Describe the movement of heat energy by the process of convection

Transfer of molecules due to density-difference resulting from increased temperature of the layers of fluid bulk leads to convection.

Explanation:

When a mass of fluid is heated form the lower layers then due to the variation of the density of the fluid at different temperature we observe the movement of molecules leading to convection.

When the lowest level of the fluid is heated it gains temperature and the molecular bulk expands on heating and its density becomes low with respect to the bulk fluid around it and hence it flows upwards to the top most layer being lighter in weight and the lowest layer is occupied by the subsequent colder and denser layer. Then again the lowest layer is heated and the process continues forming a cycle heating through the bulk transfer of fluid layers called convention.