Which of the following is NOT an air mass region?

The correct answer identifies "Desert" as a region that is not classified as an air mass. Air masses are large bodies of air that share similar temperature, humidity, and pressure characteristics. They are derived from specific geographic areas that reflect consistent climatic conditions.

Equatorial regions are characterized by warm temperatures and high humidity, leading to the formation of equatorial air masses that can influence weather patterns in the tropics. Tropical air masses typically originate from lower latitudes and are also warm, contributing to the warm, moist weather associated with these regions. Polar air masses, on the other hand, originate from high latitudes and are typically cold and dry.

While deserts have specific climates and can influence local weather, they do not represent a distinct air mass classification in meteorology based on temperature and moisture like equatorial, tropical, or polar regions do. Thus, "Desert" does not fit into the classification of air mass regions.