What is the main characteristic of an equatorial air mass?

An equatorial air mass is characterized by high humidity and warmth due to its formation in the tropics near the equator, where the sun's rays are most direct. This region experiences consistent thermodynamic conditions, leading to warm air that can hold significant amounts of moisture. Thus, when we talk about equatorial air masses, we note that they are often accompanied by characteristics like high humidity and stable, warm temperatures. This setup can lead to the development of clouds and precipitation, making them critical for various weather patterns in the regions they affect.

In contrast, other air mass types represent different characteristics. For instance, low moisture levels are typical of continental polar air masses, while cool and dry air is more indicative of polar maritime air masses. Instability and turbulence are often associated with specific weather patterns or phenomenon like thunderstorms, rather than being a defining feature of equatorial air masses. Therefore, the unique combination of warmth and high humidity distinguishes equatorial air masses from others, reinforcing the correctness of the answer.