What is a Greenhouse?

Background

Greenhouses are used extensively by botanists, commercial plant growers, and dedicated gardeners. Particularly in cool climates, greenhouses are useful for growing and propagating plants because they both allow sunlight to enter and prevent heat from escaping. The transparent covering of the greenhouse allows visible light to enter unhindered, where it warms the interior as it is absorbed by the material within. The transparent covering also prevents the heat from leaving by reflecting the energy back into the interior and preventing outside winds from carrying it away. 

Like the greenhouse covering, our atmosphere also serves to retain heat at the surface of the earth. Much of the sun's energy reaches earth as visible light. Of the visible light that enters the atmosphere, about 30% is reflected back out into space by clouds, snow and ice-covered land, sea surfaces, and atmospheric dust. The rest is absorbed by the liquids, solids, and gases that constitute our planet. The energy absorbed is eventually reemitted, but not as visible light (only very hot objects such as the sun can emit visible light). Instead, it's emitted as longer-wavelength light called infrared radiation. This is also called "heat" radiation, because although we cannot see in infrared, we can feel its presence as heat. This is what you feel when you put your hand near the surface of a hot skillet. Certain gases in our atmosphere (known as "trace" gases because they make up only a tiny fraction of the atmosphere) can absorb this outgoing infrared radiation, in effect trapping the heat energy. This trapped heat energy makes the earth warmer than it would be without these trace gases. 

The ability of certain trace gases to be relatively transparent to incoming visible light from the sun yet opaque to the energy radiated from earth is one of the best-understood processes in atmospheric science. This phenomenon has been called the "greenhouse effect" because the trace gases trap heat similar to the way that a greenhouse's transparent covering traps heat. Without our atmospheric greenhouse effect, earth's surface temperature would be far below freezing. On the other hand, an increase in atmospheric trace gases could result in increased trapped heat and rising global temperatures.

Learning Goals

1. Students will understand how greenhouses work to retain heat.
   

Alignment to National Standards

National Science Education Standards

• Unifying Concepts and Processes, Grades K to 12, pg. 117: "Models are tentative schemes or structures that correspond to real objects, events, or classes of events and that have explanatory power."
  
• Physical Science, Transfer of Energy, Grades 5 to 8, pg. 155, Item #2: "Heat moves in predictable ways flowing from warmer objects to cooler ones, until both reach the same temperature."
  
• Earth and Space Science, Grades 9 to 12, pg. 189, Item #3: "Heating of earth's surface and atmosphere by the sun drives convection within the atmosphere and oceans, producing winds and ocean currents."

Benchmarks for Science Literacy, Project 2061, AAAS

• Common Themes, Models, Grades 6 to 8, pg. 269, Item #1: "Models are often used to think about processes that happen too slowly, too quickly, or on too small a scale to observe directly, or that are too vast to be changed deliberately, or that are potentially dangerous."
  
• The Physical Setting, Energy Transformations, Grades 6 to 8, pg. 85, Item #3: "Heat can be transferred through materials by the collisions of atoms or across space by radiation. If the material is fluid, currents will set up in it that aid the transfer of heat." 

Grade Level/Time

• Grade level: 5 to 9
  
• Time:
  
  
  • Introduction by teacher: 15 minutes
    
  • Student activity (including bottle construction): 50 minutes
    

Materials

For each team of four students:

• Two two-liter plastic soda bottle "experimental chambers" (instructions to follow)
  
• Two 14- to 16-oz. plastic containers at least 4 1/2 inches in diameter at the top (sour cream, cottage cheese, or deli containers work well)
  
• Knife or scissors
  
• Tape
  
• Two thermometers
  
• One 150-watt floodlight bulb
  
• Portable reflector lamp
  
• Stand for lamp set-up
  
• Graph paper 

Experimental chamber construction

For each chamber, you will need a two-liter plastic soda bottle (with cap) and a 14- to 16-oz. plastic container for the base.

1. Remove the bottle label by soaking it in warm water.
   
2. Cut off the end of the bottle approximately 2 inches from the bottom and discard the bottom piece.
   
    
   
3. Place the capped bottle in the plastic base and the experimental chamber is ready for use.
   

Procedure

1. Organize students in teams of four.
   
2. Each team should use scissors to cut several elongated vents (1 x 4 inches) in the sides of one of the bottles.
   
   
   
3. Leave the second bottle intact.
   
4. Tape a thermometer (using cellophane tape or light-colored masking tape, not black electrical tape) to the inside of each bottle (facing out). Make sure the bulbs of the thermometers are above the top of the chamber base. (See graphic below.)
   
5. Place caps on both bottles.
   
6. Place both bottles approximately six inches away from the lamp with the thermometers facing away from the light. 
   
7. Ask students to predict which bottle will get hotter when you turn on the light.
   
8. Turn on the light and begin collecting data every minute for 20 minutes.
   
9. Have students graph data.

Observations and Questions

1. Compare and contrast the graphed data from the vented bottle and the intact bottle. What happened? How do you explain your observations?
   
2. Discuss the results with your class and develop some possible explanations (for example, the vents let cool air in).
   
3. Compare and contrast your plastic greenhouse to the greenhouse effect on earth.

Cautionary Note: The analogy between the plastic cover and the atmosphere is not a perfect one. Greenhouse covers prevent heat losses from convection (air movement carrying away the heat) as well as by radiation (direct transfer of heat energy). The atmosphere prevents only heat loss by radiation. The greenhouses used in this activity serve as a crude model of the actual atmospheric process and are only of limited use in understanding the nature and scope of the actual Greenhouse Effect.

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