E arth s atmosphere is unique. No other planet in our solar system

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1 Section Atmosphere Characteristics 1 FOCUS Section Objectives 17.1 Compare and contrast weather and climate Explain why seasonal changes occur. Reading Focus Build Vocabulary Word Part Analysis Ask students what the word sphere means. Tell them it s a three-dimensional object in which all points are equidistant from a fixed point, like a ball or globe. Tell them that atmos- comes from the Latin word meaning vapor. The word atmosphere thus refers to the spherical layer of gases, or vapors, around Earth. Have students try to figure out what the words on the vocabulary list that end in sphere mean, based on their prefixes. If they cannot figure a word out, have them look it up. (tropo- means change ; strato- means layer ; mesomeans middle ; thermo- means heat ) Reading Strategy Venn Diagram a. Northern Hemisphere: June 21 or 22; Southern Hemisphere: December 21 or 22. For the summer solstice in the Northern Hemisphere, the sun s rays are directly over the Tropic of Cancer; for the summer solstice in the Southern Hemisphere, the sun s rays are directly over the Tropic of Capricorn. b. Both summer and winter solstices represent extremes in the solar energy received by Earth. Both result from Earth s 23 1/2 degree axis tilt toward the North Star. c. Northern Hemisphere: December 21 or 22; Southern Hemisphere: June 21 or 22. For the winter solstice in the Northern Hemisphere, the sun s rays are directly over the Tropic of Capricorn; for the winter solstice in the Southern Hemisphere, the sun s rays are directly over the Tropic of Cancer. Key Concepts How does weather differ from climate? Why do seasonal changes occur? Figure 1 Buffalo, New York, was under a state of emergency in late December 01 after receiving almost 2 meters of snow. 476 Chapter 17 Vocabulary ozone troposphere stratosphere mesosphere thermosphere summer solstice winter solstice autumnal equinox spring equinox Reading Strategy Comparing and Contrasting Copy the Venn diagram below. As you read, complete the diagram by comparing and contrasting summer and winter solstices. Summer a.? b.? c.? E arth s atmosphere is unique. No other planet in our solar system has an atmosphere with the exact mixture of gases or the moisture conditions and heat needed to sustain life as we know it. The gases that make up Earth s atmosphere and the controls to which they are subject are vital to our existence. In this chapter, you will begin to examine the ocean of air in which we live. The state of the atmosphere at a given time and place is known as weather. The combination of Earth s motions and energy from the sun produce a variety of weather. As shown in Figure 1, weather strongly influences our everyday activities. Weather is constantly changing, and it refers to the state of the atmosphere at any given time and place. Climate, however, is based on observations of weather that have been collected over many years. Climate helps describe a place or region. Climate often is defined simply as average weather, but this is not a complete description. For example, farmers need to know not only the average rainfall during a growing season, but they also need to know the frequency of extremely wet and extremely dry years. The most important measurable properties of weather and climate are air temperature, humidity, type and amount of precipitation, air pressure, and the speed and direction of the wind. How does weather differ from climate? Winter 476 Chapter 17

2 Composition of the Atmosphere The composition of the atmosphere has changed dramatically over Earth s nearly 4.6 billion year history. The atmosphere is thought to have started as gases that were emitted during volcanic eruptions. Evidence indicates that oxygen did not start to accumulate in the atmosphere until about 2.5 billion years ago. The atmosphere continues to exchange material with the oceans and life on Earth s surface. Major Components Sometimes the term air is used as if it were a specific gas, which it is not. Air is a mixture of different gases and particles, each with its own physical properties. The composition of air varies from time to time and from place to place. However, if the water vapor, dust, and other variable components were removed from the atmosphere, its makeup would be very stable worldwide up to an altitude of about 80 kilometers. Look at Figure 2. Two gases nitrogen and oxygen make up 99 percent of the volume of clean, dry air. Although these gases are the most common components of air, they don t affect the weather much. