Convection is the passage of heat through a fluid by means of large-scale movements of material within the body of the fluid. If, for example, a liquid is heated from below, as in the case of water in a pan, parts close to the heat source expand and, because their density is thus reduced, rise through the liquid; near the top, they cool and begin to sink. This process continues until heat is uniformly distributed throughout the liquid. Convection in the atmosphere is responsible for many climatic effects, as in a thunderstorm (where warmer, moist air moves up and forms clouds). See also thermal conduction.