A moist, unstable air mass is characterized by which of the following?

A moist, unstable air mass typically leads to the formation of cumulus clouds, indicating vertical motion and instability. These cumulus clouds can develop into larger cumulonimbus clouds, which are associated with showery precipitation. The presence of moisture in conjunction with instability promotes the development of these dynamic cloud forms, resulting in varying intensities of showers or thunderstorms.

In contrast, the other options do not align with the characteristics of a moist, unstable air mass. For example, cloudless skies and high visibility suggest stable, dry conditions, not the instability and moisture that would create dynamic weather patterns. Continuous fog and low pressure might indicate moisture but lack the instability to generate significant cumulus development and precipitation. Heavy snowfall coupled with low humidity is also contradictory, as snow formation tends to require certain moisture levels, making it inconsistent with the definition of a moist air mass.