CHAPTER 2 (Moran and Morgan, 1997) LESSON 2 or 3 The focus of Chapter 2 in the textbook is upon electromagnetic radiation, or simply radiation, and its role in maintaining a habitable planet Earth. Specifically, the discussion focuses upon the flow of energy as radiation into and out of the earth-atmosphere system. However, before we can start to discuss this topic, several background items need to be addressed. We should explain that electromagnetic radiation is considered to be both an energy form and an energy transport mode. Therefore, we will need to define energy, a concept that is ultimately the theme of most sciences. A physicist would define energy as the ability to do work. Secondly, an energy transport mode is a means by which energy can be transmitted from a region of high energy to one having low energy. As we will see in a later chapter, radiation is one of the three energy transport mechanisms, with conduction and convection being the other two modes. Of necessity, a discussion is made of the basic properties of electromagnetic radiation, and the various types of radiation. Particular attention is paid to the solar radiation emitted from the sun that warms the planet and the terrestrial radiation that is emitted from the planet, ultimately cooling the planet. STUDY NOTES Spend some time inspecting Figure 2.1, with the intention of learning that the types of electromagnetic radiation that you may recognize really form a continuum from gamma rays to long radio waves. If you have trouble visualizing the relationships between wavelength and frequency, take a moment to look at Figure 2.2. In this diagram, a simple wave is drawn with the distance between two successive crests or troughs identified as the wavelength. If this wave were moving across the page from left to right, we could specify the speed of the wave from how fast a crest (or trough) would move from left to right, covering a particular distance in a given time interval, such as a second. We could also count the number of crests (or troughs) that pass by some reference point that you select in a second, thereby specifying a wave frequency in terms of cycles per second, a unit of frequency that is now identified as Hertz (Hz). Now visualize another wave that would have a wavelength one-half that drawn in the figure. Suppose that both waves have the same forward speed, that is, the crests (or troughs) in both waves proceed from left to right at the same speed. You would find that you would be counting the passage of crests on the shorter wavelength wave form at a faster rate than the longer wavelength train. Consequently, the shorter wavelength wave would have a higher frequency than the longer one. One additional point that should be clarified, is that in electromagnetic radiation, the shorter wavelength (or higher frequency) waves contain more energy than do longer wavelength radiation. Returning to Figure 2.1, observe that the two horizontal scales are inversely related for reasons described above. The top scale identifies the frequency in cycles per second (the notation used here is s-1) progressing from high frequency on the left to lower frequency on the right. The lower scale is the wavelength in meters, displayed from very short wavelengths on the left to extremely long wavelengths on the right. Both scales also use scientific notation, where 106 would be 1,000,000 or one million and 10-6 would be 0.0000001 or one millionth. You will not need to memorize the exact numbers associated the wavelength bands since overlaps in wavelength bands do exist over the continuum. However you should remember the names of the various types of radiation in their relative order, starting with the high energy X-rays that have a very short wavelength (very high frequency) and progressing to low energy radio waves that have a long wavelength (low frequency). For meteorological purposes, we will focus upon the portion of the electromagnetic spectrum from ultraviolet radiation through microwaves. The colored insert represents the visible portion of the spectrum, representing the "colors of the rainbow" that the normal human eye can detect. You should realize that this color sequence is ordered. You may have heard of the mnemonic, ROY G. BIV, designed to help remember the color sequence. Proceeding from right to left in the insert from the long to short wavelengths, the mnemonic is taken from Red, Orange, Yellow, Green, Blue, Indigo and Violet. Look at Figures 2.3 and 2.4, representing the radiation curves for the sun and earth, respectively. These curves are close to what are called blackbody curves, representing how radiation is emitted over various wavelength increments from an ideal radiator (or blackbody). The horizontal scale represents the wavelength, while