Convection is the movement of molecules through fluids and is the primary form of heat and mass transfer. There are two types: forced and natural, which can occur simultaneously. Convection is seen in man-made objects and natural phenomena, such as ocean currents that regulate weather patterns.
Convection is in general the movement of any molecule through any fluid state, including liquids and gases. It is the major form of heat and mass transfer on Earth and can be found in many systems around the world. Many people know it from heating their food or their homes, and it also plays a vital role in creating the weather on the planet.
There are two main types of convection: forced and natural. Forced convection occurs when the heat transfer medium moves by itself, such as when air is pushed by a fan. In this case, the heat is being delivered by the air motion of the fluid, but is not actually the cause of the motion. Sometimes forced convection is also called heat advection.
Natural convection occurs when the heat transferring medium is inspired to move by the heat itself. This occurs both because the medium expands as it heats up, as in the case of gases, and because buoyancy causes the hotter fluid to rise. This form is also sometimes referred to as free convection. The two types, forced and natural, often occur simultaneously, where the heat causes the fluid to move a bit, but it also moves due to some other force. In this case we speak of mixed convection.
Forced convection can be seen in many places around the world, both in man-made objects and in the natural world. For example, a convection oven forces heated air to circulate through the oven using a fan. While the air may move a bit on its own, the force allows for even heat disbursement. Within the human body, the circulatory system is another prime example of forced convection. Blood, among its many other uses, acts as both a warming agent and a cooling agent for the body and is forced through the circulatory system.
Natural convection can be seen most easily by placing something on top of an unvented heat source, such as a radiator. When the air directly above the heater is heated, it rises because it is more buoyant than the cold air around it. If you place a piece of tissue paper over the heater, you will see that it will be carried upward by the natural movement of the air. Similarly, in a liquid you can see the same effect at work by heating a colored liquid and placing it inside a cooler liquid of a different color. The hot liquid will rise.
Convection plays a vital role in creating ocean currents and thus helps regulate weather patterns around the world through a massive system of convection. Streams of warm, sun-warmed water rush from the equator toward the poles, and as they travel they cool steadily, helping them sink. Once there, they rise and start moving again, forming a massive conveyor belt that can take up to 1,600 years to complete its journey in some cases.
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