This Chapter delves into the intricate interactions of air masses and atmospheric systems, which play a crucial role in shaping global weather patterns. It begins with the definition of air masses, their characteristics, and their horizontal and vertical extent. This chapter explores the source regions of air masses, highlighting their relationship with temperature and moisture content, and emphasizes the significant role of various source regions, such as polar, subtropical, and continental areas. The criteria for the formation of air masses are examined, focusing on their size, homogeneity, and stagnation. Additionally, the classification of air masses based on moisture properties and origin is discussed, providing a framework for understanding different air masses’ behavior. This chapter further investigates the modification of air masses as they travel across different surfaces, leading to changes in temperature, moisture, and stability. It also provides an in-depth analysis of atmospheric fronts, cyclones, anticyclones, and tropical cyclones, which are key mechanisms driving weather systems. The role of thunderstorms, lightning, and tornadoes as dynamic and hazardous phenomena is addressed, with an emphasis on their formation, life cycle, and classification. By examining these elements, this chapter provides a comprehensive understanding of the complex interactions of air masses and atmospheric systems, which are essential for forecasting weather and comprehending climate dynamics.