CHAPTER 1 (Moran and Morgan, 1997) In this unit we will focus upon the broad features of our atmosphere, one of the components of the planet Earth. We will first distinguish between weather and climate, the principal topics of this course. We will then see how the planet Earth's atmosphere evolved to form the present mixture of gases and other suspended liquid and solid particles. The planet Earth is special because the present mixture of free nitrogen and oxygen in our atmosphere has been the result of the co-evolution of life and the atmosphere over hundreds of millions of years. We will also consider how the meteorologists have probed the atmosphere using an assortment of instruments. Based upon the measurements taken at the surface and aloft from various instrument platforms such as kites, balloons, aircraft, rockets and more recently satellites, the composition and the vertical structure of the atmosphere have been determined. The atmosphere can be described in several ways depending upon the vertical distribution of some particular weather element. In this unit we will learn that the traditional designation is based upon the vertical distribution of air temperature, since the way in which the temperature changes with height has important implications as to how the atmosphere responds to vertical motions. This topic will be discussed in further detail later. Another way of describing the vertical structure of the atmosphere is through the vertical variation in the relative concentrations of various atmospheric constituents. A variation on this theme involves the vertical variation in the concentration of electrically charged particles. Study Notes: Read through Chapter 1, As suggested at the beginning of the section entitled, "Understanding the Atmosphere", you should turn to Appendix I. Spend several minutes looking through Appendix I, the Milestones. Note the progression of meteorology as a science from Classical Greece through the Renaissance when some of the fundamental instruments were invented, then to the 19th and 20th centuries when scientific theories developed. Think about these milestone dates in terms of the general context of world history and, where appropriate, American history. STUDY NOTES Table 1.1 -- Look at this table. The intent of this table is for you to contemplate the impact that the weather has upon humans in the United States, both in terms of the human and the financial toll. You should note that in addition to these newsworthy events, lightning is responsible for nearly 100 fatalities per year. However, lightning is often underrated because only a few people usually die from one lightning event. Inspect Table 1.2, concentrating on the relative proportions of the major gases in the homosphere, or the lowest 80 km of the atmosphere. In this case, a volume of dry air is considered, without the highly variable water vapor. You do not have to memorize the exact numerical values of the percentages of each constituent by volume. However, you should be able to provide approximate values. Remember that nitrogen (N2) is the most abundant gaseous constituent, comprising of approximately 78% of the molecules in a small volume of atmospheric gas, followed by oxygen (O2) at nearly 21% and Argon (Ar) at not quite 1%. Other gases, to include the carbon dioxide (CO2), are considered trace constituents, since their concentrations are less than 0.04%. The inclusion of water vapor (H2O) with at most 4% by volume would readjust the percentages of all gases accordingly in an actual volume of air near the earth's surface. Figures 1.3 and 1.4 -- These photographs illustrate an example of a physical model developed to simulate a tornado, an atmospheric phenomenon. Figure 1.5 -- This figure is provided as an example of the numerical modeling efforts that atmospheric scientists have developed in order to understand the earth's climate as well as climatic change. In this case, a numerical model is run once with the present conditions and again using various atmospheric constituents at prescribed concentration levels that may be reached in the year 2090. The differences between computer runs are computed and compared. The results from this experiment suggest that regions in polar latitudes would experience as much as an 8 Celsius degree (14 Fahrenheit degree) temperature increase in one hundred years from their present values. Table 1.3 -- Take a moment to note the five groups of air pollutants that people in the United States release into the atmosphere, based upon various activities. You need not memorize these numeric data, but you should be aware of the various types of pollutants presently in our atmosphere. Figure 1.7 -- These two pictures show a radiosonde package and the launch of the radiosonde from one of the upper air observation stations operated by the National Weather Service. Have you ever recovered a radiosonde? Figure 1.8 -- Photographs of an operational Doppler Radar unit operated by the National Weather Service. Figure 1.9 -- Examine this figure, which represents the average vertical distribution of air temperature in the lowest 120 km of the atmosphere. The horizontal axis of this graph shows the variation in air temperature, increasing from left to right, and the vertical axis is the altitude above the earth's surface, taken for this purpose at mean sea level. Numeric values for this plot appear in the table of Appendix II as part of the data set that forms the model called the "U.S. Standard Atmosphere". Concentrate on the overall temperature pattern and the nomenclature associated with each layer. Specifically, you should realize that the temperature usually decreases with height through the atmosphere's lowest l0 km (6 miles), a layer called the troposphere. On any particular day, the vertical temperature profile may deviate especially in the lower 5 km, but the overall temperature decrease from the surface to the upper boundary of the troposphere, called the tropopause, usually remains apparent. The tropopause is at