Convection is a way for heat to move, also referred to as a heat transfer mechanism. This transfer of heat happens when a fluid such as air or water is in motion. Convection is driven by temperature differences across that fluid. When a fluid is heated, the region in closest contact with the heat source becomes less dense due to increased kinetic energy in the particles. The portion of fluid that is less dense then rises, while the denser portion of fluid sinks. The process repeats itself because the less dense fluids cool down as they move away from their heat source, making them sink, while the denser fluids heat up as they near the heat source, making them rise. This creates convection currents.
Figure 1. Air over land heats faster than air over water, leading to convection which feels like a cool ocean breeze.
Figure 2. Convection warming a room is quiet and energy efficient.
Convection plays a large role in wind patterns and in passive ventilation. The movement of wind across the globe is dependent on various spots where warm air rises and cool air sinks, creating large wind currents that affect weather. For example, air over land will typically get heated up by the sun during the day, while air over the sea will remain cool. The hot air over land will rise in the atmosphere. As it rises, it also cools down and becomes denser, causing it to sink once more. This concept is illustrated in Figure 1.
Similar to how convection works in the atmosphere, convection also causes passive ventilation (natural air movement) in a room, as shown in Figure 2. While natural convection can be used inside houses, forced convection is more common.
While natural convection can be used inside houses, forced convection is more common. This is where air currents are forced through a room by a fan. Forced convection can achieve the same effects as natural convection, the process is simply aided through devices like fans. If your house has heating vents on the bottom of your walls, this is an example of forced convection.
Figure 3. In the summer, ceiling fans should rotate counterclockwise to mix warm air and force a cool breeze downwards, creating a downdraft.
Figure 4. In the winter, ceiling fans should rotate clockwise to pull cool air up from the room and force warm air downwards, creating an updraft.