What characterizes a "marine layer"?

A marine layer is characterized by a cool, moist air mass that forms when warm air aloft traps cooler air near the ocean's surface. This typically occurs under specific atmospheric conditions, especially when the surface is significantly cooler than the air above it, often due to ocean currents or temperature differentials. The presence of this layer can lead to fog and cloud formation, particularly in coastal areas, as the moisture in the air condenses.

The other options do not accurately describe a marine layer. For example, a warm and dry air mass over the ocean does not represent the typical characteristics of a marine layer, which is inherently cool and moist. Similarly, a high-pressure system over the sea does not define a marine layer; rather, high-pressure systems may influence weather patterns but do not specifically create the conditions that define a marine layer. Lastly, while dense fog can occur due to a marine layer, it is not exclusive to nighttime; marine layers can persist throughout the day under suitable meteorological conditions.

Thus, the defining feature of a marine layer is the cool, moist air mass that is trapped between the ocean surface and the warmer atmosphere above it.