What factor primarily influences the formation of different air masses?

The formation of different air masses is primarily influenced by geographic location. Air masses develop over specific regions where the surface characteristics, such as temperature and humidity, are relatively uniform. For instance, an air mass formed over a warm ocean will be moist and warm, while one that forms over a cold land surface will be cold and dry. The geographic features, such as mountains, valleys, and bodies of water, also affect how air masses are formed and modified as they move.

Other factors like solar radiation, ocean currents, and atmospheric pressure do play roles in weather patterns and climate, but they are more indirect influences on air mass characteristics. For example, while solar radiation affects surface temperature, it's the combination of geographic features and the resultant stable conditions that truly dictate the air mass itself.