What is formed when 2 air masses come together but don't mix?

Answer 1

The boundary is called a "front" and carries an adjective referring to which air mass is advancing and pushing back the other; thus 'cold front' (cold air pushing back warm air) and 'warm front' (vice versa).

There are other more subtle situations like this, such as "outflow boundaries" - cool air left in the wake of thunderstorms and in the middle of much warmer prevailing conditions. Also "occluded fronts" - in a typical mid-latitude low pressure system, a cold front (usually aligned north to south) meets a warm front that is perpendicular to the cold front (in this case, would be aligned west/east) at the low pressure center; in some situations the cold front actually stretches northward to extend past the merge point with the warm front, so that cooler air behind a cold front is actually pushing into the cooler air to the north of the advancing warm front. The air north of the warm front in this cyclone model will normally be more humid than the air behind the cold front, thus there is a boundary between two air masses; it is just more subtle.

It should be noted that the differences in the character of the air masses that meet along a frontal boundary - mostly temperature and humidity differences - are relative to each other. The "warm air" in a frontal setup in northern Canada will often be colder than the "cold air" in a frontal setup in the Southeast U. S., for instance.