the vertical scale depicts the amount of radiation emitted at that wavelength. The exact shapes of the individual curves depend upon the absolute temperature of the object. In fact, the intervals on the bottom graph are 20 times larger than those appearing in the top panel. By necessity, the vertical scale on each graph is scaled in such a manner so as to permit both curves to be readable. From the accompanying discussion of the radiation laws in the textbook, you should realize that because the Sun and Planet Earth are at considerably different temperatures, the wavelength where the radiation curve for the sun differs from that for the earth. Specifically, in accordance with Wien's law, the wavelength of maximum emission for the sun is seen in Figure 2.3 to be at about 0.5 micrometers, which is near the center of the visible portion of the electromagnetic spectrum. In Figure 2.4, the radiation from the earth culminates near 10 micrometers, in the infrared (IR) spectral region. The area under each curve would represent the total amount of energy emitted by that object. Thus, in accordance with the Stefan-Boltzmann law, the area under the solar curve would be 160,000 times larger than that for the area under the terrestrial (or earth's) curve. You should look at both Figures 2.6 and 2.7 together. Typically, over the course of the year, the sun's rays are more vertically oriented with respect to the earth's surface over the equator and the tropics than elsewhere. Reaching polar regions, the sun's rays make a very low angle with the earth's surface, becoming nearly parallel to the surface at the poles. Consequently, the solar radiation would be more intense in the tropics than in the polar regions as a result of the differences in solar angles. While this diagram pertains to latitude or the angular distance north and south of the equator, it would also apply to the daily cycle, where the sun's rays would be more vertical near noon and nearly parallel with the horizon at sunrise or sunset. Accordingly, the solar radiation received by a horizontal surface is more intense at local solar noon than near sunrise or sunset when the rays are at a lower angle in the sky. Furthermore, by inspecting Figure 2.8, you will also see that the more vertical the sun's rays in the sky, the shorter the path that the rays take through the earth's atmosphere. As a result, the sun's rays in the tropics, in summer or near local solar noon, will have fewer molecules and aerosols in the atmosphere that would scatter, reflect or absorb the ray than if the path were longer. On the other hand, longer path lengths when the sun appears to be near the surface cause a greater depletion of the sun's rays because of added interactions of the sunlight with the additional atmospheric constituents in the longer column. Such low sun angle cases would occur in polar regions, in winter or near sunrise/sunset. Study Figure 2.9 that depicts the elliptical orbit of planet Earth around the sun. Note the time when the earth passes closest to the sun (called perihelion) is at the beginning of January and the time when the earth is farthest from the sun (called aphelion) would be in early July. For reference, the points on the orbit corresponding to the equinoxes and solstices are also included, and should be compared with Figure 2.10. As mentioned elsewhere, the variations in the solar energy received by the earth as a consequence of its revolution about the sun on its elliptical orbit are relatively small, with the entire planet receiving slightly more solar radiation in January than in July. Take time to study Figure 2.10. This diagram illustrates the primary cause for the earth's seasons, associated with the orientation of the earth's spin axis with respect to the sun at various times during the year. You should become familiar with how this diagram relates to Figures 2.11, 2.12 and 2.13. Note that in Figure 2.10, the earth's axis is inclined, causing the earth's equator to be inclined with respect to the orbital plane by an angle of approximately 23.5 degrees. Regardless of the position in the orbit, this spin axis is oriented in one direction in space. At the present geologic time, the spin axis points to near Polaris, the North Star. Now look carefully at each figure (Figures 2.11, 2.12 and 2.13) depicting the solar geometry at the equinoxes, the summer and winter solstices, respectively. You should attempt to visualize of how the orientation of the earth with respect to the sun's rays on these particular dates affects the sun angle and length of daylight at various latitudes. You should also find where the noontime sun would be directly overhead and which polar cap would experience 24 hour continuous light or 24 hour darkness. Inspect Figure 