Heat moves in three ways: conduction, convection, and radiation. In conduction, heat travels through a substance by direct contact of particles. The particles themselves do not travel. Conduction works best in solid materials. This is because they have high density. The particles are packed closely together. Dense materials also tend to retain the heat they absorb. They cool more slowly than materials that are not dense. In convection, heat is carried from one place to another by a fluid such as air or water. When heat is transferred through space and air from a warm object to a cooler one with a minimum of warming in between, the process is called radiation. This is how we receive heat from the sun. The sun's radiant energy travels through space and the atmosphere to heat the surface of the Earth. The heated earth heats the air above it. The air is not heated directly by the sun. Most of the radiant energy from the sun travels in short wavelengths as visible light. These wavelengths pass through the atmosphere and are absorbed by the dark surface of the Earth. The heated surface radiates the energy back into the air but in longer wavelengths. The atmosphere holds the longer waves, and these waves heat the air. The process just described is known as the greenhouse effect because a greenhouse is heated in this way. In the greenhouse effect, Earth's atmosphere traps infrared radiation, or heat. The short waves pass through the clear glass of the greenhouse and are absorbed by the dark objects within. These objects radiate the heat back into the air as longer wavelengths. The glass does not allow these longer wavelengths to pass through. The radiant energy is thus kept within the greenhouse. Apply it! Ieets NATEF Science Standards for understanding heat transfer by convection, radiation, and frared rays. In what three ways does heat move? In what form does most of the radiant energy from the sun travel? What is the greenhouse effect