Location: You will need to come to Dana 126 in order to check out the experimental equipment. Groups of three are the optimum size (four is the maximum allowed); you need at least one partner. You should do the experiment outside Dana, either on the loading dock on the east end, in the courtyard, or on the street side of the building. The best time to do it is between the hours of 10 am and 3 pm, so that the sun will be more overhead. You need to pick as clear a day as possible. You can use the computers in Dana 126 to analyze your data. I will be glad to answer questions and help you with Excel.

Purpose: To measure the solar insolation reaching the surface of the earth at our location.

Equipment: A black-painted aluminum block mounted in a styrofoam insulation box with a glass cover, a wooden dowel, a thermometer, and a stop watch.

Introduction: The solar insolation (brightness or intensity) at the Earth’s surface is defined as the energy per second that reaches a unit area of surface which is oriented at right angles to the direction of the sunlight. At the top of the earth's atmosphere the insolation is about 1360 W/m². This value is known as the solar constant. The atmosphere absorbs and reflects roughly one-half of the incident sunlight, so that the insolation that reaches the surface of the earth is about 680 W/m². The intensity will be different at different latitudes and during the morning or evening when the sun is lower in the sky. Cloud cover will also affect the readings.

You will determine the solar insolation in Davidson, NC on a typical day in the spring. You will measure how the temperature of a black-painted aluminum block increases when it is exposed to the sun. If the black paint is a perfect absorber (a perfect blackbody), then the amount of solar energy incident on the block is equal to the thermal energy absorbed by the block.

Definitions:        I = solar insolation
                        E = energy from the sun incident on the block
                        Q = gain of thermal energy by the block
                        A = area of block
                        T = temperature in ºC
                         c = specific heat of aluminum = 900 J/kg ºC
                         t = time
                        m = mass of aluminum block (on bottom of Styrofoam box)

By definition:                    or          

             and             ,

where D t is the time during which the block is exposed to the sun and D T is the temperature change of the block during that time.

Since        ,           then             .

We can solve this last equation for the insolation:

1. The aluminum block and its glass cover have already been enclosed in the Styrofoam insulation box, which has a cover. There is a digital thermometer and a stopwatch inside the enclosure. The wooden dowel should stick in the hole in the Styrofoam box.
2. Record the mass of the aluminum block from the bottom of the Styrofoam box.
3. Open the box while you are still in the laboratory and remove the wooden dowel, the thermometer and the stopwatch. Close the cover.
4. Take the Styrofoam box, the stopwatch and the digital thermometer outside to the courtyard, the loading dock or the sidewalk on the street side of Dana. Orient the Styrofoam box at a right angle to the sun by propping it against the cardboard half cylinder. Use the weight in combination with the half cylinder, if necessary, to keep the box from slipping. Adjust the box so that the wooden dowel does not cast a shadow; thus, the sunlight is perpendicular to the box.
5. Gently remove the probe from its clip and unwind the cable. Slide the probe as far as it will go into the hole in the cutout underneath the glass cover. Turn on the thermometer.
6. Wait for the thermometer to stabilize and then record the initial temperature in ºC.
7. Make sure that the sunlight is still perpendicular to the box. If not, readjust the box. Now, open the cover of the Styrofoam box and expose the aluminum block to the sun. Start the stopwatch when you open the cover and record the temperature every 30 seconds for 10 minutes.
8. If you think your data may have been disturbed in some way, e.g., by a cloud or person passing in front of the sun, you should repeat your measurements.
9. Bring everything back into Dana 126.
10. Measure the area of the surface of the aluminum block exposed to the sun.
11. Put everything back into the box and leave it in the same condition as you found it.

Data Analysis: