Lab 1 Solar Radiation & Seasons

Lab 1 Part 1

We look up at the sky on a clear day to see a bright yellow sphere that we know is the sun. We all have a sense of how important that ball of fire is to us: It is, after all, the primary source of energy for our planet. But do we really understand how that energy is received by the Earth and how that affects our world and our lives?

By the end of the lab, you should have gained the knowledge needed to answer these three big questions:

What is the cause of seasons?
What are the two factors that impact the amount of solar radiation the Earth receives and which one has more of an impact?
What is the relationship between latitude and surface temperature?

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Solar Radiation & Seasons Prezi Presentation by Lab Instructor

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Entering with the right mindset
Throughout this lab you will be asked to answer some questions. Those questions will come in three different varieties:

Fact based question →This will be a question with a rather clear-cut answer. That answer will be based on information (1) presented by your instructor,(2) found in background sections,or (3)determined by you from data, graphs, pictures, etc. There is more of an expectation of you providing a certain answer for a question of this type as compared to questions of the other types.

Synthesis based question → This will be a question that will require you to pull together ideas from different places in order to give a complete answer. There is still an expectation that your answer will match up to a certain response, but you should feel comfortable in expressing your understanding of how these different ideas fit together.

Hypothesis based question → This will be a question which will require you to stretch your mind little bit. A question like this will ask you to speculate about why something is the way it is, for instance. There is not one certain answer to a question of this type. This is a more open- ended question where we will be more interested in the ideas that you propose and the justification (‘I think this because . . .’) that you provide.

In the Prezi presentation that your instructor gave to you, one of the slides explained that “The amount of solar radiation a place receives during a day is a function of the … duration of the sunlight.” In this Part of the lab activity, you will explore this relationship between solar radiation and the duration of sunlight.

Click Daylight_Hours to open the file in Google™ Earth.

Experiment with the slider to see how the hours of daylight changes with a change in calendar day. Use the hand to rotate the globe. Move the slider so the date is March 21, 2007. This date is an equinox.

Q1: How does the number of daylight hours change from the equator to the Arctic Circle (i.e. 66.5 ° N)? Ignore the values at and near the North Pole.

Q2: How does the number of daylight hours change from the equator to the Antarctic Circle (i.e. 66.5 ° S or -66.5 ° )? Ignore the values at and near the South Pole.

Q3: Based on your responses to Questions 1 and 2, how would you define an equinox?

Based on the definition you just wrote, it should make sense to you that there would be another equinox on or around September 21st. Think about this and make sure you understand it before you proceed.

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Move the slider so the date is June 21, 2007. This is the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere.

Q4: How does the number of daylight hours change from the equator to the Arctic Circle?

Q5:How does the number of daylight hours change from the equator to the Antarctic Circle?

Q6: What is the range of latitudes that receive 24 hours of daylight?

Q7: What is the range of latitudes that receive zero hours of daylight? (i.e. 24 hours of darkness)?

Q8: Based on your responses to Questions 4, 5, 6, and 7, how would you define a solstice?

Move the slider so the date is December 22, 2006. This is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere.

Q9: How does the number of daylight hours change from the equator to the Arctic Circle?

Q10: How does the number of daylight hours change from the equator to the Antarctic Circle?

Q11: What is the range of latitudes that receive 24 hours of daylight?

Q12: What is the range of latitudes that receive zero hours of daylight? (i.e. 24 hours of darkness)?

Background Information

The Sun is the primary source of energy for the Earth’s climate system. Click Sun to see images of the Sun taken today by the National Aeronautics and Space Administration. The Sun is huge and hot: it has a diameter of approximately 1,400,000 km, and it has an average surface temperature of approximately 5,500° C. For comparison purposes, the Earth has a diameter of approximately 12,700 km and an average surface temperature of 14° C. Therefore, the Sun’s diameter is 109 times larger than Earth’s diameter and the Sun is nearly 400 times hotter than Earth. The Sun’s extremely high temperature means that it emits an enormous amount of radiation: the luminosity (the total amount of energy emitted) of the Sun is 3.846 x 10²⁸ W. The distance from the Sun to the Earth is approximately 150 million kilometers; at this distance, Earth intercepts a tiny amount of the radiation emitted by the Sun. Just above the Earth’s atmosphere, the average amount of solar energy intercepted by a surface perpendicular to the sunlight is approximately 1,367 W m^-2. This value is known as the solar constant. Instruments on-board NASA satellites have been measuring the solar constant since 1978; click on the left-hand image below to see an example of such a satellite. Next to the satellite image is a graph showing changes in the solar constant. You should notice at the value for the solar constant changes from year to year, but you should also recognize a long-term pattern in the values which repeats approximately every eleven years; this is notice the solar cycle (this is an important cycle because peaks in the cycle can affect communication instruments). The third image below is a graph of the solar spectrum, which shows a breakdown of the radiation making up the solar constant. Most of the solar radiation is in the visible, near-infrared, and ultraviolet portions of the electromagnetic spectrum; a representation of the electromagnetic spectrum is the fourth picture in the set below.

