An air mass is a large volume of air with relatively uniform temperature and humidity characteristics that extends horizontally over a significant region. It forms when air remains in one place long enough to acquire the temperature and moisture characteristics of the underlying surface. Air masses are classified based on their source regions, which include continental (dry) or maritime (moist) and polar (cold) or tropical (warm) origins.

The role of air masses in macro-climatic changes is crucial. They influence weather patterns and climate by bringing consistent weather conditions to different regions. When air masses move from their source regions, they carry their temperature and moisture characteristics with them, affecting the climate of areas they travel over. For example:

1. Temperature Regulation: Air masses can lead to significant temperature changes when they replace one another. A warm, tropical air mass can increase temperatures in cooler regions, while a cold, polar air mass can bring colder conditions.
2. Precipitation Patterns: Air masses also impact precipitation. Maritime air masses are moisture-laden and can lead to rainfall when they move over land and are uplifted. Conversely, continental air masses can result in dry conditions.

Overall, air masses play a key role in shaping regional climates and weather patterns by redistributing thermal and moisture conditions across large areas.