Final answer:
The type of air mass that is most likely formed over the deserts of the American Southwest is a subtropical desert air mass. Subtropical deserts are characterized by low annual precipitation and lack of monthly variation in precipitation.
Step-by-step explanation:
The type of air mass that is most likely formed over the deserts of the American Southwest is a subtropical desert air mass. Subtropical deserts are characterized by low annual precipitation, typically less than 30 cm (12 in), and a lack of monthly variation in precipitation. They are formed due to high-pressure, dry air descending at those latitudes, creating arid and semi-arid conditions.