Have you ever walked across a large parking lot on a sunny summer day and felt like you were roasting? That’s because the asphalt gets really hot in the sun! Streets, buildings and parking lots can get so hot, they raise the average temperature of urban areas by a few degrees relative to surrounding rural areas. Do you think natural materials also heat up in the sun—or only human-created materials? Try this activity to find out!
The sun is some 93 million miles (150 million kilometers) from Earth. But because its surface is so hot (about 10,000 degrees Fahrenheit), we on Earth benefit from the light it radiates—without it humans could not survive. The light radiated by our sun carries energy, part of which gets absorbed and transformed into heat when it reaches a surface. That is why places in the sun feel warmer than those in the shade. Another part of the light is reflected. That part makes surfaces look lighter and brighter in sunlight.
Earth is covered with water, soil, rocks, snow and ice as well as a wealth of human-made materials, such as concrete. When these surfaces are exposed to the sun, they warm up. This process takes time—and some materials, such as water, need more light from the sun to warm up a few degrees than do others, such as soil or rocks. Once warmed, these surfaces release heat, which warms the air above them. This process is also gradual, and some materials including water hold onto heat longer than do others, such as soil or rocks. This difference generally makes the temperature of coastal climates more moderate than those inland. But other natural factors, such as trees and plants, moderate the air temperature, too. Plants absorb water from the ground and transpire (release water into the air), which works like a natural air conditioner.
Many factors determine how much materials heat up in the sun and how they release this energy. In this activity you will test some materials yourself.
- Sunny day
- Metal wire (about one meter)
- Partially shaded area outside with at least two different ground covers (dirt, grass, rocks, concrete, etcetera)
- Find the line on the ground where an area in the sun meets an area in the shade. Place your wire on this line.
- Take a big step into the shade. What covers Earth’s surface in this particular spot?
- Find a spot in the sun where the surface is covered with the same material. Choose one that is at least one big step away from a shaded area. Does the spot in the sun look different compared with the spot in the shade? Why do you think this is the case?
- Do you think these spots will feel different, too?
- Carefully touch both places with your hand and compare how they feel. Does one feel warmer than the other? If so, which one feels warmer? Why would this be the case?
- If it’s possible and you have permission, dig up about inch of surface material in each place and feel the layer beneath it. This may not work in some locations or with some materials. (For example, you shouldn’t dig holes in a public park, and you can’t dig holes in concrete.)
- For the material in the sun, do the surface and the layer underneath feel similar or is one warmer? Do you note the same findings for the material in the shade?
- Find a different material. Can you find one spot where the surface is covered with this material in the shade and one in the sun? If so, do these spots look different? Do they also they feel different?
- If you can, repeat this investigation with a third material: include water, soil or another natural material!
- Do you see a pattern? Why would this happen?
- Go back to the wire you placed at the edge of the shade. Can you see how the edge of the shade moved? If it hasn’t moved, wait 10 minutes and then come back. Why would the edge of the shade move?
- Depending on which way the shadow moved, you now have a new area in the shade that was recently in the sun or vice versa. Investigate how this area looks and feels, compared with areas that have been in the sun or the shade for a long time. Do you notice any differences?
- Extra: If you found a pattern, test it on more materials the next time you are in the park, at a playground or an outdoor sports field—or repeat the procedure inside. Do the same differences occur? Why or why not?
- Extra: Place cups of water, dirt and/or pebbles in the shade and identical cups in the sun. Investigate how the materials in the sun look and feel compared with those in the shade. Note you cannot compare the temperatures of two different materials (for example, rocks and dirt) by feeling them with your hands, because different materials at the same temperature can feel very different. You can compare the same materials placed in the sun and shade or use an infrared thermometer to compare the surface temperature of different materials.
- Extra: Try to find out if there is a correlation between color and how much a material warms up when placed in the sun. Do dark soil or pebbles warm up more than, less than or around the same amount as light colored soil or pebbles? What about darker- and lighter-colored cement?
- Extra: Find information on the urban heat island effect and how cities try to combat it. Can you design a small-scale test to investigate if these ideas work?
Observations and results
Did the materials look brighter and lighter in the sun? Did they also feel warmer?
Our sun radiates light; some of it reaches Earth. When this light hits soil, pebbles, grass, cement or any other material that covers our planet, part of it bounces back. This reflected light makes the material look brighter and lighter. The other part of the light gets absorbed or sucked into the material and warms its surface. That is why materials in the sun feel warmer when you touch them. Warm materials heat the air surrounding them, which starts to feel warmer, too.
Maybe you noticed a material became just a little bit warmer after the sun initially shone on it—and after a while became much warmer. Warming is a process that takes time—just as when a material in the sun moves to the shade it cools down gradually.
Some materials, such as cement, dark rocks or even sand, can get very hot when left in the sun for a while. Water warms, too, but it needs to stay in the sun longer to heat up by the same amount; it also cools down more slowly than soil or cement.
More to explore
Simmering Science: Can Your House's Color Reduce Your Summer Energy Bill?, from Scientific American
What Is an Urban Heat Island?, from NASA Climate Kids
Sun's Effect on Earth, by Windows to the Universe
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies