1 Characteristics of the Atmosphere

Transcription

1 CHAPTER 22 1 Characteristics of the Atmosphere SECTION The Atmosphere KEY IDEAS As you read this section, keep these questions in mind: What are the layers of Earth s atmosphere? How has Earth s atmosphere changed over time? What Are the Parts of Earth s Atmosphere? On a clear day, the sky may seem to go on forever. However, Earth s atmosphere is actually a relatively thin layer of gas. Earth s atmosphere provides the air that living things need. It keeps temperatures from getting too high during the day and too low at night. It filters dangerous radiation from the sun. In fact, Earth s atmosphere makes life as we know it possible. The atmosphere contains all of the oxygen we breathe. However, oxygen is not the only gas in the Earth s atmosphere consists of many different gases. The two main gases in the atmosphere are nitrogen (about 78%) and oxygen (about 21%). The other gases are in very small amounts and are called trace gases. The atmosphere has several layers. Each layer has different properties. The graphs below show how thick each layer is, and how pressure and temperature vary between the layers. READING TOOLBOX Compare After you read this section, make a table comparing the different layers of Earth s Describe the location and major features of each layer. 1. Identify Which two gases make up most of Earth s atmosphere? Altitude (km) Thermosphere Mesosphere 40 Stratosphere 20 Troposphere Temperature (ºC) Temperature changes with altitude, or height above the surface, in the In some layers, temperature increases with altitude. In other layers, temperature decreases with altitude. Altitude (km) (not to scale) Thermosphere Mesosphere Stratosphere 20 Troposphere Pressure (Pa) Pressure decreases with altitude, or height above the surface, in the 2. Describe Which layer of the atmosphere is closest to Earth s surface? 3. Compare In which layer of the atmosphere are temperatures the lowest? Interactive Reader 501 The Atmosphere

2 4. Explain Why is the troposphere the densest of all the layers of the atmosphere? 5. Apply Concepts What do you think is the reason that cool air cannot rise above warm air in a temperature inversion? (Hint: Which type of air is denser?) THE TROPOSPHERE We live in the troposphere, the layer of the atmosphere that is closest to Earth s surface. Clouds, wind, rain, and snow happen mainly in the troposphere. The gas molecules in the troposphere are squeezed close together by the weight of the upper As a result, the troposphere is the densest of all the layers. Pressure is also greatest in the troposphere. The troposphere gets colder as you increase in altitude, or height above the ground. This is because light energy from the sun passes through the The ground absorbs this light energy. Then, the ground gives off some of the energy as heat. The troposphere absorbs the heat and becomes warmer. At higher altitude, air is less dense and farther away from the warmth of Earth s surface. Therefore, it is colder. The temperature in the troposphere decreases by about 6 ºC for every kilometer of altitude. The top of the troposphere is called the tropopause. The temperature at the tropopause is about 55 ºC ( 67 ºF). The low temperature at the tropopause keeps water vapor and clouds in the troposphere. The altitude of the tropopause is different at different places on Earth. At the Poles, its altitude is about 8 km. At the equator, it can be as high as 18 km. Most of the time, the troposphere is warmest close to Earth s surface. However, sometimes cool air gets trapped beneath warmer air. This condition is called a temperature inversion. When a temperature inversion occurs, the cooler air cannot rise. This cool air can trap pollution near the ground, as shown below. A temperature inversion can trap pollution near Earth s surface. Interactive Reader 502 The Atmosphere

