A mature mid-latitude cyclone is often a merging point of 2 or more air masses. The air masses north of the mid-latitude cyclone are continental polar or maritime polar air while the air masses to the south of the mid-latitude cyclone are continental tropical and/or maritime tropical. When these air masses are advected toward and into a mid-latitude cyclone they produce what are called "conveyor belts". Since these air masses have different moisture and temperature characteristics (different densities) they advect over the top or underneath each other. The advection of different air masses underneath or over the top of each other is termed "differential advection".

Rule: The denser air mass will advect underneath the less dense air mass. This same principle applies to geology: the denser oceanic crust advects under the continental crust (albeit slow!). Also, at the same temperature, moist air will advect over the top of dry air since moist air is less dense.

Using density, continental polar air is relatively dense while maritime tropical and continental tropical are relatively less dense. Because temperature is more important than moisture content at determining density, maritime polar air is more dense than continental tropical air. Therefore, from largest to smallest density the air masses are: continental polar, maritime polar, continental/maritime tropical. Continental tropical air can be less dense than maritime tropical air although the maritime tropical air has a higher moisture content. This is because the continental tropical air generally warms to a greater temperature than maritime tropical air during the day.

As air masses merge near a mature mid-latitude cyclone, the tropical air will lift above the other air masses due to it being the least dense. Since the maritime tropical air has the most water vapor content, raising mT air will produce condensation, clouds and precipitation. The continental polar air mass is dense and wants to hug right along the earth's surface. Since this cold air mass generally wants to sink, the atmosphere becomes increasingly stable behind a cold front (termed cold air advection). A mid-latitude cyclone can also tap a continental tropical airmass. When this happens, a dryline develops. In the U.S. Plains, a dryline separates continental tropical from maritime tropical. The continental tropical air sits between the cold front and the dryline. This sector of the mid-latitude cyclone is called the dry slot.

Critical point: the position and strength of a mid-latitude cyclone will determine which air masses will be drawn toward the mid-latitude cyclone. All a forecaster has to do is look at the air masses in the immediate vicinity of a mature mid-latitude cyclone to see which air masses will be drawn into the low.

Some rules of thumb: If the warm sector of the mid-latitude cyclone does not have much moisture, precipitation will not be widespread; To have a well defined dryline in association with a U.S. midlatitude cyclone, the maturing low needs to be positioned over the central or southern high plains (in late Spring / early summer); A large temperature difference between the mT and cP air masses as they are drawn toward a midlatitude cyclone will produce a strong PBL low level jet in the warm sector and will intensify the jet stream aloft; When differential advection results in dry air being lifted over mT air, the stage is set for convective instability (thermodynamic instability).