As schoolchildren learn, the sun washes Earth with every imaginable type of light wave--X-rays, ultraviolet light, visible light, infrared radiation, microwaves, radio waves, you name it. About a third of the total is reflected from clouds. Another sixth is taken in by airborne water vapor. That leaves roughly half of the incoming light--most of which is visible light, as it happens--to pass through the atmosphere. Almost all of that half is absorbed by the land, oceans, and vegetation on the surface. (A little is reflected.) Having taken in all this solar energy, the ground, water, and plants naturally warm up, which makes them emit infrared light, radiating it into the air. Most of this secondary infrared is absorbed by airborne water vapor, heating it up. Usually water vapor comprises between 1 and 4 percent of the atmosphere by weight. (The exact number changes with temperature, wind, and surface conditions.) But this relatively small quantity--1 to 4 percent--packs a big punch.