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Chapter 1 Introduction to the Atmosphere 17 Figure 1–15 When Mount St. Helens erupted in May 1980, the area shown here was buried by a volcanic mudflow. Now, plants are reestablished and new soil is forming. (...

Chapter 1 Introduction to the Atmosphere 17 Figure 1–15 When Mount St. Helens erupted in May 1980, the area shown here was buried by a volcanic mudflow. Now, plants are reestablished and new soil is forming. (Photo by Terry Donnelly/ Alamy) Composition of from the atmosphere, we would find that its makeup is very stable up to an altitude of about 80 kilometers the Atmosphere (50 miles). As you can see in Figure 1–16, two gases—nitrogen and Introduction to the Atmosphere oxygen—make up 99 percent of the volume of clean, dry air. ▸ Composition of the Atmosphere ATMOSPHERE Although these gases are the most plentiful components of the atmosphere and are of great significance to life on Earth, In the days of Aristotle, air was thought to be one of four they are of little or no importance in affecting weather fundamental substances that could not be further divided phenomena. The remaining 1 percent of dry air is mostly into constituent components. The other three substances the inert gas argon (0.93 percent) plus tiny quantities of a were fire, earth (soil), and water. Even today the term air is number of other gases. sometimes used as if it were a specific gas, which of course it is not. The envelope of air that surrounds our planet is a mixture of many discrete gases, each with its own physical properties, in which varying quantities of tiny solid and liq- Carbon Dioxide uid particles are suspended. Carbon dioxide, although present in only minute amounts (0.0391 percent, or 391 parts per million), is nevertheless a meteorologically important constituent of air. Carbon Major Components dioxide is of great interest to meteorologists because it is The composition of air is not constant; it varies from time an efficient absorber of energy emitted by Earth and thus to time and from place to place (see Box 1–3). If the water influences the heating of the atmosphere. Although the vapor, dust, and other variable components were removed proportion of carbon dioxide in the atmosphere is relatively 18 The Atmosphere: An Introduction to Meteorology Box 1–2 The Carbon Cycle: One of Earth’s Subsystems To illustrate the movement of material and energy in the Earth system, let us take a Volcanic activity Weathering brief look at the carbon cycle (Figure 1–C). Weathering of Pure carbon is relatively rare in nature. It of carbonate granite Photosynthesis rock is found predominantly in two minerals: Burning and by vegetation Respiration diamond and graphite. Most carbon is decay of by land biomass bonded chemically to other elements to organisms Burning of fossil form compounds such as carbon dioxide, fuels calcium carbonate, and the hydrocarbons found in coal and petroleum. Carbon is also the basic building block of life as it readily Burial of combines with hydrogen and oxygen to biomass form the fundamental organic compounds that compose living things. Lithosphere In the atmosphere, carbon is found CO2 mainly as carbon dioxide (CO2). Photosynthesis dissolves and respiration Deposition in seawater Atmospheric carbon dioxide is significant of marine of carbonate because it is a greenhouse gas, which organisms sediments Sediment and sedimentary rock means it is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Because CO2 entering the atmosphere many of the processes that operate on CO2 leaving the Earth involve carbon dioxide, this gas atmosphere is constantly moving into and out of the atmosphere. For example, through the FIGURE 1–C Simplified diagram of the carbon cycle, with emphasis on the flow of carbon process of photosynthesis, plants absorb between the atmosphere and the hydrosphere, geosphere, and biosphere. The colored arrows carbon dioxide from the atmosphere to show whether the flow of carbon is into or out of the atmosphere. produce the essential organic compounds needed for growth. Animals that consume these plants (or consume other animals that of respiration, return carbon dioxide to the when plants die and decay or are burned, eat plants) use these organic compounds as atmosphere. (Plants also return some CO2 this biomass is oxidized, and carbon a source of energy and, through the process to the atmosphere via respiration.) Further, dioxide is returned to the atmosphere. Concentration in parts per million (ppm) uniform, its percentage has been rising steadily for more Argon (0.934%) Neon (Ne) 18.2 than a century. Figure 1–17 is a graph showing the growth in Helium (He) 5.24 Carbon dioxide All others Methane (CH4) 1.5 atmospheric CO2 since 1958. Much of this rise is attributed (0.0391% or 391 ppm) Krypton (Kr) 1.14 to the burning of ever-increasing quantities of fossil fuels, Hydrogen (H2) 0.5 such as coal and oil. Some of this additional carbon dioxide is absorbed by the waters of the ocean or is used by plants, Oxygen but more than 40 percent remains in the air. Estimates pro- (20.946%) ject that by sometime in the second half of the twenty- first century, carbon dioxide levels will be twice as high as Nitrogen pre-industrial levels. (78.084%) Most atmospheric scientists agree that increased carbon dioxide concentrations have contributed to a warming of Earth’s atmosphere over the past several decades and will continue to do so in the decades to come. The magnitude of such temperature changes is uncertain and depends partly on the quantities of CO2 contributed by human activities in the years ahead. The role of carbon dioxide in the atmo- Figure 1–16 Proportional volume of gases composing dry air. sphere and its possible effects on climate are examined in Nitrogen and oxygen obviously dominate. more detail in Chapters 2 and 14.  Chapter 1 Introduction to the Atmosphere 19 Not all dead plant material decays remains settle to the ocean floor as In summary, carbon moves among all immediately back to carbon dioxide. A small biochemical sediment and become four of Earth’s major spheres. It is essential percentage is deposited as sediment. Over sedimentary rock. In fact, the geosphere is to every living thing in the biosphere. In the long spans of geologic time, considerable by far Earth’s largest depository of carbon, atmosphere carbon dioxide is an important biomass is buried with sediment. Under the where it is a constituent of a variety of greenhouse gas. In the hydrosphere, right conditions, some of these carbon-rich rocks, the most abundant being limestone carbon dioxide is dissolved in lakes, rivers, deposits are converted to fossil fuels—coal, (Figure 1–D). Eventually the limestone and the ocean. In the geosphere, carbon is petroleum, or natural gas. Eventually some may be exposed at Earth’s surface, where contained in carbonate-rich sediments and of the fuels are recovered (mined or pumped chemical weathering will cause the carbon sedimentary rocks and is stored as organic from a well) and burned to run factories and stored in the rock to be released to the matter dispersed through sedimentary rocks fuel our transportation system. One result of atmosphere as CO2. and as deposits of coal and petroleum. fossil-fuel combustion is the release of huge quantities of CO2 into the atmosphere. Certainly one of the most active parts of the carbon cycle is the movement of CO2 from the