Advection is the transport of material, energy, or properties by fluid motion. In meteorology, it refers to the movement of heat or humidity, while in oceanography, it describes the transport of heat, dissolved substances, or suspended matter by ocean currents. Advection is distinct from convection and can be positive or negative. It is important in weather forecasting as it affects temperature, cloud cover, precipitation, and fog. Orographic precipitation occurs when moist air is forced up and cools as it rises, causing water vapor to condense into droplets, forming clouds that can lead to rain.
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, 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 humidity. In oceanography, it is used to describe the transport of heat, dissolved substances, such as salt or suspended matter by ocean currents.
Although, in the context of heat, there may be some overlap between the two terms, advection is generally considered to be distinct from convection and is generally considered to refer to predominantly horizontal motion. An example would be the transport of heat over a distance by an air stream. By convection, on the other hand, we usually mean a mainly vertical movement of the fluid in response to the heating or cooling of a surface.
Advection can be positive or negative with respect to the property that is the object of the advection. Where the wind direction goes from higher to lower values for the property, the advection is said to be positive, as it will increase the downwind values. Conversely, a wind direction from lower to higher values results in the negative form, which decreases the leeward values. In the context of temperature, a wind blowing from a warmer region will carry warm air, raising the temperature, while a wind from a cooler region will lower it. These scenarios can be called heat advection and cold advection, respectively.
When forecasting the weather, it is extremely important to take into account advection. You can draw a map to show, for example, the temperature distribution. Lines known as isotherms would connect points of equal temperature, and meteorologists can use the map to predict possible temperature changes for a given area from the relationship between isotherms and wind direction. Where the wind, often indicated by arrows, crosses isotherms, the temperature can be expected to rise or fall, depending on whether the wind is coming from a warmer or colder area. If the wind direction is parallel to the isotherms, no temperature change 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, an advection fog can form.
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