2.14, looking first at the middle panel (B), which would be familiar to most of us who live in the midlatitudes of the Northern Hemisphere. This panel shows the seasonal paths of the sun as viewed by someone in midlatitudes of the Northern Hemisphere. Visualize how the sun moves across your local sky in March (or September), June and December. Try to reference this path with respect to local landmarks familiar to you and points of the compass. Remember that south would be located in the direction where the sun would be highest in your local sky. At the equinoxes, the sun would rise directly east of you, and then set directly to your west. In December, the sun would rise to the southeast and sink to the southwest, taking a lower path across the sky, while in summer the sun would appear to rise in the northeast quadrant of the horizon, then set to your northwest after taking a high path across your sky. Once you have become familiar with the midlatitude case, try to visualize what you would see on these three days if you were at the Equator (A) and at the North Pole (C). Scan Figure 2.15, which depicts the annual variation in the length of daylight for 5 different cities in the Northern Hemisphere at varying latitudes, or distances from the equator. These cities range from a tropical location (Caracas, Venezuela at 11 degrees North latitude) to a subpolar locale (Edmonton, AB at 54 degrees N latitude). Note that the red curve for the tropical location experiences the smallest seasonal variation, with no more than 30 minute deviation away from 12 hours of daylight while the subpolar purple curve shows the largest seasonal variation, ranging from roughly 7.5 hours to 17.5 hours. You should also note that all stations have 12 hours of daylight at the two equinoxes. Figure 2.16 -- Laws of reflection. Note that the angle of reflection equals the angle of incidence. Take a moment to study the schematic in Figure 2.18 that depicts the molecular interactions that produce two molecules of ozone (O3) from diatomic oxygen (O2) in the left-hand panel and those natural interactions that destroy the ozone molecule in the right-hand panel. You should then compare this panel with Figure 2.19 that shows a possible photochemical reaction involving chlorine released from chlorofluorocarbon compounds (CFCs). Please remember that these schematics may be simplified since the actual chemical reactions involving the production and the destruction may involve several additional complex intermediate reactions. Take a moment to notice that the average residence times for three common CFCs in Table 2.1 are at least three-quarters of a century, meaning that significant quantities of these synthetic chemicals could remain in the atmosphere well into the next century. Figure 2.20 -- Note the spatial variation of total atmospheric ozone in the Southern Hemisphere as determined from a polar orbiting NOAA satellite. The region of relatively low concentrations of ozone (identified by the magenta color) in the austral spring would correspond to the region that is often dubbed the "Antarctic ozone hole". Browse Table 2.2, comparing the surface albedo for various types of surfaces. While you should not memorize the values, you should make a comparison between these materials. For example, note that some highly reflective materials such as snow have a high albedo, reflecting between 75 to 95% of the incident visible radiation. On the other hand, other substances such as a black asphalt road or a conifer forest are poor reflectors of visible light and have a low albedo, reflecting only 5 to 10% of the incident radiation. You should also note that since albedo is a ratio, the values are dimensionless (that is, they have has no units) and can be expressed as a percentage (where the decimal equivalent is multiplied by 100). The entries describing the albedo of a water surface at various solar altitude angles is described schematically in Figure 2.21. When the sun is high in the sky, such as near solar noon, the reflectivity is low as the sunlight penetrates into the water body. Thus, the albedo decreases when the solar altitude angle becomes large (or to the right-hand side of the diagram). On the other hand, when the sun is low in the sky, as near sunset, the reflectivity becomes high as most of the sunlight is reflected off the surface. Thus the albedo of water is large for a small solar altitude angle. Study Table 2.3, the tabulation of the Global Annual Solar Radiation Budget. You should appreciate that approximately 69% of the solar radiation that reaches the top of the earth's atmosphere is absorbed by the entire planet, while the other 31% of the incoming solar radiation is reflected back to space without affecting the planet. Of the solar radiation absorbed