What Is the Relationship Between Earth s Tilt and the Seasons?

Learning Set 2 Why Are There Differences in Temperature? Review Images and Graphics While reading about Earth s tilt and the seasons, pay particular attention to the graphics included. How do they help you understand the main ideas of this section? 2.5 Read What Is the Relationship Between Earth s Tilt and the Seasons? The image of Italy below shows the dividing line between day and night on Earth. As Earth rotates on its axis, this dividing line moves across the planet, causing sunrises and sunsets. If the image were made at a different time of year, the dividing line would be at another angle because of Earth s tilted axis. The rotation of Earth causes night and day. Where there is no sunlight, people light up the night artificially. Areas of high light usage can be seen in this photo of Italy. ellipse: a shape that is a squashed or flattened circle. You used the ball and skewer to show Earth and its axis. You have begun to investigate how the tilt of Earth s axis affects the length of daylight across the planet. However, your models were not completely accurate, because Earth s orbit is not quite circular. The orbit s shape is a slightly fl attened circle called an ellipse. Because the orbit is not a perfect circle, Earth s distance from the Sun varies between 147 million km (91.4 million mi) and 152 million km (94.5 million mi). Your model did not need to take this into account, because the change in distance is only 3 percent and does not have a large effect on Earth s temperatures. Effects on temperature caused by Earth s tilted axis are much greater. Project-Based Inquiry Science WW 102

2.5 Read As you saw in your model, in June, the Northern Hemisphere tilts toward the Sun, and the Southern Hemisphere tilts away from the Sun. In December, the Northern Hemisphere tilts away from the Sun, and the Southern Hemisphere tilts toward the Sun. The hemisphere tilted toward the Sun receives more direct sunlight and also has more daylight hours. This combination of more direct sunlight and more daylight hours results in more solar energy received from the Sun. The hemisphere tilted away from the Sun receives less direct sunlight and has shorter days. The combination of less direct sunlight and fewer daylight hours results in less solar energy received from the Sun. As Earth revolves around the Sun (in one Earth year), a cycle of times when there is more incoming solar energy and times when there is less incoming solar energy is created. These changes cause the seasons. Mid-latitudes and the poles experience large variations in the amount of incoming solar energy throughout the year. The seasons in these places are very different. Latitudes near the Equator experience very little variation in the amount of incoming solar energy throughout the year, so they have little variation in their seasons. This cycle of seasons occurs each year. When you think about summer, you probably think about sunshine and hot temperatures. The Northern Hemisphere experiences the greatest number of daylight hours on or around June 21. This day is called the summer solstice. After the summer solstice, the number of daylight hours decreases each day until the winter solstice. In the Northern Hemisphere, the winter solstice is on or around December 21. The winter solstice is the day with the fewest daylight hours. After the winter solstice, the number of daylight hours increases each day until the summer solstice arrives again. summer solstice: the day with the greatest number of daylight hours. winter solstice: the day with the fewest number of daylight hours. WW 103 WEATHER WATCH

Learning Set 2 Why Are There Differences in Temperature? Stop and Think 1. Which of Earth s motions determines the length of the day? Which motion determines the length of the year? 2. Why are there more daylight hours in the United States at the summer solstice than at the winter solstice? 3. What are two reasons that the incoming solar energy is greater at the summer solstice than at the winter solstice? Monthly Average Hours of Daylight Between the summer solstice, when the number of daylight hours is greatest, and the winter solstice, when the number of daylight hours is fewest, the number of daylight hours gets smaller each day. If you look at the Monthly Average Hours of Daylight chart below, you can see that pattern. You might be able to see another pattern in the chart. Look for columns in which the average number of daylight hours is close to 12 in each location. Monthly Average Hours of Daylight Location Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Helsinki, Finland (60 N, 24 E) (hours) 6.8 9.1 11.9 14.7 17.3 18.8 18 15.6 12.8 10.1 7.5 6 Atlanta, USA (33 N, 84 W) (hours) 9.9 10.7 11.7 12.9 13.8 14.3 14.1 13.3 12.2 11.1 10.1 9.7 Quito, Ecuador (0, 78 W) (hours) Buenos Aires, Argentina (34 S, 58 W) (hours) 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 14.1 13.3 12.3 11.1 10.2 9.7 9.9 10.7 11.8 12.9 13.9 14.3 In March and September, the averages are close to 12 hours of daylight. There is one day in each of these months when there is an equal number of daylight hours and nighttime hours. When the number of daylight hours Project-Based Inquiry Science WW 104

2.5 Read equals the number of nighttime hours everywhere on Earth, it is called an equinox (similar to the word equal). There is a spring equinox and a fall equinox. In the Northern Hemisphere, the spring equinox occurs on or around March 20, and the fall equinox is on or around September 21. Look again at the table of monthly average hours of daylight. Notice the pattern of daylight hours, and the different dates places experience their summer solstice and winter solstice. equinox: one of two days in the year when the number of daylight hours equals the number of nighttime hours everywhere on Earth. Earth at the spring equinox the fall equinox the summer solstice the winter solstice. * Note: UT is an abbreviation for Universal Time, also known as Greenwich Mean Time (GMT). Stop and Think 1. Which city in the table has the most variation in day length? Why? 2. Which city has the least variation in day length? Why? 3. Why is it hard to determine the time of the equinoxes using the data in the table for Quito, Ecuador? WW 105 WEATHER WATCH