3 THE STRATOSPHERE The stratosphere is the layer above the tropopause. From the bottom of the stratosphere up to about 25 km (about 16 mi), the temperature stays close to 55 ºC ( 67 ºF). However, at 25 km, the temperature begins to increase as altitude increases. The temperature reaches about 0 ºC (32 ºF) at the top of the stratosphere. The increase in temperature occurs in the part of the stratosphere known as the ozone layer. The ozone layer is warmer because it contains a form of oxygen called ozone that absorbs solar radiation. The diagram below shows molecules of ozone and oxygen. The molecules of oxygen we breathe each have two oxygen atoms. Molecules of ozone each have three oxygen atoms. The extra oxygen atom allows ozone to absorb ultraviolet radiation from the sun. Ultraviolet radiation can damage cells and tissues in living things. By absorbing ultraviolet radiation, the ozone layer protects living things on Earth s surface from ultraviolet radiation damage. 6. Describe What happens to temperature in the stratosphere as altitude increases? 7. Compare Give one similarity and one difference between an oxygen molecule and an ozone molecule. A molecule of oxygen gas contains two oxygen atoms. A molecule of ozone contains three oxygen atoms. THE MESOSPHERE The mesosphere is the layer of the atmosphere that is above the stratosphere. The mesosphere is 50 km to 80 km (30 mi to 50 mi) above Earth s surface. Within the mesosphere, temperature decreases with altitude. Near the top of the mesosphere, temperatures fall to below 80 ºC ( 112 ºF). Interactive Reader 503 The Atmosphere

4 8. Explain Why is the temperature in the thermosphere so high? THE THERMOSPHERE Temperatures rise again in the next layer of the This layer is called the thermosphere. The thermosphere extends to an altitude of about 480 km (300 mi). The gas molecules in the thermosphere are very far apart. However, they are moving very quickly because they absorb a great deal of energy from solar radiation. Because the gas molecules are moving so quickly, the temperature in the thermosphere is very high about 980 ºC (1,800 ºF). The outermost part of the atmosphere above the thermosphere is known as the exosphere. Inthe exosphere, some gases escape from Earth s gravity and move into space. In addition, some gases in space are captured by Earth s gravity and added to Earth s THE IONOSPHERE The ionosphere is an area in the upper mesosphere and lower thermosphere in which ions form. Charged particles in the ionosphere reflect radio waves back to Earth s surface. Without the ionosphere, most radio signals would escape into space, making it difficult to receive radio broadcasts on the ground. Ionosphere 9. Describe How does the ionosphere affect radio transmissions? Ionosphere Day Night Radio signals reflect off the ionosphere, so they can be received far from their source. At night, there are fewer ions in the atmosphere to scatter the radio waves. As a result, radio transmissions can travel farther at night than during the day. Colorful light displays called auroras also take place in the ionosphere. Auroras form around the North and South Poles. They form when high-energy ions from the sun collide with ions in the ionosphere. These collisions produce light. Interactive Reader 504 The Atmosphere

5 How Has Earth s Atmosphere Changed? We know a lot about Earth s atmosphere today. However, the atmosphere has not always been the way it is today. When Earth first formed 4.56 billion years ago, it did not have an Then, beginning about 4.4 billion years ago, volcanic eruptions around the globe released large amounts of gases. This process, called outgassing, formed an atmosphere around Earth. This early atmosphere didn t have any oxygen. It would have been poisonous to most modern organisms if they had existed at that time. 10. Describe How did Earth s early atmosphere form? Earth s Early Atmosphere CO H 2 NH 2 N 3 H CO CH 4 Ar 11. Identify Name four gases in Earth s early The first life-forms on Earth were single-celled organisms that did not require oxygen to survive. About 2.5 billion years ago, some cells evolved the ability to carry out photosynthesis. During photosynthesis, energy from the sun is used to change carbon dioxide and water into food and oxygen. Therefore, during photosynthesis, cells take in carbon dioxide and release oxygen. Organisms that used photosynthesis were very successful. Therefore, they multiplied quickly. Over time, the oxygen released by these organisms built up in the By about 350 million years ago, the concentration of oxygen in the atmosphere was similar to what it is today. Earth s Atmosphere Today Ar N 2 H 2 0 O O 2 2 N 2 N 2 CO 2 CH Compare Name one gas that is common in Earth s atmosphere today but was not present in Earth s early Interactive Reader 505 The Atmosphere

