Introduction to Meteorology Chapter 1 PDF

CHAPTER 1 The Earth and Its Atmosphere ❂ I well remember a brilliant red balloon which kept me completely happy for a whole afternoon, until, while I was playing, a clumsy movement allowed it to escape....

CHAPTER 1 The Earth and Its Atmosphere ❂ I well remember a brilliant red balloon which kept me completely happy for a whole afternoon, until, while I was playing, a clumsy movement allowed it to escape. CONTENTS Overview of the Earth’s Atmosphere Composition of the Atmosphere FOCUS ON A SPECIAL TOPIC Spellbound, I gazed after it as it drifted silently away, A Breath of Fresh Air gently swaying, growing smaller and smaller until it was The Early Atmosphere Vertical Structure of the Atmosphere only a red point in a blue sky. At that moment I realized, A Brief Look at Air Pressure and Air Density for the first time, the vastness above us: a huge space Layers of the Atmosphere FOCUS ON A SPECIAL TOPIC without visible limits. It was an apparent void, full of se- The Atmospheres of Other Planets crets, exerting an inexplicable power over all the earth’s FOCUS ON AN OBSERVATION inhabitants. I believe that many people, consciously or The Radiosonde The Ionosphere unconsciously, have been filled with awe by the immen- Weather and Climate sity of the atmosphere. All our knowledge about the air, Meteorology — A Brief History A Satellite’s View of the Weather gathered over hundreds of years, has not diminished this Storms of All Sizes feeling. A Look at a Weather Map Weather and Climate in Our Lives Theo Loebsack, Our Atmosphere FOCUS ON A SPECIAL TOPIC What Is a Meteorologist? Summary Key Terms Questions for Review Questions for Thought Problems and Exercises 3 4 CH A P TER 1 ❂ Our atmosphere is a delicate life-giving blanket of air ets, along with a host of other material (comets, asteroids, that surrounds the fragile earth. In one way or an- meteors, dwarf planets, etc.), comprise our solar system. other, it influences everything we see and hear — it is Warmth for the planets is provided primarily by the sun’s intimately connected to our lives. Air is with us from birth, energy. At an average distance from the sun of nearly 150 mil- and we cannot detach ourselves from its presence. In the lion kilometers (km) or 93 million miles (mi), the earth in- open air, we can travel for many thousands of kilometers in tercepts only a very small fraction of the sun’s total energy any horizontal direction, but should we move a mere eight output. However, it is this radiant energy (or radiation)* that kilometers above the surface, we would suffocate. We may be drives the atmosphere into the patterns of everyday wind and able to survive without food for a few weeks, or without wa- weather and allows the earth to maintain an average surface ter for a few days, but, without our atmosphere, we would not temperature of about 15°C (59°F).† Although this tempera- survive more than a few minutes. Just as fish are confined to ture is mild, the earth experiences a wide range of tempera- an environment of water, so we are confined to an ocean of tures, as readings can drop below
85°C (
121°F) during a air. Anywhere we go, it must go with us. frigid Antarctic night and climb, during the day, to above The earth without an atmosphere would have no lakes or 50°C (122°F) on the oppressively hot subtropical desert. oceans. There would be no sounds, no clouds, no red sunsets. The earth’s atmosphere is a thin, gaseous envelope com- The beautiful pageantry of the sky would be absent. It would prised mostly of nitrogen and oxygen, with small amounts be unimaginably cold at night and unbearably hot during the of other gases, such as water vapor and carbon dioxide. day. All things on the earth would be at the mercy of an in- Nestled in the atmosphere are clouds of liquid water and ice tense sun beating down upon a planet utterly parched. crystals. Although our atmosphere extends upward for many Living on the surface of the earth, we have adapted so hundreds of kilometers, almost 99 percent of the atmo- completely to our environment of air that we sometimes for- sphere lies within a mere 30 km (19 mi) of the earth’s surface get how truly remarkable this substance is. Even though air is (see Fig. 1.2). In fact, if the earth were to shrink to the size tasteless, odorless, and (most of the time) invisible, it protects of a beach ball, its inhabitable atmosphere would be thinner us from the scorching rays of the sun and provides us with a than a piece of paper. This thin blanket of air constantly mixture of gases that allows life to flourish. Because we cannot shields the surface and its inhabitants from the sun’s danger- see, smell, or taste air, it may seem surprising that between ous ultraviolet radiant energy, as well as from the onslaught your eyes and the pages of this book are trillions of air mole- of material from interplanetary space. There is no definite cules. Some of these may have been in a cloud only yesterday, upper limit to the atmosphere; rather, it becomes thinner or over another continent last week, or perhaps part of the life- and thinner, eventually merging with empty space, which giving breath of a person who lived hundreds of years ago. surrounds all the planets. In this chapter, we will examine a number of important concepts and ideas about the earth’s atmosphere, many of COMPOSITION OF THE ATMOSPHERE ▼ Table 1.1 shows which will be expanded in subsequent chapters. the various gases present in a volume of air near the earth’s surface. Notice that nitrogen (N2) occupies about 78 percent and oxygen (O2) about 21 percent of the total volume of dry air. If all the other gases are removed, these percentages for Overview of the Earth’s Atmosphere nitrogen and oxygen hold fairly constant up to an elevation The universe contains billions of galaxies and each galaxy is of about 80 km (50 mi). (For a closer look at the composition made up of billions of stars. Stars are hot, glowing balls of gas of a breath of air at the earth’s surface, read the Focus section that generate energy by converting hydrogen into helium near on p. 6.) their centers. Our sun is an average size star situated near the *Radiation is energy transferred in the form of waves that have electrical and magnetic properties. The light that we see is radiation, as is ultraviolet light. More edge of the Milky Way galaxy. Revolving around the sun are on this important topic is given in Chapter 2. the earth and seven other planets (see Fig. 1.1).* These plan- †The abbreviation °C is used when measuring temperature in degrees Celsius, and *Pluto was once classified as a true planet. But recently it has been reclassified as a °F is the abbreviation for degrees Fahrenheit. More information about tempera- planetary object called a dwarf planet. ture scales is given in Appendix B and in Chapter 2. F I G U R E 1.1 The relative sizes and positions of the planets in our solar system. Pluto is included as an object called a dwarf planet. (Positions are not to scale.)  The Earth and Its Atmosphere 5 WEAT H ER WATCH When it rains, it rains pennies from heaven — sometimes. On July 17, 1940, a tornado reportedly picked up a treasure of over 1000 sixteenth-century silver coins, carried them into a thunderstorm, then dropped them on the village of Merchery in the Gorki region of Russia. At the surface, there is a balance between destruction (output) and production (input) of these gases. For example, nitrogen is removed from the atmosphere primarily by bio- logical processes that involve soil bacteria. In addition, nitro- gen is taken from the air by tiny ocean-dwelling plankton that convert it into nutrients that help fortify the ocean’s food chain. It is returned to the atmosphere mainly through the decaying of plant and animal matter. Oxygen, on the other hand, is removed from the atmosphere when organic matter decays and when oxygen combines with other substances, producing oxides. It is also taken from the atmosphere dur- ing breathing, as the lungs take in oxygen and release carbon dioxide (CO2). The addition of oxygen to the atmosphere oc- curs during photosynthesis, as plants, in the presence of sunlight, combine carbon dioxide and water to produce sugar and oxygen. The concentration of the invisible gas water vapor (H2O), however, varies greatly from place to place, and from time to time. Close to the surface in warm, steamy, tropical locations, water vapor may account for up to 4 percent of the atmo- spheric gases, whereas in colder arctic areas, its concentration may dwindle to a mere fraction of a percent (see Table 1.1). Water vapor molecules are, of course, invisible. They become visible only when they transform into larger liquid or solid particles, such as cloud droplets and ice crystals, which may grow in size and eventually fall to the earth as rain or snow. NASA The changing of water vapor into liquid water is called con- densation, whereas the process of liquid water becoming wa- F I G U R E 1. 2 The earth’s atmosphere as viewed from space. The ter vapor is called evaporation. The falling rain and snow is atmosphere is the thin blue region along the edge of the earth. ▼ TA B L E 1.1 Composition of the Atmosphere near the Earth’s Surface PERMANENT GASES VARIABLE GASES Percent (by Volume) Gas Percent Parts per Gas Symbol Dry Air (and Particles) Symbol (by Volume) Million (ppm)* Nitrogen N2 78.08 Water vapor H2O 0 to 4 Oxygen O2 20.95 Carbon dioxide CO2 0.038 385* Argon Ar 0.93 Methane CH4 0.00017 1.7 Neon Ne 0.0018 Nitrous oxide N2O 0.00003 0.3 Helium He 0.0005 Ozone O3 0.000004 0.04† Hydrogen H2 0.00006 Particles (dust, soot, etc.) 0.000001 0.01