atmosphere to the biosphere and back again. Carbon also moves from the geosphere and hydrosphere to the atmosphere and back again. For example, volcanic activity early in Earth’s history is thought to be the source of much of the carbon dioxide found in the atmosphere. One way that carbon dioxide makes its way back to the FIGURE 1–D hydrosphere and then to the solid Earth A great deal is by first combining with water to form of carbon is carbonic acid (H2CO3), which then attacks locked up in the rocks that compose the geosphere. Earth’s geosphere. One product of this chemical weathering England’s White Chalk Cliffs are an example. of solid rock is the soluble bicarbonate Chalk is a soft, porous type of ion (2HCO3–), which is carried by limestone (CaCO3) consisting mainly groundwater and streams to the ocean. of the hard parts of microscopic Here water-dwelling organisms extract this organisms called coccoliths (inset). dissolved material to produce hard parts (Photo by Prisma/SuperStock; inset (shells) of calcium carbonate (CaCO3). by Steve Gschmeissner/Photo When the organisms die, these skeletal Researchers, Inc.) Variable Components 390 Air includes many gases and particles that vary significantly from time to time and place to place. Important examples 380 include water vapor, aerosols, and ozone. Although usually present in small percentages, they can have significant ef- CO2 concentration (ppm) 370 fects on weather and climate. 360 Water Vapor The amount of water vapor in the air var- ies considerably, from practically none at all up to about 350 4 percent by volume. Why is such a small fraction of the 340 330 Figure 1–17 Changes in the atmosphere’s carbon dioxide (CO2) as measured at Hawaii’s Mauna Loa Observatory. The oscillations 320 reflect the seasonal variations in plant growth and decay in the Northern Hemisphere. During the first 10 years of this record (1958–1967), the average yearly CO2 increase was 0.81 ppm. 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 During the last 10 years (2001–2010) the average yearly increase was 2.04 ppm. (Data from NOAA) 20 The Atmosphere: An Introduction to Meteorology Box 1–3 Origin and Evolution of Earth’s Atmosphere The air we breathe is a stable mixture of gas output must have been immense. Based carbon dioxide, and sulfur dioxide, with 78 percent nitrogen, 21 percent oxygen, on our understanding of modern volcanic minor amounts of other gases and minimal nearly 1 percent argon, and small amounts eruptions, Earth’s primitive atmosphere nitrogen. Most importantly, free oxygen was of gases such as carbon dioxide and water probably consisted of mostly water vapor, not present. vapor. However, our planet’s original atmosphere 4.6 billion years ago was substantially different. Earth’s Primitive Atmosphere Early in Earth’s formation, its atmosphere likely consisted of gases most common in the early solar system: hydrogen, helium, methane, ammonia, carbon dioxide, and water vapor. The lightest of these gases, hydrogen and helium, escaped into space because Earth’s gravity was too weak to hold them. Most of the remaining gases were probably scattered into space by strong solar winds (vast streams of particles) from a young active Sun. (All stars, including the Sun, apparently experience a highly active stage early in their evolution, during which solar winds are very intense.) Earth’s first enduring atmosphere was generated by a process called outgassing, through which gases trapped in the planet’s interior are released. Outgassing from hundreds of active volcanoes still remains an important planetary function worldwide (Figure 1–E). However, early in Earth’s history, when massive heating and fluid-like FIGURE 1–E Earth’s first enduring atmosphere was formed by a process called outgassing, which motion occurred in the planet’s interior, the continues today, from hundreds of active volcanoes worldwide. (Photo by Greg Vaughn/Alamy) atmosphere so significant? The fact that water vapor is the p. 99), it absorbs or releases heat. This energy is termed source of all clouds and precipitation would be enough to ex- latent heat, which means hidden heat. As you will see in plain its importance. However, water vapor has other roles. later chapters, water vapor in the atmosphere transports Like carbon dioxide, it has the ability to absorb heat given off this latent heat from one region to another, and it is the by Earth, as well as some solar energy. It is therefore impor- energy source that drives many storms. tant when we examine the heating of the atmosphere. When water changes from one state to another, such as Aerosols The movements of the atmosphere are sufficient from a gas to a liquid or a liquid to a solid (see Figure 4–3, to keep a large quantity of solid and liquid particles sus- pended within it. Although visible dust sometimes clouds the sky, these relatively large particles are too heavy to stay Students Sometimes Ask… in the air very long. Still, many particles are microscopic and remain suspended for considerable periods of time. They Could you explain a little more about why the graph in may originate from many sources, both natural and human Figure 1–17 has so many ups and downs? made, and include sea salts from breaking waves, fine soil Sure. Carbon dioxide is removed from the air by photosynthesis, the blown into the air, smoke and soot from fires, pollen and mi- process by which green plants convert sunlight into chemical energy. croorganisms lifted by the wind, ash and dust from volcanic In spring and summer, vigorous plant growth in the extensive land areas of the Northern Hemisphere removes carbon dioxide from the eruptions, and more (Figure 1–18a). Collectively, these tiny atmosphere, so the graph takes a dip. As winter approaches, many solid and liquid particles are called aerosols. plants die or shed leaves. The decay of organic matter returns carbon Aerosols are most numerous in the lower atmosphere dioxide to the air, causing the graph to spike upward. near their primary source, Earth’s surface. Nevertheless, the upper atmosphere is not free of them, because some dust is  Chapter 1 Introduction to the Atmosphere 21 Oxygen in the Atmosphere As Earth cooled, water vapor condensed to form clouds, and torrential rains began to fill low-lying areas, which became the oceans. In those oceans, nearly 3.5 billion years ago, photosynthesizing bacteria began to release oxygen into the water. During photosynthesis, organisms use the Sun’s energy to produce organic material (energetic molecules of sugar containing hydrogen and carbon) from carbon dioxide (CO2) and water (H2O). The first bacteria probably used hydrogen sulfide (H2S) as the source of hydrogen rather than water. One of the earliest bacteria, cyanobacteria (once called blue-green algae), began to produce oxygen as a by-product of photosynthesis. Initially, the newly released oxygen was readily consumed by chemical reactions with other atoms and molecules (particularly iron) in the ocean (Figure 1–F). Once the available iron satisfied its need for oxygen and as the number of oxygen-generating FIGURE 1–F These ancient layered, iron-rich rocks, called banded iron formations, were deposited organisms increased, oxygen began to during a geologic span known as the Precambrian. Much of the oxygen generated as a by-product of build in the atmosphere. Chemical analyses photosynthesis was readily consumed by chemical reactions with