TEACHER GUIDE

TOPICS: Heat flow and capacity, weather, animal behavior

LEVEL: High School and Advanced Placement

TIME: Two class periods

PREDICTIONS: In class discussion, encourage creative, logical and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: Because the ground at night radiates heat to the sky and quickly becomes cooler than the air, heat flow is from the air to the ground, cooling the lower layer of air. Thus, the air high in a tree may be warmer on a calm night than at the base of the tree. Wind serves to mix cool and warm air; usually windy nights are significantly warmer. Large shallow depressions collect more cool air and radiate more heat into space than narrow ditches. The sides of hills are usually warmer than valley areas. Snow reflects solar energy but insulates against radiative heat loss; if there is enough snow, the ground under the snow stays warmer than the air .

PROCEDURE: This experiment works best on a clear, calm night. Try to pick an area with as much variety in vegetation and terrain as possible.  If wooded areas are not available, any covered open space can be substituted.  Students should place thermometers at different heights above the ground so that the effect of elevation can be studied.

QUESTIONS 5-11 require higher order thinking skills. We recommend that you discuss these questions in class before students write out their answers .

2-6. Answers will vary (see Heat Transfer Teacher Background) .

7. Narrow depression because cold air is collected from a smaller area and less of the exposed area is sky .

8. It is generally warmer higher up at night since cool air sinks .

9. Cold air tends to flow away from the orchard down into the valley .

10. Trees tops are warmer than the sky so they radiate more energy down .

11. Sidewalk is usually warmer than grass because it is in better thermal contact with the ground. Warmer air over the sidewalk will rise relative to the cold air over grass. The circulation is completed by air moving from the grass to the sidewalk at the surface and returning higher up .

12. The highest temperature usually is farther up and the lowest at the bottom due to the temperature inversion often found at night, especially over a snow covered surface. If it is cloudy or windy, all temperatures are likely to be the same and experimental error will be dominant. In this scenario, if the flagpole is set in pavement surrounded by grass, the cold air coming off the grass could cause the lowest temperature at the flagpole to be above the surface .

13. With wind, surface temperatures should be higher because a strong inversion is less likely to form. Note that a thermometer does not measure windchill. A bird or rabbit could be subjected to windchill so open areas might be avoided on windy nights.