Section 3: Earth and Space
Transfer of the Sunís Energy through Earthís Systems

Convection, conduction, and radiation are similar in that all of these processes are ways in which heat moves from one place to another. The difference between convection and conduction is that in convection the molecules of heated materials actually move from place to place in convection, while during conduction, heat is passed from molecule to molecule. However, the molecules themselves don’t travel.

Heat travels by conduction and radiation in solids and by convection in fluids like air and water. Radiant energy waves travel from the Sun through space and become heat energy when absorbed by a substance or material—i.e., the Earth.

We have briefly touched on radiation but now let’s look at it in more detail. Electromagnetic radiant energy is the main source of heat energy on Earth. Most of this radiant energy comes from the sun, approximately 93 million miles away from Earth. Radiation is different from conduction in that radiant energy travels from the sun (or other glowing bodies) through space in the form of invisible waves rather than transferring heat from molecule to molecule. When a solid object, like Earth, absorbs radiant energy waves, the energy becomes heat. The darker and more opaque the materials are that absorb waves of radiant energy, the faster the molecules of the material move and the hotter they become.

Much of the heat from the Sun’s radiant energy is transferred back into the atmosphere. We can sometimes see heat energy absorbed by Earth radiate back into the atmosphere because the Earth is much cooler than the sun. An example illustrating radiation from Earth is when you see the heat rising from a black-topped road in the middle of a hot summer day. Heat energy is transferred from warmer objects (such as the road) to those that are cooler (such as the air). The Sun's heat is a major influence on the kinds of weather we experience. Without the sun's heat energy, water wouldn't evaporate, then rise until it meets cooler air and condense into clouds. Without clouds, we wouldn't have rain or other types of precipitation. Plants wouldn't survive in drought conditions. High and low pressure areas that meet in frontal systems determine the amount of precipitation Earth receives. Earth's topography, with its mountains, plains, plateaus, oceans, lakes, and streams containing water, also melting snow and ice, allow the evaporation of water to take place.

The greenhouse effect is caused by radiant energy from the sun passing through the Earth’s atmosphere. Land, plants, and bodies of water absorb radiant energy as heat. Approximately one third of this heat energy is reradiated back into the atmosphere where it is mostly absorbed by water vapor and carbon dioxide. In turn, the atmosphere radiates heat energy back to Earth where it increases the Earth’s temperature. The greenhouse effect appears when some of the radiation becomes trapped in the atmosphere.

Review heat transfer at the following link.