How does advection differ from convection?

Answer 1

Atmospheric and oceanographic sciences generally differentiate between vertical convection and horizontal advection. These disciplines study the atmosphere and the oceans which are both much wider in the horizontal than they are thick in the vertical. Making the distinction between horizontal (i.e. isopycnal, along surfaces of constant density) and vertical (i.e. diapycnal, through density surfaces) therefore makes sense for meteorologists and oceanographers.

In fluid mechanics on the other hand the terminology is often used interchangeably. Their study environment is frequently a tank in a laboratory and therefore is more of a cube compared to the thin oceanic layer on the Earth. In fluid mechanics, advection and convection is the same.

Answer 2

Both advection and diffusion move the pollutant from one place to another,

but each accomplishes this differently.

The essential difference is:

- Advection goes one way (downstream);

- Diffusion goes both ways (regardless of a stream direction).

This is seen in the respective mathematical expressions:

- Advection u∂c/∂x has a first-order derivative,which means that if x is replaced by -x the term changes signs (anti-symmetry);

- Diffusion D∂2c/∂x2 has a second-order derivative,

which means that if x is replaced by -x the term does not change sign (symmetry).

Answer 3

The difference between advection and convection usually varies if the problem under consideration is related to heat and mass transfer, or in the field of atmospheric science.

In heat and mass transfer, convection is a more general term than advection. In other words, you could simply say Convection = Diffusion + Advection.