Learning Set 2 Why Are There Differences in Temperature? Reflect 1. Which state in the United States experiences the greatest number of daylight hours, and when? This state also has the coldest temperatures all year. Why is this so? Which state experiences the fewest number of daylight hours, and when? 2. Miami, Florida, has little variation in temperature and experiences warm weather year-round. It has a maximum of 13.5 hours of daylight. Chicago, Illinois, experiences cold winters, hot summers, and warm spring and fall temperatures. Chicago s longest period of daylight is about 15 hours, and its shortest is about 9 hours. Why are Miami and Chicago so different? 3. Which city in the data table is in the Southern Hemisphere? In what month does the summer solstice occur in this city? When does the winter solstice occur? Review Your Explanations You have investigated tilt and its effects on incoming solar energy, temperatures, and seasons. You developed an explanation about why there are differences in temperature between the southern and northern regions of the United States. You also developed an explanation about why it is warmer in summer than in winter and why the seasons are not the same everywhere. Now that you know more about tilt and its effects on incoming solar energy, you are ready to revise both explanations. Look at your original explanations. Think about how you originally expected tilt to affect temperature, and then review your explanations. Think about what you have read in this section about tilt and how it relates to seasons and the number of daylight hours. Think about how temperature varies with latitude. Use the following questions as a guide for reviewing your explanations. You will revise your explanations after you discuss your answers to these questions with the class. Project-Based Inquiry Science WW 106

2.5 Read 1. Review your explanation of why temperatures are different in different places on Earth. What do you know now that will allow you to make a better claim and develop a better explanation statement? Work with your group to revise your claim. 2. Review your explanation of why temperatures are different in different seasons. What do you know now that will allow you to make a better claim and develop a better explanation? Work with your group to revise your claim. 3. Suppose you had to also explain why seasons are different in the Northern and Southern Hemispheres. What is your answer to that question? What evidence and science knowledge support your answer? 4. How has your understanding of why tilt affects average temperatures changed since you started? What have you learned that has made you change your claims? What have you learned that you need to include in revised explanation statements? 5. Write a statement that combines the answers to how, why, and how much tilt affects average temperatures. Make sure to use evidence from your investigations in your statement. Revise Your Explanations Use two Create Your Explanation pages to rewrite the two explanations on your own. Then share these explanations with your group, and work as a group to develop two group explanations. Identify what else you still need to learn to make your explanations complete. WW 107 WEATHER WATCH

Learning Set 2 Why Are There Differences in Temperature? Communicate Share Your Explanations When it is your group s turn, share your explanations with the class, and tell them what else you think you need to know to make your explanations better. Revise Your Explanations As a class, select or create explanations to answer the questions from this Learning Set: Why are southern regions of the United States warmer than northern regions? and Why are temperatures different during different seasons? Use scientific terms, such as solar energy, conduction, convection, radiation, latitude, tilt, rotation, revolution, and direct and indirect sunlight, in your claims and explanation statements. Then decide together what else you still need to learn to answer those questions more fully and to fully answer the question for this Learning Set: Why are there differences in temperature? At latitudes north of the Arctic Circle and south of the Antarctic Circle, the Sun can remain visible for months without setting. However, the Sun never gets very high in the sky. Project-Based Inquiry Science WW 108

2.5 Read Update the Project Board You know a lot now about why average temperatures are different in different parts of the United States and why temperatures are different in different seasons. Add what you now know to the What are we learning? column of the Project Board. Don t forget to include evidence that supports your learning to the What is our evidence? column for each of those entries. Then, add any new questions you have identified to the What do we need to investigate? column of the Project Board. As the class Project Board is updated, make sure you record the same information on your own Project Board page. Develop a plan for responding to a severe weather event What do we think we know? What do we need to investigate? What are we learning? What is our evidence? What does it mean for the challenge or question? What s the Point? The tilt of Earth s axis causes variations in incoming solar radiation. These variations depend on latitude, which is the distance north or south of Earth s Equator. Earth s orbit is an ellipse, so Earth s distance from the Sun varies by about 3 percent. However, the effect of Earth s changing distance on incoming solar radiation is small compared to the effects caused by Earth s tilted axis. The tilt of Earth s axis also causes seasons and variations in the length of daylight across the globe. In summer, days are longer and sunlight is more direct. In winter, days are shorter and sunlight is less direct. Seasons are opposite in the Northern and Southern Hemispheres. In December, the Northern Hemisphere is tilted away from the Sun, bringing about winter in the Northern Hemisphere and summer in the Southern Hemisphere. In June, the Northern Hemisphere is tilted toward the Sun, bringing about summer in the Northern Hemisphere and winter in the Southern Hemisphere. WW 109 WEATHER WATCH