When air is heated, the molecules within the air gain energy and start moving faster. This increased molecular motion causes the air molecules to spread out, leading to the expansion of the air. As a result of this expansion, the same mass of air occupies a larger volume, which decreases its density. Because hot air is less dense than cooler surrounding air, it becomes lighter and tends to rise. This principle is the reason hot air balloons rise and is also fundamental in creating convection currents in the atmosphere. In summary:

• Air expands when heated.
• Expansion makes the air less dense.
• Less dense (lighter) hot air rises above cooler, denser air.

This effect of heated air rising is a key driver of atmospheric motions and weather patterns. The physical relationship governing this is described by the gas law PV=nRTPV=nRTPV=nRT, where heating increases temperature TTT, causing either pressure or volume (or both) to increase, leading to expansion.