An air mass is an extensive body of air that has a relatively homogeneous temperature and moisture content over a significant altitude. Air masses typically cover areas of a few hundred, thousand, or million square kilometers. A front is the boundary at which two air masses of different temperature and moisture content meet. The role of air masses and fronts in the development of weather systems was first appreciated by the Norwegian father and son team of Vilhelm and Jacob Bjerknes in the 1920s. Today, these two phenomena are still studied intensively as predictors of future weather patterns.

Air masses form when a body of air comes to rest over an area large enough for it to take on the temperature and humidity of the land or water below it. Certain locations on the earth's surface possess the topographical characteristics that favor...