How Sunlight Warms the Earth

Background Information for Teachers

The Sun is the main source of heat and light on Earth. Without it, we could not survive on our planet. The Sun is about 93 million miles (150 million kilometers) away, but because the Sun's surface is so hot (about 10000°F) and bright, we on Earth can still see the light it radiates. The Sun acts like a gigantic bonfire, sending light in all directions, including to Earth.

Image Credit: Pixabay / Creative Commons Attribution 2.0 Generic license
Figure 1. The Sun illuminates Earth.

The light radiated by the Sun carries energy. When this light reaches a surface, part of it gets absorbed and transformed into heat. That is why places in the sun feel warmer than places in the shade. Your kindergarten students know this from experience. On a cold day, they might prefer to play in the sun; on a hot day, they search for a cool spot in the shade. This lesson helps your students to explore what being in the sun does to the surfaces that cover our Earth. A follow-up lesson, Create Shade to Protect from the Sun, explores creative solutions to protect a surface from the effects of the Sun.

Earth's surface is covered with water (about 70%!), soil, rocks, snow, and ice, and a wealth of man-made materials like concrete. Note that when viewed from space, you could also say that plants (like trees and shrubs) cover the surface of the Earth. For the purposes of this project, we are concerned with the Earth's surface at ground level—so we consider larger plants and shrubs to be things that provide shade to other materials on Earth's surface.

When Earth's surfaces are exposed to the sun, they absorb some of the light and become warmer. In this lesson, your students will feel how soil, rocks, and water feel warmer after being in the sun. If you allow students to feel at different moments, they might notice that this warming effect does not happen instantly, but gradually; this process takes time. They might also notice that some materials warm less or less quickly than others.

Additional Background Links

• What is the Greenhouse Effect?, NASA
• Radiation from the Sun, Universe Today

Prep Work (10 minutes)

• In the days preceding the lesson, make sure you have an area (preferably inside) where you can leave about half of the cups in the sun, and an area where you can leave the other half in the shade, away from direct sunlight and other heat sources. Whether you can do this depends on your geographic location, the time of year, and the orientation/size of windows in your classroom as well as the weather. If you cannot get enough direct sunlight, then you can use a 60 watt or greater incandescent light bulb instead.
• Store all materials in the shade and away from a direct heat source prior to the lesson.
• Dry soil as much as possible. For example, if you use a new bag of potting soil, open it prior to the lesson. If you can, spread the soil out so the moisture can evaporate faster.
• Use a permanent marker to draw a sun on half of the cups and a cloud on the other half. Draw the symbols near the bottom edge of the cups. When filled, the soil or rocks should cover the symbols.
• Create three labels to indicate work stations. You can use a picture, a drawing, or words to indicate "rock," "soil," and "water."

Engage (15 minutes)

On sunny days, people sometimes try to fry an egg on the sidewalk, like in this video: https://www.youtube.com/watch?v=0MPNgCo9auk Create Assignment Embed Link Google Classroom Microsoft Teams Canvas Schoology Other Link copied to clipboard. What do you think when you see this video? Why do you think they were successful at frying the egg? Would it work any day, or only on a particular day? On what surface do they try to fry the egg? Would it work on any surface?

Listen to the students' questions and ideas.

In the video, they tried to fry the egg on a metal surface. The Earth around that metal surface is covered with concrete. Do you know a place where the earth is covered with concrete?

Answers might include the sidewalk, the street, or parts of the playground.

Not all of Earth's surfaces are covered with concrete. What else is at the surface of Earth? Ask students to include examples to support their claims. Optional: Use the pictures included in the slideshow as hints.

Make sure the answers include the following: Water Soil—sand is a type of soil Rocks or pebbles Answers might also include concrete, bricks and other man-made surfaces, wood chips, mulch, etc. If students mention trees, shrubs, or other large plants, explain that while these plants grow on the surface of Earth, they are not the surface of Earth (e.g. you cannot walk on them). Ask students what is under these plants that covers Earth.

Do you think we can fry an egg on any of these surfaces?

Listen to student's answers. Explain that to find out the class will examine what happens to rocks, water, and soil when they are left in the sun for a while.

How could we find this out?

Students might mention going outside, finding these surfaces, and observing what happens when they are left in the sun for a while. Explain that instead of going outside, the class will bring these materials into the classroom. They will place rocks, soil, and water in cups, then leave some in the sun and some in the shade and observe what happens.

Are there places in the class that are in the sun right now? If so, where are they, and how do you see that they are in the sun? Note: If the lights are on in the class, you might want to turn them off. This allows students to clearly see the distinction between areas receiving light from the sun and areas that are not.

Let the students point to these areas in the class. If the areas are large enough, you can have students walk over and stand in them. You can see that an area is in the sun because it receives more light, so everything looks brighter.

Now, find some places in the shade.

Explain that sunlight can go through windows but not through walls, so near the windows, we can usually find places in the sun. The pictures in the slideshow can help clarify the concept of being in the sun and in the shade.

Explore (45 minutes)

Explain that to start the experiment, students will fill cups with soil, rocks, and water, and place some in the sun and some in the shade. Later, they will be able to see and feel the effect of being in the sun.

