Explain, using convection, why a room with a single radiator under the window becomes warm all over, and why the window is the best place for the radiator. Solution: The radiator warms nearby air, which expands, becomes less dense, and rises to the ceiling. Cooler room air sinks to take its place, especially the cold air falling from the chilly window glass. This sets up a circulation loop that mixes the whole room's air. Placing the radiator under the window intercepts the cold downdraught at its source, warming it before it spreads across the floor - the most efficient arrangement.