When two unlike air masses meet what usually keeps them separate

Air mass means a large mass of air that can be characterized by their temperature and humidity within it. Air mass picks up the distinct temperature and humidity characteristics of a region when it sits over that region for several days, or longer, and, this occur when it is slowly pushed along by high-level winds. In other words, Air masses build when the air stagnates over a region for several days/weeks.

Air masses that are formed when the atmosphere remains in contact with a large, relatively uniform land or sea surface for a time can be classified according to the temperature and moisture characteristics where they develop. Heat is brought to cold areas and cools down areas that are warm because, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles.

When two large air masses meet, the boundary that separates them is called a front, and the properties of air masses are determined by the the underlying surface properties where they originate. As an air mass moves, it tends to retain the properties of the region where it formed and when different air masses meet, they don't mix. Cold air masses are denser and sink, while warm air masses are less dense and rise, which makes more clouds form, which brings precipitation when warm air rises. There are four different boundary between two air masses- Cold, Warm, Stationary, and Occluded. Cold boundary forms when a cold air mass pushes under a warm air mass, forcing the warm air to rise. While, warm boundary forms when a moist, warm air mass slides up and over a cold air mass.