18.11 Warming the Atmosphere
As you just learned, the incoming solar energy doesn't heat the atmosphere very much where we need it—in the troposphere. So how does the atmosphere near the earth's surface absorb solar energy? There are three ways. Some of the light energy reflects off the earth's surface and heads back out into space. On its way through the atmosphere, the air absorbs some of this reflected energy. But the earth's surface absorbs most of the original incoming radiant energy and converts it to thermal energy. The warmed surface radiates infrared rays back into die atmosphere. Infrared rays are the same thing as thermal radiation. Gas molecules absorb heat from thermal radiation much more easily than they do from light rays.
A second method of heat transfer is direct contact between the surface of the earth and the atmosphere. Vibrating atoms of water, rock, soil, or other materials collide with molecules of air, giving some of their n the kinetic energy to die air molecules. This process raises earth the temperature of the air in contact with these surfaces. Heat transfer by direct contact is conduction. It is the most efficient of the three methods.
The third method of heat transfer involves the vertical movement of warmed masses of air into cooler regions in the atmosphere. This process is convection. Convection is a common kind of density current similar to ones you learned exist in lakes, in the ocean, and within the mantle. Air warmed by radiation or conduction becomes less dense than the surrounding air. Cooler, denser air flows in under the warmer air, lifting it up into the atmosphere, forming a convection current. In addition to moving thermal energy to different levels of the atmosphere, convection currents are important in cloud and wind formation. You will learn about these in later chapters.