advectionadvektion

Advection, also known as advektion in some languages, is the transport of a substance or a property by the bulk motion of a fluid, such as air or water. It describes how characteristics like temperature, humidity, salinity, or momentum are carried from one location to another by the flow rather than by molecular diffusion.

In mathematical terms, advection is represented by the advection term in the scalar transport equation. For a scalar field φ, the equation is ∂φ/∂t + u·∇φ = sources − sinks + diffusion. Here, u is the velocity field of the fluid, and ∇φ is the gradient of φ. For vector fields like momentum, the advective transport is described by terms involving (u·∇)u.

Examples include heat advection by winds moving warm air into a region, and moisture advection bringing higher humidity and cloud formation. In oceans, advection can transport temperature and salinity, shaping currents and stratification. Advection is distinct from diffusion, which transports properties by random molecular or turbulent motions rather than by the bulk flow of the fluid.

In weather and climate models, advection is a fundamental mechanism for redistributing energy and moisture and for shaping fronts. It can be horizontal or vertical, though horizontal advection is often the dominant process in the free atmosphere. Advection is frequently treated together with diffusion in advection–diffusion equations.

The term advektion is used in several European languages to denote the same physical process: the transport of a quantity by moving fluid. Understanding advection is essential for forecasting weather, modeling ocean circulation, and studying environmental transport.