1. Place your materials at three stations: one with soil, one with rocks, and one with access to water. Make sure the stations are not in the sun. Each station should have a towel or paper towels for students to wipe their hands.
2. Divide the class in three groups of about equal size and assign each group to a station.
3. Explain to students that they will receive a cup and need to fill it. Show an example and explain how high you want the cup to be filled. Opaque or translucent cups need to be filled to about 1/2 inch of the brim, so the sunlight can reach the surface. Transparent cups can be filled halfway as shown in Figure 2.

Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
Figure 2. Cups with water, soil, and rocks are placed in the sun to investigate the warming effect of sunlight.

4. At each station, give half of the students a cup with a sun symbol drawn onto it, and the other half a cup without it. Ask students to fill the cup. For any station with a smaller group, fill one cup yourself so the class has an equal number of cups filled with water, soil, and rocks.
5. Direct students to store the cups in the sun or in the shade. Cups that have a sun symbol need to be stored in the sun until you observe them, and the other cups need to be in the shade. Note that the position of the sun shifts during the day, so move the cups as needed.
6. Explain to the class that all the materials they used to fill the cups started in the shade. They will come back to the cups later in the day to observe what happened. Now, have the class discuss in small groups what they think will happen.

   Tell one or two other students what you think you will see or feel when we look back in a few hours. (Provide a time indication students understand, like after lunch or recess). What will happen to the cups that we put in the sun? What will happen to the cups that stayed in the shade? Why do you think this is the case? Remember to take turns and listen to each other.

   Listen to your students' discussions without correcting them. Encourage them to give explanations.

   If your students are not familiar with a thermometer, show them a demonstration of your thermometer. Ask students what it is, what it is used for, and how it works. If one or more students know, have them explain it to the class and add to this explanation where needed. After this discussion, students should know that a thermometer can be used to measure the temperature and that the level of the liquid in the central tube rises when the thermometer heats up and falls when it cools down. To assess students' understanding, demonstrate how the liquid rises when you warm the reservoir with your hand, and ask students to explain what just happened.

7. After some time, prepare students for the observation. Note: You can feel a difference in temperature after the materials are stored for 15 minutes in the sun. However, the cooling process when removed from the sun will be fast. Storing them longer (1.5 hours or more) in the sun will create a bigger temperature difference and/or a slower cooling process. These both provide more time for observation.

   We were wondering why people would try to fry an egg on the concrete sidewalk. To find out, we decided to do an experiment and see what happens to rocks, water, and soil when these are left in the sun for a while. We filled cups with rocks, water, and soil and placed some in the sun and some in the shade. What do you think we should do next?

   Students might state, "We should go look at the cups." Help them to be more specific. At the end, the class should come to the following conclusions: We should observe (look and feel) what happened. We should concentrate on the surface of the material in the cup (The surface is the part that was directly exposed to the sun). We should compare how the materials that have been in the sun look and feel different from the ones that have been stored in the shade. If students suggest measuring the temperature with the thermometer, agree that it would be a good idea. However, explain that if you put the thermometer into the soil, you will measure the temperature inside the soil, not the temperature of the surface. You need a different kind of thermometer to measure the surface temperature of materials. Invite them to come up with different ways of probing the surface temperature.

   Label the station where students will compare the cups with rocks, the cups with soil, and the cups with water.

8. Ask students to pick up one cup and walk to the correct station. Half of the students will get a cup from the sun and half will get one from the shade. Each student will only get one material. Students with a cup of soil should go to the station labeled "soil," etc. (Note that students do not need to find the cup they filled; any cup is fine.)
9. Let students observe one material.

   Observe how the surface of the material in the cup you are holding looks and feels. (Give students time to make their observations.)
   
   If your cup has a sun, pair up with a student who has a cup without a sun. Does the surface of the material in their cup look or feel different? Tell each other what you observe, and why you think this is.

   Hide the symbol on your cup with your hand and pair up with a different student at your station. Take turns to look at and feel the material's surface in your partner's cup. Do you think this cup has been in the sun? How can you tell?

   Note: Students who have cups with water will still be able to see the symbol through the inside of the cup. Encourage them to look at and feel the surface of the water to confirm whether it was in the sun.

   Once the group work has ended, ask students to leave cups without a sun symbol at the station and place the cups with a sun back in the sun. Placing these cups back in the sun prevents these materials from cooling too much while the students fill out their worksheets. This is not needed if the materials were in the sun for a long time, but essential if they were not.

10. Ask students to draw what they observed on their worksheet.
    • Explain that the top row shows cups with a sun. It represents the cup that was placed in the sun. Find the cup that contains the materials you just observed. Color the thermometer to show how the surface of this material felt. Color all the way if it felt hot, halfway if it felt warm and only a little if it felt cold.
    • The cup just below it represents the cup with the same material that was placed in the shade. Color the thermometer to show how this material's surface felt.
11. Rotate groups to another station so that students who observed the water cups will now pick a cup with soil, etc. Students should then repeat steps 9 and 10 with their new material.
12. Repeat step 11 so all students can observe all three materials.

