Understanding the different mechanisms of heat transfer is important for physics, meteorology and geography students. For the most part, the differences are easy to understand. However, one common point of confusion is the distinction between convection and advection.
Upon first reading the definitions of both, it’s very tempting to think of the two as one and the same phenomenon, but in reality, advection is a component of the broader, more all-encompassing process of convection. Understanding this distinction is crucial if you want to describe both processes.
What Is Convection?
Convection is a multi-stage process that often results in the cyclic motion of a fluid in response to heating. Imagine the water in a pan that is over the heat: As the bottom of the pan heats up, this heat is transferred (by conduction) to the water at the bottom of the pan.
The water then heats up, causing its molecules to vibrate more and the overall density of this part of the water to decrease due to the increased motion of the molecules. The reduction in density causes the warmer portion of water to rise, being replaced by cooler, denser water from the upper part of the pan.
As the hotter water rises, it loses some of its additional energy (being transferred to other parts of the water as well as being lost as a result of the motion), causing it to cool and become denser.
At the same time, the cooler water that replaced it at the bottom of the pan has now heated up, become less dense and started to rise. The process starts over again, resulting in convection currents throughout the pan. This same process also occurs in the atmosphere, in the air near a heater, and in many other places where there is a distinct source of heat.
What Is Advection?
Advection is a more specific process, defined as the transport of something (such as temperature, moisture or a substance) from one place to another by bulk motion of a fluid, generally horizontally.
The key part of this definition is that it is solely the transport of a material or quantity by the motion of a fluid, and so the advection geography definition depends strongly on wind currents or ocean currents.
A warm ocean current, for example, moves higher-temperature water from one place to another because it is carried along with the bulk motion of the water body. In a similar way, the motion of warm and cold air bodies (i.e., hot and cold fronts) in meteorology is an example of advection.
Convection vs. Advection
The source of the confusion between convection vs advection should now be clear. In both processes, heat can be transferred from one location to another by the motion of a fluid, so in a sense there is a big overlap between the processes. The key factor, though, is that convection contains advection but also includes conductive or diffusive transfer, while advection purely refers to the transfer of the quantity through the motion of the fluid.
You can think about convection versus advection in more detail to understand the difference. At first, heat is transferred to the fluid, and this makes the molecules within it move in random directions, spreading heat through diffusion.
Then as the diffusion continues, and the density of the portion of the fluid drops, there is an advective transfer of internal energy to another location in the fluid, after which there is diffusion to other parts of the liquid.
In short, advection is specifically dependent on the currents within a fluid, while convection is a broader process that also contains diffusive heat transfer. As such, advection can only exist in gases and liquids, and it isn’t possible in a solid.
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