3 Weather and Climate

Transcription

1 CHAPTER 22 3 Weather and Climate SECTION The Atmosphere KEY IDEAS As you read this section, keep these questions in mind: What are fronts? What are some types of severe weather? How is climate different from weather? What Are Air Masses and Fronts? Many people watch weather forecasts to learn what the weather will be like. Meteorologists, or scientists who study weather, make predictions based on information about weather conditions in different areas. They plot this information on maps, such as the one shown below. Weather maps help meteorologists follow, or track, the movements of large bodies of air called air masses. All of the air in an air mass has a similar temperature and humidity. By tracking the movements of air masses, meteorologists can predict where and when they will meet. A front is a boundary between two or more air masses. There are three main types of fronts: warm fronts, cold fronts, and stationary fronts. Each type of front creates different weather conditions. In most cases, when a front moves through an area, the weather including temperature, precipitation, and wind changes. Look at the weather map shown below. Fronts are indicated on this map by curved lines. The letters H and L indicate areas of higher or lower atmospheric pressure. READING TOOLBOX Apply Concepts After you read this section, look at a current weather map from a newspaper or World Wide Web site. Find the location of fronts and air masses. Note areas of high and low pressure. Look for any severe weather. 1. Compare What is the difference between an air mass and a front? 2. Identify On the map, circle the symbols that indicate fronts. Interactive Reader 517 The Atmosphere

2 WARM FRONTS At a warm front, a warm air mass moves above a slower, denser cold air mass. The warm air mass cools as it rises above the cold air. As the temperature decreases, water vapor condenses and clouds form. The figure below shows an example of a warm front. Warm Front Cirrus 3. Clarify On the figure, color the warm air mass orange or red. Color the cold air mass blue or purple. Use a dark red line to show the warm front. Boundary of warm front Warm air Nimbostratus Altostratus Cold air Cirrostratus Cirrus and cirrostratus clouds form as a warm front begins to move over an area. As time passes, lower-lying clouds move overhead. In many cases, nimbostratus clouds release steady rain or snow for one to two days. COLD FRONTS At a cold front, a cold air mass moves quickly under a slower, warmer air mass and pushes it up. A cold front is shown in the diagram below. Notice that the edge of the cold front is steeper than the edge of a warm front. This steep edge makes the warm air rise quickly and form cumulonimbus clouds. High winds, thunderstorms, and sometimes tornadoes form at cold fronts. Cold Front 4. Clarify On the figure, color the warm air mass orange or red. Color the cold air mass blue or purple. Use a dark blue line to show the cold front. Cold air Cumulonimbus Boundary of cold front Warm air Interactive Reader 518 The Atmosphere

3 STATIONARY FRONTS A stationary front occurs when two air masses meet, but neither one pushes up over the other. The air masses at a stationary front do not move very much. As a result, stationary fronts can bring many days of similar weather conditions. The weather conditions near a stationary front are similar to those near a warm front. What Are Some Types of Stormy Weather? Inside thunderclouds, water droplets and ice crystals build up electrical charges. When charges build up, a spark may discharge, or release, the electrical energy. Lightning is a big spark that jumps between clouds or between a cloud and Earth s surface. Lightning heats up the air around it so much that the air expands faster than the speed of sound. This creates a loud booming sound thunder. Severe thunderstorms can form tornadoes, which are high-speed, rotating winds. Winds in a tornado can be as fast as 500 km/h (more than 300 mi/h). Most tornadoes happen in the Midwestern part of the United States. They generally occur in spring and early summer when cool, dry air masses and warm, humid air masses meet. Most tornadoes begin as columns of water droplets called funnel clouds. The air at the center of a funnel cloud has very low pressure. If the funnel cloud touches the ground, air rushes into the low-pressure area, forming high-speed winds. 5. Explain Why does a stationary front often bring many days of similar weather? 6. Describe When do most tornadoes form? 1. Wind moving in opposite directions causes a layer of air in the middle of a cloud to begin to spin. 2. Strong vertical winds cause the spinning column of air to turn into a vertical position. 3. The spinning column of air moves to the bottom of the cloud and forms a funnel cloud. 4. The funnel cloud becomes a tornado when it touches down on the ground. 7. Compare How is a tornado different from a funnel cloud? Interactive Reader 519 The Atmosphere

4 8. Define What is a tropical depression? 9. Identify Over what type of water do hurricanes form? HURRICANES Hurricanes are large, spinning masses of clouds, wind, and rain. They form over warm ocean water. Hurricanes are also called cyclones or typhoons. Most hurricanes occur in late summer and early fall, when oceans are warmest. Warm water evaporates and the water vapor rises, forming areas of very low pressure called tropical depressions. These tropical depressions can build strength and become hurricanes, shown below. Altitude (m) 15,000 10,000 5,000 Sea level 80 km/h winds Distance (mi) Sinking dry air 120 km/h winds Eye of the hurricane Evaporation Outward-flowing, high-level winds Heavy rains Distance (km) 50,000 40,000 30,000 20,000 10,000 Sea level Altitude (ft) 10. Describe Where does the energy in a hurricane come from? Hurricanes can be more than 600 km (375 mi) in diameter. Hurricanes move slowly, but they are extremely powerful. Wind speeds in a hurricane range from 118 km/h (73 mi/h) to greater than 250 km/h (155 mi/h). However, the eye, or center, of the hurricane is very calm. Hurricanes are powered by the energy released as water vapor condenses to form clouds. As water vapor condenses, heat energy is released into the air. The heat causes the air to expand, and the air pressure decreases. Warm, moist air continues to rise and condense, so more energy is released. This rising air creates strong winds. The energy in hurricanes comes from warm ocean water. This is why hurricanes weaken as they move over land or cool water. They also generally do not form near the poles. Interactive Reader 520 The Atmosphere

