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Advection is the transport of something within a fluid through the movement of the fluid itself, e.g., through the fluid's expansion or convection, or through gravity's effect on the fluid. The "something" transported can be a substance or a condition, e.g., pollutant particles, water vapor, gas makeup, heat energy, or electric charge. The term can be used for liquids, such as in rivers or oceans, or gases, such as in atmospheres or gas clouds.
An advection timescale is proportional to an advection length scale (scale of the distance the material is being transported) divided by an advection velocity scale (scale of the fluid velocity). It is useful for comparison with competing timescales in determining whether advection is the principal process resulting from some material's insertion or presence in a fluid.