The Greenhouse Effect: Heat Transfer in the Atmosphere
The gases in the Earth's atmosphere are warmed by heat radiated from the Earth's surface. These warmed gases surround the Earth and act like a blanket, keeping the Earth warm.
In this project, you will demonstrate the greenhouse effect. You will discover how materials of the Earth's surface affect the greenhouse effect. You will examine the relation between the greenhouse effect and surface temperatures at night. You will determine how composition and density of the atmosphere affect its ability to trap infrared energy. You will also show how cloud cover affects the surface temperatures at night.
Purpose: To demonstrate the greenhouse effect.
- two shoeboxes
- two thermometers
- colorless plastic food wrap
- Cover the bottom of each shoebox with about 2 inches (5 cm) of soil.
- Lay a thermometer on the surface of the soil in each box.
- Cover the opening of one box with a single layer of plastic wrap. Leave the other box uncovered.
- Take readings from both thermometers.
- Place both boxes side by side in a sunny place outdoors (see Figure 25.1).
- Record readings from both thermometers every 15 minutes for 1 hour.
The temperature readings show that the temperature inside the plastic-covered box was higher and increased faster.
Radiant energy from the Sun passes through the Earth's atmosphere and reaches the Earth's surface. However, about 30% of the Sun's total radiant energy reaching the Earth is reflected back into space by the atmosphere, the clouds, and the Earth's surface. About 20% is absorbed by the atmosphere, and the remaining 50% is absorbed by the Earth's surface. Radiant energy absorbed by the Earth is changed into heat or infrared rays (invisible form of radiant energy that has a heating effect). Some of this heat from the Earth warms the atmosphere above it. This heat is transferred to the atmosphere by conduction (transfer of heat by direct contact between materials), convection (transfer of heat by the movement of heated fluids-gases and liquids), and radiation (transfer of heat by emission of heat waves). Carbon dioxide and water vapor are gases in the atmosphere that help keep heat from being lost to space. They absorb heat from the Earth and reradiate (emit previously absorbed radiation) it toward the Earth. Like the plastic covering that prevents the escape of some of the infrared rays radiated from the soil, the Earth's atmosphere keeps the Earth warm. The term greenhouse effect comes from the fact that the atmosphere is similar to a greenhouse in that it helps warm the Earth's surface by trapping infrared energy.
Try New Approaches
- What effect do surface materials have on the greenhouse effect? Repeat the original experiment, preparing boxes with different surfaces by covering the soil with different materials, such as sand, rocks, and grass. A surface of water could be prepared by lining the box with plastic and filling it with about 2 inches (5 cm) of water instead of soil. Science Fair Hint: Display photographs of the various boxes with the results of the experiment. Include a display, such as the one shown in Figure 25.2, indicating the percentage of radiant energy reflected back into space. For information about reflected radiant energy, see Jack Williams, The Weather Book (New York: Vintage Books, 1992), p. 19.
- What is the relation between the greenhouse effect and surface temperatures at night, in the absence of the Sun's radiant energy? Repeat the original experiment, taking temperature readings while the boxes are in direct sunlight outdoors. Then place the boxes in a dark area indoors. Again, take readings from both thermometers every 15 minutes for 1 hour.
- Could the composition of the atmosphere affect its ability to trap infrared energy? Compare different materials for their ability to trap infrared energy. Repeat the original experiment using plastic wrap and other materials, such as waxed paper, clear Plexiglas, and glass. Science Fair Hint: Display samples of the box covers with the results of the experiment.
- Density is the mass of a substance per unit volume. The greater the density of a substance, the closer together its particles. How does the density of the atmosphere affect its ability to trap infrared energy? Repeat the previous experiment twice, first using two layers of material, then using three layers. Science Fair Hint: Compare the results of the experiment to surface environments on celestial bodies with little or no atmosphere, such as the Moon and Mars, and those with a dense atmosphere, such as Venus. Use an astronomy text to find out about the atmosphere of the different celestial bodies. Make charts showing the composition and density of their atmosphere and surface environment.