Question

What process is at work when warm air or water rises, and cool air or water sinks?

Answer 1

convection

Warmer, lighter air is more buoyant than the cooler air above it. So the warm air rises. The cooler air is denser than the air beneath it. So it sinks down. This is convection: warm air rises, and cool air sinks. Warm fluids can undergo convection as well. This is described in the chapter Plate Tectonics .