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1 Earth Science Genre Comprehension Skill Text Features Science Content Nonfiction Draw Conclusions Captions Maps Diagrams Glossary Weather Patterns Scott Foresman Science 5.8 ì<(sk$m)=bdjdhi< +^-Ä-U-Ä-U ISBN by Laura Johnson

2 Vocabulary air mass anemometer barometer Weather Patterns by Laura Johnson climate convection current front rain gauge Illustrations: 3, 5, 7, 8, 9, 10, 12, 19, 20, 22 Studio Liddell Photographs: Every effort has been made to secure permission and provide appropriate credit for photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its attention in subsequent editions. Unless otherwise acknowledged, all photographs are the property of Scott Foresman, a division of Pearson Education. Photo locators denoted as follows: Top (T), Center (C), Bottom (B), Left (L), Right (R) Background (Bkgd) Opener: Bruce Dale/NGS Image Collection; 2 Chris Noble/Getty Images; 11 A. & J. Verkaik/ Corbis; 13 A. & J. Verkaik/Corbis; 14 Annie Griffiths Belt/Corbis; 15 Reuters NewMedia Inc./ Corbis; 16 (CL) Karl Shone/ DK Images, (BC) Paul Seheult/Corbis; 17 (BL) Stephen Oliver/ DK Images, (BC) Charles O Rear/Corbis, (BR) David R. Frazier/Photo Researchers, Inc.; 18 (TL) Steve Gorton/ DK Images, (CL) University Corporation for Atmospheric Research/National Science Foundation/National Center for Atmospheric Research, (BL) Getty Images; 22 Neil Fletcher/Courtesy of Oxford University Museum/ DK Images ISBN: Copyright Pearson Education, Inc. All Rights Reserved. Printed in the United States of America. This publication is protected by Copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permissions, write to: Permissions Department, Scott Foresman, 1900 East Lake Avenue, Glenview, Illinois V

3 How does air move? Layers of Air When you take a deep breath you fill your lungs with about three liters of air. Only about 1 5 of the air we breathe is oxygen. Nearly 4 5 is a gas called nitrogen. A very small part of it is water vapor, carbon dioxide, and other gases. No other planet in our solar system has air like Earth s. Earth s atmosphere has five layers. Most weather happens in the bottom layer. Look at the diagram on the next page. It shows the layers of the atmosphere. Notice how the temperatures increase and decrease in these levels. Lower levels have greater air pressure than upper levels. At higher levels the gas particles are farther apart. Altitude: about 600 km Temperature: 1700 C Altitude: about 85 km Temperature: 90 C Exosphere Thermosphere Mesosphere Altitude: about 50 km Temperature: 0 C Stratosphere Altitude: about 815 km Temperature: 55 C Troposphere 2 3

4 Convection Currents Have you ever walked across hot sand to get to the cold water at the beach? Land warms more quickly in the sunlight than water does. At night, land cools faster than water. This causes the air above the land and the air above the water to have different temperatures. Differences in air temperature cause wind, storms, and other weather conditions. Gas particles in cool air are closer together than they are in warm air. This makes cool air heavier. So when cool air meets warm air, the cool air sinks. The warm air rises. This causes a circular movement. When solids, liquids, or gases rise and sink in a looping path, it is called a convection current. The current shown in this diagram happens near oceans and large lakes during the daytime. Six huge convection currents form around Earth. Part of the reason is that tropical areas of the world get warmer than other areas. The movement of these large currents, combined with Earth s spin, creates wind patterns we can predict. For example, winds usually blow from west to east across the United States. A jet stream is a band of very fast-moving wind. It is formed by the temperature differences between the convection currents. Jet streams form high in the sky. But their movement changes temperature, winds, and precipitation close to the ground. Cool, sinking air has high air pressure. Then it is pushed over the water by the warm air that rises behind it. Warm, rising air has low air pressure. The warm air cools as it rises. It stops rising when it is the same temperature as the air around it. When the cool air drops, the moving air is felt as wind. The warm land heats the air above it. The air above the cool water is cooler than the air above the warm land. The cool air drops below the warm air. 4 5

5 What are air masses? Kinds of Air Masses Suppose your cousins returned from a trip. They showed you some seashells they had found. You could probably use that clue to figure out that they had been to a beach. Air also carries clues about where it has been. These include temperature and amount of water vapor. When air covers an area for a while, it takes on properties of that area and becomes an air mass. An air mass is a large body of air with similar properties all through it. The temperature and amount of water vapor in an air mass stay the same for a while. They do not change even when it moves on to a new location. That s how you can tell where air has come from. There are four kinds of air masses on this map. Usually, the cause of the kind of weather you have is an air mass in your area. If it is warm and sunny where you are, it will stay that way until a new air mass moves in. Air masses are moved by winds. Some are moved by winds near the ground. Others are moved by the jet stream high above the ground. Find the continental polar air mass on the map. If the jet stream blows that air mass down from Canada, the winds will probably bring cold, dry air to the middle of the United States. Storms often occur at the edge of this air mass. Continental Polar Air The land near the North and South Poles is cold and dry. This air mass is cold and dry. Maritime Polar Air Even though the water by the North and South Poles is very cold, the water evaporates into the air. This air mass is cold and moist. Continental Tropical Air A large desert can heat the air above it. This air mass is usually warm and dry. Maritime Tropical Air An air mass over a tropical ocean or a rain forest is warm and moist, like the land below it. 6 7