0.15 Xenon Xe 0.000009 Chlorofluorocarbons (CFCs) 0.00000002 0.0002 *For CO2, 385 parts per million means that out of every million air molecules, 385 are CO2 molecules. †Stratospheric values at altitudes between 11 km and 50 km are about 5 to 12 ppm. 6 CH A P TER 1 FO CU S O N A S P E CIAL TO PI C A Breath of Fresh Air If we could examine a breath of air, we would see number of orbiting electrons, the atom as a mine the total number of stars in the universe, that air (like everything else in the universe) is whole is electrically neutral (see Fig. 1). we multiply the number of stars in a galaxy by composed of incredibly tiny particles called Most of the air particles are molecules, the total number of galaxies and obtain atoms. We cannot see atoms individually. Yet, if combinations of two or more atoms (such as 1011  1011  1022 stars in the universe. we could see one, we would find electrons whirl- nitrogen, N2, and oxygen, O2), and most of the ing at fantastic speeds about an extremely dense molecules are electrically neutral. A few, how- Therefore, each breath of air contains center, somewhat like hummingbirds darting and ever, are electrically charged, having lost or about as many molecules as there are stars in circling about a flower. At this center, or nucleus, gained electrons. These charged atoms and the known universe. are the protons and neutrons. Almost all of the molecules are called ions. In the entire atmosphere, there are nearly atom’s mass is concentrated here, in a trillionth An average breath of fresh air contains a 1044 molecules. The number 1044 is 1022 of the atom’s entire volume. In the nucleus, the tremendous number of molecules. With every squared; consequently proton carries a positive charge, whereas the neu- deep breath, trillions of molecules from the at- 1022  1022  1044 molecules tron is electrically neutral. The circling electron mosphere enter your body. Some of these in- in the atmosphere. carries a negative charge. As long as the total haled gases become a part of you, and others number of protons in the nucleus equals the are exhaled. We thus conclude that there are about The volume of an average size breath of 1022 breaths of air in the entire atmosphere. In air is about a liter.* Near sea level, there are other words, there are as many molecules in a roughly ten thousand million million million single breath as there are breaths in the atmo- (1022)† air molecules in a liter. So, sphere. Each time we breathe, the molecules we 1 breath of air ⬇ 1022 molecules. exhale enter the turbulent atmosphere. If we We can appreciate how large this number wait a long time, those molecules will eventu- is when we compare it to the number of stars ally become thoroughly mixed with all of the in the universe. Astronomers have estimated other air molecules. If none of the molecules that there are about 100 billion (1011) stars in were consumed in other processes, eventually an average size galaxy and that there may be as there would be a molecule from that single many as 1011 galaxies in the universe. To deter- breath in every breath that is out there. So, FIGURE 1 considering the many breaths people exhale in An atom has neutrons and protons at its center *One cubic centimeter is about the size of a sugar cube, their lifetimes, it is possible that in our lungs with electrons orbiting this center (or nucleus). and there are a thousand cubic centimeters in a liter. are molecules that were once in the lungs of Molecules are combinations of two or more at- †The notation 1022 means the number one followed by people who lived hundreds or even thousands oms. The air we breathe is mainly molecular nitro- twenty-two zeros. For a further explanation of this sys- of years ago. In a very real way then, we all gen (N2) and molecular oxygen (O2). tem of notation see Appendix A. share the same atmosphere. called precipitation. In the lower atmosphere, water is every- heat inside from escaping and mixing with the outside air). where. It is the only substance that exists as a gas, a liquid, and Thus, water vapor plays a significant role in the earth’s heat- a solid at those temperatures and pressures normally found energy balance. near the earth’s surface (see Fig. 1.3). Carbon dioxide (CO2), a natural component of the at- Water vapor is an extremely important gas in our atmo- mosphere, occupies a small (but important) percent of a sphere. Not only does it form into both liquid and solid cloud volume of air, about 0.038 percent. Carbon dioxide enters the particles that grow in size and fall to earth as precipitation, atmosphere mainly from the decay of vegetation, but it also but it also releases large amounts of heat — called latent comes from volcanic eruptions, the exhalations of animal life, heat — when it changes from vapor into liquid water or ice. from the burning of fossil fuels (such as coal, oil, and natural Latent heat is an important source of atmospheric energy, gas), and from deforestation. The removal of CO2 from the especially for storms, such as thunderstorms and hurricanes. atmosphere takes place during photosynthesis, as plants con- Moreover, water vapor is a potent greenhouse gas because it sume CO2 to produce green matter. The CO2 is then stored in strongly absorbs a portion of the earth’s outgoing radiant roots, branches, and leaves. The oceans act as a huge reservoir energy (somewhat like the glass of a greenhouse prevents the for CO2, as phytoplankton (tiny drifting plants) in surface  The Earth and Its Atmosphere 7 © C. Donald Ahrens F I G U R E 1. 3 The earth’s atmosphere is a rich mixture of many F I G U R E 1. 4 The main components of the atmospheric carbon gases, with clouds of condensed water vapor and ice crystals. Here, dioxide cycle. The gray lines show processes that put carbon dioxide water evaporates from the ocean’s surface. Rising air currents then into the atmosphere, whereas the red lines show processes that remove transform the invisible water vapor into many billions of tiny liquid carbon dioxide from the atmosphere. droplets that appear as puffy cumulus clouds. If the rising air in the cloud should extend to greater heights, where air temperatures are quite low, some of the liquid droplets would freeze into minute ice crystals. current value of about 385 ppm to a value near 500 ppm to- ward the end of this century. Carbon dioxide is another important greenhouse gas water fix CO2 into organic tissues. Carbon dioxide that dis- because, like water vapor, it traps a portion of the earth’s solves directly into surface water mixes downward and circu- outgoing energy. Consequently, with everything else being lates through greater depths. Estimates are that the oceans equal, as the atmospheric concentration of CO2 increases, so hold more than 50 times the total atmospheric CO2 content. should the average global surface air temperature. In fact, Figure 1.4 illustrates important ways carbon dioxide enters over the last 100 years or so, the earth’s average surface tem- and leaves the atmosphere. perature has warmed by more than 0.8°C. Mathematical cli- Figure 1.5 reveals that the atmospheric concentration mate models that predict future atmospheric conditions of CO2 has risen more than 20 percent since 1958, when it estimate that if increasing levels of CO2 (and other green- was first measured at Mauna Loa Observatory in Hawaii. house gases) continue at their present rates, the earth’s sur- This increase means that CO2 is entering the atmosphere at a face could warm by an additional 3°C (5.4°F) by the end of greater rate than it is being removed. The increase appears to this century. As we shall see in Chapter 16, the negative con- be due mainly to the burning of fossil fuels; however, defor- sequences of global warming, such as rising sea levels and the estation also plays a role as cut timber, burned or left to rot, rapid melting of polar ice, will be felt worldwide. releases CO2 directly into the air, perhaps accounting for Carbon dioxide and water vapor are not the only green- about 20 percent of the observed increase. Measurements of house gases. Recently, others have been gaining notoriety, CO2 also come from ice cores. In Greenland and Antarctica, primarily because they, too, are becoming more concentrated. for example, tiny bubbles of air trapped within the ice sheets Such gases include methane (CH4), nitrous oxide (N2O), and reveal that before the industrial revolution, CO2 levels were chlorofluorocarbons (CFCs).