Radiation Budget Lesson: Exploring Albedo

Recommended Age:
Intermediate Level (Grade 5-8)

Guiding Question:
What effect does albedo have on surface temperature?

Concepts:

Albedo is the percentage of incoming sunlight that is reflected, rather than absorbed.

Principles:

1. Albedo is represented as a percentage of the Earth's total incoming energy. Thus, an albedo of 50% would indicate that half of all incoming radiation is reflected. In general, the more radiation that is reflected usually means lower overall surface temperatures.

2. Albedo represents an important aspect of the radiation budget.

3. The Earth Radiation Budget is a concept that uses the amount of energy the Earth gets from the sun and the amount of energy the Earth sends back to space. If the Earth receives more solar energy than it sends back to space, we expect the Earth to warm. If the Earth sends more energy than it receives from the Sun, we expect the Earth to cool.

4. In general, more lightly-colored surfaces (snow and ice, for example) have a higher albedo than darker-colored ones (trees, blacktop roads, etc).

Facts:

1. The overall albedo of the Earth is thought to be about 30%.

2. ERBE instruments collect data concerning the Earth's albedo.

3. The concept of albedo explains (for example) why white robes are favored in desert regions.

Skills:

1. Experimenting and making measurements
2. Drawing conclusions

Applicable National Standards (Science, Level 5-8):

1. Standard B, "Transfer of Energy," items 1, 5, and especially 3 and 6.
2. Standard D, "Structure of the Earth System," item 10.

Materials:

The number of materials will depend on how many students and how many lab teams are working on the activity.

1. Thermometers (3 per lab team).
2. Colored paper (black) and white paper (1 sheet each per lab team).
3. One paper cup of room-temperature water per lab team.
4. Earth Radiation Budget trading card set.
5. Student Lab Sheet. (Printable version)
6. High-intensity lamps (optional for indoor activity. If used, one lamp per group is suggested.).

Room Preparation:

As most of the lesson will take place outside, no room preparation is necessary. In the absence of warm, sunny weather, the room can be set up with a number of high-intensity lamps as "suns".

Safety Precautions:

1. Students should report broken thermometers immediately - both broken glass and mercury have a high hazard potential.
2. If lamps are used, make sure students are careful not to let clothes or skin touch the bulb or metal shade (if any).

Prelab Discussion:

1. Introduce the Earth radiation budget trading cards so students will understand the concept of radiation budget, and in particular the Data card which explains albedo.

2. Ask students if they would be hotter on a sunny day wearing black or white colored clothes. Ask why they think so, by guiding them to realize that because white is "brighter" (i.e., has a higher albedo), it is correspondingly cooler; black garments reflect little sunlight and are thus warmer.

3. Review variables - independent and dependent.

Activity:

1. Distribute materials among lab teams. Each lab team should wrap one thermometer tightly in black paper. A second thermometer should be wrapped tightly in white paper, and the third thermometer should be submerged in the cup of water. All three thermometers should then be put in the Sun (or underneath the lamp).

2. The temperature readings for all three thermometers should be checked and recorded every five minutes, for a total of 10 minutes. At the end of the first 5-minute waiting period, students should rank the three materials (white paper, black paper, and water) in order, from the highest to the lowest albedo, as a working hypothesis.

3. Each of the three materials (white paper, black paper, and water) should be rated for albedo again at the end of the final 5-minute waiting period, this time using the idea that a higher albedo will yield a lower final temperature.

Discussion:

1. Which final temperature was the highest? Which was the lowest? Did your results turn out the way you expected?

2. Just in case: If the final temperature for the water proves to exceed that for the black paper, try to get the students to understand the fact that the black paper "shields" its thermometer and thus might have influenced the results. Ask for suggestions on how to redesign the experiment to account for this (an example of a more accurate method is given under "Extension Ideas," below). Have students make inference or have them state the connection between this experiment and the concept listed on the trading cards.

Closing:

Ask, "What effect does albedo have on surface temperature?"

1. Were the albedo lab activity sheets completed?
2. Did each student contribute equally to the group effort? You may wish to add a question to each activity sheet, along the lines of, "How did you divide up the work?"

1. Have students graph temperature vs. time for all three thermometers, and ask if all three warmed up at the same rate.

2. A more accurate method of determining albedo-temperature-color relationships would be to put each thermometer in a cup filled with either cola, milk, or plain water. Make sure that the starting temperatures of all three liquids are identical, and that the volumes of the three are more or less equivalent. You might wish to run the experiment this way after completing it as described above, and allow students to compare results.

3. Assume (for the sake of this experiment) that the black paper (or cola) has an albedo of 0%. Further assume that the albedo of the white paper (or milk) is 100%. Have students estimate the temperature for the Earth in general (30% albedo) under similar light conditions, based on the two end-point temperatures.

   Estimated Temperature (30%) = 100% albedo temperature + (0.3) x (0% temperature - 100 % temperature)

   The Data trading card will support further understanding of this concept.