Convection occurs in fluids that are heated. Warm fluid molecules are able rise, due to their lower density, while the cooler molecules sink. This cyclic movement of hot and cold molecules, due to differences in density, causes convection currents, aiding in heat circulation.
The repetition of warm fluid rising and cold fluid sinking, called convection, occurs due to the difference in densities between warm and cold fluids.
When heat is supplied to fluid molecules, there is an increase in their kinetic energy. The molecules closest to the heat source begin to move faster. The faster moving molecules begin to spread apart, thus reducing the density of the warm fluid. The less dense warm fluid begins to rise to the areas that have cooler fluid molecules.
The cooler fluid molecules are within closer proximity to each other than the warm fluid molecules. Therefore, a cold fluid has a higher density and will sink in less dense warm fluid. As the cold fluid begins to sink, it reaches the heat source from which the warm fluid had risen. The cold fluid molecules now begin to heat and spread apart before rising. The fluid molecules at the top have cooled down, and they begin sinking due to increased density.