According to the University of Illinois at Urbana-Champaign, convection refers to the distribution of heat through vertical motions of air. Different surfaces absorb different amounts of energy and convection occurs where a particular surface heats up rapidly. Convection includes both large- and small-scale rising and sinking of air masses and smaller air parcels.
North Carolina State University details that convection is the transfer of heat from a warmer region to a cooler one by the shifting of molecules from a heated area to an unheated area. First, the energy from the sun comes in contact with the Earth's surface and warms the air near the ground. Depending on what kind of surface, the air near the ground heats up at different rates and intensity. As the air heats up, it begins to rise and slowly cools down when it enters an area of lower pressure.
These rising columns of air are called "thermals." When this column of relatively warm air moves into cooler air above, the water vapor present in the column condenses, forming clouds. Once the air is cooled again at the higher altitude, the column gets heavier and sinks slowly back down to the surface to be heated again. The water vapor comes down as rain, and the process begins again.