Convection, animation. Convection is a process by which heat moves in fluids and gases. Hot areas are white here, progressing from red through yellow, green and blue to purple, the coolest areas. Hot fluids are less dense than cold ones, so as the fluid is heated the warm areas rise until they reach the surface top, then spread out. As they lose heat to the surrounding regions, they cool and descend again, away from the rising plume. This establishes round patterns called convection cells in the fluid.