6 When Air Masses Meet Have you ever seen a line of dark clouds move across the sky? You may have been looking at a front. A front is where the edges of two air masses meet. Most air masses move from west to east over North America. Fronts follow this path too. Fronts are named for the kind of air that they bring into an area. For example, a cold front brings colder air into an area. A warm front brings warmer air. A third kind of front is a stationary front. It does not move very much or it moves back and forth over the same area. Look at the pictures of the cold front and the warm front. Notice that both of these fronts have rising warm air. Areas of rising air near the fronts have lower air pressure than areas in the middle of the air masses. This rising warm air often causes rain or snow. When cold air moves into an area, it forces the warm air in front of it to rise quickly. Clouds form, often bringing rain or snow. Cold front Warm front With a warm front, fast-moving warm air rises over slower-moving cold air. Clouds that form move more slowly than those in a cold front. They cause precipitation to fall for a longer time. 8 9

7 What causes severe weather? Thunderstorms Severe weather can be in the form of thunderstorms, tornadoes, hurricanes, and blizzards. Even very high or very low temperatures are forms of severe weather. Different parts of the United States have different kinds of severe weather. Thunderstorms form when strong currents of moist air rise quickly. As the air rises, it cools. The moisture in the air condenses into water droplets and ice crystals. They group together to form clouds. Then, rain begins to fall. The rain pulls some air down with it. The storm now has air currents moving both up and down. Finally, all the air currents are moving down. The clouds get smaller as water droplets and ice crystals fall as rain. First stage: All air currents move upward. Second stage: Air currents are mixed Lightning is a huge electrical spark. It moves between areas of positive and negative charges. Lightning can heat air to 30,000 C in less than a second. This heat makes the air vibrate. We hear these vibrations as thunder. Final stage: All air currents move downward

8 Tornadoes A Tornado is another kind of severe weather. Many things happen as a tornado forms. Layers of wind in a storm blow at different speeds or in different directions. Then a stack of air starts to spin between these layers. It looks like a log rolling on its side. Next, upward winds lift one end of the spinning stack. Downward winds push the opposite end down. The spinning stack of air is now standing straight up and is called a funnel cloud. It is called a tornado if it touches the ground. Tornadoes usually last only a few minutes. But they can cause damage in a path many kilometers long and hundreds of meters wide. Their winds blow at hundreds of kilometers per hour. These winds can toss cars and buildings around easily. If you see or hear tornado warnings, go to a safe place right away. It is best to go to a basement. If you cannot get to a basement, go to a closet. Try to stay in rooms without windows. Why do you think it is important to stay away from windows? Air rolls between two layers of wind. 12 Upward and downward winds turn the rolling air. When it stands up straight it becomes a funnel cloud. 13

9 Hurricanes Hurricanes form over warm ocean water. Water vapor rises from the ocean and condenses as it cools. This process releases energy. Under the right conditions, this energy builds and causes hurricane winds. These winds are not as fast as the winds in a tornado. But they can often cause more damage. One reason is that hurricanes last for days, not minutes. Another reason is that hurricanes are much larger than tornadoes. Hurricanes can be hundreds of kilometers wide. Hurricanes also cause dangerous waves. These waves can damage the shoreline and flood areas near the coast. A hurricane s heavy rain can cause inland flooding too. Preparing for Hurricanes To prepare for a hurricane, board up your windows. Store food and water that will last for several days. Have flashlights and battery-powered radios close by. Put valuables in water-tight containers high off the ground. The best way to stay safe is to stay inside during the hurricane. But if you are told to leave the area, go right away. Hurricane winds rotate around a calm area in the center of the storm. The calm area is called the eye

10 How are weather forecasts made? Collecting Data To describe the weather at any one place and time, you need to describe all its parts. These include temperature, moisture, clouds, precipitation, air pressure, wind speed, and wind direction. Many different tools are used to measure all the parts of the weather. A barometer measures air pressure. Some barometers have a tube of mercury. Changes in air pressure cause the mercury to rise and fall in the tube. The kind of barometer shown here is a sealed container connected to a dial. When air pressure squeezes the container, it moves the dial. An anemometer measures wind speed. Wind makes the cups of the anemometer spin. The cups spin faster as winds move faster. A hygrometer measures moisture in the air. Some hygrometers have a pointer attached to a piece of horsehair. The hair gets shorter in drier air. This moves the pointer. A rain gauge measures rainfall. The top of the rain gauge is often wider than the bottom. This lets the tool catch more rain when only a small amount falls. It also makes it easier to see small amounts of water in the gauge. Radar can measure the winds and precipitation inside a storm. Radar stations send out energy like a radio station sends out radio signals. The energy bounces off objects it hits. Scientists can learn about the direction and speed of raindrops from the radar energy that bounces off them. Horsehair hygrometer Doppler radar tower Anemometer Barometer Rain gauge 16 17

