What is a key characteristic of a maritime polar air mass?

A maritime polar air mass is characterized by being cool and moist. This type of air mass develops over large bodies of water, such as oceans, where it picks up moisture as it moves over the water surface. As a result, maritime polar air masses are typically found in the northern latitudes and are known for bringing cooler temperatures along with significant humidity. The cool aspect is due to the fact that they originate in cooler oceanic regions, while the moisture content is derived from the water, leading to the characteristic cool and moist conditions.

The other characteristics mentioned in the options do not apply to a maritime polar air mass. For instance, hot and dry air is indicative of a desert or continental tropical air mass, while warm and humid air would be more representative of maritime tropical air masses. Additionally, cold and dry conditions are associated with continental polar or arctic air masses, which form over land and lack the moisture typically found in maritime air masses. Thus, the distinctive cool and moist nature of maritime polar air masses is what makes them unique among the different types of air masses.