Convection current round a room. Illustration showing a convection current generated by a radiator. Convection above a hot surface, such as a radiator, occurs because the heated air expands, becomes less dense, and rises (red). As it rises and moves away from the source of heat, it cools, becomes denser, and sinks (blue). The rising hot air creates low pressure which draws the cooler air towards the radiator, generating a convection current of warm and cooler air circulating around the room.