Air Masses Move
As air masses leave their source region and begin moving to other areas, they bring with them the conditions that are common in the source region. For example, an air mass that formed over a northern region such as Canada will be cold, especially if it forms during the winter. As this air mass begins to move towards the south, it brings with it the cold temperatures that are common in the northern parts of Canada. When this happens, southern parts of the continent experience much cooler weather than they are used to.
As air masses travel they slowly change, taking on the characteristics of the location over which they are changing. Thus, in our example above, as the air mass travels from northern Canada towards the south, it slowly warms. It, however, still remains cooler than the air in that region would typically be.