The border between a warm air mass and a cold air mass is called a front.

There are different types of fronts depending on which air mass is advancing:

* Cold front: When a cold air mass is advancing into a warm air mass.

* Warm front: When a warm air mass is advancing into a cold air mass.

* Stationary front: When neither air mass is advancing.

* Occluded front: When a cold front overtakes a warm front.

These fronts are important weather features as they can cause significant changes in temperature, wind, precipitation, and cloud cover.