• Solar radiationheat storm image by Nicemonkey from Fotolia.com
  
  Heat transfer is the movement of thermal energy ("heat") between substances. It occurs whenever two or more substances with different temperatures interact with each other. Because of the nature of thermal energy, which exists as vibration in molecules, heat will always spread out as evenly as it can, until everything is the same temperature. Heat transfer is the core concept in the science of thermodynamics, which investigates how thermal energy can be transferred between bodies to produce mechanical work. There are three principle modes of heat transfer.
  
  

Thermal Conduction

• Thermal conduction is the transfer of heat between adjacent molecules. All molecules vibrate according to their thermal energy levels. When two substances are in contact with each other and have different temperatures, the molecules in the warmer substance will be vibrating more energetically than the molecules in the cooler substance, and some of that "excitement" will be transferred over. Conduction will continue to occur between the substances until they are at the same temperature.
  
  Thermal conduction depends heavily on the substances involved. For instance, a carpet feels warmer to your bare foot than a tile floor. That's because the tiles are better thermal conductors than the carpet. Both are at the same temperature, but the tile has a more efficient molecular structure for transferring heat away from your foot and into itself. Thermal insulators, like carpet, will protect against heat transfer.
  
  

Thermal Convection

• Thermal convection is the transfer of heat occurring within fluids, such as liquids and gases, by internal molecular movement. Convection does not occur in solids, because in solids the internal molecules of a substance are in a fixed position and cannot move around freely. There are two basic forms of convection: advection and diffusion.
  
  In advection, heat is transferred within the fluid by large-scale motions. For instance, if you poured cream into piping hot coffee, you might then stir it in with a spoon. This motion would quickly distribute the cream throughout the coffee. As a side benefit, heat from the coffee would be more quickly transferred into the cooler cream molecules, cooling down the beverage slightly and making it ready to drink.
  
  In diffusion, heat is transferred by the random movement of individual molecules in the fluid---exactly how smoke or fog will move around even in a room with no discernible air currents. If you were to pour that cream into the coffee, but not stir it, it would still mix itself in---just not as quickly.
  
  

Thermal Radiation

• Thermal radiation is the transfer of heat that occurs when a substance becomes hot enough that it begins to emit photons. Photons are the base units of electromagnetic energy. Thus, thermal radiation is actually electromagnetic radiation that has been caused by heat.
  
  This makes it very different from conduction and convection. Heat that is transferred through radiation ceases to be thermal energy when it is transferred from molecular vibrations into photons. At this point it becomes electromagnetic energy, and it remains in this form until the photons collide with something, whereupon it may be transferred back into thermal energy. And unlike conduction and convection, thermal radiation requires no medium. It can propagate perfectly well through a vacuum. In fact, none of the heat from our sun comes from conduction or convection. It is all radiation.
  
  Another example of thermal radiation is an electric stove heating element. Once it gets hot enough, the element will begin to glow red. This is called incandescence, and is a visible demonstration of thermal radiation. Your eyes can't see it, but the heating element is emitting energy across the electromagnetic spectrum, with visible light being only one portion of that spectrum.
  
  Note that the radiation here is distinct from the strong jet of heat you can feel by placing your hand above the element. That heat comes from convection which occurs when the element heats the air, causing the air to rise. That's why the area above the burner is hot, but the area right next to it is not. The radiated heat from the stove is much lower than the convected heat. Likewise, when you hear about things like "heat radiators," those devices actually work primarily through convection. Thermal radiation only occurs when electromagnetic energy is involved.