    A sample drawing of a student's observations is shown in Figure 3.

Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
Figure 3. Example drawing of a student's observations.

Reflect (10 minutes)

What did you see or feel? Was it similar for all materials?

Students probably noticed that all three materials that were placed in the sun felt warmer than the same material stored in the shade. It might have been harder to notice the difference for water, because it did not warm up as much as the rocks or the dirt. There was probably little or no visible change in the materials. The surface of soil placed in the sun might have looked slightly dryer if the original sample was moist.

Why do you think this happens?

The Sun sends out light, which heats the surfaces of materials that it hits. In time, these materials can feel warm or even hot. Materials in the shade do not receive this light, so they do not warm up.

Was it easy to tell if a cup had been in the sun or in the shade? Was it easiest for water, soil, or rocks? Which one was the hardest? Why do you think this is the case?

Students might have noticed that it was hardest for water and easiest for soil or rocks. This is because the rocks and soil warm up more easily when placed in the sun compared to water.

Data obtained by Science Buddies staff, listed in Table 1, gives you an idea of how much difference in surface temperature to expect.

Note that soil and rocks also cool faster than water when placed in the shade after sun exposure. This should not be an issue if students put the cups back in the sun while filling in their worksheets. If you cannot put the cups back in the sun, leave them in the sun for a few hours before starting observations.

Why do you think we needed to wait a while before we could make observations? Why could we not feel the materials right away when we put them in the sun, to see if they warmed up?

It takes time for things to warm up. We had to wait a while so the materials would heat up enough for us to feel a difference.

Do you think this also happens outside? Would soil, rocks, or water in the sun feel warmer than soil, rocks, or water in the shade?

Yes. Invite students to feel next time they are outside how the soil in the garden warms up in the sun, or how the lake warms up when it is in the sun for a long time. When the sun warms up the concrete sidewalk, it is like the rocks in your project. Make sure to repeat the link between being in the sun and warming up several times during the discussion.

Did what we learned help us understand why some people try to fry an egg on the pavement on a sunny day?

Listen to the answers and see if you can detect any remaining misconceptions. Show them the video again and see if they can explain. The street cover is like the rocks. When it is in the sun for an extended period, it gets warm and sometimes it can get warm enough to fry an egg on it, as shown in the video. Note that in the video, they chose a darker surface on the street (a manhole cover) and not the concrete on the street. As darker surfaces often warm up more easily than lighter ones, it might not have worked as well on the street.

Assess

Ask students if the thermometers they colored on their worksheets show what they learned in this lesson, and why this is the case.

Ask students outside to show you the following:

• A place in the shade
• A place in the sun
• An object that is warm or will get warmer soon and why it is warm or will get warmer soon

Ask them to explain their answers.

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Environmental Scientist

Add Favorite Remove Favorite More Menu

• Read More

Career Profile

Environmental Scientists watch out for our environment. These scientists use their knowledge of the Sun's warming of surfaces on Earth to predict what will happen if we, by building a city, roads or a huge amusement park, change a large part of Earth's surface. Read more

Climate Change Analyst

Add Favorite Remove Favorite More Menu

• Read More

Career Profile

Climate Change Analysts or Climate Scientists study how overall weather conditions (like temperatures and rainfall) change over time, and how these changes affect people, animals, and the environment. They might examine how overall weather patterns are affected by the Sun warming different parts of Earth's surface. Or, for example, they might investigate whether adding lots of roads and making cities bigger could change weather conditions. Read more

Lesson Plan Variations

• Instead of having each student individually fill a cup, you can use four larger pie pans, two for soil and two for water, and a few larger pebbles. For the observation, you can use a group discussion at each station (one for each material), and skip the exercise where students observe cups and try to figure out whether they were in the sun or shade.
• Compare how color affects the warming effect of the sun by repeating the experiment with light and dark colored rocks and/or sand and potting soil.
• If you have an infrared thermometer, show it to the students and explain how this thermometer measures the temperature of surfaces. Use it to measure the temperature of the surface of the soil, water, and rocks and show how the exact measurements confirm what the students felt with their hands, or to confirm how warming takes time or how some materials warm more easily than others.
• See how warming takes time by letting the students feel the material surfaces at different intervals: instantly, after 5, 10, and 20 minutes in the sun.
• Study what happens with the materials after they have been removed from the sun and put back in the shade. Do they stay warm? Do they cool down? Do they cool fast or slowly?
• Study how moist soil dries out faster when it is left in the sun compared to when it is left in the shade.
• Let students explore what happens in more detail. Let them poke their fingers inside the water and soil. Does the material feel warm under the surface? Does that warm up as well? If you wish to use exact measurements, you can use an immersion thermometer to measure the temperature under the surface.
• Study how evaporation reduces the amount of water in the cup faster when the cup with water is left in the sun. Note it will take a considerable amount of time before a noticeable difference in water level can be observed. Leaving about 1 millimeter of water in the cup or on a plate will make it easier to observe as you can see when the water disappears.
• Use the follow-up lesson to allow students to design solutions to protect the surface of the earth from the effects of the sun.