How Do Clouds Affect Radiative Energy
One objective of the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Project is to determine the various radiative properties of different cloud types. For example, because cumulus clouds form closer to the ground than do cirrus clouds, they emit more radiation according to the Stefan-Boltzmann law. The effect of cloud type on Earth's radiation budget is one of the great unanswered questions of climatology. In this exercise, students will begin to understand the complex interactions between clouds, the atmosphere and the surface energy budget.

Objectives
  1. To begin to understand the effects of cloud type on radiative energy.
  2. To help understand how clouds affect the Earth's climate.
Notes to the teacher
  1. Obtain ARM upwelling and downwelling shortwave radiation and ARM upwelling and downwelling longwave radiation data for two different days. One day should have overcast cirrus clouds during the 24-hour period; the second should have overcast cumulus congestus or cumulonimbus clouds during the 24-hour period.
  2. Divide the class into groups of 3 or 4 students. If possible, provide each group data at different sites.
  3. You may want to assign Procedures 3, 4 and 5 as homework.
PROCEDURE

Let Day 1 be the day with cirrus cloud cover and let Day 2 be the day with cumulus cloud cover.

1. Using WxScope, graph the 24-hour upwelling and downwelling shortwave radiation at 15-minute intervals for Day 1. Make a second graph with the upwelling and downwelling longwave radiation for Day 1.

2. Repeat Procedure 1 for Day 2.

3. Create a data table with columns for the hour of the day, average upwelling shortwave, average downwelling shortwave, average upwelling longwave, and average downwelling longwave radiation.

4. For each hour, use your graphs to estimate the hourly average radiation for each type of radiation. Write each of these values on the appropriate column of the table.

5. After your table is complete, compute the total (energy per unit area) for each type of radiation. Remember that the radiation is measured in Watts per square meter, and that Watts are Joules per second.

To help you in your computations, note that the estimates you obtained in Procedure 3 are the power per square meter averaged over the hour. Hence, to obtain the energy per square meter, you will need to multiply the average power times the number of seconds in an hour. Then add the energy per square meter for every hour to get the total energy for the day.

Shortcut:

The following equation describes the same procedure as above, but you will not have to make as many computations:
Total Energy per square meter of a given type of radiation = (1 hour) x (60 minutes per hour) x (60 seconds per minute) x [Power1 + Power2 + Power3 + ... + Power24]

where Poweri = the power per square meter for hour i (from 1 to 24 hours) of the given type of radiation.

Be sure to put the appropriate units on your answers.

QUESTIONS

1. How do your results compare? How were the individual components of radiation affected under the 2 different cloud covers? Why? 

2. Does cloud type affect daily radiative energy balance? How will monthly or yearly radiative balance be affected? 

3. What other things will affect the total energy incoming or outgoing at the Earth's surface?
 PREREQUISITES
  • Knowledge of the unit Watts.
  • Ability to create and interpret graphs.
  • Completed the "Daily Radiation Balance" lesson.
MATERIALS
  • Pencil
  • Graph paper
  • Data table
  • ARM upwelling longwave radiation, downwelling longwave radiation, upwelling shortwave radiation, and downwelling shortwave radiation graphs
  • WxScope
  • Computer
VOCABULARY

Cirrus
Cumulonimbus
Cumulus congestus
Energy
Joules

CORE CURRICULUM SKILLS APPLIED IN THIS LESSON
  • Select descriptive (qualitative) or numerical (quantitative) observations in a given set of objects, organisms or events.
  • Identify qualitative and quantitative changes given conditions before, during and after an event.
  • Use appropriate Systems International (SI) units (grams, meters, liters and degrees Celsius) to measure objects, organisms or events.
  • Report data in an appropriate method.
  • Predict data points not included on a given graph.
  • Select the most logical conclusion for given experimental data.
  • Identify or create an appropriate graph or chart from collected data, table or written description.
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