Heat, at the most basic level, is the kinetic energy of atoms and molecules. Convection affects everything from the heating of your house, to the process of heat transfer within the sun.
The Relationship Between Temperature and Density
As a solid, liquid, or gas is heated, the atoms or molecules that comprise it vibrate more and more; these increased vibrations require more volume for each atom/molecule.
In a gas, this is expressed not as a "vibration" but as an increased velocity for the particles, and therefore an increased pressure on the gas's container. For this reason, most materials expand as they are heated up. This happens to the greatest extent in gases, but to a lesser extent in liquids and solids as well.
When something expands, it becomes less dense; there are fewer particles, and therefore less mass, per unit volume than there were before. But in liquids and gases (fluids), a region of lower density will rise and float above the higher density regions due to the influence of gravity. These two concepts, that heat causes a decrease in density and that fluids rise and fall according to density, combine to create the heat-transfer phenomenon of convection.
The Definition of Convection
Convection is a method of thermal energy transfer where the transfer of heat occurs via fluid motion. This fluid motion is caused by the difference in density between hotter regions of the fluid and cooler regions. These motions are called convection currents, and the convective movement of fluid continues as long as there is a temperature difference between regions.
This temperature difference is especially stark when there is a source of heat on one side of the fluid, such as a heater near the floor of a room. The warm air at the bottom continuously moves upward, while the cooler air moves downward to be heated and then subsequently also moves upward. The movement of air causes circular currents that will continue unless the air reaches an equilibrium temperature; a glass of water at room temperature will generally not have convective currents, while a glass of water with ice in it will have convective currents.
Convection is often described as a combination of two physical processes: advection and diffusion. Advection is the transport of matter by bulk motion, such as the movement of riverbed silt by the flow of the river. Diffusion is the transport of matter by particle motion from an area of high concentration to an area of low concentration, such as the movement of dye particles spreading through a glass of water.
As convection moves hotter matter higher and cooler matter lower, it does so both by moving the matter in bulk (advection) and in a particulate way (diffusion).
Convection cannot, by definition, occur in solids because of the inability to create fluid flow in solid matter (the particles cannot move relative to each other, but can only vibrate in place). Heat transfer in solids occurs instead by conduction, or the transfer of vibrational energy from one atom or molecule in a solid crystal to its neighbors. There are some exceptions to this in soft solids where the particles can move past each other.
Heating and Cooling Your House
Keeping convection in mind can help you heat or cool your house more efficiently. Since hot air tends to rise, and cool air tends to sink, it helps to put heaters closer to the floor and air conditioners higher up.
Ceiling fans can usually operate in both directions: either blowing air down from above, or blowing air up from below. Blowing air down is usually helpful in the summer so you feel the convective breeze cooling your skin; pulling air up is helpful in the winter because it helps to push the high-up hot air down and out toward the walls, without blowing directly on you.
A Freezing Lake
As water cools, it contracts and becomes more dense like most other substances. However, when it cools to about 4 degrees Celsius, it actually begins to expand slightly. Water is fairly unique in that its solid form, ice, is less dense than its liquid form. So although it generally becomes more dense as it cools, at a certain point this trend reverses, and it starts to expand until its freezing point at 0 degrees Celsius. This has an effect on how convection works in a freezing lake.
As water in a lake cools, it sinks as warmer water rises but only until the whole lake is 4 degrees Celsius. At this point convection reverses: The water that's cooler than 4 degrees Celsius is less dense than the warmer water, meaning that the top section of the lake becomes colder than the bottom and ice forms. This is why lakes freeze on top first.
The Sun (as well as most stars) undergoes internal convection with hotter plasma and cooler plasma. Within the Sun's convection zone, which stretches inward from its outer surface, heat energy is carried from the hot solar interior to the cooler outer regions via convection currents.
This creates "convection cells," which are the dark and light splotches you can see on the Sun's surface. The light spots are convection cells of hot plasma that has just risen from the interior; the dark spots are of convection cells of plasma that have cooled and will soon fall back down through the convection zone.
These dark and light spots are also sometimes called solar granules. They average about 1,000 km in diameter (about the length of the state of California) and stay at the surface for only about about eight to 20 minutes. At any one time, the Sun's surface contains about four million granules!
Other Examples of Convection
Convection is extremely important in meteorology or the study of weather. The flow of warm and cool air through the atmosphere is what creates different shapes of clouds, as well as thunderstorms, tornadoes and weather fronts.
Some ovens are able to bake by convection. Convection ovens use fans and an exhaust system that circulate the air inside the oven while it bakes, blowing hot air directly across the food. This allows the food to cook faster and more evenly than it would if it were simply placed near the oven's heating elements. It also makes the interior of the oven drier and less humid, which can be better for browning food.
The Earth's magnetic field is caused by convection currents in its outer core. At the center of the Earth is a solid inner core, surrounded by a liquid outer core that is comprised mostly of iron and nickel. Both of these metals are good conductors of electricity. The convection currents in this liquid layer create electric currents within the liquid metal, which create magnetic fields; the sum of these magnetic fields is the Earth's magnetic field, which points all compasses to the North Pole and protects the Earth from cosmic radiation.