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus tiny quantities of a number of other gases. Carbon dioxide is present in only small amounts (approximately percent), but it is an important component of air. Carbon dioxide is an active absorber of energy given off by Earth. Therefore, it plays a significant role in heating the atmosphere. Variable Components Important materials that vary in the air from time to time and place to place include water vapor, dust particles, and ozone. These components also can have significant effects on weather and climate. The amount of water vapor varies from almost none to about 4 percent by volume. Why is such a small quantity so significant? Water vapor is the source of all clouds and precipitation. Like carbon dioxide, water vapor absorbs heat given off by Earth. It also absorbs some solar energy. Movements of the atmosphere allow a large quantity of solid and liquid particles to be suspended within it. Although visible dust sometimes clouds the sky, these relatively large particles are too heavy to stay in the air for very long. Still, many particles are microscopic and remain suspended for longer periods of time. These particles include sea salts from breaking waves, fine soil blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, and ash and dust from volcanic eruptions. Argon Carbon dioxide All others Nitrogen Customize for English Language Learners Oxygen Figure 2 Volume of Clean, Dry Air Nitrogen and oxygen dominate the volume of gases composing dry air. The Atmosphere: Structure and Temperature INSTRUCT Composition of the Atmosphere Students often think that when liquid water turns into water vapor, it breaks apart into hydrogen and oxygen. Students also sometimes have trouble distinguishing between water vapor and liquid water in the atmosphere. Explain that water vapor is a gas made up of individual water molecules that are too small to see. Different amounts of water vapor in the air make the air feel dry or humid. Liquid water in the atmosphere is usually in the form of tiny droplets. Ask students to give examples of this. (clouds, fog, mist, rain, drizzle) Build Science Skills Analyzing Data Give students a brief historical background of the Farmer s Almanac, a publication that predicts long-term weather forecasts in North America by using a complex formula that considers sunspots, moon phases, and other astronomical and atmospheric conditions. Farmers have used it since early in the nineteenth century as a primary source for weather forecasts. Instruct students to visit to obtain local weather forecasts as predicted by the Farmer s Almanac. Then review newspapers or local media records for your area. Distribute your findings to students and discuss with them how accurate the almanac s forecasts are. If Web resources are not available, use printed almanacs of previous years. You could build a bar graph plotting yearly weather predictions by the almanac, and compare it to a bar graph of actual weather conditions for the same year as recorded by local weather stations. Adapt your content presentation to the less proficient ELL students. Use visual examples as much as possible to explain important concepts. For example, when discussing the components of air, emphasize the pie chart in Figure 2. When discussing how pressure varies with altitude, emphasize the graph in Figure 4. When discussing the structure of the atmosphere, refer to Figure 6. When discussing the seasons, use Figures 7 and 8. Answer to... Weather changes constantly, but climate is based on patterns of weather that have been observed over years. Climate helps describe a place or region. The Atmosphere: Structure and Temperature 477

3 Section 17.1 (continued) Students sometimes think that global warming is caused by the ozone hole. Dispel this misconception by explaining that although both phenomena occur in the atmosphere, they are otherwise not related. The depletion of ozone may lead to increases in UV radiation, which may be harmful to living things. Global warming is an increase in the temperature of the atmosphere that may be partly caused by increases in carbon dioxide. Use Visuals Figure 3 Use this diagram to discuss primary pollutants and where they come from. Ask: What is the main primary pollutant? (carbon monoxide) What is the main source of primary pollutants? (transportation) What does the term transportation refer to? (cars, trucks, trains, ships, and airplanes) What do you think the term stationary source fuel combustion refers to? (power plants, furnaces in homes, and businesses) Volatile Organics Nitrogen 13.6% Oxides 14.8% Carbon