This occurs when a mass of warm, less dense air advances, overriding a retreating mass of cold air. Because warm air is lighter, it rises smoothly and gradually over the slope of cold air, causing a slower and more continuous weather transition. It begins with high clouds (Cirrus) and evolves into low, dense clouds (Nimbostratus), resulting in moderate, continuous, and prolonged rainfall (reduced visibility or fog), followed by a rise in temperature.