The birth or creation of a front or the transition zone or interface between two air masses of different densities which may be related to temperature, pressure, and humidity.

This takes place when two adjacent air masses presenting different densities and temperatures are brought together by prevailing winds, creating a front.

This happens when either air mass, or both of them, move over a surface which strengthens their original properties; however, it occurs more often along the eastern coasts of North America and Asia, when the air mass moving out over the ocean has a weak or no distinct boundary.