by plants; the disappearance of the rest is only partly understood. Regardless, it appears that the amount of carbon dioxide in the atmosphere today is about 100 times less than it was before the GOE. Only in the past few centuries has this trend been reversed, probably due to widespread burning of fossil fuels.The Atmosphere Today. In its present form, the atmosphere acts much like the glass roof of a greenhouse. It lessens the change in temperature between day and night, summer and winter. The heat rays of the Sun penetrate the air and warm Earth's surface during the day. The overlying atmosphere traps this heat so that it escapes more slowly into space, moderating the cold of night.
The atmosphere protects life on Earth from a steady hail of meteorites. It is estimated that over 100,000 such particles strike Earth's atmosphere every 24 hours. But the friction of our atmosphere reduces all but the very largest to gas and dust before they reach ground level. The atmosphere also deflects various types of radiation and electrically charged particles from the Sun.
Because of the atmosphere, life on Earth experiences rain, wind, clouds, and other types of weather, as well as the colors of sunrise and sunset, rainbows, and the beautiful auroras, or polar lights.A Mixture of Gases. The atmosphere today consists largely of nitrogen (78 percent) and oxygen (21 percent). Other gases include argon (0.9 percent), carbon dioxide (0.04 percent), and trace amounts of neon, hydrogen, helium, ozone, methane, and nitrous oxide. The amount of water vapor in the air varies greatly depending on the place and time of measurement.
The oxygen in the atmosphere is essential in the processes of respiration and metabolism, the means by which humans and other animals derive the energy needed to sustain their existence. Oxygen is also a necessary constituent in many physical processes, such as combustion. Nitrogen, the most abundant element in the atmosphere, also plays an important role in respiration and countless other biological and physical processes.
Carbon dioxide makes up a small but important part of the atmosphere. Plants use it in photosynthesis, the process by which they both manufacture energy and produce oxygen. By contrast, animals take in oxygen and release carbon dioxide as a waste product.
The proportion of carbon dioxide in the air varies with place and time. Lightning appears to increase local concentrations of carbon dioxide. The total volume of atmospheric carbon dioxide has been increasing steadily for the past few centuries, perhaps because of the increased burning of fossil fuels.
Centered at about 16 miles (25 kilometers) above Earth lies a layer of "supercharged" oxygen known as ozone. Each ozone molecule contains three atoms of oxygen instead of the two atoms found in an ordinary oxygen molecule. The ozone layer absorbs a great deal of the Sun's radiation, and so both warms the atmosphere below it and protects life from the destructive effects of radiation. Depletion of the ozone layer due to human-made pollutants is of great concern today, and has emerged as an area of substantial scientific research.STRUCTURE OF THE ATMOSPHERE
The atmosphere is not a single mass; it consists of five layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each is characterized by a distinctive temperature distribution. The layers have been given names with the suffix -sphere, and their upper boundaries have been given names ending with the suffix -pause.Troposphere. The troposphere is the bottom layer of the atmosphere, closest to Earth's surface. The prefix tropo- means "turning or changing," because this is the layer in which the changing atmospheric conditions known as weather occur. The troposphere extends to a height of 12 miles (19 kilometers) in tropical regions, and 6 miles (10 kilometers) in polar regions.
Temperatures within this layer decrease as the height increases. The average temperature at the upper boundary of the troposphere (the tropopause) is −60° F (−51° C). Nearly all clouds are found in the lower half of the troposphere. They are composed of small water droplets and ice crystals, and they assume a great variety of forms, differing widely in vertical extent and in horizontal scale. The fair-weather, pillowy cumulus clouds of small extent can be identified by an observer on the ground. (Cumulus is Latin for "heap.") Other, more extended forms found in large storms or hurricanes may be readily identified on weather-satellite pictures taken from outside the atmosphere.