by the earth-atmosphere system, more is absorbed by the earth's surface than in the atmosphere by those atmospheric constituents, such as stratospheric ozone. Spend some time studying the schematic representation of the so-called earth's "greenhouse effect" in Figure 2.22. You should trace how a large portion of the solar radiation, as indicated by the red arrows, penetrates through the atmosphere and reaches the earth's surface, where some of the sunlight is reflected back to space. In this simple diagram, the fraction of the solar radiation that is absorbed in the atmosphere by ozone and other atmospheric constituents is neglected, but would represent approximately 23 percent of the total amount entering the planetary system. Compare this distribution of solar radiation with that of the outgoing infrared or terrestrial radiation as depicted by the green wavy arrows. Most of this radiation that leaves the surface is absorbed in the atmosphere by water vapor, carbon dioxide and other "greenhouse gases". Some of the radiation from these atmospheric constituents is emitted to space, while some is radiated back toward the surface. Inspect Figure 2.24, becoming familiar with the selective absorptivity of the earth's atmosphere to radiation of various wavelengths. This selective absorption is a result of various atmospheric constituents. This diagram considers the absorptivity of six atmospheric gases over a portion of the electromagnetic spectrum plotted as a function of wavelength. (The horizontal axis is logarithmic, because of the large range of wavelengths considered, with ultraviolet wavelengths on the left and infrared wavelengths on the right.) For each panel, the vertical axis is the absorptivity, or the fraction of radiation absorbed to the total amount incident upon the gas. Hence, in regions where the absorptivity of the gas is large would approach a value of 1, essentially no radiation at that wavelength band could pass through an atmospheric column containing that gas. Such a region is depicted with dark gray shading to indicate that the atmosphere is essentially opaque, with little transparency to radiation of that wavelength. On the other hand, in a wavelength region where the absorptivity of the gas approaches zero, nearly all of the incident radiation of that wavelength can pass through the atmosphere with no absorption. In this region, the chart would not have a gray color. Methane (CH4, also known as "swamp gas") and nitrous oxide (N2O, also known as "laughing gas") in the top two panels absorb exclusively in the IR portions of the electromagnetic spectrum at wavelengths greater than 3 micrometers. Likewise, carbon dioxide (CO2) and water (H20) also absorb in the near IR between 1 and 3 micrometers. On the other hand, oxygen (O2) and ozone (03) absorb primarily in the UV portion of the spectrum at wavelengths less than 0.3 micrometers, with some additional contributions in the IR regions. The absorptivity of a mixture of gases is basically additive, so that the absorptivity of the atmosphere near sea level contains the combined effects of all the constituents, as seen in the bottom panel of Figure 2.24. As a result, the atmosphere is relatively opaque to UV radiation (less than 0.3 micrometers) because of oxygen and ozone, transparent in the visible portion of the spectrum and relatively opaque to IR radiation (greater than 2 micrometers) because of the contributions of H20, CO2 and other constituents. Several "IR windows" are noted in the diagram at wavelengths corresponding to approximately 8 and 10.6 micrometers, where some of the terrestrial radiation can pass from the earth's surface directly to space. Weather satellites utilize these windows, since the conventional IR satellite images are meant to sense the IR radiation upwelling from the earth's surface. Figure 2.25 -- Consider the changes in concentration of atmospheric carbon dioxide over 37 years at Mauna Loa Observatory, HI. A nearly exponential increase in the carbon dioxide concentration has occurred, for reasons described in the text. As also mentioned, an annual cycle is superimposed upon this long-term increase, forming an almost saw-tooth appearance. During any one year more carbon dioxide is found in the Northern Hemisphere atmosphere during spring, just before the greening process. The smallest concentration during the year is found in late summer as much greater summertime photosynthetic activity has drawn down the carbon dioxide. Note that 350 ppm (parts per million) is equivalent to 0.035%. Look how anthropogenic emissions of carbon compounds have changed over the late 20th century in Figure 2.26. Contemplate the changes that have taken place since 1950. Table 2.4 -- Take a moment to consider how the concentrations of five