advection

Transport of a quantity by the bulk motion of a fluid (e.g. heat or moisture carried by wind):

$$\frac{\partial c}{\partial t}+\mathbf{u}\cdot \nabla c=0$$

where:

• $c$: transported quantity (concentration, temperature, …),
• $\mathbf{u}$: flow velocity,
• $\mathbf{u}\cdot \nabla c$: advection term.

Relative importance of advection and diffusion (Péclet number), with $U$, $L$ typical velocity and length scales and $D$ the diffusivity:

$$Pe=\frac{UL}{D}$$