Monoxide 49.1% What They Are Primary Pollutants Particulates 6.0% Sulfur Oxides 16.4% Figure 3 Primary Pollutants These circle graphs show major primary pollutants and their sources. Percentages are calculated by weight. Source: U.S. Environmental Protection Agency. Solid Waste Disposal 2.5% Industrial Processes Stationary 15.0% Source Fuel Combustion 27.3% Transportation 46.2% Where They Come From Miscellaneous 9.0% Another important variable component of the atmosphere is ozone. Ozone is a form of oxygen that combines three oxygen atoms into each molecule (O 3 ). Ozone is not the same as the oxygen we breathe, which has two atoms per molecule (O 2 ). There is very little ozone in the atmosphere, and it is not distributed evenly. It is concentrated in a layer located between 10 and 50 kilometers above Earth s surface. In this altitude range, oxygen molecules (O 2 ) are split into single atoms of oxygen (O) when they absorb ultraviolet (UV) radiation emitted by the sun. Ozone is then produced when a single atom of oxygen (O) and a molecule of oxygen (O 2 ) collide. This collision must happen in the presence of a third, neutral molecule that acts as a catalyst. A catalyst allows a reaction to take place without being consumed in the process. Ozone is concentrated 10 to 50 kilometers above Earth because the UV radiation from the sun is sufficient to produce single atoms of oxygen. In addition, there are enough gas molecules to bring about the required collisions. The ozone layer is crucial to life on Earth. Ozone absorbs potentially harmful UV radiation from the Sun. If ozone did not filter most UV radiation and all of the sun s UV rays reached the surface of Earth, our planet would be uninhabitable for many living organisms. Human Influence Air pollutants are airborne particles and gases that occur in concentrations large enough to endanger the health of organisms. Primary pollutants, shown in Figure 3, are emitted directly from identifiable sources. Emissions from transportation vehicles account for nearly half the primary pollutants by weight. Secondary pollutants are not emitted directly into air. They form in the atmosphere when reactions take place among primary pollutants and other substances. For example, after the primary pollutant sulfur dioxide enters the atmosphere, it combines with oxygen to produce sulfur trioxide. Then the sulfur trioxide combines with water to create sulfuric acid, an irritating and corrosive substance. Reactions triggered by strong sunlight are called photochemical reactions. For instance, when nitrogen oxides absorb solar radiation, a chain of complex reactions begins. If certain volatile organic compounds are present, secondary products form that are reactive, irritating, and toxic. This noxious mixture of gases and particles is called photochemical smog. What are secondary pollutants? 478 Chapter 17 Facts and Figures Significant improvements in air quality have occurred since the U.S. Environmental Protection Agency initiated air-pollution control programs. Between 1970 and 00, national total emissions of primary pollutants declined by about one third. This occurred during a period when the U.S. population increased by about one third, and vehicle miles traveled rose nearly 130 percent. Despite continued improvements in air quality, standards have not yet been met in large numbers of places. In 00, more than 100 million people lived in countries with unhealthy air. 478 Chapter 17

4 Height and Structure of the Atmosphere Where does the atmosphere end and outer space begin? There is no sharp boundary. The atmosphere thins as you travel away from Earth until there are too few gas molecules to detect. Pressure Changes To understand the vertical extent of the atmosphere, examine Figure 4, which shows changes in atmospheric pressure with height. Atmospheric pressure is simply the weight of the air above. At sea level, the average pressure is slightly more than 1000 millibars, or slightly more than 1 kilogram per square centimeter. One half of the atmosphere lies below an altitude of 5.6 kilometers. Above 100 kilometers, only percent of all the gases making up the atmosphere exist. Temperature Changes The pictures of snowcapped mountains rising above snow-free valleys shown in Figure 5 might remind you that Earth s atmosphere becomes colder as you climb higher. But not all layers of the atmosphere show this temperature pattern. Altitude (km) % of air lies below this altitude Mt. Everest Pressure (mb) The Atmosphere: Structure and Temperature Altitude (miles) Figure 4 Atmospheric Pressure vs. Altitude This graph shows how atmospheric pressure