Advektion

Advektion, or advection, is the transport of a substance or property by the bulk motion of a fluid. In meteorology and oceanography, advection describes how quantities such as heat, moisture, salinity, or pollutants are carried by winds or currents. Advection refers to horizontal or vertical transport by the flow, and it is distinct from diffusion, which acts through concentration gradients, and from convection, which involves buoyancy-driven vertical motion.

The mathematical treatment uses an advection term in the governing equations. For a scalar field φ transported by velocity v, the non‑conservative form of the advection equation is ∂φ/∂t + v · ∇φ = 0 for a passive scalar with no sources or sinks. In a fluid with variable density ρ, the conservative form is ∂(ρφ)/∂t + ∇·(ρφ v) = S, where S stands for sources or sinks. The material derivative Dφ/Dt = ∂φ/∂t + v · ∇φ is often used, giving Dφ/Dt = S/ρ in the general case. Advection can create sharp gradients and fronts, especially when combined with other processes like phase changes or reactions.

Practically, advection governs the transport of temperature and humidity in the atmosphere, affecting weather patterns, fronts, and moisture advection. In the ocean, advection moves salinity and nutrients with currents. In environmental science, advection transports pollutants and tracers by wind or water flow. Because it redistributes quantities without changing their total amount (in the absence of sources or sinks), advection is a fundamental mechanism in fluid dynamics alongside diffusion and chemical or phase changes.