convection. Convection is the spread of heat in a fluid, defined as a gas or liquid in which atoms and molecules are moving relatively freely. Consequently, convection can occur in the atmosphere or in bodies of water. Convection currents form when there is unequal heating of the atmosphere or water. As air or water warms, it expands and becomes less dense than the air or water above, and it rises. As air or water cools, its density increases and it sinks.