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L_0001 | the nature of science | T_0007 | The scientist must next form a conclusion. The scientist must study all of the data. What statement best explains the data? Did the experiment prove the hypothesis? Sometimes an experiment shows that a hypothesis is correct. Other times the data disproves the hypothesis. Sometimes its not possible to tell. If there is no conclusion, the scientist may test the hypothesis again. This time he will use some different experiments. No matter what the experiment shows the scientist has learned something. Even a disproved hypothesis can lead to new questions. The farmer grows crops on the two fields for a season. She finds that 2.2 times as much soil was lost on the plowed field as compared to the unplowed field. She concludes that her hypothesis was correct. The farmer also notices some other differences in the two plots. The plants in the no-till plots are taller. The soil moisture seems higher. She decides to repeat the experiment. This time she will measure soil moisture, plant growth, and the total amount of water the plants consume. From now on she will use no-till methods of farming. She will also research other factors that may reduce soil erosion. | text | null |
L_0001 | the nature of science | T_0008 | When scientists have the data and conclusions, they write a paper. They publish their paper in a scientific journal. A journal is a magazine for the scientists who are interested in a certain field. Before the paper is printed, other scientists look at it to try to find mistakes. They see if the conclusions follow from the data. This is called peer review. If the paper is sound it is printed in the journal. Other papers are published on the same topic in the journal. The evidence for or against a hypothesis is discussed by many scientists. Sometimes a hypothesis is repeatedly shown to be true and never shown to be false. The hypothesis then becomes a theory. Sometimes people say they have a theory when what they have is a hypothesis. In science, a theory has been repeatedly shown to be true. A theory is supported by many observations. However, a theory may be disproved if conflicting data is discovered. Many important theories have been shown to be true by many observations and experiments and are extremely unlikely to be disproved. These include the theory of plate tectonics and the theory of evolution. | text | null |
L_0001 | the nature of science | T_0009 | Scientists use models to help them understand and explain ideas. Models explain objects or systems in a more simple way. Models often only show only a part of a system. The real situation is more complicated. Models help scientists to make predictions about complex systems. Some models are something that you can see or touch. Other types of models use an idea or numbers. Each type is useful in certain ways. Scientists create models with computers. Computers can handle enormous amounts of data. This can more accu- rately represent the real situation. For example, Earths climate depends on an enormous number of factors. Climate models can predict how climate will change as certain gases are added to the atmosphere. To test how good a model is, scientists might start a test run at a time in the past. If the model can predict the present it is probably a good model. It is more likely to be accurate when predicting the future. | text | null |
L_0001 | the nature of science | T_0010 | A physical model is a representation of something using objects. It can be three-dimensional, like a globe. It can also be a two-dimensional drawing or diagram. Models are usually smaller and simpler than the real object. They most likely leave out some parts, but contain the important parts. In a good model the parts are made or drawn to scale. Physical models allow us to see, feel and move their parts. This allows us to better understand the real system. An example of a physical model is a drawing of the layers of Earth (Figure 1.5). A drawing helps us to understand the structure of the planet. Yet there are many differences between a drawing and the real thing. The size of a model is much smaller, for example. A drawing also doesnt give good idea of how substances move. Arrows showing the direction the material moves can help. A physical model is very useful but it cant explain the real Earth perfectly. | text | null |
L_0001 | the nature of science | T_0011 | Some models are based on an idea that helps scientists explain something. A good idea explains all the known facts. An example is how Earth got its Moon. A Mars-sized planet hit Earth and rocky material broke off of both bodies (Figure 1.6). This material orbited Earth and then came together to form the Moon. This is a model of something that happened billions of years ago. It brings together many facts known from our studies of the Moons surface. It accounts for the chemical makeup of rocks from the Moon, Earth, and meteorites. The physical properties of Earth and Moon figure in as well. Not all known data fits this model, but much does. There is also more information that we simply dont yet know. | text | null |
L_0001 | the nature of science | T_0012 | Models may use formulas or equations to describe something. Sometimes math may be the only way to describe it. For example, equations help scientists to explain what happened in the early days of the universe. The universe formed so long ago that math is the only way to describe it. A climate model includes lots of numbers, including temperature readings, ice density, snowfall levels, and humidity. These numbers are put into equations to make a model. The results are used to predict future climate. For example, if there are more clouds, does global temperature go up or down? Models are not perfect because they are simple versions of the real situation. Even so, these models are very useful to scientists. These days, models of complex things are made on computers. | text | null |
L_0001 | the nature of science | T_0013 | Accidents happen from time to time in everyday life. Since science involves an adventure into the unknown, it is natural that accidents can happen. Therefore, we must be careful and use proper equipment to prevent accidents (Figure 1.7). We must also be sure to treat any injury or accident appropriately. | text | null |