* stable at about 280 parts per million (ppm). (See Fig. 1.6.) Levels of methane, for example, have been rising over the Since the early 1800s, however, CO2 levels have increased past century, increasing recently by about one-half of one more than 37 percent. With CO2 levels presently increasing percent per year. Most methane appears to derive from the by about 0.4 percent annually (1.9 ppm/year), scientists now *Because these gases (including CO2) occupy only a small fraction of a percent in estimate that the concentration of CO2 will likely rise from its a volume of air near the surface, they are referred to collectively as trace gases. 8 CH A P TER 1 F I G U R E 1. 5 Measurements of CO2 in parts per million (ppm) at Mauna Loa Observatory, Hawaii. Higher readings occur in winter when plants die and release CO2 to the atmo- sphere. Lower read- ings occur in summer when more abundant vegetation absorbs CO2 from the atmo- sphere. The solid line is the average yearly value. Notice that the concentration of CO2 has increased by more than 20 percent since 1958. breakdown of plant material by certain bacteria in rice pad- ing gas — have been rising annually at the rate of about one- dies, wet oxygen-poor soil, the biological activity of termites, quarter of a percent. Nitrous oxide forms in the soil through and biochemical reactions in the stomachs of cows. Just why a chemical process involving bacteria and certain microbes. methane should be increasing so rapidly is currently under Ultraviolet light from the sun destroys it. study. Levels of nitrous oxide — commonly known as laugh- Chlorofluorocarbons (CFCs) represent a group of green- house gases that, up until recently, had been increasing in concentration. At one time, they were the most widely used propellants in spray cans. Today, however, they are mainly used as refrigerants, as propellants for the blowing of plastic- foam insulation, and as solvents for cleaning electronic mi- crocircuits. Although their average concentration in a volume of air is quite small (see Table 1.1, p. 5), they have an impor- tant effect on our atmosphere as they not only have the po- tential for raising global temperatures, they also play a part in destroying the gas ozone in the stratosphere, a region in the atmosphere located between about 11 km and 50 km above the earth’s surface. At the surface, ozone (O3) is the primary ingredient of photochemical smog,* which irritates the eyes and throat and damages vegetation. But the majority of atmospheric ozone (about 97 percent) is found in the upper atmosphere — in the stratosphere — where it is formed naturally, as oxygen atoms combine with oxygen molecules. Here, the concentration of ozone averages less than 0.002 percent by volume. This small F I G U R E 1. 6 Carbon dioxide values in parts per million during *Originally the word smog meant the combining of smoke and fog. Today, how- the past 1000 years from ice cores in Antarctica (blue line) and from ever, the word usually refers to the type of smog that forms in large cities, such as Mauna Loa Observatory in Hawaii (red line). (Data courtesy of Carbon Los Angeles, California. Because this type of smog forms when chemical reactions Dioxide Information Analysis Center, Oak Ridge National Laboratory.) take place in the presence of sunlight, it is termed photochemical smog.  The Earth and Its Atmosphere 9 quantity is important, however, because it shields plants, ani- mals, and humans from the sun’s harmful ultraviolet rays. It is ironic that ozone, which damages plant life in a polluted envi- ronment, provides a natural protective shield in the upper atmosphere so that plants on the surface may survive. When CFCs enter the stratosphere, ultraviolet rays break them apart, and the CFCs release ozone-destroying chlorine. Because of this effect, ozone concentration in the strato- sphere has been decreasing over parts of the Northern and Southern Hemispheres. The reduction in stratospheric ozone levels over springtime Antarctica has plummeted at such an alarming rate that during September and October, there is an ozone hole over the region. Figure 1.7 illustrates the extent of the ozone hole above Antarctica during September, 2004. Impurities from both natural and human sources are also present in the atmosphere: Wind picks up dust and soil from the earth’s surface and carries it aloft; small saltwater drops from ocean waves are swept into the air (upon evapo- rating, these drops leave microscopic salt particles suspended NASA in the atmosphere); smoke from forest fires is often carried high above the earth; and volcanoes spew many tons of fine F I G U R E 1. 7 The darkest color represents the area of lowest ash particles and gases into the air (see Fig. 1.8). Collec- ozone concentration, or ozone hole, over the Southern Hemisphere on tively, these tiny solid or liquid suspended particles of various September 22, 2004. Notice that the hole is larger than the continent of composition are called aerosols. Antarctica. A Dobson unit (DU) is the physical thickness of the ozone Some natural impurities found in the atmosphere are layer if it were brought to the earth’s surface, where 500 DU equals quite beneficial. Small, floating particles, for instance, act as 5 millimeters. surfaces on which water vapor condenses to form clouds. However, most human-made impurities (and some natural less, this poisonous gas forms during the incomplete com- ones) are a nuisance, as well as a health hazard. These we call bustion of carbon-containing fuel. Hence, over 75 percent of pollutants. For example, automobile engines emit copious carbon monoxide in urban areas comes from road vehicles. amounts of nitrogen dioxide (NO2), carbon monoxide (CO), The burning of sulfur-containing fuels (such as coal and and hydrocarbons. In sunlight, nitrogen dioxide reacts with oil) releases the colorless gas sulfur dioxide (SO2) into the air. hydrocarbons and other gases to produce ozone. Carbon When the atmosphere is sufficiently moist, the SO2 may monoxide is a major pollutant of city air. Colorless and odor- transform into tiny dilute drops of sulfuric acid. Rain con- F I G U R E 1. 8 Erupting volcanoes can send tons of particles into the atmosphere, along with vast amounts of water vapor, carbon dioxide, and sulfur dioxide. © David Weintraub/Photo Researchers 10 CH A PTER 1 taining sulfuric acid corrodes metals and painted surfaces, and turns freshwater lakes acidic. Acid rain is a major envi- BR IEF R E V IE W ronmental problem, especially downwind from major indus- Before going on to the next several sections, here is a review of trial areas. In addition, high concentrations of SO2 produce some of the important concepts presented so far: serious respiratory problems in humans, such as bronchitis and emphysema, and have an adverse effect on plant life. The earth’s atmosphere is a mixture of many gases. In a volume of dry air near the surface, nitrogen (N2) occupies about 78 THE EARLY ATMOSPHERE The atmosphere that originally percent and oxygen (O2) about 21 percent. surrounded the earth was probably much different from the Water vapor, which normally occupies less than 4 percent in a air we breathe today. The earth’s first atmosphere (some 4.6 volume of air near the surface, can condense into liquid cloud billion years ago) was most likely hydrogen and helium — the droplets or transform into delicate ice crystals. Water is the two most abundant gases found in the universe — as well as only substance in our atmosphere that is found naturally as a hydrogen compounds, such as methane (CH4) and ammonia gas (water vapor), as a liquid (water), and as a solid (ice). (NH3). Most scientists feel that this early atmosphere escaped Both water vapor and carbon dioxide (CO2) are important into space from the earth’s hot surface. greenhouse gases. A second, more dense atmosphere, however, gradually Ozone (O3) in the stratosphere protects life from harmful ultra- enveloped the earth as gases from molten rock within its hot violet (UV) radiation. At the surface, ozone is the main ingredi- interior escaped through volcanoes and steam vents. We as- ent of photochemical smog. sume that volcanoes spewed out the same gases then as they do today: mostly water vapor (about 80 percent), carbon di- The majority of water on our planet is believed to have come oxide (about 10 percent), and up to a few percent nitrogen. from its hot interior through outgassing. These gases (mostly water vapor and carbon dioxide) prob- ably created the earth’s second atmosphere. As millions of years passed, the constant outpouring of gases from the hot interior — known as outgassing — pro- vided a rich supply of water vapor, which formed into Vertical Structure of the Atmosphere clouds.