iron to produce these rocks. (Photo by of rocks suggest that a significant amount John Cancalosi/Photolibrary) of oxygen appeared in the atmosphere as early as 2.2 billion years ago and increased Another significant benefit of the “oxygen For the first time, Earth’s surface was steadily until it reached stable levels about explosion” is that oxygen molecules (O2) protected from this type of solar radiation, 1.5 billion years ago. Obviously, the readily absorb ultraviolet radiation and which is particularly harmful to DNA. availability of free oxygen had a major rearrange themselves to form ozone (O3). Marine organisms had always been impact on the development of life and vice Today, ozone is concentrated above the shielded from ultraviolet radiation by versa. Earth’s atmosphere evolved together surface in a layer called the stratosphere, the oceans, but the development of the with its life-forms from an oxygen-free where it absorbs much of the ultraviolet atmosphere’s protective ozone layer made envelope to an oxygen-rich environment. radiation that strikes the upper atmosphere. the continents more hospitable. carried to great heights by rising currents of air, and other its distribution is not uniform. In the lowest portion of the particles are contributed by meteoroids that disintegrate as atmosphere, ozone represents less than 1 part in 100 million. they pass through the atmosphere. It is concentrated well above the surface in a layer called the From a meteorological standpoint, these tiny, often stratosphere, between 10 and 50 kilometers (6 and 31 miles). invisible particles can be significant. First, many act as sur- In this altitude range, oxygen molecules (O2) are split faces on which water vapor may condense, an important into single atoms of oxygen (O) when they absorb ultraviolet function in the formation of clouds and fog. Second, aero- radiation emitted by the Sun. Ozone is then created when sols can absorb or reflect incoming solar radiation. Thus, a single atom of oxygen (O) and a molecule of oxygen (O2) when an air-pollution episode is occurring or when ash fills collide. This must happen in the presence of a third, neu- the sky following a volcanic eruption, the amount of sun- tral molecule that acts as a catalyst by allowing the reaction light reaching Earth’s surface can be measurably reduced. to take place without itself being consumed in the process. Finally, aerosols contribute to an optical phenomenon we Ozone is concentrated in the 10- to 50-kilometer height range have all observed—the varied hues of red and orange at because a crucial balance exists there: The ultraviolet radia- sunrise and sunset (Figure 1–18b). tion from the Sun is sufficient to produce single atoms of oxygen, and there are enough gas molecules to bring about Ozone Another important component of the atmosphere the required collisions. is ozone. It is a form of oxygen that combines three oxygen The presence of the ozone layer in our atmosphere is atoms into each molecule (O3). Ozone is not the same as the crucial to those of us who are land dwellers. The reason is oxygen we breathe, which has two atoms per molecule (O2). that ozone absorbs the potentially harmful ultraviolet (UV) There is very little ozone in the atmosphere. Overall, it rep- radiation from the Sun. If ozone did not filter a great deal of resents just 3 out of every 10 million molecules. Moreover, the ultraviolet radiation, and if the Sun’s UV rays reached 22 The Atmosphere: An Introduction to Meteorology Dust storm Air pollution (a) (b) Figure 1–18 (a) This satellite image from November 11, 2002, shows two examples of aerosols. First, a large dust storm is blowing across northeastern China toward the Korean Peninsula. Second, a dense haze toward the south (bottom center) is human-generated air pollution. (b) Dust in the air can cause sunsets to be especially colorful. (Satellite image courtesy of NASA; photo by elwynn/ Shutterstock) the surface of Earth undiminished, land areas on our planet protected life on the planet. However, over the past half cen- would be uninhabitable for most life as we know it. Thus, tury, people have unintentionally placed the ozone layer in anything that reduces the amount of ozone in the atmo- jeopardy by polluting the atmosphere. The most significant sphere could affect the well-being of life on Earth. Just such of the offending chemicals are known as chlorofluorocarbons a problem is described in the next section. (CFCs). They are versatile compounds that are chemically stable, odorless, nontoxic, noncorrosive, and inexpensive to produce. Over several decades many uses were developed Concept Check 1.6 for CFCs, including as coolants for air-conditioning and 1 Is air a specific gas? Explain. refrigeration equipment, as cleaning solvents for electronic components, as propellants for aerosol sprays, and in the 2 What are the two major components of clean, dry air? What production of certain plastic foams. proportion does each represent? 3 Why are water vapor and aerosols important constituents of Earth’s atmosphere? Students Sometimes Ask… 4 What is ozone? Why is ozone important to life on Earth? Isn’t ozone some sort of pollutant? Yes, you’re right. Although the naturally occurring ozone in the stratosphere is critical to life on Earth, it is regarded as a pollutant Ozone Depletion— when produced at ground level because it can damage vegetation and be harmful to human health. Ozone is a major component A Global Issue in a noxious mixture of gases and particles called photochemical smog. It forms as a result of reactions triggered by sunlight that The loss of ozone high in the atmosphere as a consequence occur among pollutants emitted by motor vehicles and industries. of human activities is a serious global-scale environmental Chapter 13 provides more information about this. problem. For nearly a billion years Earth’s ozone layer has  Chapter 1 Introduction to the Atmosphere 23 No one worried about how CFCs might affect the atmo- of maximum depletion is confined to the Antarctic region sphere until three scientists, Paul Crutzen, F. Sherwood by a swirling upper-level wind pattern. When this vortex Rowland, and Mario Molina, studied the relationship. In weakens during the late spring, the ozone-depleted air is no 1974 they alerted the world when they reported that CFCs longer restricted and mixes freely with air from other lati- were probably reducing the average concentration of ozone tudes where ozone levels are higher. in the stratosphere. In 1995 these scientists were awarded A few years after the Antarctic ozone hole was discov- the Nobel Prize in chemistry for their pioneering work. ered, scientists detected a similar but smaller ozone thin- They discovered that because CFCs are practically inert ning in the vicinity of the North Pole during spring and (that is, not chemically active) in the lower atmosphere, a early summer. When this pool breaks up, parcels of ozone- portion of these gases gradually makes its way to the ozone depleted air move southward over North America, Europe, layer, where sunlight separates the chemicals into their and Asia. constituent atoms. The chlorine atoms released this way, through a complicated series of reactions, have the net effect of removing some of the ozone. Effects of Ozone Depletion Because ozone filters out most of the damaging UV radia- The Antarctic Ozone Hole tion in sunlight, a decrease in its concentration