Daily Radiation "Balance"
If we consider the entire Earth-atmosphere system, then the amount of radiation entering the system must equal to the amount leaving, or the system would continually heat or cool. In this case, because radiation is the only method for energy transfer between the Earth-atmosphere system and space, this balance is purely a radiative balance. However, within the system, there is not radiative balance. In fact, there is a net radiative cooling in the atmosphere, which is balanced in the "system" by latent and sensible heating.

Objectives
  1. To learn to calculate the total amount of incoming and outgoing radiation over a day.
  2. To determine how "balanced" the radiative system is over a particular location.
Notes to the teacher
  1. Obtain ARM upwelling and downwelling shortwave radiation and ARM upwelling and downwelling longwave radiation data for a clear 24-hour period of time.
  2. Divide the class into groups of 3 or 4 students. If possible, provide each group data at different sites, preferably on the same clear day.
  3. You may want to assign Procedures 2, 3 and 4 as homework.
PROCEDURE

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

2. Use the data table provided by your teacher to record your results. Complete the hour column of the data table for the day of your observations. For example, write in 12 AM, 1 AM, ..., 11 PM.

3. 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.

4. 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 much total incoming radiative energy (shortwave and longwave) did you compute on this day? 

2. How much total outgoing radiative energy (shortwave and longwave) did you compute? 

3. Which total energy (from Questions 1 and 2) is larger? By how much? What does this mean? What might have happened to this energy? (Optional) Are your answers consistent with what you know about the radiative balance of the atmosphere? Why or why not? (Optional) How much energy is this compared to the amount used in your home on a similar day?

4. Compare the answers to Questions 1, 2 and 3 among all the groups. Is there a consistency among the groups' results?

5. List the factors that can affect the total energy incoming or outgoing at the earth's surface?

 PREREQUISITES
  • Knowledge of power, energy and radiation
  • Ability to create and interpret graphs
MATERIALS VOCABULARY

Energy
Downwelling radiation
Joules
Upwelling radiation
Watts 

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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