Convection transfer refers to a form of heating where molecules are warmed, become lighter or less dense, and move away from the heat source. Common examples include warm air rising when heated by the ground, or water moving upward in a pan when heated from below. Another type of heat transfer is conduction, where heat moves through an object such as when metal is heated by a flame at one end and becomes hot at the other.
A heat source is required for convection transfer to occur, whether it a pan sitting on a stove or ground or trees heated by the sun. As heating occurs, molecules closest to the heat source gain energy, which causes them to move faster than surrounding molecules. If nothing further occurred, heat transfer would occur very slowly, because heat would have to move primarily by conduction.
In general, heat moves more quickly in convection transfer because an increase in energy results in a change in density of a fluid, which can be a gas or liquid. The lower density fluid will rise until it cools and then returns to a lower level. In the atmosphere, warm air rises until it reaches an altitude where clouds form, which releases heat and air can cool again. Warm water will rise in a heated pan until it reaches the surface, where heat transfer can occur to air above the pan, and the water will cool.
The movement of fluid through warming and cooling cycles causes the total fluid volume to warm more quickly. This is why air temperatures can rise quickly on a sunny day, because convection transfer is causing movement of the air and warming occurs more quickly than on a cloudy day. It is important to note that convection transfer in air does not significantly contribute to wind, which is primarily an effect of pressure differences between low and high pressure areas.
Air convection does cause thermals, or bubbles of rising warm air, that air travelers may experience as bumpiness or turbulence. Glider pilots can take advantage of air convection by circling in areas of rising air to gain altitude without the use of a motor. Convection can create severe weather if rising air occurs in conditions where water droplets can form as the air cools. A change from water vapor to liquid raindrops is called condensation, which releases heat and causes further heating of surrounding air. The result can be an air mass thunderstorm, which results from the convection transfer of warm air and precipitation.
Convection is not a very efficient way to mix fluids, because movement only occurs from changing densities. That is why industrial processes usually include mixing, to bring fluids more rapidly to a constant temperature. Mixing causes fluid to contact heated surfaces and then rapidly moves away and is replaced with cooler material. Along with better heating efficiency, mixing also prevents hot spots or areas of excessive heat that can damage chemical reactions.