varies with altitude. Comparing How do changes in air pressure at low altitudes compare with air pressure changes at high altitudes? Figure 5 In Jasper National Park in Alberta, Canada, snowy mountaintops contrast with warmer, snow-free lowlands below. Height and Structure of the Atmosphere Build Science Skills Using Models To help students understand how and why air pressure decreases as you go up through the atmosphere, have them work in groups to build models of the atmosphere. Have each group make a stack of at least 6 identical heavy books. Have them place a sheet of scrap paper between each layer, with part of the paper sticking out the same amount. Have students then try to pull out each sheet of paper from between the layers. Ask: Which sheet was the hardest to pull out? (the one on the bottom) Why was it so hard to pull out? (It had the most layers of books piled on it, and thus the most weight.) Which sheet was the easiest to pull out? (the one on the top) Why was it so easy to pull out? (It had only one book piled on it, and thus the least weight.) Integrate Physics Units of Air Pressure Students may be confused by the many different units used to measure air pressure. Newspaper and television weather reports often use the non-metric unit inches of mercury, which is related to the height of a column of mercury in a mercury barometer. The average air pressure at sea level is about inches of mercury. The metric (SI) unit for air pressure is the pascal (Pa), the equivalent of one newton per square meter (N/m 2 ), which is force per unit area. Meteorologists use the unit millibar (mb), which equals 100 Pa. The average air pressure at sea level is about 101,325 Pa. Ask: How many millibars would 101,325 Pa be? ( mb) Answer to... Figure 4 Air pressure decreases rapidly near Earth s surface and more gradually at greater heights. Secondary pollutants are pollutants that form when reactions take place among primary pollutants and other substances. The Atmosphere: Structure and Temperature 479

5 Section 17.1 (continued) Build Reading Literacy Refer to p. 186D in Chapter 7, which provides the guidelines for relating text and visuals. Relate Text and Visuals Have students read the text on p. 480 that describes the features of the layers of the atmosphere. Have students use Figure 6 to describe how temperature changes as you go up through the atmosphere. (It decreases with altitude in the troposphere; it remains constant in the lower part of the stratosphere and then gradually increases until 50 km above Earth; it decreases with height in the mesosphere; and it increases again in the thermosphere.) Visual Use Visuals L1 L1 Figure 6 Use this diagram to discuss layers of the atmosphere. Ask: What is the lowest level of the atmosphere, and what happens there? (troposphere; essentially all important weather phenomenon occur) What is the secondlowest level of the atmosphere, and what important gas is found there? (stratosphere; ozone) What are the third and upper levels of the atmosphere? (mesosphere; thermosphere) Figure 6 This diagram illustrates the thermal structure of the atmosphere. Interpret How do air temperatures change with height in the mesosphere? For: Links on the layers of the atmosphere Visit: Web Code: cjn-6171 Height (km) Aurora Temperature THERMOSPHERE Meteor Mesopause MESOSPHERE Stratopause STRATOSPHERE Maximum ozone Tropopause Mt. Everest TROPOSPHERE C F Temperature The atmosphere can be divided vertically into four layers based on temperature. Figure 6 illustrates these layers. The bottom layer, where temperature decreases with an increase in altitude, is the troposphere. It is in this layer that essentially all important weather phenomena occur. The thickness of the troposphere is not the same everywhere. It varies with latitude and the season. On average, the temperature drop continues to a height of about 12 kilometers, where the outer boundary of the troposphere, called the tropopause, is located. Beyond the tropopause is the stratosphere. In the stratosphere, the temperature remains constant to a height of about kilometers. It then begins a gradual increase in temperature that continues until the stratopause, at a height of nearly 50 kilometers above Earth s surface. Temperatures increase in the stratosphere because the atmosphere s ozone is concentrated here. Recall that ozone absorbs ultraviolet radiation from the sun. As a result, the stratosphere is heated. In the third layer, the mesosphere, temperatures again decrease with height until the mesopause. The mesopause is more than 80 kilometers above the surface and the temperatures approach 90 