Final answer:
An air mass must sit over a region for several hours to acquire its characteristics, such as moisture and temperature.
Step-by-step explanation:
For an air mass to acquire the characteristics of a region, it must sit over that region for several hours. During this time, the air mass interacts with the land or water surface and exchanges heat and moisture, which allows it to acquire the properties of the region.
For example, when an air mass sits over an ocean, it may acquire the moisture and warmth of the ocean, resulting in a maritime air mass.
Similarly, when an air mass sits over a desert, it may become dry and hot, acquiring the characteristics of a desert air mass.