Albedo and Angle of Incidence




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Albedo and Angle of Incidence Measurements



STEM




Albedo and Angle of Incidence

Have you ever observed bright sunlight reflecting from the surface of materials on Earth’s surface? On a bright, sunny day, have you ever walked barefoot across some surfaces that felt cool and other surfaces that felt too hot to walk on?


Question 1: What types of materials reflect a lot of sunlight?
Question 2: What factors might influence how much sunlight is absorbed as heat on the surface of Earth?
Many polar researchers are trying to understand why temperatures in the Polar Regions are increasing more rapidly than in other regions of the world. This activity explores two factors that influence the climate of Polar Regions and the rate of change of climate in Polar Regions.

  • Part I of this activity explore how the relationship between the albedo and heat absorption.

  • Part II of this activity explores the affect of the angle of incidence of sunlight on heat absorption.


Part I. Albedo
Albedo is a measurement of the fraction of incident sunlight that is reflected from the surface of a material back to space. The albedo of snow and ice is much higher that the albedo of water. Fresh snow can have an albedo higher than 0.90. That means that more that 90% of the incident sunlight is reflected from the surface of the fresh snow. The albedo of ice and old snow typically range from 0.20 to 0.45.
Question 3: Why would the albedo of old snow be less than the albedo of fresh snow?
Land and ocean have albedos that range from 0.10 to 0.40. You can easily observe effects of effect of differences in albedo on Earth’s surface. For example, if you clear some snow from a section of a dark colored driveway, the snow closest to the cleared area melts more quickly than the snow further away. The cleared surface of the driveway absorbs more solar energy and increases the temperature of the nearby snow, speeding up the melting process.


Materials for an Albedo Investigation
You can use the following materials to explore the relationship between the albedo and heat absorption.


The Experimental Procedure


  1. Using a plate as a template to draw circles with the same diameter on a sheet of black paper and a sheet of white paper. Use scissors to carefully cut out the circles.

  2. Tape each circle onto the top of a plate. Leave an opening to insert a thermometer under the paper and parallel to the plate. Insert the thermometer so that you can read the temperature.

  3. Position the heat lamp so that it will shine parallel to the table top and just above it.

  4. Measure 30 cm from the lamp and label that position with masking tape and a pen.

  5. Hold the black covered plate at the 30 cm mark so that the plate is perpendicular to the table so that the light from the lamp can shine directly onto the black paper. (See Figure 1 below.)



Figure 1. Light rays from the lamp are perpendicular to the paper.


  1. Record the temperature at the start and at 1 minute intervals until you have 5 readings.

  2. Repeat this process with the white plate.


Question 4: Which plate had the larger temperature change per minute?
Question 5: What was the average temperature increase per minute for each plate?
Question 6: How might your experimentation help you understand how the loss of ice and snow in the Polar Regions would contribute to climate change, especially in Polar Regions?

Part II. Angle of Incidence and Heat Absorption
The sun follows an interesting path across the sky each day. It is low on the horizon early in the morning. Because Earth’s rotates on its axis, the sun appears to move from east to west across the sky. The sun reaches its highest point in the sky at midday. The sun then continues to move to the west until it is low on the horizon at the end of the day.

Because the sun rises and sets, the rays of sunlight reach Earth’s surface at different angles. That angle is called the angle of incidence. You can investigate how changes in the angle of incidence of sunlight affect the rate at which sunlight is absorbed as heat.


Materials for an Angle of Incidence Investigation
You can use the following materials to explore the relationship between the angle of incidence of light and heat absorption.

  • 2 paper or Styrofoam 8 inch plates

  • Black and white construction paper

  • Scotch or masking tape

  • Scissors

  • Ruler and protractor

  • 2 thermometers

  • Heat lamp


The Experimental Procedure
If you have done the albedo experiment in Part I, and the black covered plate has cooled to room temperature, you can use that plate and go directly to Step 8.

  1. Using a plate as a template to draw circles with the same diameter on a sheet of black paper and a sheet of white paper. Use scissors to carefully cut out the circles.

  2. Tape each circle onto the top of a plate. Leave an opening to insert a thermometer under the paper and parallel to the plate. Insert the thermometer so that you can read the temperature.

  3. Position the heat lamp so that it will shine parallel to the table top and just above it.

  4. Measure 30 cm from the lamp and label that position with masking tape and a pen.

  5. Hold the black covered plate at the 30 cm mark so that the plate is perpendicular to the table so that the light from the lamp can shine directly onto the black paper. (See Figure 1 below.)



Figure 1. Light rays from the lamp are perpendicular to the paper.


  1. Record the temperature at the start and at 1 minute intervals until you have 5 readings.

The Experimental Procedure (continued)


  1. Let the thermometer and plate cool until they are at room temperature (or use another plate and thermometer.)

  2. Now place the plate flat on table, with its center at the 30 cm mark. Position the lamp so that its rays strike the plate at a 20 angle (see Figure 2 shown below.)

  3. Record the temperature at the start and at 1 minute intervals until you have 5 readings.


(sin 20) A = 0.342 A

Figure 2: The figures above show a side view of the experiment when the angle of incidence of light rays is 20.

Question 5: What is the average temperature increase per minute?
Question 6: Which plate had the larger temperature change per minute?
Question 7: How do you interpret your result?
A Few More Challenging Questions:
Measure the area of the circular plate that you have been using. The sin of an angle of 20º is equal to 0.342. Multiply the final temperature from Step 9 times the sin of the angle of 20º. You can use the sin button on most calculators to do that calculation.
Question 8: Is that value that you have calculated using the sin of 20º similar to the final temperature at the end of Step 6?
Question 9: How would you explain the difference between the results of Step 6 and Step 9?
Question 10: How might the results of your experimentation explain why the Polar Regions are colder than the lower Latitudes?


Positive Feedback
Have someone ever said “Very good job.” or “Very nicely done.” after you have finished a homework assignment or improved your scores on science tests? Those statements are examples of “positive feedback”. Many people are motivated to do even better work when they get some positive feedback.

The term “positive feedback” can also be used to describe some processes that occur in Earth’s climate system. One example of positive feedback seems to be happening in the Arctic Ocean.



Data that polar researchers are collecting seem to indicate that the polar climate is changing more rapidly than the climate at lower Latitudes. As sea ice melts and more open water appears, more energy is absorbed and warming accelerates. A similar effect occurs as snow and ice on land are replaced by vegetation or bare ground.
Question 11: How would you use the results of your investigation in Part I and Part II of this activity to explain why the loss of sea ice on the Arctic Sea could lead to an even more rapid loss of more sea ice?







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