Students know that the uneven heating of Earth causes air movements and that oceans and the water cycle influence weather. They also know that heat energy is transferred by radiation, conduction, and convection and that radiation from the Sun is responsible for winds and ocean currents, which in turn influence the weather and climate. They should have learned the concept of density and that warm, less-dense fluids rise and cooler, denser fluids sink (see Standard Set 8, "Density and Buoyancy," for grade eight in Chapter 4). Students who have completed courses in chemistry and physics know that water has high heats of crystallization and evaporation and high specific heat (see Standard 7.d for chemistry in this chapter). Others will have to be introduced to these concepts. This knowledge provides a foundation of physical principles for a fuller understanding of energy flow through Earth's system.