If you have ever grabbed the metal handle of pot being heated over a campfire, you have painfully experienced heat transfer. There are four ways in which heat is transferred from one object to another: conduction, radiation, convection and advection. Heat almost always flows from the higher temperature object to the lower, changing the internal energy of both objects in the process. The primary difference between convection and advection heat transfers is the direction of the exchange.
Convection heat transfer involves the transfer of heat through the movement of the medium’s particles. This medium must be a gas or liquid, thereby allowing for movement. Convection always transfers heat in the vertical plane. This movement is driven by variations in the medium’s density and, therefore, buoyancy. Heated particles expand, causing them to decrease in density; these particles become more buoyant than surrounding particles, causing them to rise. As they rise, their heat is transferred to cooler portions of the medium located above them.
Convection heat transfer takes place when a pot of water is heated. As the water molecules closest to the heat source warm, they expand. This expansion lowers their density and they begin to rise; this is what causes water in a pot to boil. The atmosphere also provides an example of convection heat transfer. When a packet of air is warmed by solar energy -- radiation heat transfer -- the air packet expands, lowering its density. This increases its buoyancy and causes it to rise in the atmosphere. This produces an unstable atmosphere with a vertical flow of air.
Advection heat transfer differs from convection in that the movement of heat is confined to the horizontal plane. This type of heat transfer is not powered by variations in density, but rather requires an outside force, such as wind or currents, to displace the particles of the medium. As the particles move horizontally into systems that are hotter or colder, heat is transferred.
The primary example of advection heat transfer is the movement of meteorological fronts. These fronts represent air masses of cold or warm air that are moved horizontally over the surface by winds; as these air masses encounter warmer or cooler air, heat is exchanged between the systems. Ocean currents are another example of advection heat transfer. Rather than vertically, currents move warm or cold water in horizontal directions. As these waters interact with warmer or cooler areas of water, heat is exchanged between them.
The remaining types of heat transfers are conduction and radiation. Conduction transfers heat from one object to another with no movement; teat is transferred from molecule to molecule. This type of heat transfer only occurs in solids; the handle of a hot pot is an example of conduction. Radiation heat transfer involves the transfer of heat by electromagnetic waves of energy. An example of radiation is sunlight; when these waves strike other particles, they cause them to vibrate, or warm.