permits Although ozone depletion by CFCs occurs worldwide, mea- more of these harmful wavelengths to reach Earth’s surface. surements have shown that ozone concentrations take an What are the effects of the increased ultraviolet radiation? especially sharp drop over Antarctica during the Southern Each 1 percent decrease in the concentration of stratospheric Hemisphere spring (September and October). Later, during ozone increases the amount of UV radiation that reaches November and December, the ozone concentration recovers Earth’s surface by about 2 percent. Therefore, because ul- to more normal levels (Figure 1–19). Between 1980, when traviolet radiation is known to induce skin cancer, ozone it was discovered, and the early 2000s, this well-publicized depletion seriously affects human health, especially among ozone hole intensified and grew larger until it covered an area fair-skinned people and those who spend considerable time roughly the size of North America (Figure 1–20). in the sun. The hole is caused in part by the relatively abundant The fact that up to a half million cases of these cancers ice particles in the south polar stratosphere. The ice boosts occur in the United States annually means that ozone deple- the effectiveness of CFCs in destroying ozone, thus caus- tion could ultimately lead to many thousands more cases ing a greater decline than would otherwise occur. The zone each year.* In addition to raising the risk of skin cancer, an increase in damaging UV radiation can negatively impact the human immune system, as well as promote cataracts, a clouding of the eye lens that reduces vision and may cause 30 blindness if not treated. The effects of additional UV radiation on animal and plant life are also important. There is serious concern that 25 Area of North America crop yields and quality will be adversely affected. Some Extent of scientists also fear that increased UV radiation in the Ant- Million square kilometers 2006 arctic will penetrate the waters surrounding the continent 20 ozone hole and impair or destroy the microscopic plants, called phy- toplankton, that represent the base of the food chain. A 15 decrease in phytoplankton, in turn, could reduce the popu- Extent of lation of copepods and krill that sustain fish, whales, pen- 2010 10 ozone hole guins, and other marine life in the high latitudes of the Southern Hemisphere. 5 Montreal Protocol 0 Aug Sep Oct Nov Dec What has been done to protect the atmosphere’s ozone layer? Realizing that the risks of not curbing CFC emissions were difficult to ignore, an international agreement known as the Figure 1-19 Changes in the size of the Antarctic ozone hole Montreal Protocol on Substances That Deplete the Ozone Layer during 2006 and 2010. The ozone hole in both years began was concluded under the auspices of the United Nations in to form in August and was well developed in September and late 1987. The protocol established legally binding controls October. As is typical, each year the ozone hole persisted through November and disappeared in December. At its maximum, the area of the ozone hole was about 22 million square kilometers in * For more on this, see Severe and Hazardous Weather: “The Ultraviolet 2010, an area nearly as large as all of North America. Index,” p. 49. 24 The Atmosphere: An Introduction to Meteorology 30 Area of North 25 America 20 Million square kilometers 15 Area of Antarctica 10 5 Extent of ozone hole 0 1980 1985 1990 1995 2000 2005 2010 2015 1979 Ozone (Dobson Units) 2010 Year 110 220 330 440 550 Figure 1–20 The two satellite images show ozone distribution in the Southern Hemisphere on the days in September 1979 and 2010 when the ozone hole was largest. The dark blue shades over Antarctica correspond to the region with the sparsest ozone. The ozone hole is not technically a “hole” where no ozone is present but is actually a region of exceptionally depteted ozone in the stratosphere over the Antarctic that occurs in the spring. The small graph traces changes in the maximum size of the ozone hole, 1980–2010. (NOAA) on the production and consumption of gases known to cause ozone depletion. As the scientific understanding of Vertical Structure ozone depletion improved after 1987 and substitutes and of the Atmosphere alternatives became available for the offending chemicals, the Montreal Protocol was strengthened several times. More Introduction to the Atmosphere ▸ Extent of the Atmosphere/Thermal Structure of the Atmosphere than 190 nations eventually ratified the treaty. ATMOSPHERE The Montreal Protocol represents a positive international To say that the atmosphere begins at Earth’s surface and ex- response to a global environment problem. As a result of the tends upward is obvious. However, where does the atmo- action, the total abundance of ozone-depleting gases in the sphere end and where does outer space begin? There is no atmosphere has started to decrease in recent years. Accord- sharp boundary; the atmosphere rapidly thins as you travel ing to the U.S. Environmental Protection Agency (U.S. EPA), away from Earth, until there are too few gas molecules to the ozone layer has not grown thinner since 1998 over most detect. of the world.* If the nations of the world continue to follow the provisions of the protocol, the decreases are expected to continue throughout the twenty-first century. Some offend- ing chemicals are still increasing but will begin to decrease in Pressure Changes coming decades. Between 2060 and 2075, the abundance of To understand the vertical extent of the atmosphere, let us ozone-depleting gases is projected to fall to values that exist- examine the changes in atmospheric pressure with height. ed before the Antarctic ozone hole began to form in the 1980s. Atmospheric pressure is simply the weight of the air above. At sea level the average pressure is slightly more than 1000 millibars. This corresponds to a weight of slightly more than Concept Check 1.7 1 kilogram per square centimeter (14.7 pounds per square 1 What are CFCs, and what is their connection to the ozone inch). Obviously, the pressure at higher altitudes is less problem? (Figure 1–21). 2 During what time of year is the Antarctic ozone hole well One-half of the atmosphere lies below an altitude of developed? 5.6 kilometers (3.5 miles). At about 16 kilometers (10 miles), 90 percent of the atmosphere has been traversed, and above 3 Describe three effects of ozone depletion. 