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Studying Heat Budgets of Lakes

All lakes store and release heat energy. As lakes exchange latent heat with the atmosphere they affect the surrounding air temperature. In this lesson you use WOW data to calculate the heat budget of a lake. You also explore factors that play a role in the lake/weather relationship.

Knowledge Base
Consider your experiences with air and water temperatures around lakes. During which months does the water feel warmer than the air? At what times of day does water feel coldest? Warmest?

You may need to review how to calculate water volumes for each one meter layer of a lake. You also need to recall that it takes 1 calorie to raise 1 cubic centimeter of water one degree (at 20°C) and 1 million calories to raise 1 cubic meter of water 1 degree if no changes of state are involved.

Experimental Design

Data Collection

1. Use water temperature data collected at 1 meter intervals at 6 a.m., 2 p.m., 6 p.m., and 2 a.m. Record the temperatures in Table 1.
2. List the surface area for each 1-meter deep layer in the lake (See Ice Lake Total Heat Budget). Fill in that information in column 2 of Table 2.
3. Calculate the water volume in each 1-meter layer of the entire lake. Fill that information in column 3 of Table 2.

Figure 1. Lake Surface Areas at 1 Meter Intervals

 Ice Lake Lake Independence Depth Intervals (m) Area at surface of this depth (m2) Depth Intervals (m) Area at surface of this depth (m2) 1 m 1.600E+05 1 m 3.290E+06 2 m 1.481E+05 2 m 2.875E+06 3 m 1.362E+05 3 m 2.625E+06 4 m 1.244E+05 4 m 2.275E+06 5 m 1.126E+05 5 m 1.925E+06 6 m 1.008E+05 6 m 1.675E+06 7 m 8.890E+04 7 m 1.350E+06 8 m 7.710E+04 8 m 9.750E+05 9 m 6.520E+04 9 m 7.000E+05 10 m 5.640E+04 10 m 4.200E+05 11 m 4.160E+04 11 m 2.050E+05 12 m 2.970E+04 12 m 1.560E+05 13 m 1.790E+04 13 m 1.310E+05 14 m 6.300E+03 14 m 1.100E+05 15 m 4.000E+03 15 m 9.000E+04 16 m 1.000E+02 16 m 7.000E+04 17 m 4.000E+04 18 m 1.000E+04

Data Management and Analysis

1. To raise the temperature of water, energy is required. In lakes, what is the source of energy?

2. For each layer of the lake, calculate the change in temperature between 6 a.m. and 2 p.m. by subtracting the 6 o'clock in the morning temperature from the temperature at 2 o'clock in the afternoon. Fill this information in Table 2, Column 4 (Temperature change).

3. Calculate how many calories were involved in changing the water temperature between 6 a.m. and 2 p.m. Fill that information in Table 2, column 5 (Calories used).

4. Calculate the temperature change between 6 p.m. in the evening and 2 a.m. at night for each layer of the lake. Fill in that information in Table 2, column 6 (Temperature change).

5. Calculate how many calories were involved in changing the water temperature between 6 .pm. and midnight. Fill those numbers in Table 2, column 7 (Calories used).

Table 1. Temperature Measurements for Ice Lake.

 Date data was collected by RUSS: ____________ Water layer 6:00 a.m. 2:00 p.m. 6:00 p.m. 2:00 a.m. 0-1 m depth 1-2 m depth 2-3 m depth 3-4 m depth 4-5 m depth 5-6 m depth

Table 2. Water Volume and Energy Changes for Ice Lake

 Date data was collected by RUSS: ____________ Water layer Surface area of this layer (m2) Water Volume (m3) Temp. change, (°C) 6am-2pm Calories used Temp. change, (°C) 6pm-2am Calories released 0-1 m depth 1-2 m depth 2-3 m depth 3-4 m depth 4-5 m depth 5-6 m depth

Interpretation of Results

1. What happened to the energy that was given off when the lake's surface cooled?
2. What happened to the energy given off by layers below the surface?
3. How much total energy was released from the lake between 6 p.m. and 2 a.m?
4. How do the temperature changes in layers below the surface of the lake compare to temperature changes in the surface layer?
5. How can the energy released from lakes affect local weather patterns and the surrounding environment?
6. How might the size of the lake affect its impact on the weather and surrounding environment?
7. Identify a large lake in Minnesota that probably affects the local weather. Describe how and why you think this lake would affect the local weather.
8. Identify an example of a large lake outside the U.S. that might affect local weather. Describe how you could find out more about the affects of this lake on the surrounding environment.
9. A cubic centimeter of water requires 540 calories to boil (change from a liquid to a gas at 100°C). This is an example of the different amounts of energy required when matter changes state. How much energy would be required to boil off the top meter of the lake beginning at 2pm? Where would you get that much energy?
10. When a cubic centimeter of water freezes (changes from a liquid to a solid at 0°C), 80 calories of energy are released. (When water changes state — liquid to solid — the amount of energy involved in the change is different than the amount involved in incremental changes in temperature while water is in the same state.) How much energy would be given off for the top meter of the lake to freeze if the freezing began at 2 am? [Hint, you have to calculate the energy released for each degree of temperature drop, PLUS the 80 calories, for each cubic centimeter.]
11. How might the energy released from a lake when it freezes affect the surrounding environment?

Reporting Results