Energy Transfer in the Atmosphere
Heat is transferred between materials in three main ways: radiation, conduction, and convection. Heat is energy transported between objects of two different temperatures. In general, heat is transferred from a warmer object to a cooler one. When heat is transferred to an object, the molecules in that object move faster and increase the temperature.
Radiation transfers energy by electromagnetic waves (remember the electromagnetic spectrum includes visible light, as well as heat, UV radiation, microwaves, and gamma rays). When electromagnetic radiation strikes an object, the energy carried by the wave is transferred to the object, causing the particle motion within that object to increase. For example, a microwave emits microwave radiation to transfer heat to food. Similarly, the reason you feel heat from the sun is due to the transfer of heat by radiation. All matter emits and absorbs electromagnetic radiation, but some materials are better at absorbing radiation than others. Black objects tend to absorb a lot of radiation, but shiny surfaces reflect more radiation than they absorb.
Conduction transfers heat through direct contact between two objects. When two objects are placed in contact with each other, heat flows from the warmer to the cooler object. When the faster-moving particles in the warmer object collide with the slower-moving particles in the cooler object, the faster particles transfer some of their energy to the cooler ones. For example, when a hot pan is placed on a counter, the counter increases in temperature as the faster-moving particles of the pan collide with and increase the motion of the molecules that make up the counter. At the same time, the particles of the pan slow down and the temperature drops. Some materials, such as metals, are good conductors of heat, while other materials, such as glass, wood, plastic, and air, are not. Materials that are not good at transferring heat by the process of conduction are known as insulators.
Convection transfers heat through the movement of fluids or gases in circulation cells. This is what happens in Earth's mantle as rocks are heated and rise and then cooled, so they sink. Convection is what drives the movement of Earth's tectonic plates. A pot of water heated on a stove is another example. The pot itself, and then the water at the bottom, becomes heated by conduction. When water is heated, it expands, becomes less dense, and rises up through the surrounding cooler water. The cooler, denser water then sinks to the bottom of the pot, where it in turn is heated. The convection current - the circulating path of hot water heating & rising and cold water cooling & sinking - transfers heat by actually moving the warmer water to a new area. It forces the hot water to mix with the cooler water and increases conduction by bringing the cool water to the bottom of the pot. This same process occurs in the atmosphere, as air is heated near the Earth's surface, becomes less dense and rises through the surrounding cooler air.
The following interactive activity illustrates these three methods of heat transfer.
Last modified: Tuesday, 7 February 2012, 6:16 PM