Final answer:
The boundary between warm and cold air masses is sloped because warm air is less dense and rises above the denser cold air, forming an inclined boundary.
Step-by-step explanation:
The boundary between warm and cold air masses slopes upwards due to differences in air density and temperature. Warm air, being less dense than cold air, rises above the cold air. The slope results from the cold air mass being denser and thus staying closer to the surface, while the warm air mass rises above it, leading to an inclined boundary between the two. As warm air rises, it expands and cools, and when it reaches the saturation point, water vapor condenses into clouds, leading to precipitation, particularly across mountain ranges which can cause a rain shadow effect.