A body in the physical world is made up of mass. The body may be made up of liquid, solid or gas. The mass is not distributed evenly within the body. Parts of the body are denser than the other parts in the same body. This difference in density causes rising or falling currents. This occurs in fluids. When fluids are heated, the warmer particles in the fluid become warmer. They expand and rise while denser particles flow downwards. This movement of warmer and colder particles causes currents and is called convection. The moving body of fluid is called convection cell.
Convection is a method of heat transfer and is used in many appliances that require heating – convection ovens, radiators. Convection occurs naturally in the earth – in the atmosphere, in the water bodies, and even in the tectonic plates in the earth’s crust. Convection causes rain and temperature changes. While convection occurs naturally in the earth, it can also be forced and this is used in many appliances. In forced convection, a pump or other mechanism forces the heated fluid to move. This can be seen in ovens, in refrigerators and in air conditioners. These appliances work by moving hot air away from the room or space, thereby causing cooling. Only the heat can be transferred, not the cold. So these appliances do not infuse cold air, they move the hot air out. The human heart is also a type of convection pump. It forces the blood through the blood vessels to the skin surface where it is cooled by evaporation.
Convection cells create a circular pattern of movement of fluids caused by rising warm fluids and sinking cool fluids. They can be as small as a few millimetres or as big as the earth itself. Convective cells are seen on the surface of the sun also where the brighter spots are heated fluids which become cooler as they rise to the surface and sink again as they cool, only to be heated again.
Sea breeze in coastal areas is also caused by convection. During the day time, the sun heats up the land faster than the sea water. The air over the land becomes warmer and rises up. The cooler air over the sea rushes in to take the place of the warmer air and this causes sea breeze. In the night, the reverse is the case, when the air over the land cools much faster than the air over the water and the breeze moves from the land into the sea. This is called land breeze.
Clouds are also formed on account of convection. A cumulonimbus cloud or thundercloud is a typical convection cell. In the oceans, convection cause ocean currents which are important factors in the climate of a region. Ocean currents also help in fishing and large amounts of fish are found at points where the warm and cool currents meet.
The formation and functioning of convection cells is indeed an important phenomenon in the formation of landforms and wind movements on this planet.