For the Earth as a whole, only about half the solar radiation present at the top of the atmosphere reaches Earth’s surface. The solar radiation must pass through Earth’s atmosphere where it is absorbed by gases and particles and reflected back into space by clouds and particles. For example, the next lab focuses on the absorption of solar radiation by a gas, ozone, in a layer of the atmosphere known as the stratosphere. The left-side portion of the image below shows the absorption and reflection of solar radiation by the atmosphere and Earth’s surface. Note that the incoming solar radiation is 341.3 W m^-2, rather than the solar constant value of 1365.2 W m^-2. The solar constant is averaged across the entire surface of Earth; therefore, the value is divided by four. The 341.3 W m^-2 value is the total solar radiation reaching the top of the atmosphere per second (174,226,942,644,300,000 W) divided by the surface area of Earth (510,000,000,000,000 m²).

At the present time, Earth’s axis of rotation is tilted 23.5 °away from the perpendicular to its orbital plane. This is what causes changes in solar radiation received by locations on Earth over the course of a year. Examine the image below to visualize this tilt. Therefore, the Earth’s tilt is the cause of the seasons. It takes one day for Earth to complete a full rotation, and the Earth orbits around the Sun, which is typically 150 million kilometers away, once every 365.2564 solar days (i.e. one year). The axis remains tilted in the same direction towards the stars (particularly the North star Polaris) throughout a year. The tilt causes day-today changes in the duration and intensity of solar radiation at all latitudes.

Play the video below which shows the Earth revolving around the Sun and how the tilt of the Earth determines what latitudes receive the most solar radiation.

The seasons pertain to the Northern Hemisphere. Watch the animation several times and notice how the Earth’s axis is always pointed the same direction (i.e. it is always pointed towards the North Star). The distance from the Earth to the Sun is not the cause of the seasons: Earth has a nearly circular orbit with perihelion (i.e. Earth closest to the Sun) occurring on January 5 and aphelion (i.e. Earth farthest from the Sun) occurring on July 5.

When solar radiation intercepts a portion of Earth at a lower angle (i.e. the Sun is closer to the horizon), the energy is spread over a larger area, and is thus weaker than if the Sun were higher overhead with solar energy concentrated on a smaller area. Examine image below help visualize the effect of this ‘beam spreading’. A location with the Sun directly overhead (i.e. an altitude or angle of incidence of 90°) receives 40% more solar radiation per square meter (e.g., W m^-2) than will a location with the Sun at an altitude of 45°. The lower Sun angle (45°) causes the solar radiation to be received over a much larger surface area, which decreases the total amount of solar radiation in W m^-2.

Below is a graph showing the change in the intensity of solar radiation with a change in angle of incidence. Notice how solar radiation is 40% more intense (i.e. 100 units compared to 70 units) when the Sun is directly overhead as opposed to being at an altitude of 45°.

Another factor to consider is daylight hours; more daylight hours leads to more solar radiation. The intensity and duration of sunlight is greatest during the summer and least during the winter.

In order to help you better visualize the relationship between the intensity of solar radiation and light intensity, you are going to explore an on-line animation. Click Seasons_Interactive (courtesy of Astronomy Education at the University of Nebraska-Lincoln). By properly manipulating this tool, you will be able to do the following: (1) see the Earth’s position relative to the Sun on every day of the year (left-side panel); (2) identify where solar radiation is most intense (upper-right panel); (3) and visualize the intensity of solar radiation at any latitude (lower-right panel). You should play with this tool for a few minutes before answering the questions below to insure that you can get the most out of it. Also, you are strongly encouraged to examine the four annotated screen shots below for June 21 for information on exactly what is shown in the different windows of the tool.

You are going to revisit the location you saw in the beginning of the lab that received 24 hours of solar radiation on June 21. The video is below for you to view again. This time pay attention to the highest altitude the sun reaches. This is the noon altitude of the sun. You can figure out the value of the latitude by moving back and forth between the video and the interactive tool. In the interactive tool, you will need to change the date to June 21 and move the observer latitude to the latitude you determined with your solar-motion demonstrator earlier in the lab. Make sure you have “orbit view,” “view from sun,” and “sunlight angle” selected.

Q13: What is the Sun’s altitude at the latitude of the location in the video 24 Hours? This altitude is in degrees is equivalent to the noon altitude of the Sun.

Q14: How does the Sun’s altitude calculated using the Season Interactive Tool tool compare to the altitude you determined from the 24 Hour video and the Solar Motion Demonstrator during the Prezi?

Q15: What will be the Sun’s altitude on the other solstice?

