Frontal Systems and Advection

Frontal Systems

The final lifting mechanism which we will discuss is the overriding of air at frontal boundaries. Previous readings introduced the various categories of air masses and the different types of fronts that form when these air masses meet. An air mass has certain temperature and moisture characteristics. In fact, these characteristics are the basis of categorizing air masses. For instance, a continental-polar air mass (cP) originates from a continent near a polar region. Continental air tends to be dry while polar air is relatively cold. If a cP air mass meets up with a continental tropical mass (cT), which is dry and warm, the warm tropical air will override or rise above the polar air. Likewise, moist air, because it is less dense, will override dry air. But how is this different from convergence? In the case of convergence, the lifting results from air molecules pushing one another upward, like pushing two small piles of sand together with your hands, forcing a larger pile to form. When two frontal boundaries meet, the lifting that occurs is due to the relative buoyancy of the two air masses. The more buoyant air mass will override the lesser buoyant air mass. The buoyancy is determined by the characteristics of the air masses (i.e., temperature and moisture content).

Advection means the transportation of air by the mean wind, typically as horizontal movement. Temperature advection is the transport of heat to or from a region by the wind. Vertical motion is also called advection if the movement is caused by the mean wind, but the vertical motion of air is called convection if it moves due to buoyancy.

In advection, the situation is one of cooler air moving over a warmer surface. This is not to be confused with daytime heating of the surface, rather the movement of cold air through advection over a warmer surface, lifting the warmer air as it moves.