THE HEAT BUDGET OF THE EARTH: A HANDS-ON INVESTIGATION

One of the best known areas of confusion for students at all levels are the causes of the seasons. A substantial fraction believe that distance from the sun explains the seasonal cycle, with the Earth being further from the sun during the Northern Hemisphere winter and closer during the summer. We have developed a new lab which gives a direct hands-on appreciation of the causes of the seasons, as well as the equator-pole temperature gradient, the role of albedo, and the continent vs. ocean heat capacity. Students begin by measuring the area of a projected spot of light on a screen at different distances from the source. They then replace the screen with a solar cell and measure the current output as a function of distance. Together, these activities convey the concept of the root mean square decrease of intensity with distance. This result is then used to compare the solar flux at aphelion vs. perihelion. Using a globe, students then determine the change in the solar angle as a function of latitude and of season. The solar cell is now used to determine the effect of angle on the intensity of the radiation. Students quickly learn that the effect of changing solar angle greatly exceeds that of distance. In the second part of the lab, they use heat lamps to measure the relative rates of heating of foam blocks covered with white and black paper. They also measure the rates of heating and cooling of cans containing equal masses of water ands sand. These labs are integrated with lecture materials covering the climate of the Earth. They replace earlier pencil and paper labs which students found tedious and uninformative.