Density of Warm Air vs. Cool Air
As air is heated it expands (moving atoms apart). This reduces the density of air in unconfined space. As a result warm air rises. Conversely, as air cools, it condenses (moving atoms together) and increases it's density in unconfined space. As a result cold air sinks. Because the atmosphere is unconfined, dense cool air will sink and flow to displace warm air in another location (Figure 8.12).
Density of Moist Air vs. Dry Air
Air saturated with water vapor is less dense than dry air. As a result, moist air will rise relative to dry air if air temperatures and pressures are the same.