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Dimming the Sun

Classroom Activity


Activity Summary
Students will set up pan evaporation devices and for four uninterrupted days will measure and record pan evaporation amounts, air temperature, relative humidity, wind speed, solar radiation, and precipitation.

Learning Objectives
Students will be able to:

  • collect, interpret, and analyze weather variable data.

  • describe atmospheric variables that affect evaporation.

Materials for teacher
  • copy of the "Class Charts" teacher sheet (PDF or HTML)

Materials for each team
  • copy of the "Investigating Evaporation" student handout (PDF or HTML)
  • copy of the "Data Sheet" student handout (PDF or HTML)
  • copy of the "Graphing Data" student handout 1 (PDF)
  • copy of the "Graphing Data" student handout 2 (PDF)
  • pencil or pen
  • calculator
  • graph paper

Materials for Pan Setups

  • 2 stainless steel or galvanized pans of equal size (at least 25 cm x 25 cm x 6 cm)
  • 2 wood blocks (same or greater perimeter as pan bottom)
  • two 2L containers
  • room temperature water
  • masking tape
  • 2 metric rulers with millimeter graduations
  • level
  • 2 rain gauges with millimeter graduations
  • 1 opaque shading device, if necessary
  • waterproof marker
  • 1 thermometer (optional)
  • 2 anemometers (optional)
  • 2 hygrometers (optional)
  • magnifying glass (optional)
  • graph paper (optional)

Earth's temperature rises when the sun heats the surface and when sun rays reflected off the planet are absorbed and re-emitted by greenhouse gas molecules in the atmosphere. While naturally occurring atmospheric greenhouse gases such as water vapor and carbon dioxide provide a beneficial warming effect, some scientists believe that human-generated greenhouse gases are contributing to increased global warming. Some scientists fear that a continued accelerated warming of Earth's surface may lead to climate changes that melt polar ice caps, raise sea levels, change rainfall, and impact local climate conditions.

Other scientists believe they have found evidence that shows Earth is cooling. Global dimming, as it is known, is a phenomenon believed to cause a reduction in the amount of sunlight reaching Earth's surface. This is thought to occur when pollution particles create dense clouds that prevent sunlight from getting through and reflect sunlight back into space. Some scientists believe global dimming is altering rainfall patterns worldwide and is masking the true impact of global warming. There is still much scientific debate about the extent and impact of both global warming and global dimming.

One of the ways that scientists determine energy changes in the environment is by looking at water evaporation rates. Evaporation rates can be measured by calculating the change of depth of water in a container over a given time period. Evaporation rates are mainly influenced by sunlight, relative humidity, and wind speed.

Evaporation rates have been calculated worldwide for hundreds of years and are used to determine the approximate amount of water loss from lakes or crops. They are also used to help calculate the amount of water needed for irrigation. Many U.S. weather stations determine evaporation rates using a U.S. Class-A pan, which is a stainless steel pan, 25.4 centimeters high and 1.2 meters in diameter. The pan, which is normally installed on a wooden platform set on the ground in a grassy open area, is filled with water to within 6.35 centimeters of the top and left exposed. The pan evaporation rate is simply the amount of water that evaporates from the pan in a given period of time. This rate is measured by manual readings or using an evaporation gauge. Water is added to the pan to bring it back to its original level each day.

In the 1990s, scientists worldwide started noticing the pan evaporation rate was falling, despite an overall rise in global temperature. The decrease in the pan evaporation rate is a key piece of evidence used to support the hypothesis of global dimming. In this activity, students will set up their own pan evaporation experiments and determine which variables affect evaporation.

Key Terms

climate: The weather conditions in a given location averaged over a long period of time.

pan evaporation rate: The amount of water that evaporates from a pan in a given period of time.

relative humidity: The amount of water in the air compared with how much water the air can hold at a given temperature.

weather: Meteorological conditions such as temperature, wind speed, relative humidity, and precipitation at a given time and place.

