In meteorology, convection refers primarily to atmospheric motions in the vertical direction. As the earth is heated by the sun, different surfaces absorb different amounts of energy and convection may occur where the surface heats up very rapidly. As the surface warms, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise.
A simple demonstration of condensation through convection can be performed by placing a pot of water on a heated stove. The burner represents the heating of the earth's surface by the sun, while the water and the air above it represent the atmosphere. As the bottom of the pot (earth's surface) begins to heat the water (lower atmosphere), warmer and less dense water evaporates and rises (thermal) into the drier, colder air above the pot (middle atmosphere). This causes the thermals to cool and water vapor within to condense, forming a small cloud, or steam, that is visible above the pot of heated water.
Photo by: Bramer
This same process occurs in the real atmosphere as the water vapor within rising thermals condenses to form a cloud, as occurred in the example shown above.