Air masses are primarily defined by what characteristic?

Air masses are primarily defined by their source regions. This characteristic is crucial because the source region determines the overall properties of the air mass, including its temperature and humidity. Air masses form over large areas with uniform conditions, such as land or water bodies, and their characteristics are influenced by the nature of those surface conditions.

For example, an air mass that originates over a cold ocean will typically be cold and moist, while one that develops over a warm desert will be hot and dry. Understanding the source regions helps meteorologists predict the behavior and movement of air masses as they influence weather patterns in different areas.

The other characteristics, such as movement, temperature, and pressure, are indeed important in meteorology but do not define what an air mass is. Movement refers to how air masses travel and can change their characteristics, while temperature and pressure describe current conditions rather than the defining aspects of the air mass itself.