* Rain fell upon the earth for many thousands of A vertical profile of the atmosphere reveals that it can be years, forming the rivers, lakes, and oceans of the world. Dur- divided into a series of layers. Each layer may be defined in ing this time, large amounts of CO2 were dissolved in the a number of ways: by the manner in which the air tempera- oceans. Through chemical and biological processes, much of ture varies through it, by the gases that comprise it, or even the CO2 became locked up in carbonate sedimentary rocks, by its electrical properties. At any rate, before we examine such as limestone. With much of the water vapor already these various atmospheric layers, we need to look at the condensed and the concentration of CO2 dwindling, the at- vertical profile of two important variables: air pressure and mosphere gradually became rich in nitrogen (N2), which is air density. usually not chemically active. It appears that oxygen (O2), the second most abundant A BRIEF LOOK AT AIR PRESSURE AND AIR DENSITY Ear- gas in today’s atmosphere, probably began an extremely slow lier in this chapter we learned that most of our atmosphere is increase in concentration as energetic rays from the sun split crowded close to the earth’s surface. The reason for this fact water vapor (H2O) into hydrogen and oxygen during a pro- is that air molecules (as well as everything else) are held near cess called photodissociation. The hydrogen, being lighter, the earth by gravity. This strong invisible force pulling down probably rose and escaped into space, while the oxygen re- on the air above squeezes (compresses) air molecules closer mained in the atmosphere. together, which causes their number in a given volume to This slow increase in oxygen may have provided enough increase. The more air above a level, the greater the squeezing of this gas for primitive plants to evolve, perhaps 2 to 3 billion effect or compression. years ago. Or the plants may have evolved in an almost Gravity also has an effect on the weight of objects, in- oxygen-free (anaerobic) environment. At any rate, plant cluding air. In fact, weight is the force acting on an object due growth greatly enriched our atmosphere with oxygen. The to gravity. Weight is defined as the mass of an object times the reason for this enrichment is that, during the process of pho- acceleration of gravity; thus tosynthesis, plants, in the presence of sunlight, combine car- Weight  mass  gravity. bon dioxide and water to produce oxygen. Hence, after plants evolved, the atmospheric oxygen content increased more An object’s mass is the quantity of matter in the object. rapidly, probably reaching its present composition about Consequently, the mass of air in a rigid container is the same several hundred million years ago. everywhere in the universe. However, if you were to instantly *It is now believed that some of the earth’s water may have originated from nu- travel to the moon, where the acceleration of gravity is much merous collisions with small meteors and disintegrating comets when the earth less than that of earth, the mass of air in the container would was very young. be the same, but its weight would decrease.  The Earth and Its Atmosphere 11 When mass is given in grams (g) or kilograms (kg), vol- ume is given in cubic centimeters (cm3) or cubic meters (m3). Near sea level, air density is about 1.2 kilograms per cubic meter (nearly 1.2 ounces per cubic foot). The density of air (or any substance) is determined by the masses of atoms and molecules and the amount of space be- tween them. In other words, density tells us how much matter is in a given space (that is, volume). We can express density in a variety of ways. The molecular density of air is the number of molecules in a given volume. Most commonly, however, density is given as the mass of air in a given volume; thus mass Density =. volume Because there are appreciably more molecules within the same size volume of air near the earth’s surface than at higher levels, air density is greatest at the surface and decreases as we move up into the atmosphere. Notice in Fig. 1.9 that, be- cause air near the surface is compressed, air density normally F I G U R E 1. 9 Both air pressure and air density decrease with decreases rapidly at first, then more slowly as we move farther increasing altitude. The weight of all the air molecules above the earth’s away from the surface. surface produces an average pressure near 14.7 lbs/in.2 Air molecules are in constant motion. On a mild spring day near the surface, an air molecule will collide about 10 bil- pressure is inches of mercury (Hg), which is commonly used lion times each second with other air molecules. It will also in the field of aviation and on television and radio weather bump against objects around it — houses, trees, flowers, the broadcasts. At sea level, the standard value for atmospheric ground, and even people. Each time an air molecule bounces pressure is against a person, it gives a tiny push. This small force (push) divided by the area on which it pushes is called pressure; thus 1013.25 mb  1013.25 hPa  29.92 in. Hg. force Billions of air molecules push constantly on the human Pressure . body. This force is exerted equally in all directions. We are not area crushed by it because billions of molecules inside the body If we weigh a column of air 1 square inch in cross section, push outward just as hard. Even though we do not actually extending from the average height of the ocean surface (sea feel the constant bombardment of air, we can detect quick level) to the “top” of the atmosphere, it would weigh nearly changes in it. For example, if we climb rapidly in elevation, 14.7 pounds (see Fig. 1.9). Thus, normal atmospheric pres- our ears may “pop.” This experience happens because air col- sure near sea level is close to 14.7 pounds per square inch. If lisions outside the eardrum lessen. The popping comes about more molecules are packed into the column, it becomes more as air collisions between the inside and outside of the ear dense, the air weighs more, and the surface pressure goes up. equalize. The drop in the number of collisions informs us On the other hand, when fewer molecules are in the column, that the pressure exerted by the air molecules decreases with the air weighs less, and the surface pressure goes down. So, height above the earth. A similar type of ear-popping occurs the surface air pressure can be changed by changing the mass as we drop in elevation, and the air collisions outside the of air above the surface. eardrum increase. Pounds per square inch is, of course, just one way to ex- Air molecules not only take up space (freely darting, press air pressure. Presently, the most common unit found on twisting, spinning, and colliding with everything around surface weather maps is the millibar* (mb) although the hec- topascal (hPa) is gradually replacing the millibar as the pre- WE ATHE R WATCH ferred unit of pressure on surface charts. Another unit of The air density in the mile-high city of Denver, Colorado, is *By definition, a bar is a force of 100,000 newtons (N) acting on a surface area of normally about 15 percent less than the air density at sea level. As 1 square meter (m2). A newton is the amount of force required to move an object the air density decreases, the drag force on a baseball in flight also with a mass of 1 kilogram (kg) so that it increases its speed at a rate of 1 meter per decreases. Because of this fact, a baseball hit at Denver’s Coors second (m/sec) each second. Because the bar is a relatively large unit, and because Field will travel farther than one hit at sea level. Hence, on a surface pressure changes are usually small, the unit of pressure most commonly found on surface weather maps is the millibar, where 1 bar  1000 mb. The unit warm, calm day, a baseball hit for a 340-foot home run down the of pressure designed by the International System (SI) of measurement is the pascal left field line at Coors Field would simply be a 300-foot out if hit (Pa), where 1 pascal is the force of 1 newton acting on a surface of 1 square meter. at Camden Yards Stadium in Baltimore, Maryland. A more common unit is the hectopascal (hPa), as 1 hectopascal equals 1 millibar. 