L_0001 | the nature of science | T_0014 | If you work in the science lab, you may come across dangerous materials or situations. Sharp objects, chemicals, heat, and electricity are all used at times in science laboratories. With proper protection and precautions, almost all accidents can be prevented (Figure 1.8). If an accident happens, it can be dealt with appropriately. Below is a list of safety guidelines to follow when doing labs: Follow directions at all times. A science lab is not a play area. Be sure to obey all safety guidelines given in lab instructions or by the lab supervisor. Be sure to use the correct amount of each material. Tie back long hair. Wear closed shoes with flat heels. Shirts should have no hanging sleeves, hoods, or drawstrings. Use gloves, goggles, or safety aprons as instructed. Be very careful when you use sharp or pointed objects, such as knives. Clean up broken glass quickly with a dust pan and broom. Never touch broken glass with your bare hands. Never eat or drink in the science lab. Table tops and counters could have dangerous substances on them. Keep your work area neat and clean. A messy work area can lead to spills and breakage. Completely clean materials like test tubes and beakers. Leftover substances could interact with other sub- stances in future experiments. If you are using flames or heat plates, be careful when you reach. Be sure your arms and hair are kept far away from heat sources. Use electrical appliances and burners as instructed. Know how to use an eye wash station, fire blanket, fire extinguisher, and first aid kit. Alert the lab supervisor if anything unusual occurs. Fill out an accident report if someone is hurt. The lab supervisor must know if any materials are damaged or discarded. | text | null |
L_0001 | the nature of science | T_0015 | Many Earth science investigations are conducted in the field (Figure 1.9). Field work needs some additional precautions: Be sure to wear appropriate clothing. Hiking requires boots, long pants, and protection from the Sun, for example. Bring sufficient supplies like food and water, even for a short trip. Dehydration can occur rapidly. Take along first aid supplies. Let others know where you are going, what you will be doing, and when you will be returning. Take a map with you if you dont know the area and leave a copy of the map with someone at home. Try to have access to emergency services and some way to communicate. Beware that cell phones may not have coverage in all locations. Be sure that you are accompanied by a person familiar with the area or is familiar with field work. | text | null |
L_0005 | erosion and deposition by wind | T_0045 | Dust storms like the one in Figure 10.20 are more common in dry climates. The soil is dried out and dusty. Plants may be few and far between. Dry, bare soil is more easily blown away by the wind than wetter soil or soil held in place by plant roots. | text | null |
L_0005 | erosion and deposition by wind | T_0046 | Like flowing water, wind picks up and transports particles. Wind carries particles of different sizes in the same ways that water carries them. You can see this in Figure 10.21. Tiny particles, such as clay and silt, move by suspension. They hang in the air, sometimes for days. They may be carried great distances and rise high above the ground. Larger particles, such as sand, move by saltation. The wind blows them in short hops. They stay close to the ground. Particles larger than sand move by traction. The wind rolls or pushes them over the surface. They stay on the ground. | text | null |
L_0005 | erosion and deposition by wind | T_0047 | Did you ever see workers sandblasting a building to clean it? Sand is blown onto the surface to scour away dirt and debris. Wind-blown sand has the same effect. It scours and polishes rocks and other surfaces. Wind-blown sand may carve rocks into interesting shapes. You can see an example in Figure 10.22. This form of erosion is called abrasion. It occurs any time rough sediments are blown or dragged over surfaces. Can you think of other ways abrasion might occur? | text | null |
L_0005 | erosion and deposition by wind | T_0048 | Like water, when wind slows down it drops the sediment its carrying. This often happens when the wind has to move over or around an obstacle. A rock or tree may cause wind to slow down. As the wind slows, it deposits the largest particles first. Different types of deposits form depending on the size of the particles deposited. | text | null |
L_0005 | erosion and deposition by wind | T_0049 | When the wind deposits sand, it forms small hills of sand. These hills are called sand dunes. For sand dunes to form, there must be plenty of sand and wind. Sand dunes are found mainly in deserts and on beaches. You can see examples of sand dunes in Figure 10.23. | text | null |
L_0005 | erosion and deposition by wind | T_0049 | When the wind deposits sand, it forms small hills of sand. These hills are called sand dunes. For sand dunes to form, there must be plenty of sand and wind. Sand dunes are found mainly in deserts and on beaches. You can see examples of sand dunes in Figure 10.23. | text | null |
L_0005 | erosion and deposition by wind | T_0050 | What causes a sand dune to form? It starts with an obstacle, such as a rock. The obstacle causes the wind to slow down. The wind then drops some of its sand. As more sand is deposited, the dune gets bigger. The dune becomes the obstacle that slows the wind and causes it to drop its sand. The hill takes on the typical shape of a sand dune, shown in Figure 10.24. | text | null |
L_0005 | erosion and deposition by wind | T_0051 | Once a sand dune forms, it may slowly migrate over the land. The wind moves grains of sand up the gently sloping side of the dune. This is done by saltation. When the sand grains reach the top of the dune, they slip down the steeper side. The grains are pulled by gravity. The constant movement of sand up and over the dune causes the dune to move along the ground. It always moves in the same direction that the wind usually blows. Can you explain why? | text | null |
L_0005 | erosion and deposition by wind | T_0052 | When the wind drops fine particles of silt and clay, it forms deposits called loess. Loess deposits form vertical cliffs. Loess can become a thick, rich soil. Thats why loess deposits are used for farming in many parts of the world. You can see an example of loess in Figure 10.25. | text | null |
L_0005 | erosion and deposition by wind | T_0053 | Its very important to control wind erosion of soil. Good soil is a precious resource that takes a long time to form. Covering soil with plants is one way to reduce wind erosion. Plants and their roots help hold the soil in place. They also help the soil retain water so it is less likely to blow away. Planting rows of trees around fields is another way to reduce wind erosion. The trees slow down the wind, so it doesnt cause as much erosion. Fences like the one in Figure 10.26 serve the same purpose. The fence in the figure is preventing erosion and migration of sand dunes on a beach. | text | null |
L_0013 | history of earths life forms | T_0113 | There are over 1 million species of plants and animals living on Earth today. Scientists think that there are millions more that have not yet been discovered. | text | null |
L_0013 | history of earths life forms | T_0114 | Each organism has the ability to survive in a specific environment. Dry desert environments are difficult to live in. Desert plants have special stems and leaves to conserve water. Animals have other ways to live in the desert. The Namib Desert receives only 1.5 inches of rainfall each year. The Namib Desert beetle lives there. How do the beetles get enough water to survive? Early morning fog deposits water droplets. The droplets collect on a beetles wings and back. The beetle tilts its rear end up. When the droplet is heavy enough, it slides forward. It lands in the beetles mouth. There are many other environments that need unique approaches for survival (Figure 12.10). | text | null |
