What is the primary characteristic of the air mass behind a cold front?

The primary characteristic of the air mass behind a cold front is that it is cooler and drier. When a cold front moves through an area, it typically replaces the warmer air that was present before the front with a mass of cooler, denser air. This process leads to a significant drop in temperature and a reduction in humidity levels.

Cold fronts are associated with a rapid change in weather conditions, and the cooler air tends to be more stable, leading to clearer skies and generally better flying conditions after the front passes. The arrival of such air can also contribute to the development of showers and thunderstorms right at the front, but once the front has passed, the air mass stabilizes and becomes clearer and drier in comparison to the warm, humid air that was present prior to the front.

The other options present characteristics that do not align with the general behavior of air masses behind cold fronts. For instance, warmer and more humid air typically precedes a cold front rather than follows it, unstable and turbulent conditions are often associated with the cold front itself during its approach, and stagnant and calm conditions are not representative of the dynamics often seen after a cold front moves through.