100 kilometers (62 miles) only 0.00003 percent of all the 4 What is the Montreal Protocol? gases composing the atmosphere remain. At an altitude of 100 kilometers the atmosphere is so thin that the density of air is less than could be found in the * U.S. EPA, Achievements in Stratospheric Ozone Protection, Progress Report. most perfect artificial vacuum at the surface. Nevertheless, EPA-430-R-07-001, April 2007, p. 5. the atmosphere continues to even greater heights. The truly  Chapter 1 Introduction to the Atmosphere 25 Kathy Orr, Broadcast Meteorologist not the ‘rip and read’ of years gone by. We take data from the supercomputers in Wash- ington or models by the Navy and make our own forecasts. There are some services that provide forecasts locally and nationally, but they’re not located where we are. I can look out the window and tell whether those fore- casts are going to be accurate or not.” As a weathercaster, Orr has worked to promote education in science and math. For three years, she led a community program called Kidcasters. By offering children a chance to present the weather on TV, Orr hoped to interest elementary school children in science and math. For the past nine sum- mers, she has conducted a similar program called Orr at the Shore. Each program highlights environmental issues along the New Jersey coast. KATHY ORR is an award-winning broadcast meteorologist in Philadelphia. (Photo courtesy of Kathy Orr) My job is to explain complicated ideas to people in an Kathy Orr is a trusted and familiar face couldn’t see how to combine her two major uncomplicated way. on the airwaves of Philadelphia. As chief interests. “There weren’t any women doing meteorologist for CBS3, Orr has kept the the weather on television back then. There Orr continues to promote science literacy City of Brotherly Love abreast of the weather were also not a lot of meteorologists on TV; by volunteering for the American Meteoro- for 18 years and earned 10 regional Emmy it was less about the science and more for logical Society’s DataStreme Atmosphere awards in the process. comic relief,” she says. Project. As a DataStreme mentor, she has vis- She majored in broadcasting at Syracuse ited dozens of schools to train teachers in the Orr calls being a television University and went on to earn a second science of meteorology. The teachers then weathercaster a dream come true. degree in meteorology at the State University promote the use of weather lessons in their of New York at Oswego. There she learned districts to pique student interest in science, Orr calls being a television weather- the basis for the snow squalls that transfixed mathematics, and technology. Orr considers caster a dream come true. Growing up in her as a girl. “These kinds of phenomena are her forecasts educational as well. “My job is Syracuse, New York, Orr operated her own associated with being on the downwind side to explain complicated ideas to people in an miniature weather station and marveled at of a Great Lake. When wind comes along, uncomplicated way.” the snow squalls that howled across Lake the lake acts like a snowmaking machine.” Being a weathercaster, Orr says, is Ontario. “It could be a sunny afternoon, then While still in school, Orr landed a job as demanding but also exhilarating. “In TV, the wind would blow over the lake. All of the weathercaster on a Syracuse station’s the hours are crazy. If you work mornings, a sudden there was a blinding blizzard,” brand-new morning show. She’s remained a you’re up at 2 AM; if nights, you’re up until she says. television meteorologist ever since. midnight. So you really have to love it. But if When not watching the skies, Orr stayed Today, Orr says, being a trained meteorol- you do, you’ll find a way. And I feel blessed glued to her family’s TV set. At the time, she ogist “is definitely a competitive advantage. It’s to have done this for so long.” rarefied nature of the outer atmosphere is described very sea level, an atom or molecule, on the average, undergoes well by Richard Craig: about 7 3 109 such collisions each second; near 600 km, this number is reduced to about 1 each minute.* The earth’s outermost atmosphere, the part above a few hundred kilometers, is a region of extremely low density. The graphic portrayal of pressure data (Figure 1–21) shows Near sea level, the number of atoms and molecules in a that the rate of pressure decrease is not constant. Rather, cubic centimeter of air is about 2 3 1019; near 600 km, it is pressure decreases at a decreasing rate with an increase only about 2 3 107, which is the sea-level value divided by in altitude until, beyond an altitude of about 35 kilometers a million million. At sea level, an atom or molecule can be (22 miles), the decrease is negligible. expected, on the average, to move about 7 3 1026 cm before colliding with another particle; at the 600-km level, this *Richard Craig, The Edge of Space: Exploring the Upper Atmosphere (New distance, called the “mean free path,” is about 10 km. Near York: Doubleday & Company, Inc., 1968), p. 130. 26 The Atmosphere: An Introduction to Meteorology 36 22 32 20 Capt. Kittinger, USAF 1961 31.3 km (102,800 ft) 18 28 Air pressure = 9.6 mb 16 24 14 Altitude (miles) Altitude (km) 20 12 Air pressure at 16 top of Mt. Everest 10 (29,035 ft) is 314 mb 50% of 8 12 atmosphere lies below 6 this altitude 8 4 4 2 This jet is cruising at an altitude of 10 kilometers (6.2 miles). (Photo by inter- 0 200 400 600 800 1000 light/Shutterstock) Pressure (mb) Question 1 Refer to the graph in Figure 1–21. What is the approximate air pressure where the jet is flying? Figure 1–21 Atmospheric pressure changes with altitude. Question 2 About what percentage of the atmosphere is below the jet The rate of pressure decrease with an increase in altitude is not (assuming that the pressure at the surface is 1000 millibars)? constant. Rather, pressure decreases rapidly near Earth’s surface and more gradually at greater heights. Although measurements had not been taken above a height of about 10 kilometers (6 miles), scientists believed Put another way, data illustrate that air is highly that the temperature continued to decline with height to a compressible—that is, it expands with decreasing pressure value of absolute zero (–273°C) at the outer edge of the atmo- and becomes compressed with increasing pressure. Conse- sphere. In 1902, however, the French scientist Leon Philippe quently, traces of our atmosphere extend for thousands of Teisserenc de Bort refuted the notion that temperature kilometers beyond Earth’s surface. Thus, to say where the atmosphere ends and outer space begins is arbitrary and, to a large extent, depends on what phenomenon one is study- ing. It is apparent that there is no sharp boundary. Figure 1–22 Temperatures drop with an increase in altitude In summary, data on vertical pressure changes show in the troposphere. Therefore, it is possible to have snow on a mountaintop and warmer, snow-free lowlands below. (Photo by that the vast bulk of the gases making up the atmosphere David Wall/Alamy) is very near Earth’s surface and that the gases gradually merge with the emptiness of space. When compared with the size of the solid Earth, the envelope of air surrounding our planet is indeed very shallow. Temperature Changes By the early twentieth century much had been learned about the lower atmosphere. The upper atmosphere was partly known from indirect methods. Data from balloons and kites had revealed that the air temperature dropped with increas- ing height above Earth’s surface. This phenomenon is felt by anyone who has climbed a high mountain and is obvious in pictures of snow-capped mountaintops rising above snow- free lowlands (Figure 1–22).  Chapter 1 Introduction to the Atmosphere 27 decreases continuously with an increase in 90 altitude. In studying the results of more than 140 200 balloon launchings, Teisserenc de Bort found that the temperature stopped decreas- 130 Aurora 80 ing and leveled off at an altitude between 8 120 and 12 kilometers (5 and 7.5 miles). This sur- THERMOSPHERE prising discovery was at first doubted, but 110 70 subsequent data-gathering confirmed his findings. Later, through the use of balloons 100 60 and rocket-sounding techniques, the temper- 90 ature structure of the atmosphere up to great heights became clear. Today the atmosphere 80 Mesopause 50 Height (km) Height (miles) is divided vertically into four layers on the Tem p 70 Meteor erat basis of temperature (Figure 1–23). ure MESOSPHERE 40 60 Troposphere The bottom layer in which we live, where temperature decreases with 50 Stratopause 30 an increase in altitude, is the troposphere. 