5 STAYING SAFE DURING SEVERE WEATHER Severe weather can be very dangerous. You can reduce the danger to yourself by preparing before storms hit, and by following safety rules during severe weather. The following list describes how to prepare for severe weather: 1. Stay aware of weather reports so that you know where and when a storm may occur. 2. Have your family plan to meet in a certain place in case you get separated. 3. Keep a survival kit with water, nonperishable food, a flashlight, and a battery-powered radio. During a major storm, electricity may go out and water may be turned off. Traveling may be difficult or impossible. Therefore, it is important to keep a survival kit in case you are not able to leave your home for several days. The survival kit should contain items such as water, food, batteries, bandages, and any medicines you may need. It is also important to follow safety rules during a storm. Each type of severe weather has different dangers. The chart below shows some of the things you should do in order to stay safe during lightning, tornadoes, and hurricanes. Discuss In a small group, discuss the different kinds of severe weather that are common in the area in which you live. Brainstorm materials you should keep in a survival kit to help you stay safe during severe weather. 11. Explain Why is it important to have extra food and water in case of severe weather? Type of weather Lightning Tornadoes Hurricane Safety tips Get inside a building. Stay away from windows. Do not talk on the phone. If you are caught outside, stay away from water and tall objects. Crouch down on the ground. Move to a basement or storm cellar. If the building you are in does not have a basement or storm cellar, move to a first-floor room with few windows. If you are outside, lie in a ditch or low-lying area and cover your head with your hands. Stay in the innermost part of the building. Stay away from windows. Listen to a radio for flood warnings. If you are ordered to leave the area, do so promptly and calmly. 12. Describe What should you do if you are caught outside during a tornado? Interactive Reader 521 The Atmosphere

6 13. Define What is climate? How Is Weather Related to Climate? You know that the weather can change from day to day. However, climate does not change as quickly. Climate is the average weather of a region over many years. For example, the climate near the equator is hot and humid, and does not change much during the seasons. In contrast, the climate in much of the United States is temperate, with warm summers and cold winters. Many different factors affect the climate of an area. LATITUDE An area s latitude is one of the most important factors affecting its climate. Different parts of Earth s surface receive different amounts of solar energy. The diagram below shows how sunlight strikes different latitudes. Each of the light bars represents the same amount of solar energy. 14. Explain Why is solar energy more concentrated at the equator? Earth s surface is curved. Therefore, solar energy is more concentrated at the equator than at the poles. Equator The solar energy that strikes near the equator is concentrated in a small area. Therefore, temperatures are higher near the equator. The solar energy that strikes farther from the equator is spread out over a larger area. Therefore, temperatures are lower at high latitudes. 15. Explain Why may the windward side of a mountain have a wetter climate than the other side? TOPOGRAPHY The topography, or surface features, of an area also affect climate. For example, the climate on the windward, or wind-facing, side of a mountain may be very wet. On the other side of the mountain, the climate may be much drier. This effect occurs because air must rise to move over a mountain. As the air rises, it cools, and clouds form. The clouds release water vapor as precipitation on the windward side. The other side of the mountain is said to be in a rain shadow. Interactive Reader 522 The Atmosphere

7 SEASONS Most places in the United States have four distinct seasons: winter, spring, summer, and fall. Many people think that this happens because Earth moves closer to and farther from the sun in its orbit. However, that is not the reason. Actually, Earth has seasons because its axis of rotation is tilted. This tilt causes different areas to receive different amounts of sunlight at different times of year. The figure below shows how Earth s orbit and rotation cause the seasons. In June, represented by point A, the Northern Hemisphere is tilted toward the sun. As a result, the temperatures are higher and the Northern Hemisphere experiences summer. At the same time, the Southern Hemisphere is tilted away from the sun, causing it to receive less sunlight. As a result, the temperatures are lower and the Southern Hemisphere experiences winter. Position C in the diagram shows Earth in December. At this time, the Northern Hemisphere is tilted away from the sun. The Southern Hemisphere is tilted toward the sun. As a result, the Northern Hemisphere experiences winter, and the Southern Hemisphere experiences summer. 16. Explain Why does Earth have seasons? D C A Sun Earth s orbit 23.5 o North South B 17. Describe Which hemisphere experiences summer when Earth is at position C? LONG-TERM CLIMATE CHANGE Earth s climate changes slowly due to factors such as plate tectonics and changes in the tilt of Earth s axis. Faster changes can be caused by volcanic eruptions. Fast climate change also seems to be happening now, as the global average temperature is rising. Most scientists think that this change is caused by increases in greenhouse gases from burning fossil fuels. Scientists are not yet sure how this will affect Earth in the future. Interactive Reader 523 The Atmosphere

8 Section 3 Review SECTION VOCABULARY air mass a large body of air throughout which temperature and moisture content are similar climate the average weather conditions in an area over a long period of time front the boundary between air masses of different densities and usually different temperatures topography the size and shape of the land surface features in a region, including its relief 1. Compare How is climate different from weather? 2. Describe What happens at a warm front? 3. Explain What causes thunder and lightning? 4. Explain Why don t hurricanes generally form at high latitudes? 5. Identify List two factors that can affect an area s climate. 6. Infer If Earth s axis were tilted at a larger angle than it actually is, would the differences between the seasons be smaller or greater? Explain your answer. 7. Describe What should you do if you are caught outside during a lightning storm? Interactive Reader 524 The Atmosphere