24.3 Solar Energy and Winds

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Section 24.3 (continued) Global Winds Use Visuals Figure 13 Identify 0, 30 S, 60 S, 30 N, and 60 N. Explain that the global wind patterns shown in this figure remain fairly constant throughout the year, though the change of seasons does have some effect on the circulation of the atmosphere. Heating patterns change as the seasons change, and patterns of air pressure (and thus wind) change as a result. Point out that convection cells are vertical and reach to the top of the troposphere. Point to the convection cell between 0 and 30 S and ask, Which direction is this wind pattern felt on the ground? (Northwest) In which regions of Earth are the global winds moving generally from the east in a westerly direction? (0°–30° N, 0°–30° S, 60° N–90° N, and 60° S–90° S) Why are the arrows in the 0–30 S region (over South America) curving to the left? (The northerly winds curve westward as the result of Earth’s rotation—or the Coriolis effect.) This figure can be used to help explain the movements of fronts and storms in Section 24.5 and global climate patterns in Section 24.7. Visual Convection Cells 758 Chapter 24 L1 L2 Purpose Students observe a convection cell produced in water. Materials 1-L beaker, water, hot plate or Bunsen burner, 5–10 drops of food coloring Procedure Fill the beaker with approximately 750 mL water. Place the beaker on a hot plate or Bunsen burner and heat water until some steam is rising, but before boiling. Turn off the heat source. Explain that convection cells are common phenomena in both air and water on many scales, from global to local winds, and from ocean currents to coffee cups. Slowly add the drops of food coloring (they will spread quickly, so only add enough to make the convection cell visible). Ask students to explain what is causing the convection cell. Expected Outcome The food coloring will reveal the circulation of water in a convection cell before dissipating. This cell is produced when warm water touches surface air, cools, and sinks again to the bottom. Visual, Logical Warm air rises at the equator until it reaches the top of the troposphere. The circulating air patterns are called “convection cells.” Figure 13 Earth is surrounded by a set of global wind belts. Figure 14 For hundreds of years, sailing ships have relied on global winds to transport cargo across the oceans. Interpreting Visuals Which band of global winds would a sailing ship use to move cargo from Canada to Europe? 758 Facts and Figures Dry air sinks over the world’s deserts. Where the Wind Dies While trade winds and westerlies occur where convection cells blow across the surface, there are also areas where the wind dies out. These occur where the convection cells produce areas of rising or sinking air, such as at the equator, at 30 north and south latitude, and at 60 north and south latitude. The area of low winds at the equator is referred to as the doldrums, and the low Trade winds Westerlies Westerlies Earth’s rotation Doldrums Trade winds Polar easterlies Very cold air sinks at the poles and flows outward, creating winds called polar easterlies. The area where the trade winds die out is known as the doldrums. Global Winds Winds that blow over long distances from a specific direction are called global winds. These winds are part of a worldwide pattern of air circulation. Global winds are caused by the unequal heating of Earth’s surface across a large region. Convection Cells Global winds move in a series of huge bands called convection cells. As you can see in Figure 13, these bands look like loops from the side. These bands are caused by temperature variations across Earth’s surface. At the equator, for example, temperatures tend to be warmer than at other latitudes. Warm air rises at the equator, creating a low-pressure region. This warm air is replaced by cooler air brought by global winds blowing near the surface. Higher in the atmosphere, air blows away from the equator toward the poles. Similar convection cells cover large bands of latitude across Earth. The trade winds are wind belts just north and south of the equator. In the Northern Hemisphere, they blow from the northeast to the southwest. The prevailing westerlies occur between 30° and 60° latitude in both hemispheres. These winds generally blow from west to east over much of North America. The polar easterlies extend from 60° latitude to the poles in both hemispheres. Trade winds, westerlies, and polar easterlies are examples of global winds. wind regions at 30 north and south latitude are called the horse latitudes. This name has a gruesome historical origin. Colonial sailors traversing the Atlantic would frequently get stuck around 30 N when the wind died. To survive at sea with a limited supply of fresh water on board (and to lighten the ship’s weight) the sailors would throw a few horses
