Final answer:
An unstable parcel of air rises due to buoyancy until it reaches an altitude where its density and temperature match the surrounding air, following the principles of Charles's Law and the laws of thermodynamics.
Step-by-step explanation:
When considering an unstable parcel of air, if the surrounding air has a different density and temperature, the parcel will respond by moving to reach equilibrium. Since the density of air decreases with altitude, Charles's Law helps us understand that as a parcel of air warms up, its volume increases and hence its density decreases. This heated, less dense parcel then demonstrates buoyancy, which causes it to rise in the atmosphere. According to the laws of thermodynamics, this air parcel will continue to rise until it reaches an altitude where the surrounding air has similar density and temperature to the parcel itself.