12 CH A PTER 1 earth), the air pressure would be about 300 mb. The summit is above nearly 70 percent of all the air molecules in the at- mosphere. At an altitude approaching 50 km, the air pressure is about 1 mb, which means that 99.9 percent of all the air molecules are below this level. Yet the atmosphere extends upwards for many hundreds of kilometers, gradually becom- ing thinner and thinner until it ultimately merges with outer space. (Up to now, we have concentrated on the earth’s atmo- sphere. For a brief look at the atmospheres of the other plan- ets, read the Focus section on pp. 14–15.) LAYERS OF THE ATMOSPHERE We have seen that both air pressure and density decrease with height above the earth — rapidly at first, then more slowly. Air temperature, however, has a more complicated vertical profile.* Look closely at Fig. 1.11 and notice that air temperature normally decreases from the earth’s surface up to an altitude of about 11 km, which is nearly 36,000 ft, or 7 mi. This de- crease in air temperature with increasing height is due pri- marily to the fact (investigated further in Chapter 2) that F I G U R E 1.1 0 Atmospheric pressure decreases rapidly with sunlight warms the earth’s surface, and the surface, in turn, height. Climbing to an altitude of only 5.5 km, where the pressure is warms the air above it. The rate at which the air temperature 500 mb, would put you above one-half of the atmosphere’s molecules. decreases with height is called the temperature lapse rate. The average (or standard) lapse rate in this region of the lower atmosphere is about 6.5°C for every 1000 m or about 3.6°F them), but — as we have seen — these same molecules have for every 1000 ft rise in elevation. Keep in mind that these weight. In fact, air is surprisingly heavy. The weight of all the values are only averages. On some days, the air becomes air around the earth is a staggering 5600 trillion tons, or colder more quickly as we move upward. This would increase about 5.136  1018 kg. The weight of the air molecules acts or steepen the lapse rate. On other days, the air temperature as a force upon the earth. The amount of force exerted over would decrease more slowly with height, and the lapse rate an area of surface is called atmospheric pressure or, simply, air would be less. Occasionally, the air temperature may actually pressure.* The pressure at any level in the atmosphere may increase with height, producing a condition known as a tem- be measured in terms of the total mass of air above any point. perature inversion. So the lapse rate fluctuates, varying from As we climb in elevation, fewer air molecules are above us; day to day and season to season. hence, atmospheric pressure always decreases with increasing The region of the atmosphere from the surface up to height. Like air density, air pressure decreases rapidly at first, about 11 km contains all of the weather we are familiar with then more slowly at higher levels, as illustrated in Fig. 1.9. on earth. Also, this region is kept well stirred by rising and Figure 1.10 also illustrates how rapidly air pressure de- descending air currents. Here, it is common for air molecules creases with height. Near sea level, atmospheric pressure is to circulate through a depth of more than 10 km in just a few usually close to 1000 mb. Normally, just above sea level, at- days. This region of circulating air extending upward from mospheric pressure decreases by about 10 mb for every 100 the earth’s surface to where the air stops becoming colder meters (m) increase in elevation — about 1 inch of mercury with height is called the troposphere — from the Greek tro- for every 1000 feet (ft) of rise. At higher levels, air pressure pein, meaning to turn or change. decreases much more slowly with height. With a sea-level Notice in Fig. 1.11 that just above 11 km the air tempera- pressure near 1000 mb, we can see in Fig. 1.10 that, at an al- ture normally stops decreasing with height. Here, the lapse titude of only 5.5 km (3.5 mi), the air pressure is about 500 rate is zero. This region, where, on average, the air tempera- mb, or half of the sea-level pressure. This situation means ture remains constant with height, is referred to as an isother- that, if you were at a mere 5.5 km (about 18,000 ft) above the mal (equal temperature) zone.† The bottom of this zone earth’s surface, you would be above one-half of all the mole- marks the top of the troposphere and the beginning of an- cules in the atmosphere. other layer, the stratosphere. The boundary separating the At an elevation approaching the summit of Mt. Everest (about 9 km, or 29,000 ft — the highest mountain peak on *Air temperature is the degree of hotness or coldness of the air and, as we will see in Chapter 2, it is also a measure of the average speed of the air molecules. *Because air pressure is measured with an instrument called a barometer, atmo- †In many instances, the isothermal layer is not present, and the air temperature spheric pressure is often referred to as barometric pressure. begins to increase with increasing height.  The Earth and Its Atmosphere 13 F I G U R E 1.1 1 Layers of the atmosphere as related to the average profile of air temperature above the earth’s surface. The heavy line illustrates how the average temperature varies in each layer. troposphere from the stratosphere is called the tropopause. Even though the air temperature is increasing with height, The height of the tropopause varies. It is normally found at the air at an altitude of 30 km is extremely cold, averaging less higher elevations over equatorial regions, and it decreases in than
46°C. At this level above polar latitudes, air tempera- elevation as we travel poleward. Generally, the tropopause is tures can change dramatically from one week to the next, as higher in summer and lower in winter at all latitudes. In a sudden warming can raise the temperature in one week by some regions, the tropopause “breaks” and is difficult to lo- more than 50°C. Such a rapid warming, although not well cate and, here, scientists have observed tropospheric air mix- understood, is probably due to sinking air associated with ing with stratospheric air and vice versa. These breaks also circulation changes that occur in late winter or early spring as mark the position of jet streams — high winds that meander well as with the poleward displacement of strong jet stream in a narrow channel, like an old river, often at speeds exceed- winds in the lower stratosphere. (The instrument that mea- ing 100 knots.* sures the vertical profile of air temperature in the atmosphere From Fig. 1.11 we can see that, in the stratosphere, the air temperature begins to increase with height, producing a tem- WE ATHE R WATCH perature inversion. The inversion region, along with the lower isothermal layer, tends to keep the vertical currents of the If you are flying in a jet aircraft at 30,000 feet above the earth, the troposphere from spreading into the stratosphere. The inver- air temperature outside your window would typically be about sion also tends to reduce the amount of vertical motion in the
60°F. Due to the fact that air temperature normally decreases stratosphere itself; hence, it is a stratified layer. with increasing height, the air temperature outside your window may be more than 110°F colder than the air at the surface directly *A knot is a nautical mile per hour. One knot is equal to 1.15 miles per hour (mi/ below you. hr), or 1.9 kilometers per hour (km/hr). 14 CH A PTER 1 FO CU S O N A S P E CIAL TO PI C The Atmospheres of Other Planets Earth is unique. Not only does it lie at just the right distance from the sun so that life may flourish, it also provides its inhabitants with an atmosphere rich in nitrogen and oxygen — two gases that are not abundant in the atmospheres of either Venus or Mars, our closest planetary neighbors. The Venusian atmosphere is mainly car- bon dioxide (95 percent) with minor amounts of water vapor and nitrogen. An opaque acid- cloud deck encircles the planet, hiding its sur- face. The atmosphere is quite turbulent, as in- NASA NASA struments reveal twisting eddies and fierce winds in excess of 125 mi/hr. This thick dense F I G U R E 2 A portion of Jupiter extending F I G U R E 3 The Great Dark Spot on Nep- atmosphere produces a surface air pressure of from the equator to the southern polar latitudes. tune. The white wispy clouds are similar to the about 90,000 mb, which is 90 times greater The Great Red Spot, as well as the smaller ones, high wispy cirrus clouds on earth. However, on than that on earth. To experience such a pres- are spinning eddies similar to storms that exist in Neptune, they are probably composed of methane sure on earth, one would have to descend in the earth’s atmosphere. ice crystals. the ocean to a depth of about 900 m (2950 ft). Moreover, this thick atmosphere of CO2 pro- thin cold atmosphere, there is no liquid water accompanied by winds of several hundreds of duces a strong greenhouse effect, with a on Mars and virtually no cloud cover — only a kilometers per hour. These winds carry fine dust scorching hot surface temperature of 480°C barren desertlike landscape. In addition, this around the entire planet. The dust gradually set- (900°F). thin atmosphere produces an average surface tles out, coating the landscape with a thin red- The atmosphere of Mars, like that of Ve- air pressure of about 7 mb, which is less than dish veneer. nus, is mostly carbon dioxide, with only small one-hundredth of that experienced at the sur- The atmosphere of the largest planet, Jupi- amounts of other gases. Unlike Venus, the face of the earth. Such a pressure on earth ter, is much different from that of Venus and Martian atmosphere is very thin, and heat es- would be observed above the surface at an alti- Mars. Jupiter’s atmosphere is mainly hydrogen capes from the surface rapidly. Thus, surface tude near 35 km (22 mi). (H2) and helium (He), with minor amounts of temperatures on Mars are much lower, averag- Occasionally, huge dust storms develop methane (CH4) and ammonia (NH3). A promi- ing around