40 The term was coined in 1908 by Teisserrenc de Bort and literally means the region where 30 STRATOSPHERE 20 air “turns over,” a reference to the apprecia- Maximum ozone ble vertical mixing of air in this lowermost 20 10 zone. 10 Tropopause The temperature decrease in the tropo- Mt. Everest TROPOSPHERE sphere is called the environmental lapse rate. Its average value is 6.5°C per kilome- –100 – 90 – 80 –70 – 60 – 50 – 40 – 30 – 20 –10 0 10 20 30 30 50˚C ter (3.5°F per 1000 feet), a figure known as –140 –120 –100 – 80 – 60 – 40 – 20 0 20 40 60 80 100 120˚F the normal lapse rate. It should be empha- 32 Temperature sized, however, that the environmental lapse rate is not a constant but rather can be highly variable and must be regularly mea- Figure 1–23 Thermal structure of the atmosphere. sured. To determine the actual environmen- tal lapse rate as well as gather information about vertical changes in air pressure, wind, The troposphere is the chief focus of meteorologists and humidity, radiosondes are used. A radiosonde is an because it is in this layer that essentially all important instrument package that is attached to a balloon and trans- weather phenomena occur. Almost all clouds and certainly mits data by radio as it ascends through the atmosphere all precipitation, as well as all our violent storms, are born (Figure 1–24). The environmental lapse rate can vary dur- in this lowermost layer of the atmosphere. This is why the ing the course of a day with fluctuations of the weather, troposphere is often called the “weather sphere.” as well as seasonally and from place to place. Sometimes shallow layers where temperatures actually increase with Stratosphere Beyond the troposphere lies the height are observed in the troposphere. When such a rever- stratosphere; the boundary between the troposphere and sal occurs, a temperature inversion is said to exist.* the stratosphere is known as the tropopause. Below the The temperature decrease continues to an average tropopause, atmospheric properties are readily transferred height of about 12 kilometers (7.5 miles). Yet the thick- by large-scale turbulence and mixing, but above it, in the ness of the troposphere is not the same everywhere. It stratosphere, they are not. In the stratosphere, the tempera- reaches heights in excess of 16 kilometers (10 miles) in the ture at first remains nearly constant to a height of about 20 tropics, but in polar regions it is more subdued, extend- kilometers (12 miles) before it begins a sharp increase that ing to 9 kilometers (5.5 miles) or less (Figure 1–25). Warm continues until the stratopause is encountered at a height of surface temperatures and highly developed thermal mix- about 50 kilometers (30 miles) above Earth’s surface. Higher ing are responsible for the greater vertical extent of the tro- temperatures occur in the stratosphere because it is in this posphere near the equator. As a result, the environmental layer that the atmosphere’s ozone is concentrated. Recall lapse rate extends to great heights; and despite relatively that ozone absorbs ultraviolet radiation from the Sun. Con- high surface temperatures below, the lowest tropospheric sequently, the stratosphere is heated by the Sun. Although temperatures are found aloft in the tropics and not at the the maximum ozone concentration exists between 15 and 30 poles. kilometers (9 and 19 miles), the smaller amounts of ozone above this height range absorb enough UV energy to cause *Temperature inversions are described in greater detail in Chapter 13. the higher observed temperatures. 28 The Atmosphere: An Introduction to Meteorology 30 Pole Tropopause 27 24 Equator 21 Tropical tropopause 18 Altitude (km) Middle latitude 15 tropopause 12 Polar tropopause 9 6 3 0 –70 –60 –50 –40 –30 –20 –10 0 10 20 Temperature (˚C) Figure 1–25 Differences in the height of the tropopause. The variation in the height of the tropopause, as shown on the small inset diagram, is greatly exaggerated. Figure 1–24 A lightweight instrument package, the radiosonde, is suspended below a 2-meter-wide weather balloon. As the radiosonde is carried aloft, sensors measure pressure, temperature, and relative humidity. A radio transmitter sends the measurements to a ground receiver. By tracking the radiosonde in flight, information on wind speed and direction aloft is also lowest-orbiting satellites. Recent technical developments are obtained. Observations where winds aloft are obtained are called just beginning to fill this knowledge gap. “rawinsonde” observations. Worldwide, there are about 900 upper-air observation stations. Through international agreements, Thermosphere The fourth layer extends outward from data are exchanged among countries. (Photo by Mark Burnett/ the mesopause and has no well-defined upper limit. It Photo Researchers, Inc.) is the thermosphere, a layer that contains only a tiny frac- tion of the atmosphere’s mass. In the extremely rarified air of this outermost layer, temperatures again increase, due to Mesosphere In the third layer, the mesosphere, temper- the absorption of very shortwave, high-energy solar radia- atures again decrease with height until at the mesopause, tion by atoms of oxygen and nitrogen. some 80 kilometers (50 miles) above the surface, the aver- Temperatures rise to extremely high values of more than age temperature approaches 290°C (2130°F). The coldest 1000°C (1800°F) in the thermosphere. But such temperatures temperatures anywhere in the atmosphere occur at the me- are not comparable to those experienced near Earth’s sur- sopause. The pressure at the base of the mesosphere is only face. Temperature is defined in terms of the average speed about one-thousandth that at sea level. At the mesopause, at which molecules move. Because the gases of the thermo- the atmospheric pressure drops to just one-millionth that at sphere are moving at very high speeds, the temperature sea level. Because accessibility is difficult, the mesosphere is very high. But the gases are so sparse that collectively is one of the least explored regions of the atmosphere. The they possess only an insignificant quantity of heat. For this reason is that it cannot be reached by the highest-flying reason, the temperature of a satellite orbiting Earth in the airplanes and research balloons, nor is it accessible to the thermosphere is determined chiefly by the amount of solar  Chapter 1 Introduction to the Atmosphere 29 radiation it absorbs and not by the high temperature of the almost nonexistent surrounding air. If an astronaut inside were to expose his or her hand, the air in this layer would not feel hot. Concept Check 1.8 1 Does air pressure increase or decrease with an increase in altitude? Is the rate of change constant or variable? Explain. 2 Is the outer edge of the atmosphere clearly defined? Explain. 3 The atmosphere is divided vertically into four layers on the basis of temperature. List these layers in order from lowest to highest. In which layer does practically all of our weather occur? 4 Why does temperature increase in the stratosphere? 