What do you call a boundary between two different air masses, neither of which is strong enough to replace the other?

A boundary between two different air masses, where neither is strong enough to replace the other, is known as a stationary front. In this situation, the air masses remain in place without significant movement, which often leads to prolonged periods of cloudy skies and precipitation as the air masses interact.

Stationary fronts occur when a warm air mass and a cold air mass collide, causing the front to stagnate. This can result in various weather conditions depending on the characteristics of the air masses involved. In contrast, warm fronts, cold fronts, and occluded fronts involve the movement of air masses, with one air mass actively displacing the other.

By understanding the stationary front's behavior, one can better predict the associated weather patterns that may arise from the situation where two contrasting air masses meet but do not advance.