Advection is the transport of a substance (and its properties) by bulk motion. During advection, a fluid transports some conserved quantity or material via bulk motion. The fluid’s motion is described mathematically as a vector field, and the transported material is described by a scalar field showing its distribution over space. Advection requires currents in the fluid, and so cannot happen in rigid solids. It does not include transport of substances by molecular diffusion.
Advection is sometimes confused with the more encompassing process of convection which is the combination of advective transport and diffusive transport.
Advection is a general term related to fluid motion. It simply means the transport of material, energy, or a property such as temperature or humidity by the movement of a fluid, which can be a liquid or a gas. The term can, however, have a number of more specific meanings depending on the context. It is most commonly used in the field of meteorology, where it usually refers to the movement of heat or moisture. In oceanography, it is used to describe the transport of heat, dissolved substances such as salt, or suspended material by ocean currents.
Although, in the context of heat, there may be some overlap between the two terms, advection is generally considered distinct from convection and is generally considered to refer to predominantly horizontal motion. An example would be the transport of heat away from an air stream. Convection, on the other hand, is usually understood to refer to predominantly vertical movement of fluid in response to heating or cooling of a surface.
Advection can be positive or negative with respect to the property being advected. Where the wind direction ranges from higher to lower values for the property, advection is said to be positive, since it will increase downwind values. Conversely, a wind direction from lower to higher values results in the negative form, which decreases downwind values. In the context of temperature, a wind blowing from a warmer region will carry warm air, increasing the temperature, while wind from a colder region will lower it. These scenarios can be called heat advection and cold advection, respectively.
In weather forecasting, it is extremely important to account for advection. A map can be drawn to show, for example, the temperature distribution. Lines known as isotherms would connect points of equal temperature, and forecasters can use the map to predict any temperature changes for a given area from the relationship between the isotherms and wind direction. Where the wind, often indicated by arrows, crosses the isotherms, the temperature can be predicted to increase or decrease, depending on whether the wind is coming from a warmer or cooler area. If the wind direction is parallel to the isotherms, no change in temperature would be expected.
Moisture can also be transported by air movement and this is important in predicting cloud cover, precipitation, and fog. Where there is airflow from an area of high moisture content, such as a warm ocean, this can be expected to produce clouds and precipitation. This is especially true if the air is forced to rise to a colder level by the local topography, a phenomenon known as orographic precipitation. A typical example would be wind blowing from an ocean across a coastal mountain range: moist air is forced up and cools as it rises, causing water vapor to condense into droplets, forming clouds that can lead to rain. When moist air is cooled below the dew point by a cold surface, advection fog can form.