The sun acting on the earth's atmosphere creates our climate and weather conditions. During the day, the sun's energy heats the atmosphere, the land, and the sea. At night, much of this heat is released back into space. The warmth of the sun moves air and moisture across the earth's surface to give us seasonal and daily weather patterns.
Solar energy is the source of almost all of our energy resources. Ultraviolet (UV) radiation from the sun triggers photosynthesis in green plants, which produces the oxygen we breathe, the plants we eat, and the fuels we use for heat and power. Ultraviolet wavelengths make up only about 1 percent of the sun's rays that reach sea level, and are too short to be visible. About half of the energy in sunlight that reaches the earth arrives as visible wavelengths. The remainder is infrared (IR) wavelengths, which are longer than visible light, and which carry the sun's heat.
Plants combine the sun's energy with water and turn it into sugars, starches, and proteins through photosynthesis, giving us food to eat, which in turn builds and fuels our bodies. Humans and other animals breathe in oxygen and exhale carbon dioxide. Plants supply us with this oxygen by taking carbon dioxide from the air and giving back oxygen. Besides its roles in food supply and oxygen production, photosynthesis also produces wood for construction, fibers for fabrics and paper, and landscape plantings for shade and beauty.
Plants transfer the sun's energy to us when we eat them, or when we eat plant-eating animals. That energy goes back to plants when animal waste decomposes and releases nitrogen, phosphorus, potassium, carbon, and other elements into the soil and water. Animals or microorganisms break down dead animals and plants into basic chemical compounds, which then reenter the cycle to nourish plant life.
The heat of the sun evaporates water into the air, purifying it by distillation. The water vapor condenses as it rises and then precipitates as rain and snow, which clean the air as they fall to earth. Heavier particles fall out of the air by gravity, and the wind dilutes and distributes any remaining contaminants when it stirs up the air.
The sun warms our bodies and our buildings both directly and by warming the air around us. We depend on the sun's heat for comfort, and design our buildings to admit sun for warmth. Passive and active solar design techniques protect us from too much heat and cool our buildings in hot weather.
During the day, the sun illuminates both the outdoors and, through windows and skylights, the indoors. Direct sunlight, however, is often too bright for comfortable vision. When visible light is scattered by the atmosphere, the resulting diffuse light offers an even, restful illumination. Under heavy clouds and at night, we use artificial light for adequate illumination.
Sunlight disinfects surfaces that it touches, which is one reason the old-fashioned clothesline may be superior to the clothes dryer. Ultraviolet radiation kills many harmful microorganisms, purifying the atmosphere, and eliminating disease-causing bacteria from sunlit surfaces. It also creates vitamin D in our skin, which we need to utilize calcium.
Sunlight can also be destructive. Most UV radiation is intercepted by the high-altitude ozone layer, but enough gets through to burn our skin painfully and even fatally. Over the long term, exposure to UV radiation may result in skin cancer. Sunlight contributes to the deterioration of paints, roofing, wood, and other building materials. Fabric dyes may fade, and many plastics decompose when exposed to direct sun, which is an issue for interior designers when specifying materials.
All energy sources are derived from the sun, with the exception of geothermal, nuclear, and tidal power. When the sun heats the air and the ground, it creates currents that can be harnessed as wind power. The cycle of evaporation and precipitation uses solar energy to supply water for hydroelectric power. Photosynthesis in trees creates wood for fuel. About 14 percent of the world's energy comes from biomass, including firewood, crop waste, and even animal dung. These are all considered to be renewable resources because they can be constantly replenished, but our demand for energy may exceed the rate of replenishment.
Our most commonly used fuels—coal, oil, and gas—are fossil fuels. As of 1999, oil provided 32 percent of the world's energy, followed by natural gas at 22 percent, and coal at 21 percent. Huge quantities of de caying vegetation were compressed and subjected to the earth's heat over hundreds of millions of years to create the fossilized solar energy we use today. These resources are clearly not renewable in the short term.
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