This lecture discusses air masses and fronts, including their defining characteristics. An air mass is a large body of air with similar properties that modifies the areas it moves over and is modified in turn. Air masses are classified by their moisture content (continental dry or maritime humid) and temperature source region (polar, tropical, arctic, or equatorial). Fronts occur at the boundaries between differing air masses, and the main types of fronts are cold, warm, stationary, and occluded fronts, which are distinguished by the movement and interaction of warm and cold air masses.