What do we call the boundary between warm and cold air masses?

The boundary between warm and cold air masses is referred to as a front. In meteorology, a front represents a transition zone that marks the change in air masses, typically resulting from variations in temperature and density. This term encompasses different types of fronts, such as cold fronts, warm fronts, and stationary fronts, each indicating specific atmospheric conditions and weather changes associated with the movement of these air masses.

While "stationary front" refers to a specific type of front where there is little to no movement of the air masses, "thermal front" and "weather front" are not standard terminology used in meteorology. Thus, the term "front" effectively captures the general concept of the boundary between warm and cold air masses. Understanding this concept is essential for interpreting weather patterns and forecasting atmospheric behavior.