60°C (
76°F). Because of its near the Martian surface. Such storms may be nent feature on Jupiter is the Great Red up to an elevation sometimes exceeding 30 km [100,000 ft] is molecules to a much greater degree. Moreover, much of the the radiosonde. More information on this instrument is solar energy responsible for the heating is absorbed in the given in the Focus section on p. 16.) upper part of the stratosphere and, therefore, does not reach The reason for the inversion in the stratosphere is that down to the level of ozone maximum. And due to the low air the gas ozone plays a major part in heating the air at this alti- density, the transfer of energy downward from the upper tude. Recall that ozone is important because it absorbs ener- stratosphere is quite slow. getic ultraviolet (UV) solar energy. Some of this absorbed Above the stratosphere is the mesosphere (middle energy warms the stratosphere, which explains why there is an sphere). The boundary near 50 km, which separates these inversion. If ozone were not present, the air probably would layers, is called the stratopause. The air at this level is ex- become colder with height, as it does in the troposphere. tremely thin and the atmospheric pressure is quite low, aver- Notice in Fig. 1.11 that the level of maximum ozone con- aging about 1 mb, which means that only one-thousandth of centration is observed near 25 km (at middle latitudes), yet all the atmosphere’s molecules are above this level and 99.9 the stratospheric air temperature reaches a maximum near percent of the atmosphere’s mass is located below it. 50 km. The reason for this phenomenon is that the air at The percentage of nitrogen and oxygen in the mesosphere 50 km is less dense than at 25 km, and so the absorption of is about the same as at sea level. Given the air’s low density in intense solar energy at 50 km raises the temperature of fewer this region, however, we would not survive very long breathing  The Earth and Its Atmosphere 15 Spot — a huge atmospheric storm about three of hot hydrogen. Energy from this lower region Spot. The white wispy clouds in the photograph times larger than earth — that spins counter- rises toward the surface; then it (along with Ju- are probably composed of methane ice crystals. clockwise in Jupiter’s southern hemisphere (see piter’s rapid rotation) stirs the cloud layer into Studying the atmospheric behavior of other Fig. 2 p. 14). Large white ovals near the Great more or less horizontal bands of various colors. planets may give us added insight into the Red Spot are similar but smaller storm systems. Swirling storms exist on other planets, workings of our own atmosphere. (Additional Unlike the earth’s weather machine, which is too, such as on Saturn and Neptune. In fact, information about size, surface temperature, driven by the sun, Jupiter’s massive swirling the large dark oval on Neptune (Fig. 3) appears and atmospheric composition of planets is clouds appear to be driven by a collapsing core to be a storm similar to Jupiter’s Great Red given in Table 1.) ▼ TA B L E 1 Data on Planets and the Sun DIAMETER AVERAGE DISTANCE FROM SUN AVERAGE SURFACE TEMPERATURE MAIN ATMOSPHERIC COMPONENTS Kilometers Millions of Kilometers °C °F Sun 1,392  103 5,800 10,500 — Mercury 4,880 58 260* 500 — Venus 12,112 108 480 900 CO2 Earth 12,742 150 15 59 N2, O2 Mars 6,800 228
60
76 CO2 Jupiter 143,000 778
110
166 H2, He Saturn 121,000 1,427
190
310 H2, He Uranus 51,800 2,869
215
355 H2, CH4 Neptune 49,000 4,498
225
373 N2, CH4 Pluto 3,100 5,900
235
391 CH4 *Sunlit side. here, as each breath would contain far fewer oxygen molecules parts of the body. Also, given the low air pressure, the blood in than it would at sea level. Consequently, without proper one’s veins would begin to boil at normal body temperatures. breathing equipment, the brain would soon become oxygen- The air temperature in the mesosphere decreases with starved — a condition known as hypoxia. Pilots who fly above height, a phenomenon due, in part, to the fact that there is 3 km (10,000 ft) for too long without oxygen-breathing appa- little ozone in the air to absorb solar radiation. Consequently, ratus may experience this. With the first symptoms of hypoxia, the molecules (especially those near the top of the meso- there is usually no pain involved, just a feeling of exhaustion. sphere) are able to lose more energy than they absorb, which Soon, visual impairment sets in and routine tasks become dif- results in an energy deficit and cooling. So we find air in the ficult to perform. Some people drift into an incoherent state, mesosphere becoming colder with height up to an elevation neither realizing nor caring what is happening to them. Of near 85 km. At this altitude, the temperature of the atmo- course, if this oxygen deficiency persists, a person will lapse sphere reaches its lowest average value,
90°C (
130°F). into unconsciousness, and death may result. In fact, in the me- The “hot layer” above the mesosphere is the thermo- sosphere, we would suffocate in a matter of minutes. sphere. The boundary that separates the lower, colder meso- There are other effects besides suffocating that could be sphere from the warmer thermosphere is the mesopause. In experienced in the mesosphere. Exposure to ultraviolet solar the thermosphere, oxygen molecules (O2) absorb energetic energy, for example, could cause severe burns on exposed solar rays, warming the air. Because there are relatively few 16 CH A PTER 1 FO CU S O N A N O B S E RVAT I O N The Radiosonde The vertical distribution of temperature, pres- vide a vertical profile of winds.* (When winds sure, and humidity up to an altitude of about are added, the observation is called a rawin- 30 km can be obtained with an instrument sonde.) When plotted on a graph, the vertical called a radiosonde.* The radiosonde is a small, distribution of temperature, humidity, and wind lightweight box equipped with weather instru- is called a sounding. Eventually, the balloon ments and a radio transmitter. It is attached to bursts and the radiosonde returns to earth, its a cord that has a parachute and a gas-filled bal- descent being slowed by its parachute. loon tied tightly at the end (see Fig. 4). As the At most sites, radiosondes are released balloon rises, the attached radiosonde measures twice a day, usually at the time that corre- air temperature with a small electrical thermom- sponds to midnight and noon in Greenwich, eter — a thermistor — located just outside the England. Releasing radiosondes is an expensive box. The radiosonde measures humidity electri- operation because many of the instruments are cally by sending an electric current across a never retrieved, and many of those that are re- carbon-coated plate. Air pressure is obtained by trieved are often in poor working condition. To a small barometer located inside the box. All of complement the radiosonde, modern satellites this information is transmitted to the surface by (using instruments that measure radiant en- radio. Here, a computer rapidly reconverts the ergy) are providing scientists with vertical tem- various frequencies into values of temperature, perature profiles in inaccessible regions. pressure, and moisture. Special tracking equip- ment at the surface may also be used to pro- © C. Donald Ahrens *A radiosonde that is dropped by parachute from an air- *A modern development in the radiosonde is the use of craft is called a dropsonde. satellite Global Positioning System (GPS) equipment. Radiosondes can be equipped with a GPS device that provides more accurate position data back to the com- F I G U R E 4 The radiosonde with parachute puter for wind computations. and balloon. atoms and molecules in the thermosphere, the absorption of lower orbit. (For this reason, the spacecraft Solar Max fell to