C. The fourth layer extends outward from the mesopause and has no welldefined upper limit. It is the thermosphere, a layer that contains only a tiny fraction of the atmosphere s mass. Temperatures increase in the thermosphere because oxygen and nitrogen absorb short-wave, highenergy solar radiation Height (miles) 480 Chapter 17 Facts and Figures Download a worksheet on layers of the atmosphere for students to complete, and find additional teacher support from NSTA SciLinks. The rate of temperature decrease in the troposphere is called the environmental lapse rate. Its average value is 6.5 C/km. This figure is known as the normal lapse rate. The environmental lapse rate is not a constant but can be highly variable. To determine the actual environmental lapse rate, as well as gather information about vertical changes in pressure, wind, and humidity, meteorologists use radiosondes. A radiosonde is an instrument package that is attached to a balloon, and it transmits data by radio as it ascends through the atmosphere. 480 Chapter 17

6 Earth-Sun Relationships Nearly all of the energy that drives Earth s variable weather and climate comes from the sun. Earth absorbs only a tiny percentage of the energy given off by the sun less than one two-billionth. This may seem insignificant, but the amount is several hundred thousand times the electrical-generating capacity of the United States. Solar energy is not distributed evenly over Earth s surface. The amount of energy received varies with latitude, time of day, and season of the year. As you will see, the variations in solar heating are caused by the motions of Earth relative to the sun and by variations in Earth s land and ocean surface. It is the unequal heating of Earth that creates winds and drives the ocean s currents. These movements transport heat from the tropics toward the poles in an attempt to balance energy differences. The results of these processes are the phenomena we call weather. Earth s Motions Earth has two principal motions rotation and revolution. Rotation is the spinning of Earth about its axis. The axis is an imaginary line running through the north and south poles. Our planet rotates once every 24 hours, producing the daily cycle of daylight and darkness. Revolution is the movement of Earth in its orbit around the sun. Earth travels at nearly 113,000 kilometers per hour in an elliptical orbit about the sun. Earth s Orientation We know that it is colder in the winter than in the summer. But why? Length of day and a gradual change in the angle of the noon sun above the horizon affect the amount of energy Earth receives. Seasonal changes occur because Earth s position relative to the sun continually changes as it travels along its orbit. Earth s axis is not perpendicular to the plane of its orbit around the sun. Instead it is tilted 23.5 degrees from the perpendicular, as shown in Figure 7. Because the axis remains pointed toward the North Star as Earth moves around the sun, the position of Earth s axis to the sun s rays is constantly changing. If the axis were not tilted, we would not have seasonal changes. The orientation of Earth relative to the sun and the constant movement of Earth cause the angle of the noon sun to vary by up to 47 degrees ( 23.5 degrees to 23.5 degrees) for many locations during the year. For example, a mid-latitude city like New York, located about 40 degrees north latitude, has a maximum noon sun angle of 73.5 degrees when the sun s vertical rays reach their farthest northward location in June. Six months later, New York has a minimum noon sun angle of 26.5 degrees. Sun's rays Atmosphere S 231/ 2 N / / 2 Figure 7 Tilt of Earth s Axis Earth s axis always points toward the North Star as it revolves around the sun. 30 Earth-Sun Relationships Angles and Seasons Purpose Students observe how differences in the angle of the sun s rays affect heating of Earth s surface. Materials narrow-beam flashlight, globe on stand Procedure Dim the room lights. Position the globe similarly to how Earth is shown in Figure 7. First, shine the light around 23 degrees S so it makes a tight circle. Explain that this represents summer in the Southern Hemisphere. Then move the flashlight and beam up to about 23 degrees N (keeping the flashlight horizontal). Ask students to describe how the area covered by the light changed, and what season this represents in the Northern Hemisphere. Expected Outcome The light spreads out to cover a greater area, which represents winter in the Northern Hemisphere. Make sure students understand that the light from the flashlight represents a fixed amount of energy. When it spreads out, it may look as if more energy is hitting the surface of the globe. Actually, the same amount of energy is being spread out over a larger area. In which direction does Earth s axis point? The Atmosphere: Structure and Temperature 481 Answer to... Figure 6 Air temperatures decrease with height in the mesosphere. Earth s axis points toward the North Star. The Atmosphere: Structure and Temperature 481