L_0013 | history of earths life forms | T_0115 | Organisms must be able to get food and avoid being food. Hummingbirds have long, thin beaks that help them drink nectar from flowers. Some flowers are tubular to fit hummingbird beaks. The battle between needing food and being food plays out in the drama between lions and zebras. When a herd of zebras senses a lion, the animals run away. The zebras dark stripes confuse the lions. It becomes hard for them to focus on just one zebra. The zebras may get away. But lions are swift and agile. A lion may be able to get a zebra, maybe one thats old or sick. | text | null |
L_0013 | history of earths life forms | T_0116 | Every organism is different from every other organism. Every organisms genes are different, too. | text | null |
L_0013 | history of earths life forms | T_0117 | There are variations in the traits of a population. For example, there are lots of variations in the color of human hair. Hair can be blonde, brown, black, or even red. Hair color is a trait determined by genes. | text | null |
L_0013 | history of earths life forms | T_0118 | At some point, the variation probably came from a mutation. A mutation is a random change in an organisms genes. Mutations are natural. Some are harmful, but many are neutral. If the trait from the mutation is beneficial, that organism may have a better chance to survive. An organism that survives is likely to have offspring. If it does, it may pass the mutation on to its offspring. The offspring may be more likely to survive. | text | null |
L_0013 | history of earths life forms | T_0119 | Some of the characteristics an organism has may help it survive. These characteristics are called adaptations. Some adaptations are better than others. Adaptations develop this way. Think about a population of oak trees. Imagine that a fungus has arrived from Asia to North America. Most of the North American are killed by the fungus. But a few oak trees have a mutation that allows them to survive the fungus. Those oak trees are better adapted to the new environment than the others. Those trees have a better chance of surviving. They will probably reproduce. The trees may pass on the favorable mutation to their offspring. The other trees will die. Eventually, the population of oak trees will change. Most of the trees will have the trait to survive the fungus. This is an adaptation. Over time, traits that help an organism survive become more common. Traits that hinder survival eventually disappear. | text | null |
L_0013 | history of earths life forms | T_0120 | Adaptations in a species add up. If the environment is stable, the species wont change. But if the environment is changing, the species will need to adapt. Many adaptations may be necessary. In time, the species may change a lot. The descendants will be very different from their ancestors. They may even become a new species. This process is called evolution. Evolution happens as a species changes over time. Organisms alive today evolved from earlier life forms. We can learn about this from fossils. For example, horse fossils from 60 million years ago are very different from modern horses. Ancient horses were much smaller than they are today (Figure 12.12). The horses teeth and hooves have also changed. The horses evolved because of changes in their environment. | text | null |
L_0013 | history of earths life forms | T_0121 | Most of the organisms that once lived on Earth are now extinct. Earths environment has changed many times. Many organisms could not adapt to the changes. They died out. The organisms that did survive passed traits on to their offspring. The changes added up, eventually producing the species we see today. We study fossils to see the organisms that lived at certain times. We can see how those organisms changed with time. We can see how they evolved. | text | null |
L_0013 | history of earths life forms | T_0122 | The Phanerozoic Eon is divided into three eras the Paleozoic, the Mesozoic, and the Cenozoic (Table 12.1). They span from about 540 million years ago to the present. We live now in the Cenozoic Era. Earths climate changed numerous times during the Phanerozoic Eon. Just before the beginning of the Phanerozoic Eon, much of the Earth was covered with glaciers. As the Phanerozoic Eon began, the climate became a warm and humid tropical climate. During the Phanerozoic, Earths climate has gone through at least 4 major cycles between times of cold glaciers and times of warm tropical seas. Some organisms survived environmental changes in the climate; others became extinct when the climate changed beyond their capacity to cope with it. | text | null |
L_0013 | history of earths life forms | T_0123 | The warm, humid climate of the early Cambrian allowed life to expand and diversify. This brought the Cambrian Explosion. Life exploded both in diversity and in quantity! By the beginning of the Paleozoic, organisms had developed shells. Shells could hold their soft tissues together. They could protect the organisms from predators and from drying out. Some organisms evolved external skeletons, called exoskeletons. Organisms with hard parts also make good fossils. Fossils from the Cambrian are much more abundant than fossils from the Precambrian. There was much more diversity, so complex ecosystems could develop (Figure 12.14). All of this was in the seas. | text | null |
L_0013 | history of earths life forms | T_0124 | Paleozoic life was most diverse in the oceans. Paleozoic seas were full of worms, snails, clams, trilobites, sponges, and brachiopods. Organisms with shells were common. The first fish were simple, armored, jawless fish. Fish have internal skeletons. Some, like sharks, skates, and rays, have skeletons of cartilage. More advanced fish have skeletons of bones. Fish evolved jaws and many other adaptations for ocean life. Figure 12.13 shows some of the diversity of Earths oceans. | text | null |
L_0013 | history of earths life forms | T_0125 | An organism that lives in water is supported by the water. It does not need strong support structures. It also does not need to be protected against drying out. This is not true of land. Moving from the seas to land required many adaptations. Algae had covered moist land areas for millions of years. By about 450 million years ago, plants began to appear on land. Once there were land plants, animals had a source of food and shelter. To move to land, animals needed strong skeletons. They needed protection from drying out. They needed to be able to breathe air. Eventually they had skeletons, lungs and the other the adaptations they needed moved onto the land. One group of fish evolved into amphibians. Insects and spiders were already land dwellers by the time amphibians appeared. | text | null |
