Warm air rises because when air is heated, its molecules move faster and spread apart, making the air less dense compared to the surrounding cooler air. This decrease in density makes warm air buoyant, causing it to rise above the denser cold air. This process, known as convection, drives the movement of heat in the atmosphere and explains why warm air tends to rise while cooler air sinks.

The rising of warm air continues until it reaches a point where its density matches the surrounding air, achieving stability. This phenomenon is fundamental in weather patterns and is also the reason why upper floors in buildings tend to be warmer than lower floors.