7 Section 17.1 (continued) Students may think that summer begins before June 21, and winter begins before December 21. Explain that the seasons are officially defined by astronomers using the dates of the solstices and equinoxes as the first day of each season. Meteorologists sometimes prefer to refer to summer as June, July, and August, and winter as December, January, and February. These definitions match the actual weather conditions associated with the seasons more closely. ASSESS 3 Evaluate Understanding Ask students to explain what causes the seasons. Suggest that they can draw diagrams to help them understand the seasons. Reteach Use Figure 6 to review the layers of the atmosphere and their characteristics. L1 Arctic Circle Tropic of Cancer Equator Tropic of Capricorn June Sun vertical at Latitude 23 1 /2 N Orbit Sun Equinox September Sun vertical at equator Figure 8 s and equinoxes are important events in Earth s yearly weather cycle. Equinox March Sun vertical at equator Section 17.1 Assessment 23 1 /2 December Sun vertical at Latitude 23 1 /2 S s and Equinoxes On June 21 or 22 each year the axis is such that the Northern Hemisphere is leaning 23.5 degrees toward the sun. This date, shown on the left side of Figure 8, is known as the summer solstice, or the first official day of summer. Six months later, in December, when Earth has moved to the opposite side of its orbit, the Northern Hemisphere leans 23.5 degrees away from the sun. December 21 or 22 is the winter solstice, the first day of winter. On days between these extremes, Earth s axis is leaning at amounts less than 23.5 degrees to the rays of the sun. The equinoxes occur midway between the solstices. September 22 or 23 is the date of the autumnal equinox in the Northern Hemisphere. March 21 or 22 is the date of the spring equinox for the Northern Hemisphere. On these dates, the vertical rays of the sun strike the equator (0 degrees latitude) because Earth is in a position in its orbit so that the axis is tilted neither toward nor away from the sun. Length of Daylight The length of daylight compared to darkness also is determined by Earth s position in orbit. All latitudes receive 12 hours of daylight during the vernal and autumnal equinoxes (equal night). The length of daylight on the summer solstice in the Northern Hemisphere is greater than the length of darkness. The farther you are north of the equator on the summer solstice, the longer the period of daylight. When you reach the Arctic Circle, at 66.5 degrees N latitude, the length of daylight is 24 hours. The combination of Earth s tilt and its revolution around the sun cause the solstices and equinoxes to be opposite in the Northern Hemisphere and Southern Hemisphere. In the Northern Hemisphere, the maximum energy from the sun is received on the summer solstice, June 21 or 22; in the Southern Hemisphere, the maximum energy is received on the winter solstice, December 21 or 22. Reviewing Concepts 1. Compare and contrast weather and climate. 2. Why do seasonal changes occur? 3. How much of Earth s atmosphere is located below about 5.6 kilometers? 4. How do ozone molecules form in the stratosphere? 5. In which layers of the atmosphere does temperature increase with increasing height? 482 Chapter 17 Critical Thinking 6. Applying Concepts Explain what would happen to air temperatures in the troposphere if carbon dioxide were removed from air. Connecting Concepts Using Figure 8, explain why solstices and equinoxes are opposite for the Northern and Southern hemispheres. Section 17.1 Assessment 1. Climate is a description of aggregate weather conditions for an area, and weather is the state of the atmosphere at a given moment in time. 2. Seasonal changes occur because Earth s axis is tilted. Earth s orientation to the sun continually changes as it travels along its orbit. 3. About 50 percent of Earth s atmosphere is below about 5.6 km. 4. Ozone forms when a single atom of oxygen (O) collides with a diatomic molecule of oxygen (O 2 ). 5. Temperature increases with height in the stratosphere and thermosphere. 6. Because carbon dioxide absorbs heat from Earth efficiently, air temperatures would decrease if it were removed from the troposphere. 482 Chapter 17