Page 102 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

66 part I The energy–atmosphere System
Earth’s atmosphere is a unique reservoir of life- sustaining gases, the product of 4.6 billion years of development. Some of the gases are crucial compo- nents in biological processes; some protect us from hos- tile radiation and particles from the Sun and beyond. As shown in Geosystems Now, when humans venture away from the lower regions of the atmosphere, they must wear elaborate protective spacesuits that provide services, which the atmosphere performs for us all the time.
In this chapter: We examine the modern atmosphere using the criteria of composition, temperature, and func- tion. Our look at the atmosphere also includes the spa- tial impacts of both natural and human-produced air pollution. We all interact with the atmosphere with each breath we take, the energy we consume, the travelling we do, and the products we buy. Human activities cause stratospheric ozone losses and the blight of acid deposi- tion on ecosystems. These matters are essential to physi- cal geography, for they are influencing the atmospheric composition of the future.
Atmospheric Composition, Temperature, and Function
The modern atmosphere probably is the fourth general atmosphere in Earth’s history. A gaseous mixture of an- cient origin, it is the sum of all the exhalations and in- halations of life interacting on Earth throughout time. The principal substance of this atmosphere is air, the medium of life as well as a major industrial and chemi- cal raw material. Air is a simple mixture of gases that is naturally odourless, colourless, tasteless, and form- less, blended so thoroughly that it behaves as if it were a single gas.
As a practical matter, we consider the top of our at- mosphere to be around 480 km above Earth’s surface, the same altitude we used in Chapter 2 for measuring the solar constant and insolation received. Beyond that altitude is the exosphere, which means “outer sphere,” where the rarefied, less dense atmosphere is nearly a vacuum. It contains scarce lightweight hydrogen and he- lium atoms, weakly bound by gravity as far as 32 000 km from Earth.
Atmospheric Profile
Think of Earth’s modern atmosphere as a thin envelope of imperfectly shaped concentric “shells” or “spheres” that grade into one another, all bound to the planet by gravity. To study the atmosphere, we view it in layers, each with distinctive properties and processes. Figure 3.1 charts the atmosphere in a vertical cross-section profile, or side view. Scientists use three atmospheric criteria— composition, temperature, and function—to define layers for distinct analytical purposes. These criteria are dis- cussed just ahead, after a brief consideration of air pres- sure. (As you read the criteria discussions, note that they repeatedly follow the path of incoming solar radiation as it travels through the atmosphere to Earth’s surface.)
Air pressure changes throughout the atmospheric profile. Air molecules create air pressure through their motion, size, and number, exerting a force on all surfaces they come in contact with. The pressure of the atmo- sphere (measured as force per unit area) pushes in on all of us. Fortunately, that same pressure also exists inside us, pushing outward; otherwise, we would be crushed by the mass of air around us.
Earth’s atmosphere also presses downward under the pull of gravity and therefore has weight. Gravity compresses air, making it denser near Earth’s surface (Figure 3.2). The atmosphere exerts an average force of approximately 1 kg·cm−1 at sea level. With increasing altitude, density and pressure decrease—this is the “thinning” of air that humans feel on high mountain summits as less oxygen is carried in each breath a per- son takes. This makes breathing more difficult at the top of Mount Everest, where air pressure is about 30% of that on Earth’s surface. More information on air pressure and the role it plays in generating winds is in Chapter 6.
Over half the total mass of the atmosphere, com- pressed by gravity, lies below 5.5 km altitude. Only 0.1% of the atmosphere remains above an altitude of 50 km, as shown in the pressure profile in Figure 3.2b (percent- age column is farthest to the right).
At sea level, the atmosphere exerts a pressure of 1013.2 mb (millibar, or mb; a measure of force per square metre of surface area) of mercury (symbol, Hg), as measured by a barometer. In Canada and certain other
Georeport 3.1 Earth’s Evolving Atmosphere
The first atmosphere on earth was probably formed from outgassing, or the release of gases trapped within earth’s interior. We still see outgassing today in the form of volcanic activity. This atmosphere was high in sulfuric gases, low in
nitrogen, and devoid of oxygen. The second atmosphere formed when earth cooled and vapour condensed to form clouds and rain. Oceans formed, nitrogen increased, but oxygen was still not present in the atmosphere. as oceanic life evolved, bacteria began the process of photosynthesis, using the Sun’s energy to convert atmospheric carbon dioxide to oxygen. Oxygen became significant in the atmosphere about 2.2 billion years ago, but it took another billion years before atmospheric oxygen levels were stable. Our modern atmosphere formed when oxygen molecules absorbed sunlight and formed ozone, the protective layer in the stratosphere that shields all life from ultraviolet radiation.