selected "Greenhouse Gases" have changed since the pre-Industrial Revolution of the early 19th century. SPECIAL TOPICS (Why is the Sky Blue) Pages 46 and 47 -- Read through this topic, learning about the two forms of scattering of sunlight. From this discussion, you should learn that the relative size of the matter suspended in the atmosphere determines the type of scatter. Specifically, tiny molecules would preferentially scatter the shorter wavelength blue light in a type of scattering process called Raleigh scatter. On the other hand, the larger aerosols and cloud droplets scatter radiation in a manner called Mie scatter that shows no preference to wavelength. SPECIAL TOPICS (The Hazards of Overexposure to UV Radiation) Pages 48-53 -- Read through this Special Topic. You should gain an appreciation for how prolonged exposure to UV radiation can be detrimental to your health. As of this writing, the UVI forecasts for selected cities throughout the United States appear in some newspapers, on the Internet and also on "The Weather Channel" during the summer recreation season. Table 1 is provided only as a guide to show the significance of the numbers that would appear in the UVI forecasts. Read Weather Fact (Greenhouse Effect on Mars and Venus) on page 61, noting differences between the constituents in atmospheres on Mars and Venus, as well as between these two planets and planet Earth. Consider why these differences may occur. MATHEMATICAL NOTE -- Page 64. This note is intended for the mathematically inclined and describes the specific mathematical relationships developed by physicists at the turn of the twentieth century to describe the behavior of an ideal radiator (or blackbody). CHAPTER 2 (Moran and Morgan, 1997) RADIATION Electromagnetic radiation is described in terms of its basic properties and spectrum of types. Earth intercepts solar radiation as the planet rotates on its axis and revolves about the sun. Changes in solar altitude and length of day that accompany the march of the seasons produce a regular annual cycle in the amount of solar radiation that reaches any locality on the surface of the planet. As solar radiation travels through the atmosphere, some is reflected and scattered back to space and some is absorbed (that is, converted to heat). Absorption of ultraviolet (UV) radiation during the natural formation and destruction of ozone in the stratosphere shields us from potentially lethal intensities of UV. Nonetheless, some UV reaches the Earth's surface and over-exposure may cause sunburn or even skin cancer. Solar radiation that strikes the Earth's surface is either reflected or absorbed, depending on the surface albedo. In response to solar radiational heating, the Earth-atmosphere system emits infrared radiation. The so-called greenhouse effect, however, slows the loss of infrared radiation to space and causes the Earth's surface to be considerably warmer than the upper atmosphere. Water vapor is the principal greenhouse gas. Upward trends in the concentrations of other infrared-absorbing gases, especially carbon dioxide, may enhance the greenhouse effect and trigger global warming. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: identify the principal characteristics of electromagnetic radiation and the electromagnetic spectrum. distinguish among the various forms of electromagnetic radiation. explain how solar altitude influences the intensity of solar radiation received at the Earth's surface. describe what causes the astronomical seasons. define the solar constant. describe the interactions that take place as solar radiation travels through the atmosphere. explain the significance of the stratospheric ozone layer for life on identify the principal threat to the stratospheric ozone shield. describe the interactions that take place when solar radiation strikes the Earth's surface. explain the role of the oceans in the global solar radiation budget. contrast solar radiation with terrestrial infrared radiation. identify the gases responsible for the greenhouse effect. explain how a buildup of greenhouse gases might lead to global warming. 2 Radiation 32 Electromagnetic Radiation 33 Radiation Laws 35 Input of Solar Radiation 37 Solar Radiation and the Atmosphere 43 The Stratospheric Ozone Shield 45 Solar Radiation and the Earth's Surface 56 Solar Radiation Budget 57 Infrared Response and the Greenhouse Effect 57 Radiation Measurement 63 Conclusions 64 Special Topic: Why Is the Sky Blue? 46 Special Topic: The Hazards of Overexposure to UV Radiation 48 Weather Fact: Greenhouse Effect on Mars and Venus 61 Mathematical Note: Blackbody Radiation Laws 64 Key Terms 65 Summary Statements 65 Review Questions 66 Quantitative Questions 66 Questions for Critical Thinking 66 Selected Readings 66 2