When two air masses collide, the formation is called a front.

Here's a breakdown:

* Front: A boundary separating two air masses of different temperatures, humidity, and density.

There are different types of fronts, each with its own characteristics:

* Cold Front: A boundary where a cold air mass is displacing a warmer air mass.

* Warm Front: A boundary where a warm air mass is displacing a colder air mass.

* Stationary Front: A boundary where two air masses are neither advancing nor retreating.

* Occluded Front: A boundary where a cold front overtakes a warm front.

The collision of air masses is a key driver of weather patterns and can lead to various weather phenomena, including precipitation, thunderstorms, and changes in wind direction.