the level where the atmosphere no longer cools with height. The value of 11 km is an average value since seasonal and latitudinal variations in the height of the tropopause are observed. The layer above the tropopause is called the stratosphere where the temperature first remains relatively constant with height then increases to a maximum value of approximately 0 degrees Celsius at approximately 50 km. This warmest level at the top boundary of the stratosphere is called the stratopause. The mesosphere and thermosphere lay above, separated by the extremely cold mesopause. The reason for this type of complex vertical temperature structure appearing in Figure 1.9 is based upon the combined effects of the atmospheric composition and the heating of this atmosphere. Briefly, the atmosphere is relatively transparent to sunlight and this sunlight heats the ground. Consequently, the earth's surface serves as a surface heat source that ultimately heats the lowest layers of the atmosphere from below. Cooling occurs as one ascends into the lower troposphere because one is moving away from the surface heat source. The relatively warm temperatures found at the stratopause, near 50 km altitude, result from the presence of ozone, a form of oxygen that absorbs ultraviolet radiation coming in from the sun. Absorption of ultraviolet radiation heats the atmospheric gases at levels between 40 and 50 km. Above the mesopause, the temperature increases rapidly in the thermosphere. In this region, the low number of molecules, atoms and charged particles are excited by the high-energy solar radiation impinging on this rarefied region of the atmosphere. As we will see in Chapter 3, the temperature of a substance is a measure of the average molecular motion, and the rapid speeds of a few molecules give the appearance of a high temperature. Look at Figure 1.11, along with the discussion in the Special Topic below, to see how the ionosphere may affect radio waves. Figure 1.12 -- A photograph showing the northern lights. Have you ever seen such a display? In Figure 1.13, the magnetosphere acts as a "wind sock" for the solar wind, becoming drawn out for a long distance from the earth on the side facing away from the sun. The magnetic field lines indicated on the diagram appear to converge near two locations on the earth (the blue sphere) called the "geomagnetic poles" (to where your simple magnetic compass points). As described in the text, charged particles emitted from the sun are ducted by the magnetic field lines toward these poles and produce the occasional auroral displays that you may have the fortunate to witness as northern (or southern) lights. Figure 1.14 -- This picture is a photograph of the sun using a special camera and film. In this case, the solar flare (pointing to the lower right of the disk) extends outward from the sun approximately 320,000 km from the sun's surface. Read the Weather Fact (Severe Weather Frequency) page 13. Special Topic (The Martian Atmosphere) Page 26 -- Read this topic to see how scientists have sampled the Martian atmosphere using spacecraft and then determined the composition of the Martian atmosphere. Since the writing of this edition of the textbook, a newer space mission called Mars Pathfinder with the Sojourner rover made a highly successful exploratory mission to Mars in July 1997. Also note that many of the proposed explanations as to how the Martian climate has evolved are speculative at this point, but may be revised or even discarded as newer information becomes available. Special Topic (The Ionosphere and Radio Transmission) Page 26 -- Quickly read this topic, noting how radio waves interact with the Ionosphere. You should also see how the ionosphere is maintained. The layer terminology is provided here, but is of importance primarily to radio interests. CHAPTER 1 (Moran and Morgan, 1997) ATMOSPHERE: ORIGIN, COMPOSITION, AND STRUCTURE We begin our study of the atmosphere by distinguishing between weather and climate, the principal topics of this book. The atmosphere is the arena in which weather and climate take place. Our present atmosphere is the product of a lengthy evolutionary process and consists of a mixture of gases (mostly nitrogen and oxygen) in which is suspended a variety of tiny solid and liquid particles (collectively called aerosols). Much of what is known about the atmosphere's composition and structure is derived from direct measurements by instruments borne by kites, balloons, aircraft, rockets, and Earth-orbiting satellites. One finding of these efforts is that the atmosphere can be subdivided into layers based on the vertical profile of average air temperature. The lowest of these layers, the troposphere, is the site of virtually all weather. A portion of the upper atmosphere, known as the ionosphere, contains a relatively high concentration of electrically charged particles. This region is also the site of the aurora. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: distinguish between weather and climate. describe the scientific method. explain the value of models in scientific investigations. describe the principal events in the evolution of the Earth's atmosphere. distinguish between the heterosphere and the homosphere. explain why the significance of an atmospheric gas or aerosol is not necessarily related to its relative abundance. sketch the average vertical temperature profile of the atmosphere. describe the origin and significance of the ionosphere. explain how the ionosphere influences long-distance radio transmissions. 1 Atmosphere: Origin, Composition, and Structure 10 Understanding the Atmosphere 13 Evolution of the Atmosphere 16 Probing the Atmosphere 21 Temperature Profile of the Atmosphere 24 The Ionosphere and the Aurora 25 Conclusions 29 Weather Fact: Severe Weather Frequency 13 Special Topic: The Martian Atmosphere 18 Special Topic: The Ionosphere and Radio Transmission 26 Key Terms 29 Summary Statements 29 Review Questions 30 Questions for Critical Thinking 30 Selected Readings 30 4