L_0013 | history of earths life forms | T_0125 | An organism that lives in water is supported by the water. It does not need strong support structures. It also does not need to be protected against drying out. This is not true of land. Moving from the seas to land required many adaptations. Algae had covered moist land areas for millions of years. By about 450 million years ago, plants began to appear on land. Once there were land plants, animals had a source of food and shelter. To move to land, animals needed strong skeletons. They needed protection from drying out. They needed to be able to breathe air. Eventually they had skeletons, lungs and the other the adaptations they needed moved onto the land. One group of fish evolved into amphibians. Insects and spiders were already land dwellers by the time amphibians appeared. | text | null |
L_0013 | history of earths life forms | T_0126 | The Mesozoic Era is the age of reptiles. Mostly we think of it as the age of dinosaurs. Earth was populated by an enormous diversity of reptiles. Some were small and some were tremendously large. Some were peaceful plant eaters. Some were extremely frightening meat eaters. Some dinosaurs developed protection, such as horns, spikes, tail clubs, and shielding plates. These adaptations were defense against active predators. Most dinosaurs lived on land. Still, pterosaurs flew the skies. Plesiosaurs and ichthyosaurs swam in the oceans (Figure 12.15). Feathered dinosaurs gave rise to birds. | text | null |
L_0013 | history of earths life forms | T_0127 | The Cenozoic Era is the age of mammals. The Cenozoic began with the extinction of every land creature larger than a dog. The most famous victims were the dinosaurs. Mammals have the ability to regulate body temperature. This is an advantage, as Earths climate went through sudden and dramatic changes. Mastodons, saber tooth tigers, hoofed mammals, whales, primates and eventually humans all lived during the Cenozoic Era (Figure 12.16). Table 12.1 shows some of the life forms that developed during the Phanerozoic Eon. Life gradually became more diverse and new species appeared. Most modern organisms evolved from species that are now extinct. Era Cenozoic Millions of Years Ago 0.2 (200,000 years ago) 35 Mesozoic 130 150 200 Major Forms of Life First humans First grasses; grasslands begin to dominate the land First plants with flowers First birds on Earth First mammals on Earth Paleozoic 300 360 400 475 First reptiles on Earth First amphibians on Earth First insects on Earth First plants and fungi begin growing on land First fish on Earth 500 | text | null |
L_0013 | history of earths life forms | T_0128 | The eras of the Phanerozoic Eon are separated by mass extinctions. During these events, large numbers of organisms became extinct very rapidly. There have been several extinctions in the Phanerozoic but two stand out more than the others. | text | null |
L_0013 | history of earths life forms | T_0129 | Between the Paleozoic Era and the Mesozoic Era was the largest mass extinction known. At the end of the Permian, nearly 95% of all marine species died off. In addition, 70% of land species became extinct. No one knows the cause of this extinction. Some scientists blame an asteroid impact. Other scientists think it was a gigantic volcanic eruption. | text | null |
L_0013 | history of earths life forms | T_0130 | The most famous mass extinction was 65 million years ago. Between the Mesozoic Era and the Cenozoic Era, about 50% of all animal species died off. This mass extinction is when the dinosaurs became extinct. Most scientists think that the extinction was caused by a giant meteorite that struck Earth. The impact heated the atmosphere until it became as hot as a kitchen oven. Animals roasted. Dust flew into the atmosphere and blocked sunlight for a year or more. This caused a deep freeze and ended photosynthesis. Sulfur from the impact mixed with water in the atmosphere. The result was acid rain. The rain dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced, animals starved. | text | null |
L_0024 | air movement | T_0240 | Air movement takes place in the troposphere. This is the lowest layer of the atmosphere. Air moves because of differences in heating. These differences create convection currents and winds. Figure 15.19 shows how this happens. Air in the troposphere is warmer near the ground. The warm air rises because it is light. The light, rising air creates an area of low air pressure at the surface. The rising air cools as it reaches the top of the troposphere. The air gets denser, so it sinks to the surface. The sinking, heavy air creates an area of high air pressure near the ground. Air always flows from an area of higher pressure to an area of lower pressure. Air flowing over Earths surface is called wind. The greater the difference in pressure, the stronger the wind blows. | text | null |
L_0024 | air movement | T_0241 | Local winds are winds that blow over a limited area. They are influenced by local geography. Nearness to an ocean, lake or mountain range can affect local winds. Some examples are found below. | text | null |
L_0024 | air movement | T_0242 | Ocean water is slower to warm up and cool down than land. So the sea surface is cooler than the land in the daytime. It is also cooler than the land in the summer. The opposite is also true. The water stays warmer than the land during the night and the winter. These differences in heating cause local winds known as land and sea breezes. Land and sea breezes are illustrated in Figure 15.20. A sea breeze blows from sea to land during the day or in summer. Thats when air over the land is warmer than air over the water. The warm air rises. Cool air from over the water flows in to take its place. A land breeze blows from land to sea during the night or in winter. Thats when air over the water is warmer than air over the land. The warm air rises. Cool air from the land flows out to take its place. | text | null |
L_0024 | air movement | T_0243 | Monsoons are like land and sea breezes, but on a larger scale. They occur because of seasonal changes in the temperature of land and water. In the winter, they blow from land to water. In the summer, they blow from water to land. In regions that experience monsoons, the seawater offshore is extremely warm. The hot air absorbs a lot of the moisture and carries it over the land. Summer monsoons bring heavy rains on land. Monsoons occur in several places around the globe. The most important monsoon in the world is in southern Asia, as shown in Figure 15.21. These monsoons are important because they carry water to the many people who live there. | text | null |
L_0024 | air movement | T_0244 | Global winds are winds that occur in belts that go all around the planet. You can see them in Figure 15.22. Like local winds, global winds are caused by unequal heating of the atmosphere. | text | null |
