Warm air rises at the equator and travels to around 30° North where it cools and sinks to the surface, before returning to the tropics. This movement is known as the Hadley cell.

• A. Warm; cools; Hadley
• B. Cold; warms; Ferrel
• C. Warm; warms; Hadley
• D. Cold; sinks; Polar

Answer: (D)

The key idea is a tropical circulation loop driven by strong heating at the equator. Warm air rises there because heat makes air buoyant. As this air rises, it expands and cools. The cooler air then sinks around about 30° latitude, and the surface flow returns toward the tropics, completing the Hadley cell. So the best description is warm air rising, then cooling as it moves away from the surface heat, and sinking at subtropical latitudes, which is characteristic of the Hadley cell. This isn’t describing the polar cell, where cold air sinks at high latitudes, nor the Ferrel cell, which operates between 30° and 60°.