Advection is the horizontal transport or transfer of a quality such as heat and cold from one point to another. Advective transfers occur either in the oceans by currents of seawater or by large-scale movement in the atmosphere where humidity (atmospheric moisture) is another important property. In both cases a major example is the transport of cold air or water masses from the polar regions to lower latitudes.

Another good example for advection is advection fog. Advection fog forms as relatively warm and moist air is moved over a colder surface, either a much cooler sea current or cold land-masses in winter. The cold surface chills air to dewpoint, resulting in low clouds and fog. Vertical transport is more localized and is known as convection.

Related Links:
Weather Fact: Dew Point
Expert maps: Low cloud cover over British Isles
Marine Weather: Water Temperatures