From a human perspective, probably the most important aspect of Earth's atmosphere is that we can breathe it. Air is a mixture of gases, the most common of which are nitrogen (78 percent by volume), oxygen (21 percent), argon (0.9 percent), and carbon dioxide (0.03 percent). Water vapor is a variable constituent, making up anywhere from 0.1 percent to 3 percent, depending on location and climatic conditions. The presence of a large amount of oxygen makes our atmosphere unique in the solar system, and the presence of even trace amounts of water and carbon dioxide plays a vital role in the workings of our planet.
Figure 7.2 shows a cross section of our planet's atmosphere. Compared with Earth's overall dimensions, the extent of the atmosphere is not great. Half of it lies within 5 km of the surface, and all but 1 percent of it is found below 30 km. The portion of the atmosphere below about 12 km is called the troposphere. Above it, extending up to an altitude of 40 to 50 km, lies the stratosphere. Between 50 and 80 km from the surface lies the mesosphere. Above about 80 km, in the ionosphere, the atmosphere is kept partly ionized by solar ultraviolet radiation. These various atmospheric regions are distinguished from one another by the temperature gradient (decreasing or increasing with altitude) in each.
Figure 7.2 Diagram of Earth's atmosphere, showing the changes of temperature and pressure from the surface to the bottom of the ionosphere.
Atmospheric density decreases steadily with increasing altitude, and as the right-hand vertical axis in Figure 7.2 shows, so does pressure. Climbing even a modest mountain—4 or 5 km high, say—clearly demonstrates the thinning of the air in the troposphere. Climbers must wear oxygen masks when scaling the tallest peaks on Earth.
The troposphere is the region of Earth's (or any other planet's) atmosphere where convection occurs, driven by the heat of Earth's warm surface. Convection is the constant upwelling of warm air and the concurrent downward flow of cooler air to take its place, a process that physically transfers heat from a lower (hotter) to a higher (cooler) level. In Figure 7.3(a), part of Earth's surface is heated by the Sun. The air immediately above the warmed surface is heated, expands a little, and becomes less dense. As a result, the hot air becomes buoyant and starts to rise. At higher altitudes, the opposite effect occurs: the air gradually cools, grows denser, and sinks back to the ground. Cool air at the surface rushes in to replace the hot buoyant air. In this way, a circulation pattern is established. These convection cells of rising and falling air not only contribute to atmospheric heating but are also responsible for surface winds. This constant churning motion is responsible for all the weather we experience.
Figure 7.3 Convection occurs whenever cool fluid overlies warm fluid. The resulting circulation currents are familiar to us as the winds in Earth's atmosphere caused by the solar-heated ground. Hot air rises, cools, and falls repeatedly. Eventually, steady circulation patterns with rising and falling currents are established and maintained, provided that the source of heat (the Sun in the case of the atmosphere) remains intact.
Atmospheric convection can also create clear-air turbulence—the bumpiness we sometimes experience on aircraft flights. Ascending and descending parcels of air, especially below fluffy clouds (themselves the result of convective processes), can cause a choppy ride. For this reason, passenger aircraft tend to fly above most of the turbulence, at the top of the troposphere or in the lower stratosphere, where the atmosphere is stable and the air is calm.
Straddling the boundary between the stratosphere and the mesosphere is the ozone layer where, at an altitude of around 50 km, incoming solar ultraviolet radiation is absorbed by atmospheric oxygen, ozone, and nitrogen. (Ozone is a form of oxygen, consisting of three oxygen atoms combined into a single molecule.) The ozone layer is one of the insulating spheres that serve to shield life on Earth from the harsh realities of outer space. Not so long ago, scientists judged space to be hostile to advanced life forms because of what is missing out there—breathable air and a warm environment. Now, most scientists regard outer space harsh because of what is present out there—fierce radiation and energetic particles, both of which are injurious to human health. Without the protection of the ozone layer, advanced life (at least on Earth's surface) would be at best unlikely and at worst impossible.
Above about 100 km, in the ionosphere, the atmosphere is significantly ionized by the high-energy portion of the Sun's radiation spectrum, which breaks down molecules into atoms and atoms into ions. The degree of ionization increases with altitude. The presence of many free electrons makes this region of the upper atmosphere a good conductor of electricity, and this conductivity renders the ionosphere highly reflective to certain radio wavelengths. 
(Sec. 3.3) The reason that AM radio stations can be heard well beyond the horizon is that their signals bounce off the ionosphere before reaching the receiver. FM signals cannot be received from stations over the horizon, because the ionosphere is transparent to the somewhat shorter wavelengths of radio waves in the FM band.
Much of the Sun's radiation manages to penetrate Earth's atmosphere, eventually reaching the ground. Most of this energy takes the form of visible radiation—ordinary sunlight. Essentially all of the solar radiation not absorbed by or reflected from clouds in the upper atmosphere is absorbed by Earth's surface. The result is that our planet's surface and most objects on it heat up considerably during the day. Earth cannot absorb this solar energy indefinitely, however. If it did, the surface would soon become hot enough to melt, and life on our planet would not exist.
