Credits
Barbara Liukkonen developed this lesson.

Goals
Students will investigate how much energy is stored in lake water as latent heat, calculate how much latent heat is exchanged with the atmosphere, and consider the effects of heat gain and heat loss on the surrounding environment.

Introduction
Water has unique properties that affect its ability to store and release heat energy. In this activity students develop an understanding of latent heat. They calculate heat budgets for lakes and explore the effects of lake temperatures on the surrounding environment.

Students can meet the goals for this lesson by completing either a directed study or student inquiry lesson.

The "Studying Heat Budgets of Lakes" lesson uses a worksheet approach to prepare students for class discussions on the subject. Students need to print the lesson to complete it as a worksheet.

The directed study lesson is found in the student section of WOW under the title: "Studying Heat Budgets of Lakes."

The student inquiry lesson provides students with an assignment as a television meteorologist. The end result of the television assignment is an oral presentation, written paper, TV script, video, or multi-media presentation about heat budgets, depending on the instructor’s requirements.

The student inquiry lesson is found in the student section of WOW under the title: "Investigating Heat Budgets of Lakes."

Outcomes
Students will:

1. Review how to estimate lake volume.
2. Calculate how much latent heat is stored in lake water.
3. Calculate how much energy is released to the atmosphere as a lake cools at night.
4. Calculate how much energy is required to freeze the lake surface.
5. Consider how latent heat stored in and released from a lake can affect local weather conditions.

Keywords
Calorie, temperature, conduction, latent energy, states of matter

Prerequisites
Students should have a basic understanding of calories as units of energy. It is also helpful if they have had prior experience calculating lake volumes.

Materials/Resources/Software

1. A copy of the "Studying Heat Budgets of Lakes" lesson for student groups doing the directed study lesson.
2. Access to WOW temperature profile data for Ice Lake or Independence Lake at different times during the day. (4-6 hour intervals of data are useful.) Students could be told to find this data through the WOW web site, or the data could be provided as a handout.

Time Required
Two — three class periods

Curriculum Connections
Physics - calorie, temperature, radiation, conduction, latent energy, states of matter

WOW Curriculum Links
Diel Temperature Variation

Procedure

Knowledge Base
The "Understanding" section of the WOW website includes information about heat budgets for some WOW lakes (see Figure 1). You may want to display information such as this for the students. This could be done either during your initial discussions for this lesson, or as part of the discussion and closure for the lesson.

Figure 1. Heat Budget for Ice Lake (click for full figure)   

Directed Study
Discuss students’ experiences with air and water temperatures around lakes. What have students noticed about the differences in air and water temperatures during the day? What are air and water temperatures like at night?

Student Inquiry
Students play the role of a television meteorologist. Their television station has asked them to develop a series about heat budgets of lakes. Students should review daily and seasonal air and lake water temperature changes. Ask students to write a one paragraph script that includes the following details:

Is air or water usually warmer in the early spring?

Does air or water retain heat longer in the fall?

Does air or water warm up more quickly early in the morning?

Does air or water retain heat longer as night falls?

Notes: Water temperature changes lag behind the changing seasons. For example, during warm spring days, the water temperature is much cooler than the air temperature. During cool, early fall days the water temperature is much warmer than the air temperature. There is also a daily temperature lag; air temperatures warm up during the day before the surface of the lake. The night air also cools much more quickly than the surface temperature of a lake.

Experimental Design

Directed Study
Review how to calculate water volumes for each one meter layer of a lake. Review the basic concepts of calories and energy transfer. Energy is required to raise the temperature of water. Ask students to hypothesize about the source of energy for lakes.

Notes: Energy from the sun is the primary source of increasing temperatures in a lake.

It takes 1 calorie to raise one cubic centimeter of water one degree (at 20°C). How many calories does it take to raise 1 cubic meter of water one degree? When a cubic centimeter of water drops one degree, energy is released. How many calories are given off when a cubic meter of water drops one degree? (Have students describe and explain how they arrived at their answers.)

Notes: 1 million calories are required to change one cubic meter of water by one degree (Celsius), if no changes of state are involved.

Divide the class into groups of two before handing out the "Studying Heat Budgets of Lakes" lesson. Each member of the group should have a responsibility. If one member of the group is the "recorder" and responsible for writing answers on the worksheet, then the other member could be the "research technician," responsible for finding the data to insert into the tables and doing the necessary calculations. When students are half done with the worksheet they should switch responsibilities.

Student Inquiry
Ask students to examine the depth and area calculations for Ice Lake and Lake Independence. Students need to develop an explanation of how to calculate heat budgets (in calories). They should also demonstrate that the heat budget changes in each lake throughout the day.

Data Collection

Directed Study
The worksheet format of the "Studying Heat Budgets for Lakes" will guide students through the data collection.

Student Inquiry
Students should collect the data they need to calculate heat budgets during a 24 hour period from Lake Independence and Ice Lake. Students need to develop a table or other appropriate format for recording their data.

Data Management and Analysis

Directed Study
Students are required to complete several calculations for questions 4-8 on their worksheet. Encourage them to think about whether their calculations are "reasonable".

Student Inquiry
Ask students to calculate the amount of heat lost or gained (in calories) between the periods when the RUSS gathered temperature data in lakes. As they complete the calculations they should consider the following questions:

What was the source of energy for heat gains?

Where did the heat go when the lake heat budget demonstrated a loss of heat energy?

How many calories were involved in the heat budget changes between each period of measurement?

Interpretation of Results

Directed Study
Students should complete questions 9-19 of the worksheet before participating in a class discussion. Discuss how lakes might affect the temperature of the surrounding environment. Discuss variables that the students feel may affect the lake temperature/air temperature relationship. Conclude with a review of how to calculate the information needed for this activity (lake volume and changes in temperature for layers of a lake.)

Student Inquiry
Television viewers responded to initial reports on heat budgets with many questions. Ask students to reflect on the viewers’ questions as they begin to prepare their presentation.

Reporting Results

Directed Study
When the worksheets are done, discuss the results and how the students arrived at their results.

Student Inquiry
Students should develop a television broadcast based on their analysis of WOW lake data and investigation of heat budgets. The broadcast can be an oral presentation, a TV script, video, or multi-media presentation.

Extensions
1. Compare temperature changes in a lake on different days during the same season or different days in different seasons.

2. If students had been at the lake on the morning this data was collected, would there have been fog above the surface of the water? Why?

3. Calculate the Birgean Heat Budget for this lake - Energy/year = E(midsummer) - E(midwinter)

4. Compare the heat budget for this lake to four lakes in other ecoregions in Minnesota.