There is far more water vapor and carbon dioxide in the troposphere than in any other layer. These two gases play an important role in Earth's heat balance by absorbing and trapping much of the infrared radiation that comes from the Sun before and after it reaches Earth's surface. The troposphere is therefore heated by infrared radiation reflected back from the ground as well as from the Sun. Its temperature generally drops about 19° F per mile (6.5° C per kilometer) of altitude gained.
The troposphere is also the densest part of the atmosphere, containing 75 percent of its mass. It therefore has the greatest air pressure. We do not ordinarily think of air as having weight. But due to gravity, the atmosphere exerts a pressure of about 14.7 pounds per square inch at ground level. Air pressure decreases with increasing altitude because there are fewer molecules of air at higher altitudes. This is why airplanes that travel at great heights above the surface must be built with artificially pressurized cabins or other oxygen-delivery systems.Stratosphere. Above the tropopause lies the layer of air called the stratosphere. The air here is considerably "thinner" (less dense) than in the underlying troposphere, and very dry. Other than water vapor, the stratosphere contains essentially the same gases as the troposphere below it—only in smaller amounts.
The stratosphere is named for its strata, or sections, of temperature gradations. In the lower part of the stratosphere, the temperature is nearly constant with height, but in the upper part of the strastosphere, temperature increases with height. The stratosphere is especially important for its concentrated amount of ozone gas. This ozone layer shields life on the ground from deadly amounts of ultraviolet radiation. As it absorbs ultraviolet radiation, the ozone layer grows warm. As a result, the stratosphere is much warmer at its center and top—where ozone occurs—than at its base. Maximum temperatures of about 45° F (7° C) are reached at a height of about 30 miles (48 kilometers), which marks the stratopause. The lower stratosphere is the typical cruising level for airliners.
Clouds are rare in the stratosphere, but it has a system of winds that includes the jet streams of the lower stratosphere. These fast, world-circling air currents vary with the seasons, but their circulation patterns are quite persistent.
In the Northern Hemisphere during the winter, a jet stream occurs at an altitude of about 6 miles (10 kilometers) above the southern United States, blowing from the west at speeds of more than 80 miles (130 kilometers) per hour. Another jet stream forms a vast winter whirlwind above the Arctic, also flowing from west to east, but at a height of about 18 miles (30 kilometers) or more.
In the spring, a sudden warming of the Arctic stratosphere takes place, and the circumpolar circulation reverses direction, so that it blows from east to west. In the middle latitudes, the jet streams shift poleward during the summer, maintaining their west-to-east direction, but losing much of their winter velocities.
The above-described seasonal changes in the stratospheric jet streams are thought to profoundly affect weather in the troposphere below. For instance, jet streams may bring warmth to the poles in spring. But jet streams may also prevent warm air from approaching the poles during winter.Mesosphere. Above the stratopause lies the mesosphere (meso- means "middle"). It holds the coldest section of the atmosphere: at a height of 50 miles (80 kilometers), temperatures may drop to about −184° F (−120° C). The mesosphere is the location of meteor showers, and is the lower limit for orbiting spacecraft. Because it has been explored only by sounding rockets, the mesosphere is the least understood layer of the atmosphere.
Fascinating phenomena occur in the mesosphere. Cosmic rays from outer space shatter atmospheric atoms, creating showers of secondary atomic particles that may penetrate Earth. On some dark evenings after sunset, so-called noctilucent, or "night-shining," cloudlike formations appear at the top of the mesosphere. Illuminated by the just-set Sun, these rippling ribbons are thought to consist of water vapor or meteor dust. Airglow—sunlight reradiated by heated atmospheric particles—occurs at about the same height as noctilucent clouds. And the auroras—northern and southern lights—sometimes extend down into the mesosphere from the atmospheric layer above.Thermosphere. Above the mesopause lies the thermosphere. The prefix thermo-, meaning "heat," is quite appropriate for this layer. The thermosphere is an intensely hot layer of atmosphere. The temperature increases with height until it reaches about 2,250° F (1,232° C) at a height of 300 miles (480 kilometers). Temperatures in the thermosphere probably also vary by several hundred degrees between day and night. The space shuttle maintained its orbit in the upper thermosphere. The air in the thermosphere is dense enough to burn up fast-moving meteors, whose fiery trails have been observed at altitudes as high as 186 miles (300 kilometers).
Intense radiative energy from space tends to break thermospheric gases into individual atoms and their electrically charged particles. This is very important for communications because the ions reflect radio waves back toward Earth, permitting worldwide communication. During solar flares and intense sunspot activity, atmospheric ionization is increased. This sometimes causes radio waves to be absorbed rather than reflected, and radio communication falters.Exosphere. Above 300 miles (480 kilometers) lies the fringe region known as the exosphere, the last atmospheric layer before space. There Earth's atmosphere merges with the gases of interplanetary space. Exo means "outside" or "external." The exosphere is home to orbiting space observatories including the Hubble Space Telescope. At times a rather faint glow can be seen radiating from the exosphere. Known as zodiacal light, or gegenschein, the glow derives from sunlight reflected off the countless particles of meteoritic dust that swarm near Earth. The location of the exopause is unknown, although some consider it to be about halfway to the Moon, or some 120,000 miles (190,000 kilometers).
Electrical Regions. The atmosphere also may be divided into regions on the basis of electrical properties. The regions include the ionosphere, the magnetosphere, and the Van Allen radiation belts.
The ionosphere is roughly located from a height of 50 to 300 miles (80 to 480 kilometers). The ionosphere contains many electrically charged particles, or ions, as well as neutral molecules. The atmosphere just below the ionosphere consists mainly of neutral molecules of air whose motions are controlled by pressure forces and by Earth's gravitational field. This sets up a vertical voltage difference amounting to about 400,000 volts between Earth and the ionosphere. Thunderstorms occurring all over the world generate and maintain this potential difference. In addition, the ionosphere is the region of auroral displays, and is also used in radio communications for reflecting radio signals over long distances.
Above a height of 300 miles lies a region of the atmosphere called the magnetosphere. It is so named because the behavior of the ions and atomic particles is controlled almost entirely by Earth's magnetic field.
Between 2,000 and 20,000 miles (3,200 and 32,000 kilometers) high lies a radiation zone containing many high-energy ionized particles. Called the Van Allen radiation belts, the zone was first detected by James A. Van Allen of the United States and Sergei N. Vernov of the Soviet Union through the analysis of data that had been obtained from artificial satellites. The belt comprises two main sections—a stable inner belt and a dynamic outer belt. A third, temporary section occasionally forms within the outer belt. First detected in 2012, this transitory belt persisted for only a few weeks before being destroyed by a burst of solar wind. Scientists continue to study the data gathered during this surprising discovery.STUDYING THE ATMOSPHERE
Every day of the year, scientists gather information on air pressure, temperature, and humidity by releasing small, gas-filled balloons called radiosondes. Although radiosondes rise to an altitude of about 40 miles (65 kilometers), they are used primarily to gather data about the lower two layers of the atmosphere: the troposphere and the stratosphere.
As the radiosonde rises through the troposphere, an attached radio transmitter relays weather information that meteorologists can use to develop their daily forecasts. The battery-operated radio transmitter is remarkably light and compact.
More-stable weather balloons, built to remain for long periods at heights of 1 to 15 miles (2 to 25 kilometers), have also been launched. These devices keep track of atmospheric conditions over the long term, enabling scientists to gain a better understanding of the atmosphere.
Atmospheric scientists have also used rockets to aid their studies. They sometimes launch chemical-filled shells, and then study the reactions of these chemicals in different layers of the atmosphere.
Satellites have become increasingly important tools in studying Earth's atmosphere. Sophisticated sensors aboard the satellites transmit data to computers at ground level. The resulting maps portray areas of the atmosphere according to their temperatures, humidities, or various chemical compositions. Satellite imaging has been especially crucial in the recent study of human climate modification and of global ozone depletion.THE OZONE HOLE
As explained earlier, the stratosphere contains within it a layer of ozone that acts as a "sunscreen," shielding life on Earth's surface from the Sun's harmful ultraviolet radiation. Today human activities are severely depleting this ozone shield. The extent of this depletion—and what can be done about it—are now the subjects of much study. (Ozone depletion should not be confused with ozone pollution, which occurs at ground level. Ironically, car exhaust and other human activities produce an overabundance of ozone in the lower atmosphere, where it damages both plants and human health.)
In the late 1970s, scientists first noticed the appearance of an ozone hole over Antarctica. They determined that some thinning of the ozone layer was natural during the Antarctic spring. But this thinning had been greatly increased by the presence of human-made chemicals in the upper atmosphere.
The main culprit was, and still is, the chlorine molecules found in industrial chemicals known as halogenated hydrocarbons. The best known are the chlorofluorocarbons, or CFCs, used in aerosol sprays, refrigeration, and air-conditioning systems. In the stratosphere, ultraviolet light breaks down CFCs so that they release their destructive chlorine.
The chlorine molecules then react with ozone molecules. Each chlorine molecule can destroy as many as 100,000 ozone molecules. It does so by speeding a reaction in which ozone (O3) is converted into ordinary oxygen (O2). Chlorine does the most damage in areas of the atmosphere where clouds of ice or volcanic dust further speed its reaction with ozone.
In 1987, many nations of the world agreed to reduce the emissions of CFCs by 50 percent by 1998. Then, after the danger of ozone depletion became even more apparent, the nations agreed to an outright ban on CFCs by 2000. But scientists warn this is not enough. Experts are urging a complete ban of chlorine-emitting chemicals by 2030. Bromine and nitric oxide also deplete stratospheric ozone.
Meanwhile, depletion of the ozone layer continues to worsen, causing an increase in the amount of ultraviolet radiation that reaches Earth. NASA's Total Ozone Mapping Spectrometers (TOMS) aboard several satellites have reported that the fluctuating ozone hole over Antarctica measures some 10 million square miles (25.9 million square kilometers)—larger than the area of North America. Thinning has been recorded over every continent. The elevated levels of ultraviolet radiation may already be harming wildlife and contributing to the increased occurrence of skin cancer.
Atmospheric scientists continue to monitor ozone levels with TOMS and other sophisticated instruments aboard aircraft and balloons and on the ground, as part of Mission to Planet Earth—NASA's long-term effort to study Earth as a global environmental system. Researchers also investigate the complex ways in which ozone-destroying chemicals reach the stratosphere and alter the ozone there. And there are new concerns. One is the increasing number of rocket launches in the world's growing space programs. Every type of rocket engine causes some ozone loss. Experts warn that rocket launches must be restricted to prevent continued ozone destruction. Another concern is climate change. Ozone depletion worsens when the stratosphere becomes colder. But because global warming traps heat in the troposphere, less heat reaches the stratosphere, which makes it colder. Global warming and unregulated rocket launches could accelerate ozone destruction far beyond the damage caused by CFCs.THE GREENHOUSE EFFECT
Research has shown that human activities can change the climate in other ways as well. The burning of oil, coal, and natural gas has produced a steady increase in the amount of carbon dioxide in the atmosphere. Like water vapor, carbon dioxide and certain other gases absorb and hold heat in a sort of blanket around the world. This phenomenon has been termed the greenhouse effect because the gases act like the heat-trapping glass of a greenhouse.
While natural greenhouse gases are vital to prevent our world from freezing over, a marked increase may spell big trouble. Indeed, there is disturbing evidence of climate change: melting glaciers; forest fires; drought; heat waves; more-severe storms; and the loss of certain plant and animal species.
Melting polar ice can result in rising sea levels, which could impact coastal communities. Climatologists say that fossil-fuel use has led to the fastest warming trend in Earth's history. In fact, the three warmest years on record have all occurred since 1998. Studies suggest that Earth's average temperature could increase 2° to 10° F (1° to 6° C) by 2100 if steps are not taken to reduce greenhouse-gas emissions.