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Photosynthetically Available Radiation (PAR) Measurements
Part 1. Calculating The Solar Constant Using A TI-8*

data | hook | main | background & resources | student

Author Contact Information

Barbara Schulz
Biolab.org
Seattle, Washington
bschulz@biolab.org

Linda Wygoda
Sam Houston High School
Lake Charles, Louisiana
wygoda@tea.rice.edu

Overview
The Earth’s climate system constantly tries to maintain a balance between the energy that reaches the Earth from the sun and the energy that leaves the Earth and goes back out into space. The clouds, the Earth’s surface, and the atmosphere all influence this balance. The radiation budget is critical to understanding climate change and the energy relationships of food chains. Does the amount of photosynthetically available radiation influence the food web in the dry valley lakes? Can one compare the PAR from various locations with that from the LTER McMurdo Dry Valleys lakes?

The major determiner of climate is solar radiation. The amount of solar energy that strikes the Earth is a measurable value that is important in the study of food chains and the radiation budget of climates. The solar input is the amount of power a given cross-sectional area receives from the sun when the sun is directly over that area. To determine the solar input, the amount of heat absorbed by a known mass of water exposed to a known cross-sectional area of sunlight is calculated in this experiment using a graphing calculator/CBL system and temperature probe.

The student will

  • measure and calculate the solar input
  • discuss the variables influencing their measurements
  • discuss the reasons why solar input might vary using library and internet resources
  • measure and calculate solar input at their watershed testing site
  • compare data with the LTER-MDV data, the demo data and other available data
  • discuss reasons for the differences

    Grade Level/Discipline
    11-12 Advanced or AP Biology or AP Environmental Science. Could be adapted for 9 honors biology.

    National Standards

    Pre-activity set-up

  • Use black spray paint to coat the surfaces of the 50mL flat culture bottles.
  • Drill holes in the lids to these bottles to accommodate the temperature probes.
  • Large Styrofoam cups cut so that the culture bottle will fit in a side notch.

    Materials

  • TI-8* calculator
  • CBL system
  • 2 - temperature probes
  • 1- 50 mL culture bottle (spray painted black)
  • 1 - styrofoam cup (pre-cut mold)
  • distilled water (cooled to10-15 C below the outside air temperature)
  • graduated cylinder
  • masking tape
  • meter stick

    Time Frame
    1 - sunny day to make measurements in the field (50 minute class period) 1 - class period to analyze data 1 - 2 class periods to explore internet resources on this topic. 1 – class period to measure the PAR at the watershed testing site

    Engagement and Exploration (Student Inquiry Activity
    Have students view pictures from the glacier web site of Antarctica.

  • As a class, list adjectives on the board that describe the Antarctic climate they see (bright, sunny, white) Compare these with adjectives they have heard describing a jungle or rainforest (dark, shady, hot).

  • Ask "What is the relationship between white and cold and hot and dark? They might think about white clothing versus dark clothing; or different colored automobiles etc. The general relationship that white is reflecting radiant energy and black is absorbing energy should be formed.

  • To reinforce this relationship, connect two temperature probes to the CBL system. Place one probe under a black sheet of construction paper and the other under a white sheet of paper. Position both probes so that they both receive an equal amount of light from a gooseneck lamp. Ask students to hypothesize about which should heat up faster. Test the hypothesis with the equipment.

  • The polar ice sheets represent a large part of the Earth’s surface. Ask "How does the ice sheet influence the solar radiation available for use by living systems?"

    Ask students if they think all areas on the Earth receive the same amount of solar radiation? Why or why not? What variables will effect the amount of solar radiation absorbed? What variables will effect the amount of specific wavelengths of solar radiation absorbed by producers? Students are now ready to do the activity. Refer to the student activity masters for their directions.

    Check out this web site ../../invitation/1_seasons1.html

    Explanation (Discussing)
    The accepted value of the solar input is approximately 1000 watt/m2 on a clear day at sea level. Above the Earth’s atmosphere, this value is1353 watt/m2 ± 2%. The clouds, the Earth’s surface, and the atmosphere all influence this balance. Seasons also affect the amount of radiation that the Earth receives. Other important factors include cloud cover, albedo, fluctuations in the sun’s energy output, and the position of the Earth in relation to the Sun (aphelion and perihelion). The LTER-McMurdo Dry Valley site is located at 77*.37’S( latitude), 163*.07’E( longitude).

    A radiation budget is one way of accounting for the solar energy entering and leaving the Earth system. This total energy can be absorbed or reflected. The reflected energy is accounted for in the measurement of albedo. The solar input measurement accounts for the energy absorbed. Photosynthetic rates reflect the radiation captured by producers and become available for living systems. Refer students to the primary productivity activity in which students measures the net amount of carbon fixed in a 24-hour period.

    Elaboration (Polar Applications)
    Students discuss the variables influencing their measurements in their lab reports for this activity. These variables include: the amount of water, the color of the plastic bottle, the sun’s angle, the area of the sunlight striking the bottle, and the rate of temperature change of the water. Students should understand how increasing or decreasing each of these variables affects their measurements.

    Using library and internet resources students research and discuss the reasons why solar input might vary. A useful starting point is: http://home.larc.nasa.gov/education/Facts.html http://home.larc.nasa.gov/education/whatare.html http://huey.colorado.EDU/LTER http://LTERnet.edu Students hypothesize, research and compare radiation energy for different regions of the world including Antarctica. Antarctica is a major area in which energy is reflected back into space. This characteristic of Antarctica makes it a very important piece in any study of global climate and ecosystems.

    Exchange (Students Draw Conclusions)
    Student lab groups should give short seminar presentations on the results of their experiments and their library/internet research

    Evaluation (Assessing Student Performance)

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