L_0024 | air movement | T_0245 | Earth is hottest at the equator and gets cooler toward the poles. The differences in heating create huge convection currents in the troposphere. At the equator, for example, warm air rises up to the tropopause. It cant rise any higher, so it flows north or south. By the time the moving air reaches 30 N or S latitude, it has cooled. The cool air sinks to the surface. Then it flows over the surface back to the equator. Other global winds occur in much the same way. There are three enormous convection cells north of the equator and three south of the equator. | text | null |
L_0024 | air movement | T_0246 | Earth is spinning as air moves over its surface. This causes the Coriolis effect. Winds blow on a diagonal over the surface, instead of due north or south. From which direction do the northern trade winds blow? Without Coriolis Effect the global winds would blow north to south or south to north. But Coriolis makes them blow northeast to southwest or the reverse in the Northern Hemisphere. The winds blow northwest to southeast or the reverse in the southern hemisphere. The wind belts have names. The Trade Winds are nearest the equator. The next belt is the westerlies. Finally are the polar easterlies. The names are the same in both hemispheres. | text | null |
L_0024 | air movement | T_0247 | Jet streams are fast-moving air currents high in the troposphere. They are also the result of unequal heating of the atmosphere. Jet streams circle the planet, mainly from west to east. The strongest jet streams are the polar jets. The northern polar jet is shown in Figure 15.23. | text | null |
L_0026 | changing weather | T_0262 | An air mass is a large body of air that has about the same conditions throughout. For example, an air mass might have cold dry air. Another air mass might have warm moist air. The conditions in an air mass depend on where the air mass formed. | text | null |
L_0026 | changing weather | T_0263 | Most air masses form over polar or tropical regions. They may form over continents or oceans. Air masses are moist if they form over oceans. They are dry if they form over continents. Air masses that form over oceans are called maritime air masses. Those that form over continents are called continental air masses. Figure 16.6 shows air masses that form over or near North America. An air mass takes on the conditions of the area where it forms. For example, a continental polar air mass has cold dry air. A maritime polar air mass has cold moist air. Which air masses have warm moist air? Where do they form? | text | null |
L_0026 | changing weather | T_0264 | When a new air mass goes over a region it brings its characteristics to the region. This may change the areas temperature and humidity. Moving air masses cause the weather to change when they contact different conditions. For example, a warm air mass moving over cold ground may cause an inversion. Why do air masses move? Winds and jet streams push them along. Cold air masses tend to move toward the equator. Warm air masses tend to move toward the poles. Coriolis effect causes them to move on a diagonal. Many air masses move toward the northeast over the U.S. This is the same direction that global winds blow. | text | null |
L_0026 | changing weather | T_0265 | When cold air masses move south from the poles, they run into warm air masses moving north from the tropics. The boundary between two air masses is called a front. Air masses usually dont mix at a front. The differences in temperature and pressure cause clouds and precipitation. Types of fronts include cold, warm, occluded, and stationary fronts. | text | null |
L_0026 | changing weather | T_0266 | A cold front occurs when a cold air mass runs into a warm air mass. This is shown in Figure 16.7. The cold air mass moves faster than the warm air mass and lifts the warm air mass out of its way. As the warm air rises, its water vapor condenses. Clouds form, and precipitation falls. If the warm air is very humid, precipitation can be heavy. Temperature and pressure differences between the two air masses cause winds. Winds may be very strong along a cold front. As the fast-moving cold air mass keeps advancing, so does the cold front. Cold fronts often bring sudden changes in the weather. There may be a thin line of storms right at the front that moves as it moves. In the spring and summer, these storms may be thunderstorms and tornadoes. In the late fall and winter, snow storms may occur. After a cold front passes, the cold air mass behind it brings cooler temperatures. The air is likely to be less humid as well. Can you explain why? | text | null |
L_0026 | changing weather | T_0267 | When a warm air mass runs into a cold air mass it creates a warm front. This is shown in Figure 16.8. The warm air mass is moving faster than the cold air mass, so it flows up over the cold air mass. As the warm air rises, it cools, resulting in clouds and sometimes light precipitation. Warm fronts move slowly and cover a wide area. After a warm front passes, the warm air mass behind it brings warmer temperatures. The warm air is also likely to be more humid. | text | null |
L_0026 | changing weather | T_0268 | With an occluded front, a warm air mass becomes trapped between two cold air masses. The warm air is lifted up above the cold air as in Figure 16.9. Cloudy weather and precipitation along the front are typical. | text | null |
L_0026 | changing weather | T_0269 | Sometimes two air masses stop moving when they meet. These stalled air masses create a stationary front. Such a front may bring clouds and precipitation to the same area for many days. | text | null |
L_0026 | changing weather | T_0270 | Cold air is dense, so it sinks. This creates a center of high pressure. Warm air is less dense so it rises. This creates a center of low pressure. Air always flows from higher to lower pressure. As the air flows, Earths surface rotates below it causing Coriolis effect. So while the wind blows into the low pressure, it revolves in a circular pattern. This wind pattern forms a cyclone. The same happens while the wind blows out of a high pressure. This forms an anticyclone. Both are shown in Figure 16.10. A cyclone is a system of winds that rotates around a center of low pressure. Cyclones bring cloudy, wet weather. An anticyclone is a system of winds that rotates around a center of high pressure. Anticyclones bring fair, dry weather. | text | null |
L_0027 | storms | T_0271 | A storm is an episode of severe weather caused by a major disturbance in the atmosphere. Storms can vary a lot in the time they last and in how severe they are. A storm may last for less than an hour or for more than a week. It may affect just a few square kilometers or thousands. Some storms are harmless and some are disastrous. The size and strength of a storm depends on the amount of energy in the atmosphere. Greater differences in temperature and air pressure produce stronger storms. Types of storms include thunderstorms, tornadoes, hurricanes, and winter storms such as blizzards. | text | null |
