Convection is broadly the movement of any molecules through any fluid state, including liquids and gasses. It is the primary form of heat and mass transfer on Earth, and can be found in many systems around the world. Many people are familiar with it from heating their food or their houses, and it also plays a pivotal role in creating the weather conditions on the planet.
There are two main types of convection: forced and natural. Forced convection occurs when the medium transferring the heat is moving on its own, such as when air is being pushed by a fan. In this case, the heat is disbursed by the movement of the air of fluid, but it isn’t actually causing the movement. Sometimes forced convection will be referred to as heat advection, as well.
Natural convection occurs when the medium transferring the heat is being inspired to move by the heat itself. This happens both because the medium expands as it heats up, as in the case of gases, and also because buoyancy causes the warmer fluid to rise. This form is also sometimes referred to as free convection. The two types, forced and natural, often occur at the same time, where the heat is causing the fluid to move somewhat, but it is also moving because of some other force. In this case, it is referred to as mixed convection.
Forced convection can be seen in many places in 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 by using a fan. Although the air might move somewhat on its own, the force allows for an even disbursement of heat. Within the human body, the circulatory system is another great example of forced convection. Blood, among its many other uses, acts as both a heating agent and cooling agent for the body, and is forced through the circulatory system.
Natural convection can be seen most easily by placing something above a non-fanned heat source, like a radiator heater. As 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 above the heater, you’ll see it get carried upwards by the natural air movement. Similarly, in a liquid you can see the same effect at work by heating up one liquid that is colored and placing it within a colder liquid of a different color. The warm liquid will rise up.
Convection plays a pivotal role in creating the oceanic currents, and thereby helps to regulate the weather patterns around the world through a massive convection system. Warm water currents, heated by the sun, head from the equator towards the poles, and as they travel they steadily cool, helping them sink. Once there they upwell, and begin to move again, forming a massive conveyor belt that in some cases can take upwards of 1,600 years to complete its journey.