  1. Before class, identify level, outside locations in an open area (such as a field or blacktop) where the pan evaporation devices can be set up and left undisturbed for four uninterrupted days. Pan 1 will be placed on wood in an unsheltered location; pan 2 will be placed on wood in a shaded location. Try to place the pans close to one another. If there is no appropriate shaded location, use a shading device, such as an umbrella or an opaque fabric or cardboard overhang, to shelter the pan.

  2. In the classroom, set up the pans. Use a permanent marker to number each pan and place a mark two centimeters from the top of the pan.

  3. Organize the class into four teams and number them one through four. Distribute and review the handouts with teams. Have students write their team number on their "Data Sheet" handout. Tell students that they will collect data from an unsheltered and sheltered pan for four uninterrupted days to learn about the variables that affect pan evaporation.

  4. Make a schedule, and assign Teams 1 and 2 to take morning measurements (at about the same time but Team 1 always measures first) for both pans and Teams 3 and 4 to take afternoon measurements (at about the same time but Team 3 always goes first) for both pans. Try to have students take measurements as early and as late in the day as possible.

  5. After taking the daily morning measurement, Team 2 needs to refill the pans to the exact level of the original water mark with room temperature water (and/or remove any rainwater so that the water is at the original starting line). After taking the late afternoon measurement, Team 4 needs to refill the pans in the same manner.

  6. Using the "Class Charts" teacher handout as a guide, transfer the "Measurements Chart," the "Daily Averages Chart," and the "Averages for Graphing Chart" onto chart paper so students can enter their data, and the results can be analyzed and graphed by the class. Students will be entering their data into the all-class "Measurements Chart" in the correct space next to their teams' number each day.

  7. Demonstrate how to read the instruments and have teams practice until they are proficient. Have teams practice using the ruler to measure water loss from a pan. Emphasize the importance of taking an accurate ruler reading (eyes looking straight at the ruler rather than looking at an angle) when measuring water evaporation. Students should measure water depth from the same place in the pan each time. Practice refilling the pan with students. If an anemometer and/or a hygrometer are not available, students can obtain wind speed and humidity data from NOAA's National Weather Service Web site (see sidebar) or local news outlets.

  8. Take the pans, containers of room temperature water, wood blocks, and instruments to the predetermined locations with students the day before the first measurements will be taken. Fill each pan exactly to the marked line with water. Make sure the pans are level and cannot be easily bumped or disturbed. Measure the initial pan water level to the nearest millimeter. Have students write this number on their "Data Sheet" handout. Place a rain gauge next to each pan and place a thermometer next to the sheltered (shaded) pan. (Air temperature is measured in the shade.) Make sure that the water in the pan is at the original starting level at the time that Team 4 would be taking its afternoon measurement.

  9. Have students collect and record their data for four days, entering their data into the class chart daily. Monitor measurements daily and consider deleting outliers when the data is averaged. At the end of the data collection period, and after all student data has been posted, hang the "Daily Averages Chart" and the "Averages for Graphing Chart" next to the "Measurements Chart."

  10. Have Teams 1 and 2 calculate a.m. averages and Teams 3 and 4 calculate p.m. averages for sheltered and unsheltered locations for the four variables—air temperature, relative humidity, average wind speed, solar radiation (amount of shade for sheltered, clouds or sun for unsheltered) and for the evaporation amounts. Enter these into the "Daily Averages Chart."

  11. Then, as a class, calculate daily averages for both unsheltered and sheltered locations for the four variables (air temperature will be the same for both) and enter the averages into the "Averages for Graphing Chart." Add together the a.m. and p.m. evaporation amounts for each day and enter these amounts into the "Averages for Graphing Chart." Round up all numbers to one decimal point for graphing.

  12. Have teams work together to complete the graphs on their "Graphing Data" handouts or have students use graph paper to create their graphs. There are eight graphs for each location. The evaporation results that students obtained for the Unsheltered Location will be the same for all four graphs. The evaporation results that students obtained for the Sheltered Location will be the same for all four graphs for that location.