L_0027 | storms | T_0272 | Thunderstorms are are known for their heavy rains and lightning. In strong thunderstorms, hail and high winds are also likely. Thunderstorms are very common. Worldwide, there are about 14 million of them each year! In the U.S., they are most common and strongest in the Midwest. | text | null |
L_0027 | storms | T_0273 | Thunderstorms occur when the air is very warm and humid. The warm air rises rapidly to create strong updrafts. When the rising air cools, its water vapor condenses. The updrafts create tall cumulonimbus clouds called thunder- heads. You can see one in Figure 16.12. | text | null |
L_0027 | storms | T_0274 | During a thunderstorm, some parts of a thunderhead become negatively charged. Other parts become positively charged. The difference in charge creates lightning. Lightning is a huge release of electricity. Lightning can jump between oppositely charged parts of the same cloud, between one cloud and another, or between a cloud and the ground. You can see lightning in Figure 16.13. Lightning blasts the air with energy. The air heats and expands so quickly that it explodes. This creates the loud sound of thunder. Do you know why you always hear the boom of thunder after you see the flash of lightning? Its because light travels faster than sound. If you count the seconds between seeing lightning and hearing thunder, you can estimate how far away the lightning was. A lapse of 5 seconds is equal to about a mile. | text | null |
L_0027 | storms | T_0275 | Severe thunderstorms have a lot of energy and strong winds. This allows them to produce tornadoes. A tornado is a funnel-shaped cloud of whirling high winds. You can see a tornado in Figure 16.14. The funnel moves along the ground, destroying everything in its path. As it moves it loses energy. Before this happens it may have gone up to 25 kilometers (16 miles). Fortunately, tornadoes are narrow. They may be only 150 meters (500 feet) wide. | text | null |
L_0027 | storms | T_0276 | The winds of a tornado can reach very high speeds. The faster the winds blow, the greater the damage they cause. Wind speed and damage are used to classify tornadoes. Table 16.1 shows how. F Scale F0 (km/hr) 64-116 (mph) 40-72 F1 117-180 73-112 Damage Light - tree branches fall and chimneys may col- lapse Moderate - mobile homes, autos pushed aside F Scale F2 (km/hr) 181-253 (mph) 113-157 F3 254-332 158-206 F4 333-419 207-260 F5 420-512 261-318 F6 >512 >318 Damage Considerable - roofs torn off houses, large trees up- rooted Severe - houses torn apart, trees uprooted, cars lifted Devastating - houses lev- eled, cars thrown Incredible - structures fly, cars become missiles Maximum tornado wind speed | text | null |
L_0027 | storms | T_0277 | Look at the map in Figure 16.15. It shows where the greatest number of tornadoes occur in the U.S. Tornadoes can happen almost anywhere in the U.S. but only this area is called tornado alley. Why do so many tornadoes occur here? This is where warm air masses from the south run into cold air masses from the north. | text | null |
L_0027 | storms | T_0278 | Tornadoes may also come from hurricanes. A hurricane is an enormous storm with high winds and heavy rains. Hurricanes may be hundreds of kilometers wide. They may travel for thousands of kilometers. The storms wind speeds may be greater than 251 kilometers (156 miles) per hour. Hurricanes develop from tropical cyclones. Hurricanes form over warm very ocean water. This water gives them their energy. As long as a hurricane stays over the warm ocean, it keeps growing stronger. However, if it goes ashore or moves over cooler water, it is cut off from the hot water energy. The storm then loses strength and slowly fades away. | text | null |
L_0027 | storms | T_0279 | At the center of a hurricane is a small area where the air is calm and clear. This is the eye of the hurricane. The eye forms at the low-pressure center of the hurricane. You can see the eye of a hurricane in Figure 16.16. | text | null |
L_0027 | storms | T_0280 | Like tornadoes, hurricanes are classified on the basis of wind speed and damage. Table 16.2 shows how. Category 1 (weak) Kph 119-153 Mph 74-95 2 (moderate) 154-177 96-110 3 (strong) 178-209 111-130 Damage Above normal; no real damage to structures Some roofing, door, and window damage, consid- erable damage to vegeta- tion, mobile homes, and piers Some buildings damaged; mobile homes destroyed Category 4 (very strong) Kph 210-251 Mph 131-156 5 (devastating) >251 >156 Damage Complete roof failure on small residences; major erosion of beach areas; major damage to lower floors of structures near shore Complete roof failure on many residences and in- dustrial buildings; some complete building failures | text | null |
L_0027 | storms | T_0281 | Some of the damage from a hurricane is caused by storm surge. Storm surge is very high water located in the low pressure eye of the hurricane. The very low pressure of the eye allows the water level to rise above normal sea level. Storm surge can cause flooding when it reaches land. You can see this in Figure 16.17. High winds do a great deal of damage in hurricanes. High winds can also create very big waves. If the large waves are atop a storm surge, the high water can flood the shore. If the storm happens to occur at high tide, the water will rise even higher. | text | null |
L_0027 | storms | T_0282 | Like hurricanes, winter storms develop from cyclones. But in the case of winter storms, the cyclones form at higher latitudes. In North America, cyclones often form when the jet stream dips south in the winter. This lets dry polar air pour south. At the same time, warm moist air from the Gulf of Mexico flows north. When the two air masses meet, the differences in temperature and pressure cause strong winds and heavy precipitation. Two types of winter storms that occur in the U.S. are blizzards and lake-effect snow storms. | text | null |
L_0027 | storms | T_0283 | A blizzard is a snow storm that has high winds. To be called a blizzard, a storm must have winds greater than 56 kilometers (35 miles) per hour and visibility of 14 mile or less because of wind-blown snow. You can see a blizzard in Figure 16.18. Blizzards are dangerous storms. The wind may blow the snow into deep drifts. Along with the poor visibility, the snow drifts make driving risky. The wind also makes cold temperatures more dangerous. The greater the wind speed, the higher the windchill. Windchill is what the temperature feels like when the wind is taken into account. It depends on air temperature and wind speed, as you can see in Figure 16.19. Higher windchill will cause a person to suffer frostbite and other harmful effects of cold sooner than if the wind isnt blowing. | text | null |
L_0027 | storms | T_0284 | Some places receive very heavy snowfall just about every winter. If they are near a lake, they may be getting lake- effect snow. Figure 16.20 shows how lake-effect snow occurs. Winter winds pick up moisture as they pass over the relatively warm waters of a large lake. When the winds reach the cold land on the other side, the air cools. Since there was so much moisture in the air it can drop a lot of snow. More than 254 centimeters (100 inches) of snow may fall in a single lake-effect storm! | text | null |