As it heats up, Earth's surface reradiates much of its absorbed energy. This reemitted radiation follows the usual blackbody curve discussed in Chapter 3. (Sec. 3.4) As the surface temperature rises, the amount of energy radiated increases rapidly, according to Stefan's law. Eventually, Earth radiates as much energy back into space as it receives from the Sun, and a stable balance is struck. In the absence of any complicating effects, this balance would be achieved at an average surface temperature of about 250 K (—23°C). At that temperature, Wien's law tells us that most of the reemitted energy is in the form of infrared (heat) radiation.
But there are complications. Infrared radiation is partially blocked by Earth's atmosphere. The primary reason for this is the presence of molecules of water vapor and carbon dioxide, which absorb very efficiently in the infrared portion of the spectrum. Even though these two gases are only trace constituents of our atmosphere, they manage to absorb a large fraction of all the infrared radiation emitted from the surface. Consequently, only some of that radiation escapes back into space. The remainder is trapped within our atmosphere, causing the temperature to increase.
This partial trapping of solar radiation is known as the greenhouse effect. The name comes from the fact that a very similar process operates in a greenhouse. Sunlight passes relatively unhindered through glass panes, but much of the infrared radiation reemitted by the plants is blocked by the glass and cannot get out. Consequently, the interior of the greenhouse heats up, and flowers, fruits, and vegetables can grow even on cold wintry days.*
*Although this process does contribute to warming the interior of a greenhouse, it is not the most important effect. A greenhouse works mainly because its glass panes prevent convection from carrying heat up and away from the interior. Nevertheless, the name "greenhouse effect" to describe the heating effect due to Earth's atmosphere has stuck.
The radiative processes that determine the temperature of Earth's atmosphere are illustrated in Figure 7.4. Earth's greenhouse effect makes our planet almost 40 K hotter than would otherwise be the case.
Figure 7.4 The greenhouse effect. Sunlight that is not reflected by clouds reaches Earth's surface, warming it up. Infrared radiation reradiated from the surface is partially absorbed by water vapor and carbon dioxide in the atmosphere, causing the overall surface temperature to rise.
The magnitude of the greenhouse effect is very sensitive to the concentration of so-called greenhouse gases (that is, gases that absorb infrared radiation efficiently) in the atmosphere. Carbon dioxide and water vapor are the most important of these, although other atmospheric gases also contribute. The amount of carbon dioxide in Earth's atmosphere is increasing, largely as a result of the burning of fossil fuels (principally oil and coal) in the industrialized world. Carbon dioxide levels have increased by over 20 percent in the last century, and they are continuing to rise at a present rate of 4 percent per decade. In Chapter 9 we will see how a runaway increase in carbon dioxide levels in the atmosphere of the planet Venus has radically altered conditions on its surface, causing its temperature to rise to over 700 K. Although no one is predicting that Earth's temperature will ever reach that of Venus, many scientists now believe that this increase, if left unchecked, may result in global temperature increases of several kelvins over the next half-century—enough to cause dramatic, and possibly catastrophic, changes in our planet's climate.
Why is our atmosphere made up of its present constituents? Why is it not composed entirely of nitrogen, say, or of carbon dioxide, like the atmospheres of Venus and Mars? The origin and development of Earth's atmosphere was a fairly complex and lengthy process.
When Earth first formed, any primary atmosphere it might have had would have consisted of the gases most common in the early solar system. These were light gases, such as hydrogen, helium, methane, ammonia, and water vapor—a far cry from the atmosphere we enjoy today. Almost all this light material, and especially any hydrogen or helium, escaped into space during the first half-billion or so years after Earth was formed. (For more information on how planets retain or lose their atmospheres, consult More Precisely 8-1.)
Subsequently, Earth developed a secondary atmosphere, which was outgassed from the planet's interior as a result of volcanic activity. Volcanic gases are rich in water vapor, methane, carbon dioxide, sulfur dioxide, and compounds containing nitrogen (such as nitrogen gas, ammonia, and nitric oxide). Solar ultraviolet radiation decomposed the lighter, hydrogen-rich gases, allowing the hydrogen to escape, and liberated much of the nitrogen from its bonds with other elements. As Earth's surface temperature fell, the water vapor condensed and oceans formed. Much of the carbon dioxide and sulfur dioxide became dissolved in the oceans or combined with surface rocks. Oxygen is such a reactive gas that any free oxygen that appeared at early times was removed as quickly as it formed. An atmosphere consisting largely of nitrogen slowly appeared.
The final major development in the story of our planet's atmosphere is known so far to have occurred only on Earth. Life appeared in the oceans more than 3.5 billion years ago, and organisms eventually began to produce atmospheric oxygen. The ozone layer formed, shielding the surface from the Sun's harmful radiation. Eventually, life spread to the land and flourished. The fact that oxygen is a major constituent of the present-day atmosphere is a direct consequence of the evolution of life on Earth.