5 Why are temperatures in the thermosphere not strictly comparable to those experienced near Earth’s surface? Vertical Variations in Composition In addition to the layers defined by vertical variations in temperature, other layers, or zones, are also recognized in the atmosphere. Based on composition, the atmosphere is often divided into two layers: the homosphere and the heterosphere. From Earth’s surface to an altitude of about 80 kilometers (50 miles), the makeup of the air is uniform in When this weather balloon was launched, the surface temperature was terms of the proportions of its component gases. That is, the 17°C. It is now at an altitude of 1 kilometer. (Photo by David R. Frazier/ composition is the same as that shown earlier, in Figure 1–16. Photo Researchers, Inc.) This lower uniform layer is termed the homosphere, the zone Question 1 What term is applied to the instrument package being car- of homogeneous composition. ried aloft by the balloon? In contrast, the very thin atmosphere above 80 kilometers Question 2 In what layer of the atmosphere is the balloon? is not uniform. Because it has a heterogeneous composition, Question 3 If average conditions prevail, what air temperature is the the term heterosphere is used. Here the gases are arranged instrument package recording? How did you figure this out? into four roughly spherical shells, each with a distinctive composition. The lowermost layer is dominated by molec- Question 4 How will the size of the balloon change, if at all, as it rises through the atmosphere? Explain. ular nitrogen (N2), next, a layer of atomic oxygen (O) is encountered, followed by a layer dominated by helium (He) atoms, and finally a region consisting largely of hydrogen (H) atoms. The stratified nature of the gases making up the heterosphere varies according to their weights. Molecular absorb high-energy shortwave solar energy. In this process, nitrogen is the heaviest, and so it is lowest. The lightest gas, each affected molecule or atom loses one or more electrons hydrogen, is outermost. and becomes a positively charged ion, and the electrons are set free to travel as electric currents. Although ionization occurs at heights as great as Ionosphere 1000 kilometers (620 miles) and extends as low as perhaps Located in the altitude range between 80 to 400 kilometers 50 kilometers (30 miles), positively charged ions and nega- (50 to 250 miles), and thus coinciding with the lower portions tive electrons are most dense in the range of 80 to 400 kilo- of the thermosphere and heterosphere, is an electrically meters (50 to 250 miles). The concentration of ions is not charged layer known as the ionosphere. Here molecules of great below this zone because much of the short-wavelength nitrogen and atoms of oxygen are readily ionized as they radiation needed for ionization has already been depleted. 30 The Atmosphere: An Introduction to Meteorology In addition, the atmospheric density at this level results its Southern Hemisphere counterpart, the aurora australis in a large percentage of free electrons being swiftly cap- (southern lights), appear in a wide variety of forms. Some- tured by positively charged ions. Beyond the 400-kilometer times the displays consist of vertical streamers in which there (250-mile) upward limit of the ionosphere, the concentra- can be considerable movement. At other times the auroras tion of ions is low because of the extremely low density of appear as a series of luminous expanding arcs or as a quiet the air. Because so few molecules and atoms are present, glow that has an almost foglike quality. relatively few ions and free electrons can be produced. The occurrence of auroral displays is closely correlated The electrical structure of the ionosphere is not uni- in time with solar-flare activity and, in geographic location, form. It consists of three layers of varying ion density. From with Earth’s magnetic poles. Solar flares are massive mag- bottom to top, these layers are called the D, E, and F lay- netic storms on the Sun that emit enormous amounts of ers, respectively. Because the production of ions requires energy and great quantities of fast-moving atomic particles. direct solar radiation, the concentration of charged parti- As the clouds of protons and electrons from the solar storm cles changes from day to night, particularly in the D and E approach Earth, they are captured by its magnetic field, zones. That is, these layers weaken and disappear at night which in turn guides them toward the magnetic poles. Then, and reappear during the day. The uppermost layer, or as the ions impinge on the ionosphere, they energize the F layer, on the other hand, is present both day and night. atoms of oxygen and molecules of nitrogen and cause them The density of the atmosphere in this layer is very low, and to emit light—the glow of the auroras. Because the occur- positive ions and electrons do not meet and recombine as rence of solar flares is closely correlated with sunspot activ- rapidly as they do at lesser heights, where density is higher. ity, auroral displays increase conspicuously at times when Consequently, the concentration of ions and electrons in sunspots are most numerous. the F layer does not change rapidly, and the layer, although weak, remains through the night. The Auroras Concept Check 1.9 As best we can tell, the ionosphere has little impact on our 1 Distinguish between the homosphere and the heterosphere. daily weather. But this layer of the atmosphere is the site 2 What is the ionosphere? Where in the atmosphere is it located? of one of nature’s most interesting spectacles, the auroras (Figure 1–26). The aurora borealis (northern lights) and 3 What is the primary cause of the auroras? Figure 1–26 Aurora borealis (northern lights) as seen from Alaska. The same phenomenon occurs toward the South Pole, where it is called the aurora australis (southern lights). (Photo by agefotostock/ SuperStock)  Chapter 1 Introduction to the Atmosphere 31 Give It Some Thought 1. Determine which statements refer to weather and greenhouse gases have increased global average which refer to climate. (Note: One statement includes temperatures. aspects of both weather and climate.) b. One or two studies suggest that hurricance intensity a. The baseball game was rained out today. is increasing. b. January is Omaha’s coldest month. 5. Refer to Figure 1–21 to answer the following questions. c. North Africa is a desert. a. If you were to climb to the top of Mount Everest, d. The high this afternoon was 25°C. how many breaths of air would you have to take at e. Last evening a tornado ripped through central that altitude to equal one breath at sea level? Oklahoma. b. If you are flying in a commercial jet at an altitude f. I am moving to southern Arizona because it is of 12 kilometers, about what percentage of the warm and sunny. atmosphere’s mass is below you? g. Thursday’s low of –20°C is the coldest temperature 6. If you were ascending from the surface of Earth to ever recorded for that city. the top of the atmosphere, which one of the following h. It is partly cloudy. would be most useful for determining the layer of the 2. After entering a dark room, you turn on a wall switch, atmosphere you were in? Explain. but the light does not come on. Suggest at least three a. Doppler radar hypotheses that might explain this observation. b. Hygrometer (humidity) 3. Making accurate measurements and observations is c. Weather satellite a basic part of scientific inquiry. The accompanying d. Barometer (air pressure) radar image, showing the distribution and intensity