L_0028 | weather forecasting | T_0285 | Weather is very difficult to predict. Thats because its very complex and many factors are involved. Slight changes in even one factor can cause a big change in the weather. Still, certain rules of thumb generally apply. These rules help meteorologists forecast the weather. For example, low pressure is likely to bring stormy weather. So if a center of low pressure is moving your way, you can expect a storm. | text | null |
L_0028 | weather forecasting | T_0286 | Predicting the weather requires a lot of weather data. Technology is used to gather the data and computers are used to analyze the data. Using this information gives meteorologists the best chance of predicting the weather. | text | null |
L_0028 | weather forecasting | T_0287 | Weather instruments measure weather conditions. One of the most important conditions is air pressure, which is measured with a barometer. Figure 16.23 shows how a barometer works. There are also a number of other commonly used weather instruments (see Figure 16.24): A thermometer measures temperature. An anemometer measures wind speed. A rain gauge measures the amount of rain. A hygrometer measures humidity. A wind vane shows wind direction. A snow gauge measures the amount of snow. | text | null |
L_0028 | weather forecasting | T_0288 | Weather instruments collect data from all over the world at thousands of weather stations. Many are on land but some float in the oceans on buoys. You can see what a weather station looks like in Figure 16.25. Theres probably at least one weather station near you. Other weather devices are needed to collect weather data in the atmosphere. They include weather balloons, satellites, and radar. You can read about them in Figure 16.25. Weather stations contain many instruments for measuring weather conditions. The weather balloon in Figure | text | null |
L_0028 | weather forecasting | T_0288 | Weather instruments collect data from all over the world at thousands of weather stations. Many are on land but some float in the oceans on buoys. You can see what a weather station looks like in Figure 16.25. Theres probably at least one weather station near you. Other weather devices are needed to collect weather data in the atmosphere. They include weather balloons, satellites, and radar. You can read about them in Figure 16.25. Weather stations contain many instruments for measuring weather conditions. The weather balloon in Figure | text | null |
L_0028 | weather forecasting | T_0289 | What do meteorologists do with all that weather data? They use it in weather models. The models analyze the data and predict the weather. The models require computers. Thats because so many measurements and calculations are involved. | text | null |
L_0028 | weather forecasting | T_0290 | You may have seen weather maps like the one in Figure 16.26. A weather map shows weather conditions for a certain area. The map may show the actual weather on a given day or it may show the predicted weather for some time in the future. Some weather maps show many weather conditions. Others show a single condition. | text | null |
L_0028 | weather forecasting | T_0291 | The weather map in Figure 16.26 shows air pressure. The lines on the map connect places that have the same air pressure. Air pressure is measured in a unit called the millibar. Isobars are the lines that connect the points with the same air pressure. The map also shows low- and high-pressure centers and fronts. Find the cold front on the map. This cold front is likely to move toward the northeast over the next couple of days. How could you use this information to predict what the weather will be on the East Coast? | text | null |
L_0028 | weather forecasting | T_0292 | Instead of air pressure, weather maps may show other weather conditions. For example, a temperature map might show the high and low temperatures of major cities. The map may have isotherms, lines that connect places with the same temperature. | text | null |
L_0029 | climate and its causes | T_0293 | Climate is the average weather of a place over many years. It includes average temperatures. It also includes average precipitation. The timing of precipitation is part of climate as well. What determines the climate of a place? Latitude is the main factor. A nearby ocean or mountain range can also play a role. | text | null |
L_0029 | climate and its causes | T_0294 | Latitude is the distance north or south of the equator. Its measured in degrees, from 0 to 90 . Several climate factors vary with latitude. | text | null |
L_0029 | climate and its causes | T_0295 | Temperature changes with latitude. You can see how in Figure 17.2 At the equator, the Suns rays are most direct. Temperatures are highest. At higher latitudes, the Suns rays are less direct. The farther an area is from the equator, the lower is its temperature. At the poles, the Suns rays are least direct. Much of the area is covered with ice and snow, which reflect a lot of sunlight. Temperatures are lowest here. | text | null |
L_0029 | climate and its causes | T_0296 | Global air currents affect precipitation. How they affect it varies with latitude. You can see why in Figure 17.3. | text | null |
L_0029 | climate and its causes | T_0297 | Global air currents cause global winds. Figure 17.4 shows the direction that these winds blow. Global winds are the prevailing, or usual, winds at a given latitude. The winds move air masses, which causes weather. The direction of prevailing winds determines which type of air mass usually moves over an area. For example, a west wind might bring warm moist air from over an ocean. An east wind might bring cold dry air from over a mountain range. Which wind prevails has a big effect on the climate. What if the prevailing winds are westerlies? What would the climate be like? | text | null |
L_0029 | climate and its causes | T_0298 | When a place is near an ocean, the water can have a big effect on the climate. | text | null |
L_0029 | climate and its causes | T_0299 | Even places at the same latitude may have different climates if one is on a coast and one is inland. On the coast, the climate is influenced by warm moist air from the ocean. A coastal climate is usually mild. Summers arent too hot, and winters arent too cold. Precipitation can be high due to the moisture in the air. Farther inland, the climate is influenced by cold or hot air from the land. This air may be dry because it comes from over land. An inland climate is usually more extreme. Winters may be very cold, and summers may be very hot. Precipitation can be low. | text | null |
L_0029 | climate and its causes | T_0300 | Ocean currents carry warm or cold water throughout the worlds oceans. They help to even out the temperatures in the oceans. This also affects the temperature of the atmosphere and the climate around the world. Currents that are near shore have a direct impact on climate. They may make the climate much colder or warmer. You can see examples of this in Figure 17.5. | text | null |
