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Advection is a general term relating 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 — this may be a liquid or 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 can be some overlap between the two terms, advection is usually regarded as distinct from convection and is generally taken to refer to a mainly horizontal movement. An example would be the transport of heat over a distance by a flow of air. Convection, on the other hand, usually means a mainly vertical movement of fluid in response to the heating or cooling of a surface.
Advection can be positive or negative with regard to the property being advected. Where the wind direction is from higher to lower values for the property, advection is said to be positive, as it will increase the values downwind. Conversely, a wind direction from lower to higher values results in the negative form, which decreases the downwind values. In the context of temperature, a wind blowing in from a warmer region will transport warm air, increasing the temperature, while wind from a colder region will lower the temperature. These scenarios can be called heat advection and cold advection, respectively.
In weather forecasting, it is extremely important to take advection into account. A map can be drawn to show, for example, temperature distribution. Lines known as isotherms would connect points of equal temperature, and meteorologists can use the map to predict any likely temperature changes for a given area from the relationship between isotherms and wind direction. Where the wind, often indicated by arrows, crosses the isotherms, the temperature can be expected to rise or fall, depending on whether the wind is flowing from a warmer or colder area. If the wind direction is parallel to the isotherms, no temperature change would be expected.
Moisture or humidity can also be transported by air movement and this is important in forecasting cloud cover, precipitation and fog. Where there is a flow of air from an area high in moisture — for example, a warm ocean — this can be expected to produce clouds and precipitation. This is especially the case if the air is forced to rise to a colder level by the local topography, a phenomenon known as orographic rainfall. A typical example would be wind blowing from an ocean across a range of coastal mountains: the moist air is forced to rise and cools as it rises, causing the water vapor to condense into droplets, forming clouds that may lead to rain. When advected moist air is cooled below its dew point by a cold surface, an advection fog can form.