e. Thermometer (temperature) of precipitation associated with a storm, provides 7. The accompanying photo provides an example of one example. Identify three additional images in this interactions among different parts of the Earth system. chapter that illustrate ways in which scientific data are It is a view of a mudflow that was triggered by gathered. Suggest advantages that might be associated extraordinary rains. Which of Earth’s four “spheres” with each example. were involved in this natural disaster that buried a small town on the Philippine island of Leyte? Describe how each contributed to the mudflow. (Photo by AP Photo/Pat Roque) 8. Where would you expect the thickness of the troposphere (that is, the distance between Earth’s (Image by National Weather Service) surface and the tropopause) to be greater: over Hawaii 4. During a conversation with your meteorology professor, or Alaska? Why? Do you think it is likely that the she makes the two statements listed below. Which can be thickness of the troposphere over Alaska is different considered a hypothesis? Which is more likely a theory? in January than in July? If so, why? a. After several decades, the science community has determined that human-generated  36 The Atmosphere: An Introduction to Meteorology Earth–Sun Relationships winds and drives the ocean’s currents, which in turn trans- port heat from the tropics toward the poles in an unending Heating Earth’s Surface and Atmosphere attempt to balance energy inequalities. ATMOSPHERE ▸ Understanding Seasons The consequences of these processes are the phenomena we call weather. If the Sun were “turned off,” global winds The amount of solar energy received at any location varies and ocean currents would quickly cease. Yet as long as the with latitude, time of day, and season of the year. Contrasting Sun shines, winds will blow and weather will persist. So to images of polar bears and perpetual ice and palm trees along understand how the dynamic weather machine works, we a tropical beach serve to illustrate the extremes (Figure 2–1). must know why different latitudes receive different quan- The unequal heating of Earth’s land–sea surface creates tities of solar energy and why the amount of solar energy received changes during the course of a year to produce the seasons. Earth’s Motions Earth has two principal motions—rotation and revolution. Rotation is the spinning of Earth on its axis that produces the daily cycle of day and night. The other motion, revolution, refers to Earth’s move- ment in a slightly elliptical orbit around the Sun. The dis- tance between Earth and Sun averages about 150 million kilometers (93 million miles). Because Earth’s orbit is not perfectly circular, however, the distance varies during the course of a year. Each year, on about January 3, our planet is about 147.3 million kilometers (91.5 million miles) from the Sun, closer than at any other time—a position called perihe- lion. About six months later, on July 4, Earth is about 152.1 million kilometers (94.5 million miles) from the Sun, farther away than at any other time—a position called aphelion. Although Earth is closest to the Sun and receives up to 7 percent more energy in January than in July, this difference plays only a minor role in producing seasonal temperature variations, as evidenced by the fact that Earth is closest to (a) the Sun during the Northern Hemisphere winter. What Causes the Seasons? If variations in the distance between the Sun and Earth do not cause seasonal temperature changes, what does? The gradual but significant change in day length certainly accounts for some of the difference we notice between summer and winter. Furthermore, a gradual change in the angle (altitude) of the Sun above the horizon is also a major contributing fac- tor (Figure 2–2). For example, someone living in Chicago, Illi- nois, experiences the noon Sun highest in the sky in late June. But as summer gives way to autumn, the noon Sun appears lower in the sky, and sunset occurs earlier each evening. The seasonal variation in the angle of the Sun above the horizon affects the amount of energy received at Earth’s sur- face in two ways. First, when the Sun is directly overhead (at a 90° angle), the solar rays are most concentrated and thus most intense. At lower Sun angles, the rays become more spread out and less intense (Figure 2–3). You have probably (b) experienced this when using a flashlight. If the beam strikes Figure 2–1 An understanding of Earth-Sun relationships is a surface perpendicularly, a small intense spot is produced. basic to an understanding of weather and climate. (a) In tropical By contrast, if the flashlight beam strikes at any other angle, latitudes, temperature contrasts during the year are modest. (Photo the area illuminated is larger—but noticeably dimmer. by Maria Skaldina/Shutterstock) (b) In polar regions, seasonal Second, but of less significance, the angle of the Sun de- temperature contrasts can be dramatic. (Photo by Michael Collier) termines the path solar rays take as they pass through the  Chapter 2 Heating Earth’s Surface and Atmosphere 37 June 21-22 Longest day March 21-22 Day and September 22-23 night equal E Sun E angle Sun 73 1/2° angle 50° N S N S W W (a) Summer solstice at 40°N latitude (b) Spring or fall equinox at 40°N latitude December 21-22 Shortest day June 21-22 Noon 24 hours of Sun daylight E E Midnight Sun Sun Sun angle angle N 26 1/2° S N 33 1/2° S W W (c) Winter solstice at 40°N latitude (d) Summer solstice at 80°N latitude Figure 2–2 Daily paths of the Sun for a place located at 40° north latitude for the (a) summer solstice; (b) spring or fall equinox, and (c) winter solstice and for a place located at 80° north latitude at the (d) summer solstice. atmosphere (Figure 2–4). When the Sun is directly overhead, a distance roughly equal to the thickness of 11 atmospheres the rays strike the atmosphere at a 90° angle and travel the (Table 2–1). The longer the path, the greater the chance that shortest possible route to the surface. This distance is re- sunlight will be dispersed by the atmosphere, which re- ferred to as 1 atmosphere. However, rays entering the atmos- duces the intensity at the surface. These conditions account phere at a 30° angle must travel twice this distance before for the fact that we cannot look directly at the midday Sun reaching the surface, while rays at a 5° angle travel through but we enjoy gazing at a sunset. 1 unit 1 un it 1u nit 90˚ 45˚ 30˚ 1 unit 1.4 units 2 units Figure 2–3 Changes in the Sun’s angle cause variations in the amount of solar energy reaching Earth’s surface. The higher the angle, the more intense the solar radiation. 38 The Atmosphere: An Introduction to Meteorology 231/2 ˚ N In summary, the most important reasons for variations in Atmosphere the amount of solar energy reaching a particular location are the 90˚ seasonal changes in the angle at which the Sun’s rays strike the surface and changes in the length of daylight. 661/2˚ Sun's Earth’s Orientation rays 30˚ What causes fluctuations in Sun angle and length of day- light during the course of a year? Variations occur because Earth’s orientation to the Sun continually changes. Earth’s 0˚ axis (the imaginary line through the poles around which Earth rotates) is not perpendicular to the plane of its orbit 231/2 ˚ around the Sun—called the plane of the ecliptic. Instead, it is tilted 23 1/2° from the perpendicular, called the