L_0029 | climate and its causes | T_0301 | Did you ever hike or drive up a mountain? Did you notice that it was cooler near the top? Climate is not just different on a mountain. Just having a mountain range nearby can affect the climate. | text | null |
L_0029 | climate and its causes | T_0302 | Air temperature falls at higher altitudes. You can see this in Figure 17.6. Why does this happen? Since air is less dense at higher altitudes, its molecules are spread farther apart than they are at sea level. These molecules have fewer collisions, so they produce less heat. Look at the mountain in Figure 17.7. The peak of Mount Kilimanjaro, Tanzania (Africa, 3 south latitude) is 6 kilometers (4 miles) above sea level. At 3 S its very close to the equator. At the bottom of the mountain, the temperature is high year round. How can you tell that its much cooler at the top? | text | null |
L_0029 | climate and its causes | T_0303 | Mountains can also affect precipitation. Mountains and mountain ranges can cast a rain shadow. As winds rise up a mountain range the air cools and precipitation falls. On the other side of the range the air is dry and it sinks. So there is very little precipitation on the far (leeward) side of a mountain range. Figure 17.8 shows how this happens. | text | null |
L_0032 | ecosystems | T_0324 | An ecosystem is a group of living things and their environment. The word ecosystem is short for ecological system. Like any system, an ecosystem is a group of parts that work together. You can see examples of ecosystems in Figure 18.1. The forest pictured is a big ecosystem. Besides trees, what living things do you think are part of the forest ecosystem? The dead tree stump in the same forest is a small ecosystem. It includes plants, mosses, and fungi. It also includes insects and worms. | text | null |
L_0032 | ecosystems | T_0325 | Abiotic factors are the nonliving parts of ecosystems. They include air, sunlight, soil, water, and minerals. These are all things that are needed for life. They determine which living things and how many of them an ecosystem can support. Figure 18.2 shows an ecosystem and its abiotic factors. | text | null |
L_0032 | ecosystems | T_0326 | Biotic factors are the living parts of ecosystems. They are the species of living things that reside together. | text | null |
L_0032 | ecosystems | T_0327 | A species is a unique type of organism. Members of a species can interbreed and produce offspring that can breed (they are fertile). Organisms that are not in the same species cannot do this. Examples of species include humans, lions, and redwood trees. Can you name other examples? Each species has a particular way of making a living. This is called its niche. You can see the niche of a lion in Figure 18.3. A lion makes its living by hunting and eating other animals. Each species also has a certain place where it is best suited to live. This is called its habitat. The lions habitat is a grassland. Why is a lion better off in a grassland than in a forest? | text | null |
L_0032 | ecosystems | T_0328 | All the members of a species that live in the same area form a population. Many different species live together in an ecosystem. All their populations make up a community. What populations live together in the grassland in Figure | text | null |
L_0032 | ecosystems | T_0329 | All ecosystems have living things that play the same basic roles. Some organisms must be producers. Others must be consumers. Decomposers are also important. | text | null |
L_0032 | ecosystems | T_0330 | Producers are living things that use energy to make food. Producers make food for themselves and other living things. There are two types of producers: By far the most common producers use the energy in sunlight to make food. This is called photosynthesis. Producers that photosynthesize include plants and algae. These organisms must live where there is plenty of sunlight. Which living things are producers in Figure 18.3? Other producers use the energy in chemicals to make food. This is called chemosynthesis. Only a very few producers are of this type, and all of them are microbes. These producers live deep under the ocean where there is no sunlight. You can see an example in Figure 18.4. | text | null |
L_0032 | ecosystems | T_0331 | Consumers cant make their own food. Consumers must eat producers or other consumers. Figure 18.5 lists the three main types of consumers. Which type are you? Consumers get their food in different ways Figure 18.6. Grazers feed on living organisms without killing them. A rabbit nibbles on leaves and a mosquito sucks a drop of blood. Predators, like lions, capture and kill animals for food. The animals they eat are called prey. Even some plants are consumers. Pitcher plants trap insects in their sticky fluid in their pitchers. The insects are their prey. Scavengers eat animals that are already dead. This hyena is eating the remains of a lions prey. Decomposers break down dead organisms and the wastes of living things. This dung beetle is rolling a ball of dung (animal waste) back to its nest. The beetle will use the dung to feed its young. The mushrooms pictured are growing on a dead log. They will slowly break it down. This releases its nutrients to the soil. | text | null |
L_0032 | ecosystems | T_0332 | All living things need energy. They need it to power the processes of life. For example, it takes energy to grow. It also takes energy to produce offspring. In fact, it takes energy just to stay alive. Remember that energy cant be created or destroyed. It can only change form. Energy changes form as it moves through ecosystems. | text | null |
L_0032 | ecosystems | T_0333 | Most ecosystems get their energy from the Sun. Only producers can use sunlight to make usable energy. Producers convert the sunlight into chemical energy or food. Consumers get some of that energy when they eat producers. They also pass some of the energy on to other consumers when they are eaten. In this way, energy flows from one living thing to another. | text | null |
L_0032 | ecosystems | T_0334 | A food chain is a simple diagram that shows one way energy flows through an ecosystem. You can see an example of a food chain in Figure 18.7. Producers form the base of all food chains. The consumers that eat producers are called primary consumers. The consumers that eat primary consumers are secondary consumers. This chain can continue to multiple levels. At each level of a food chain, a lot of energy is lost. Only about 10 percent of the energy passes to the next level. Where does that energy go? Some energy is given off as heat. Some energy goes into animal wastes. Energy also goes into growing things that another consumer cant eat, like fur. Its because so much energy is lost that most food chains have just a few levels. Theres not enough energy left for higher levels. | text | null |
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