If Earth were not rotating on its axis, global winds would move in roughly straight paths from the poles to the equator. However, because Earth rotates, global winds move in a curved path between the poles and the equator. Notice in Figure 13 that global winds curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The curving effect that Earth’s rotation has on all freemoving objects, including global winds, is called the Coriolis effect. If Earth were not rotating, a rocket launched from the North Pole toward the equator would move in a straight line, as shown in Figure 15. However, because Earth is rotating underneath the rocket, the rocket would appear to an observer on Earth to curve to the right. Similary, Earth’s rotation causes global winds to curve. Monsoons Seasonal changes in the heating of Earth’s surface affect the circulation of the atmosphere. A monsoon is a wind system that is characterized by seasonal reversal of direction. Monsoons are similar to land and sea breezes except that they occur on a much wider scale and longer time frame. For example, the summer monsoon that occurs over much of South and Southeast Asia blows warm, humid air from the ocean onto land. As this air rises over land, it cools and brings heavy rainfall to parts of that region. In winter the monsoon reverses, blowing from land onto water and bringing drier weather. Jet Stream Global wind patterns are also affected by fast-moving streams of air at high altitudes. A belt of high-speed wind in the upper troposphere is called a jet stream. Jet streams are caused by great differences in air pressure that develop at high altitudes. Section 24.3 Assessment Reviewing Concepts 1. What happens to solar energy when it reaches Earth’s atmosphere? 2. Explain how energy is transferred within the troposphere. 3. What causes the wind to blow? 4. How are local winds and global winds similar? How are they different? 5. What are monsoons, and what causes them? Critical Thinking 6. Relating Cause and Effects How does the Coriolis effect influence global wind patterns? Section 24.3 Assessment 1. Solar energy is distributed in three ways: some is reflected, some is absorbed by the atmosphere, and some is absorbed by Earth’s surface. 2. Energy is transferred within the troposphere by radiation, convection, and conduction. 3. Wind is caused by differences in air pressure between different locations. 4. Both local and global winds are produced by differences in air pressure that result from A Non-rotating Earth Movement of rocket Movement of rocket Equator Equator 7. Applying Concepts You and your family vacation at a cabin on the shore of a large lake. At night, you notice that a breeze blows over your cabin toward the lake. Explain what causes the wind to blow in that direction. Thermal Energy Recall what you learned in Chapter 16 about the specific heat of water. Use this information to explain why land and sea breezes undergo daily reversals in direction. B Direction of Earth’s rotation Rotating Earth Figure 15 The Coriolis effect causes free-moving objects, such as rockets and global winds, to move in a curved path. A A rocket launched from the North Pole toward the equator would move in a straight line if Earth were not rotating. B The Coriolis effect causes such a rocket to appear to curve to the right. Similarly, the Coriolis effect causes global winds to curve to the right in the Northern Hemisphere. Weather and Climate 759 the unequal heating of Earth’s surface. For local winds, this unequal heating takes place within a small region, and the resulting wind blows over only a short distance. In contrast, global winds are caused by unequal heating across a large area, and blow over long distances for extended periods of time. 5. Monsoons are large wind systems that have seasonal reversals of direction. They are caused by air pressure differences that result from the unequal heating of air above the ocean and land. 3 ASSESS Evaluate Understanding Ask students to create a flowchart describing how local winds are created. Reteach L2 L1 Use Figure 13 to explain convection cells and the Coriolis effect. Water has a high specific heat, higher than that of land. As a result, water warms up slower during the day and cools down slower at night than land. This affects the temperature of the air above the water and land. Land and sea breezes are created by this daily cycle of unequal heating of air in coastal regions. If your class subscribes to the Interactive Textbook, use it to review key concepts in Section 24.3. Answer to . . . Figure 14 A sailing ship would use the westerlies to move cargo from Canada to Europe. 6. The Coriolis effect causes global winds to turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. 7. When night falls, the lake water remains warm as the land cools off quickly. The warmer air over the water has a lower pressure than the cooler air over the land. The result is a land breeze, which is when the cooler air over land moves toward water. Weather and Climate 759
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24.3 Solar Energy and Winds

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