a small amount of energetic solar energy can cause a large earth in December, 1989, as did the Russian space station, increase in air temperature. Furthermore, because the amount Mir, in March, 2001.) The amount of drag is related to the of solar energy affecting this region depends strongly on solar density of the air, and the density is related to the tempera- activity, temperatures in the thermosphere vary from day to ture. Therefore, by determining air density, scientists are able day (see Fig. 1.12). The low density of the thermosphere to construct a vertical profile of air temperature. also means that an air molecule will move an average distance At the top of the thermosphere, about 500 km (300 mi) (called mean free path) of over one kilometer before colliding above the earth’s surface, molecules can move distances of with another molecule. A similar air molecule at the earth’s 10 km before they collide with other molecules. Here, many surface will move an average distance of less than one mil- of the lighter, faster-moving molecules traveling in the right lionth of a centimeter before it collides with another mole- direction actually escape the earth’s gravitational pull. The cule. Moreover, it is in the thermosphere where charged region where atoms and molecules shoot off into space is particles from the sun interact with air molecules to produce sometimes referred to as the exosphere, which represents the dazzling aurora displays. (We will look at the aurora in more upper limit of our atmosphere. detail in Chapter 2.) Up to this point, we have examined the atmospheric lay- Because the air density in the upper thermosphere is so ers based on the vertical profile of temperature. The atmo- low, air temperatures there are not measured directly. They sphere, however, may also be divided into layers based on its can, however, be determined by observing the orbital change composition. For example, the composition of the atmo- of satellites caused by the drag of the atmosphere. Even sphere begins to slowly change in the lower part of the ther- though the air is extremely tenuous, enough air molecules mosphere. Below the thermosphere, the composition of air strike a satellite to slow it down, making it drop into a slightly remains fairly uniform (78 percent nitrogen, 21 percent oxy-  The Earth and Its Atmosphere 17 gen) by turbulent mixing. This lower, well-mixed region is known as the homosphere (see Fig. 1.12). In the thermo- sphere, collisions between atoms and molecules are infre- quent, and the air is unable to keep itself stirred. As a result, diffusion takes over as heavier atoms and molecules (such as oxygen and nitrogen) tend to settle to the bottom of the layer, while lighter gases (such as hydrogen and helium) float to the top. The region from about the base of the thermosphere to the top of the atmosphere is often called the heterosphere. THE IONOSPHERE The ionosphere is not really a layer, but rather an electrified region within the upper atmosphere where fairly large concentrations of ions and free electrons exist. Ions are atoms and molecules that have lost (or gained) one or more electrons. Atoms lose electrons and become F I G U R E 1.1 3 At night, the higher region of the ionosphere (F positively charged when they cannot absorb all of the energy region) strongly reflects AM radio waves, allowing them to be sent over transferred to them by a colliding energetic particle or the great distances. During the day, the lower D region strongly absorbs and sun’s energy. weakens AM radio waves, preventing them from being picked up by dis- The lower region of the ionosphere is usually about tant receivers. 60 km above the earth’s surface. From here (60 km), the ionosphere extends upward to the top of the atmosphere. Hence, the bulk of the ionosphere is in the thermosphere, as in the higher reaches of the ionosphere, such waves bounce illustrated in Fig. 1.12. repeatedly from the ionosphere to the earth’s surface and back The ionosphere plays a major role in AM radio communi- to the ionosphere again. In this way, standard AM radio waves cations. The lower part (called the D region) reflects standard are able to travel for many hundreds of kilometers at night. AM radio waves back to earth, but at the same time it seriously Around sunrise and sunset, AM radio stations usually weakens them through absorption. At night, though, the D make “necessary technical adjustments” to compensate for region gradually disappears and AM radio waves are able to the changing electrical characteristics of the D region. Be- penetrate higher into the ionosphere (into the E and F re- cause they can broadcast over a greater distance at night, gions — see Fig. 1.13), where the waves are reflected back to most AM stations reduce their output near sunset. This re- earth. Because there is, at night, little absorption of radio waves duction prevents two stations — both transmitting at the same frequency but hundreds of kilometers apart — from interfering with each other’s radio programs. At sunrise, as the D region intensifies, the power supplied to AM radio transmitters is normally increased. FM stations do not need to make these adjustments because FM radio waves are shorter than AM waves, and are able to penetrate through the ionosphere without being reflected. BR IEF R E V IE W We have, in the last several sections, been examining our atmo- sphere from a vertical perspective. A few of the main points are: Atmospheric pressure at any level represents the total mass of air above that level, and atmospheric pressure always decreases with increasing height above the surface. The rate at which the air temperature decreases with height is called the lapse rate. A measured increase in air temperature with height is called an inversion. The atmosphere may be divided into layers (or regions) accord- ing to its vertical profile of temperature, its gaseous composi- tion, or its electrical properties. F I G U R E 1.1 2 Layers of the atmosphere based on temperature (red line), composition (green line), and electrical properties (dark blue The warmest atmospheric layer is the thermosphere; the cold- line). (An active sun is associated with large numbers of solar est is the mesosphere. Most of the gas ozone is found in the eruptions.) stratosphere. 18 CH A PTER 1 We live at the bottom of the troposphere, which is an atmo- 2 million years or so, we would see the ice advance and retreat spheric layer where the air temperature normally decreases with several times. Of course, for this phenomenon to happen, the height. The troposphere is a region that contains all of the average temperature of North America would have to de- weather we are familiar with. crease and then rise in a cyclic manner. The ionosphere is an electrified region of the upper atmosphere Suppose we could photograph the earth once every thou- that normally extends from about 60 km to the top of the sand years for many hundreds of millions of years. In time- atmosphere. lapse film sequence, these photos would show that not only is the climate altering, but the whole earth itself is changing as well: Mountains would rise up only to be torn down by ero- We will now turn our attention to weather events that sion; isolated puffs of smoke and steam would appear as vol- take place in the lower atmosphere. As you read the remain- canoes spew hot gases and fine dust into the atmosphere; and der of this chapter, keep in mind that the content serves as a the entire surface of the earth would undergo a gradual trans- broad overview of material to come in later chapters, and that formation as some ocean basins widen and others shrink.