Maritime air masses form ____.

Maritime air masses form over water, which is the defining characteristic of these air masses. They typically originate over oceans or large lakes, where they acquire moisture from the water surface. As a result, maritime air masses tend to be humid and can bring significant precipitation when they move into other regions and interact with different air masses or geographical features.

The formation process emphasizes the importance of the surface over which an air mass develops; water bodies provide the necessary moisture that defines maritime air masses, distinguishing them from continental air masses, which develop over land and generally exhibit drier characteristics. Thus, the correct answer accurately reflects the conditions under which maritime air masses are formed.