<em>Convection currents form because a heated fluid expands, becoming less dense. The less-dense heated fluid rises away from the heat source. As it rises, it pulls cooler fluid down to replace it. This fluid in turn is heated, rises and pulls down more cool fluid. This cycle establishes a circular current that stops only when heat is evenly distributed throughout the fluid. For instance, a hot radiator heats the air immediately around it. The air rises toward the ceiling, pulling cooler air down from the ceiling into the radiator to be heated. This process repeats until the air in the room is evenly heated. Convection drives the Gulf Stream and other currents that turn over and mix up the waters in the world’s oceans. Cold polar water is drawn down from higher latitudes and sinks to the ocean bottom, pulled down toward the equator as lighter, warmer water rises to the ocean’s surface. The warmer water is pulled northward to replace the cold water that’s been pulled southward. This process distributes heat and soluble nutrients around the world. Thermal energy also moves within the ocean and within the atmosphere through the process of convection. During convection, cooler water or air sinks, and warmer water or air rises. This movement causes currents. Ocean currents are like rivers of water moving through the sea. The convection happens because hot water has a lower density and thus rises up to the surface where it cools and sinks down to the bottom again. When water starts boiling there is the additional effect of the formation of water vapor bubbles that have a much stronger buoyancy than hot water.</em>
