What phenomenon occurs when warm air rises and cool air descends?

The phenomenon described occurs due to the process of convection. Convection involves the movement of heat through fluids (which can be liquids or gases) where warmer, less dense air rises and cooler, denser air descends. This cycle of rising and sinking creates a circulation pattern that can lead to temperature equilibrium within the fluid.

In meteorology, convection is a crucial process because it contributes to cloud formation, weather patterns, and the distribution of heat in the atmosphere. When warm air rises, it can expand and cool, which may eventually lead to condensation and cloud formation if conditions are right.

The other concepts listed do not fit this description: conduction refers to the transfer of heat through direct contact, radiation is the transfer of heat in the form of electromagnetic waves, and condensation is the process of a gas turning into a liquid. Therefore, convection aptly describes the rising and descending movement of air due to temperature differences, making it the correct answer.