* many of the concepts and ideas you encounter are designed In summary, the earth and its atmosphere are dynamic to familiarize you with items you might read about in a news- systems that are constantly changing. While major transfor- paper or magazine, or see on television. mations of the earth’s surface are completed only after long spans of time, the state of the atmosphere can change in a matter of minutes. Hence, a watchful eye turned skyward will Weather and Climate be able to observe many of these changes. Up to this point, we have looked at the concepts of When we talk about the weather, we are talking about the weather and climate without discussing the word meteorol- condition of the atmosphere at any particular time and place. ogy. What does this term actually mean, and where did it Weather — which is always changing — is comprised of the originate? elements of: METEOROLOGY — A BRIEF HISTORY Meteorology is the 1. air temperature — the degree of hotness or coldness of the study of the atmosphere and its phenomena. The term itself air goes back to the Greek philosopher Aristotle who, about 2. air pressure — the force of the air above an area 340 b.c., wrote a book on natural philosophy entitled Meteo- 3. humidity — a measure of the amount of water vapor in the rologica. This work represented the sum of knowledge on air weather and climate at that time, as well as material on as- 4. clouds — a visible mass of tiny water droplets and/or ice tronomy, geography, and chemistry. Some of the topics cov- crystals that are above the earth’s surface ered included clouds, rain, snow, wind, hail, thunder, and 5. precipitation — any form of water, either liquid or solid hurricanes. In those days, all substances that fell from the sky, (rain or snow), that falls from clouds and reaches the and anything seen in the air, were called meteors, hence the ground term meteorology, which actually comes from the Greek word 6. visibility — the greatest distance one can see meteoros, meaning “high in the air.” Today, we differentiate 7. wind — the horizontal movement of air between those meteors that come from extraterrestrial sources If we measure and observe these weather elements over a outside our atmosphere (meteoroids) and particles of water specified interval of time, say, for many years, we would and ice observed in the atmosphere (hydrometeors). obtain the “average weather” or the climate of a particular In Meteorologica, Aristotle attempted to explain atmo- region. Climate, therefore, represents the accumulation of spheric phenomena in a philosophical and speculative man- daily and seasonal weather events (the average range of ner. Even though many of his speculations were found to be weather) over a long period of time. The concept of climate erroneous, Aristotle’s ideas were accepted without reservation is much more than this, for it also includes the extremes of for almost two thousand years. In fact, the birth of meteorol- weather — the heat waves of summer and the cold spells of ogy as a genuine natural science did not take place until the winter — that occur in a particular region. The frequency of invention of weather instruments, such as the thermometer at these extremes is what helps us distinguish among climates the end of the sixteenth century, the barometer (for measur- that have similar averages. ing air pressure) in 1643, and the hygrometer (for measuring If we were able to watch the earth for many thousands of humidity) in the late 1700s. With observations from instru- years, even the climate would change. We would see rivers of ments available, attempts were then made to explain certain ice moving down stream-cut valleys and huge glaciers — sheets weather phenomena employing scientific experimentation of moving snow and ice — spreading their icy fingers over and the physical laws that were being developed at the time. large portions of North America. Advancing slowly from As more and better instruments were developed in the Canada, a single glacier might extend as far south as Kansas 1800s, the science of meteorology progressed. The invention and Illinois, with ice several thousands of meters thick cover- *The movement of the ocean floor and continents is explained in the widely ac- ing the region now occupied by Chicago. Over an interval of claimed theory of plate tectonics.  The Earth and Its Atmosphere 19 of the telegraph in 1843 allowed for the transmission of rou- tine weather observations. The understanding of the con- cepts of wind flow and storm movement became clearer, and in 1869 crude weather maps with isobars (lines of equal pres- sure) were drawn. Around 1920, the concepts of air masses and weather fronts were formulated in Norway. By the 1940s, daily upper-air balloon observations of temperature, humid- ity, and pressure gave a three-dimensional view of the atmo- sphere, and high-flying military aircraft discovered the exis- tence of jet streams. Meteorology took another step forward in the 1950s, when high-speed computers were developed to solve the mathematical equations that describe the behavior of the at- mosphere. At the same time, a group of scientists in Prince- ton, New Jersey, developed numerical means for predicting the weather. Today, computers plot the observations, draw NOAA the lines on the map, and forecast the state of the atmosphere at some desired time in the future. F I G U R E 1.1 4 Doppler radar image showing the heavy rain and After World War II, surplus military radars became avail- able, and many were transformed into precipitation-measur- hail of a severe thunderstorm (dark red area) over Indianapolis, Indi- ana, on April 14, 2006. ing tools. In the mid-1990s, these conventional radars were replaced by the more sophisticated Doppler radars, which have the ability to peer into a severe thunderstorm and unveil equator is 0°, whereas the latitude of the North Pole is 90°N its winds and weather, as illustrated in Fig. 1.14. and that of the South Pole is 90°S. Most of the United States In 1960, the first weather satellite, Tiros I, was launched, is located between latitude 30°N and 50°N, a region com- ushering in space-age meteorology. Subsequent satellites pro- monly referred to as the middle latitudes. vided a wide range of useful information, ranging from day and night time-lapse images of clouds and storms to images Storms of All Sizes Probably the most dramatic spectacle that depict swirling ribbons of water vapor flowing around in Fig. 1.15 is the whirling cloud masses of all shapes and the globe. Throughout the 1990s, and into the twenty-first sizes. The clouds appear white because sunlight is reflected century, even more sophisticated satellites were developed to back to space from their tops. The largest of the organized supply computers with a far greater network of data so that cloud masses are the sprawling storms. One such storm more accurate forecasts — perhaps up to two weeks or shows as an extensive band of clouds, over 2000 km long, more — will be available in the future. west of the Great Lakes. Superimposed on the satellite image With this brief history of meterology we are now ready to is the storm’s center (indicated by the large red L) and its observe weather events that occur at the earth’s surface. adjoining weather fronts in red, blue, and purple. This middle-latitude cyclonic storm system (or extratropical cy- A SATELLITE’S VIEW OF THE WEATHER A good view of clone) forms outside the tropics and, in the Northern Hemi- the weather can be seen from a weather satellite. Figure 1.15 sphere, has winds spinning counterclockwise about its center, is a satellite image showing a portion of the Pacific Ocean and which is presently over Minnesota. the North American continent. The image was obtained from A slightly smaller but more vigorous storm is located over a geostationary satellite situated about 36,000 km (22,300 mi) the Pacific Ocean near latitude 12°N and longitude 116°W. above the earth. At this elevation, the satellite travels at the This tropical storm system, with its swirling band of rotating same rate as the earth spins, which allows it to remain posi- clouds and surface winds in excess of 64 knots* (74 mi/hr), is tioned above the same spot so it can continuously monitor known as a hurricane. The diameter of the hurricane is about what is taking place beneath it. 800 km (500 mi). The tiny dot at its center is called the eye. The solid black lines running from north to south on the Near the surface, in the eye, winds are light, skies are generally satellite image are called meridians, or lines of longitude. clear, and the atmospheric pressure is lowest. Around the eye, Since the zero meridian (or prime meridian) runs through however, is an extensive region where heavy rain and high Greenwich, England, the longitude of any place on earth is surface winds are reaching peak gusts of 100 knots. simply how far east or west, in degrees, it is from the prime Smaller storms are seen as white spots over the Gulf of meridian. North America is west of Great Britain and most of Mexico. These spots represent clusters of towering cumulus the United States lies between 75°W and 125°W longitude. clouds that have grown into thunderstorms, that is, tall The solid black lines that parallel the equator are called churning clouds accompanied by lightning, thunder, strong parallels of latitude. The latitude of any place is how far north or south, in degrees, it is from the equator. The latitude of the *Recall from p. 13 that 1 knot equals 1.15 miles per hour. 20 CH A PTER 1 F I G U R E 1.1 5 This satellite image (taken in visible reflected light) shows a variety of cloud patterns and storms in the earth’s atmosphere. NOAA/National Weather Service gusty winds, and heavy rain. If you look closely at Fig. 1.15, A Look at a Weather Map We can obtain a better picture you will see similar cloud forms in many regions. There were of the middle-latitude storm system by examining a simpli- probably thousands of thunderstorms occurring throughou