11 The first weather satellite was launched in The first kind of weather balloon carried instruments in Weather Forecasts You have probably noticed some weather patterns. For example, you might have noticed that temperatures usually rise during the day, then fall in the evening. They also rise and fall with the seasons of the year. Weather forecasters study patterns of weather change. They look at how air, land, and the water cycle work together to make weather systems. Then they use what they learn to predict the weather. Forecasters also study weather patterns of the past. They compare these patterns to weather conditions now. They try to see how the weather will change. Weather maps give information about the weather now and predicted weather. Look at the weather map on the next page. The map key tells what each symbol means. On the weather map, fronts are shown by curved lines. The triangles and half-circles on the lines point in the direction that the fronts are moving. Notice that they show movement from west to east, which is most common in the United States. Warm fronts bring warmer weather. Cold fronts bring cooler weather. Fronts are always found in places of low pressure. They often bring clouds to an area. Clear skies are usually found in areas of high pressure, away from fronts. Area of high air pressure Area of low air pressure Warm front The telegraph was invented in It was soon used to send weather reports. Cold front Stationary front Snow Rain Thunderstorms 19

12 What is climate? Weather and Climate Weather and climate are different. Weather is the temperature, moisture, clouds, precipitation, air pressure, and wind in one area at one time. Climate is the average of all these conditions over a period of about thirty years. Climates usually change much more slowly than weather. Landforms Affect Climate Tops and bottoms of mountains often have different climates. Because of their height, the air at the top is usually cooler than the air at the base. Opposite sides of mountains can also have different climates. The Cascade Mountains are in the western United States. The west side of these mountains has a wet climate. By the time the air crosses the mountains, most of the moisture has fallen out of it. So, the climate on the east side is very dry. Oceans Affect Climate Air over oceans does not change temperature as quickly as air over land. In the winter, air near the ocean is often warmer than the air a few miles inland. This is because the ocean absorbs heat all summer, then stays warmer longer than the land. In the summer, air near the ocean is cooler than the air inland because the water takes longer to heat up after winter. Warm or cool ocean currents can affect climates. The Gulf Stream and the North Atlantic Drift are warm currents. They heat the air above them. This air flows northward and makes Europe s climate warmer. A cold current flows from Alaska to California. It brings cold air southward. This makes the climate along the California coast a bit cooler. The air is drier after the rain or snow has fallen. The air that sinks down the side of the mountain no longer has enough moisture in it to make clouds or rain. Moist winds blow in from the ocean. Clouds form as this moist air cools over the mountains. Water in the clouds falls as rain or snow

13 Past Climates About 400 years ago, the climates of North America and Europe were colder and wetter than they are now. This time is called the Little Ice Age. The Earth holds clues about climate changes. For example, huge glaciers moved across the continents during major ice ages. Their movement formed hills and lakes. By making maps of these hills and lakes, researchers can follow the paths that the glaciers took. Fossils also give scientists clues about climate changes. Suppose a scientist finds a fossil of a fern in a desert. Ferns grow only in wet places, so the fossil is a clue that the area once had a wetter climate. Fossils of animals can offer the same kinds of clues. Fossils of ferns are found in places that were probably warm and wet in the past. The white areas on the map show places that were covered with glaciers during the last Major Ice Age. How Climates Change What events might have cooled a climate in the past? Volcanic eruptions and asteroids hitting Earth may have quickly cooled climates in the past. These events might have put dust and other materials into the upper atmosphere. There, they blocked the sunlight or reflected it back into space. This would have caused the climate to cool. Carbon dioxide, methane, and water vapor can make climates warmer. Humans produce these gases by burning coal and gasoline. These gases are also produced naturally through decaying matter, forest fires, volcanoes, and the water cycle. Many different events help form and change a climate. Scientists continue to question and learn about this process. Clouds of dust caused by volcanic eruptions may have blocked sunlight and cooled the climate. This crater was formed when a meteor crashed to Earth

14 Glossary air mass anemometer barometer climate convection current front rain gauge a large body of air with similar properties all through it a tool used to measure wind speed a tool used to measure air pressure the average of weather conditions over a long time when solids, gases, or liquids rise and sink in a looping path the place where two air masses meet a tool used to measure how much rain has fallen What did you learn? 1. What causes convection currents to form? 2. What is the difference between weather and climate? 3. How does an anemometer measure wind speed? 4. Several events need to happen in a certain order for a tornado to form. On your own paper, write to explain the steps that leads to the formation of a tornado. Include details from the book to support your answer. 5. Draw Conclusions Suppose it was warm and sunny when you arrived at school in the morning. When you leave school to go home, it is cool and rainy. What conclusion could you draw about the change in the weather? 24