A convection cell is a system in which a fluid is warmed, loses density and is forced into a region of greater density. The cycle repeats and a pattern of motion forms. Convection cells in Earth's atmosphere are responsible for the blowing of wind, and can be found in a variety of other natural and manmade phenomena.
Convection, along with conduction and radiation, is one of the three methods of heat transfer. Convection takes place through the actual movement of matter. This means that convection can only take place in gases, liquids and plasma--not solid matter. A good example of convection is in a hot air balloon. As the air in the balloon is heated, the molecules of which it is composed spread out. This leads to an increase in the air's volume, which leads to a decrease in density. Dense matter moves into less dense matter whenever it has a chance. The warm air in the balloon is pushed upward by the cooler air of the surrounding atmosphere, taking the balloon with it.
Natural and Forced Convection
Natural convection takes place when the motion is entirely due to density differences between warm and cold matter. Forced convection happens when another force, such as a fan or pump, contributes to the motion.
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A source of heat is required for a convection cell to form. Fluid is warmed by the heat source and is pushed away. The fluid then begins to lose heat, and inevitably cools. This cooler, denser matter is forced back toward the initial heat source by the flow of newly heated matter. A system of motion forms, called a convection cell. The fluid will continue to move for as long as the heat source is present.
Convection Cells in the Atmosphere
Convection cells occur in Earth's atmosphere on both small and large scales. A sea breeze, for example, can be the result of a convection cell. Water holds heat better than land. This means that when the sun rises, the air on land warms more quickly than the air above the water. A low-density area forms over the land. Higher-density air from the water seeks to replace it, creating an ocean breeze. At night the same thing happens, but in reverse. On a larger scale, air is warmed by higher temperatures at the equator, rises and spreads north and south toward the poles, where it is cooled.
Other Convection Cells
Convection cells are responsible for making macaroni rise and sink in a pot of boiling water. One of the forces that contributes to lava erupting from a volcano is convection. Convection cells can even be found on the sun.