6 13. Describe What are aerobic organisms? AEROBIC ORGANISMS As the concentration of oxygen in Earth s atmosphere increased, new types of organisms evolved. These organisms could use oxygen to obtain energy from food. These organisms, called aerobic organisms, were very successful. In fact, most types of organisms today are aerobic organisms. The flowchart below shows how aerobic organisms and photosynthetic organisms keep oxygen concentrations in our atmosphere stable. Photosynthetic organisms, such as plants, take in carbon dioxide and water. They use energy from sunlight to convert carbon dioxide and water into food and oxygen. They give off oxygen. The carbon dioxide enters the atmosphere, where it can be taken up by photosynthetic organisms. The oxygen enters the atmosphere, where it can be taken up by aerobic organisms. 14. Identify Name a type of organism that is both photosynthetic and aerobic. Aerobic organisms, including plants and animals, take in oxygen. They use the oxygen to release energy from food. One of the products of this process is carbon dioxide. Aerobic organisms give off carbon dioxide. How Have Humans Changed the Atmosphere? Since the Industrial Revolution in the 1800s, humans have caused changes in the Many of these changes have been harmful to the atmosphere and to life on Earth. 15. Explain Why have most countries banned the use of CFCs? CHEMICALS THAT CAN HARM THE OZONE LAYER Recall that the stratosphere contains a layer of ozone molecules that absorb harmful ultraviolet radiation from the sun. In the 1980s, scientists found that the ozone layer was becoming thin in places. Other scientists found that chemicals called chlorofluorocarbons, or CFCs, destroy ozone molecules. CFCs were commonly used in refrigerators and spray cans. Now, most countries have banned their use. Hopefully, the ozone layer will slowly restore itself now that fewer CFCs are being released. Interactive Reader 506 The Atmosphere

7 THE GREENHOUSE EFFECT AND GLOBAL WARMING Have you ever opened a car door on a sunny day? You may have noticed that the air inside the car was much warmer than the air outside. This happens because some of the sunlight that enters the car becomes trapped inside as heat energy. In a similar way, some atmospheric gases keep Earth much warmer than it would be without an The process by which gases trap heat in Earth s atmosphere is called the greenhouse effect. First, energy released by the sun warms Earth s surface. Then, some of this heat is radiated back toward space. Some gases, such as carbon dioxide and water, absorb some of this heat energy. As a result, the atmosphere becomes warmer. These gases are called greenhouse gases. Without the greenhouse effect, Earth would be much colder. However, if too much energy is absorbed by the atmosphere, global temperatures will rise. This global warming could cause problems such as rising ocean levels and droughts. The graph below shows how average temperature has been rising during the past decades. Discuss Learn more about the greenhouse effect and greenhouse gases. Which gases are greenhouse gases? Which are the most important greenhouse gases? Share the information you learn with a partner or a small group. The average yearly temperature on Earth has risen in the last 100 years. 16. Describe What was the approximate variation from normal temperature in the year 2000? Give your answer to the nearest tenth of a degree. Since the late 1800s, people have been burning coal, oil, and natural gas for energy. When these fossil fuels are burned, carbon dioxide enters the Carbon dioxide is a greenhouse gas. Many scientists now think that this increase in carbon dioxide is the major reason that temperatures have risen so much in the last 100 years. Interactive Reader 507 The Atmosphere

8 Section 1 Review SECTION VOCABULARY greenhouse effect the warming of the surface and lower atmosphere of Earth that occurs when carbon dioxide, water vapor, and other gases in the atmosphere absorb and reradiate infrared radiation mesosphere the coldest layer of the atmosphere, between the stratosphere and the thermosphere, in which temperature decreases as altitude increases stratosphere the layer of the atmosphere that lies between the troposphere and the mesosphere and in which temperature increases as altitude increases; contains the ozone layer thermosphere the uppermost layer of the atmosphere, in which temperature increases as altitude increases; includes the ionosphere troposphere the lowest layer of the atmosphere, in which temperature drops at a constant rate as altitude increases; the part of the atmosphere where weather conditions exist 1. Explain Why does temperature increase with altitude in the stratosphere? 2. Apply Concepts There is very little water vapor in the stratosphere, mesosphere, and thermosphere. What do you think is the reason for this? (Hint: What happens to water vapor at the tropopause?) 3. Identify What caused the concentration of oxygen in Earth s atmosphere to increase starting about 2.5 billion years ago? 4. Describe How does pressure change as altitude increases in the atmosphere? 5. Explain How is ozone in the upper stratosphere helpful to living things? 6. Describe How does the greenhouse effect work? Interactive Reader 508 The Atmosphere