Q16: How many of hours of solar radiation does the location receive on the other solstice? You can use Daylight_Hours to determine this, but it is not necessary.

Q17: What will be the Sun’s altitude on either of the equinoxes?

Q18: How many hours of solar radiation does the location receive on either of the equinoxes? You can use Daylight_Hours to determine this, but it is not necessary. In the upper-right panel of the Seasons Interactive tool, you currently have it set for ‘view from sun’. Take one last look at the features of the Earth and Sun in this setting. Now switch to the ‘view from side’ setting. Look again at the features of the Earth and Sun. You should notice a significant flaw in one of these features in this representation.

Q19: What is that flaw in the representation and how could that flaw lead to a misunderstanding about the cause of the seasons?

Click Atlanta&Barrow to open the file in Google™ Earth. You will be exploring the number of daylight hours, the noon altitude of the sun, and total solar radiation at Atlanta, Georgia USA and Barrow, Alaska USA. Within Google™ Earth, go to Tools and Options… and make sure Decimal Degrees is selected (it is in in the Show Lat/Long box).

Q20: What is the latitude (in decimal degrees) of Atlanta? Double-click the Atlanta “bullseye” to zoom into downtown Atlanta. The latitude, longitude, and elevation of downtown Atlanta are shown at the bottom of the screen.

Type “Barrow, Alaska” in the “Fly to” space in order to fly to Barrow. Double-click the Barrow “bullseye” to zoom into Barrow. The latitude, longitude, and elevation of Barrow are shown at the bottom of the screen.

Q21: What is the latitude (in decimal degrees) of Barrow? (You should recognize that the latitude of Barrow is similar to the latitude of the location in the 24 hour video.)

Q22: What is the difference in latitude — to the nearest degree — between Atlanta and Barrow?

Q23: Which location do you expect to have more differences in the solar radiation and in noon altitude of the sun throughout the year? Why?

Below is a graph of solar intensity for Atlanta, GA and Barrow, AK over a one year period. Solar intensity is the amount of solar power (energy from the sun per unit time) per unit area reaching a location of interest. Use the graph to answer the question below.

Q24: On all days of the year, how much higher is the angle of the Sun over Atlanta compared to Barrow?

Below is a graph of Solar Duration, i.e the number of daylight hours per day. Use the graph in answering questions 25-26

Q25: Why did both places have approximately the same number of daylight hours on March 21 and September 21?

Q26: How many more daylight hours did Barrow have in June than did Atlanta?

Q27: Based on both the noon altitude of the Sun (i.e. intensity of solar radiation) and the number of daylight hours (i.e. duration of solar radiation), what place do you think receives more solar radiation in June and why?

Once you have answer this question, click on Total_Solar_Radiation to check your answer. If your answer is wrong, think about why you answered the question the way you did.

Q28: What place received more solar radiation in June and what does this indicate about the relative importance of intensity and duration with respect to total solar radiation?

View average monthly temperatures at Atlanta and Barrow.

Q29: What is the major reason for Atlanta being warmer than Barrow during each month?

Click January_Temperature to open the file in Google™ Earth. The image shows mean land-surface temperatures from 2000-2008 derived from data acquired from a MODIS instrument aboard NASA’s Terra satellite. Also notice that if you move your cursor to the Southern Hemisphere it will have a negative latitude and if you move your cursor to the Western Hemisphere it will have a negative longitude. For example, the location of Rio de Janeiro, Brazil is -22.9° latitude and -43.2° longitude.

Q30: What is the general relationship between latitude and temperature in the Northern Hemisphere?

Q31: What is the warmest land mass in January and why?

Q32: You may have noticed that the color bands representing similar temperatures did not always follow exactly the lines of latitude. This indicates that there are factors other than latitude which affect temperature; identify two or three other factors which you think might affect temperature.

Lab Essential Questions

Q33: What is the cause of seasons?

a. the change throughout the year in the distance between the Sun and Earth

b. a change throughout the year in the amount of solar radiation received at a location

c. a change throughout the year in the Earth’s tilt

d. a change throughout the year in the number of sunspots

Q34: What are the two factors that impact the amount of solar radiation the Earth receives and which one has more of an impact?

a. intensity and duration of sunlight; intensity is more important

b. intensity and duration of sunlight; duration is more important

c. intensity and quality of sunlight; quality is more important

d. duration and quality of sunlight; quality is more important

Q35: What is the relationship between latitude and surface temperature? Keep in mind that latitudes have positive values in the Northern Hemisphere and negative values in the Southern Hemisphere.

a. in the Northern Hemisphere, there is an increase in temperature with an increase in latitude

b. in the Southern Hemisphere, there is a decrease in temperature with an increase in latitude

c. in the Western Hemisphere, there is an decrease in temperature with an increase in latitude

d. in the Eastern Hemisphere, there is an increase in temperature with an increase in latitude