    Unsheltered Graphs
    average air temperature
    1b: evaporation amount

    2a: average relative humidity
    2b: evaporation amount

    3a: average wind speed
    3b: evaporation amount

    4a: average solar radiation
    4b: evaporation amount

    Sheltered Graphs
    average air temperature
    1b: evaporation amount

    2a: average relative humidity
    2b: evaporation amount

    3a: average wind speed
    3b: evaporation amount

    4a: average solar radiation
    4b: evaporation amount

  13. Analyze the data with students and discuss the results. Based on the data, which variables seem to most affect water evaporation? Ask students how results might differ if they were collected in more locations and/or over a longer period of time.

  14. As an extension, have students work in teams and design and write an indoor pan evaporation experiment.

Activity Answer

Student results should reflect that differences in relative humidity, wind speed, and sunlight affect the pan evaporation. In general, when relative humidity increases, the pan evaporation decreases. It is also common to see an increase in pan evaporation with an increase in wind speed, especially if the wind is moving away moist air and bringing in dry air Solar radiation (the amount of sunlight) has the greatest impact on pan evaporation. Though students may see little change in pan evaporation due to changes in air temperature, measuring solar radiation includes a consideration of day length, air temperature, and cloud cover. Results will vary depending on local weather conditions.

Student Handout Questions

  1. How did the pan evaporation amounts change in relation to air temperature? Relative humidity? Wind speed? Solar radiation? Answers will vary some depending upon location. In general, when relative humidity increases, the pan evaporation decreases. When wind speed increases, the pan evaporation increases. When solar radiation increases, evaporation increases.

  2. Which variables seem to most affect the evaporation amounts? Explain your answer. Student results should reveal that wind speed, relative humidity, and solar radiation most affected evaporation amounts. Sample results showed that wind speed and relative humidity affected evaporation amounts even in the sheltered area.

  3. What difference, if any, was there in the amount of evaporation that occurred in the unsheltered location compared to the sheltered location? What might account for any differences? Students will most likely find more evaporation in the unsheltered location on sunny days (due to more solar radiation). This may vary depending on humidity levels—if humidity is high, evaporation may be less.

  4. If the amount of sunlight reaching Earth's surface were to decrease, would you expect the evaporation rate on Earth's surface to increase, decrease, or stay the same? Why? If the amount of sunlight were to decrease, the evaporation rate would likely decrease.

  5. What are the limitations of the experiment? Some limitations include lack of sophisticated instrumentation, reader error, and water loss due to unmeasured variables such as air pressure differences or animals drinking from the pan. In addition, data used from a local weather source may not reflect microclimate conditions at pan setup locations.

  6. What could the class do to have more confidence in the conclusions drawn from this experiment? Carry out the experiment in more locations for a longer period of time.

Links and Books

Web Sites

NOVA—Dimming the Sun
Find out more about aircraft contrails and their impact, read the producer's take on why some people remain skeptical about global warming, see a time line of global dimming discoveries, and learn about tools to reduce and prevent air pollution.

U.S. Global Change Research Information Office
Features general information, resources, and links to other organizations dealing with global change. Also includes an e-mail service, Ask Dr. Global Change, where readers can send questions about global environmental change.

What's Up With the Weather?
Provides interviews with proponents and skeptics of global warming, shows how ice cores reveal climate data, takes a look at home energy consumption, and reviews alternative energy sources.


Global Warming
by Fred Pearce and John Gribben. Dorling Kindersley, 2002.
Examines the causes and effects of global warming.

by Brian Cosgrove. Dorling Kindersley, 2004.


The "Investigating Evaporation" activity aligns with the following National Science Education Standards (see and the Principles and Standards for School Mathematics (see

Grades 5-8
Science Standard A

Science as Inquiry
Abilities necessary to do scientific inquiry

Mathematics Standard

Grades 9-12
Science Standard A

Science as Inquiry
Abilities necessary to do scientific inquiry

Mathematics Standard

Classroom Activity Author

Margy Kuntz has written and edited educational materials for 20 years. She has authored numerous educational supplements, basal text materials, and trade books on science, math, and computers.

Teacher's Guide
Dimming the Sun

Video is not required for this activity

Koch Foundation

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