The sun is our most important source of heat. The sun's energy is the driving force for many of the processes that occur around us on a daily basis. Weather patterns, wind, ocean currents, plant growth, and the water cycle are just a few of the processes the sun influences.
Only a small fraction of the energy released by the sun reaches the Earth's atmosphere. The spectrum (range) of solar radiation is shown in the following figure. Visible light forms only a small part of the entire spectrum. Incoming solar radiation is selectively absorbed and reflected to warm the Earth and atmosphere and to maintain a temperature friendly to life.
meteorite Mass of stone or metal that has passed through the atmosphere and has struck the Earth's surface.
meteoroid Mass of stone or metal traveling through space.
photosynthesis Process by which plants use chlorophyll and energy from sunlight to manufacture food from carbon dioxide.
pollution Waste substances added to the environment.
respiration The process by which oxygen combines with food to release energy and carbon dioxide.
scattering The bending of light rays in all directions by gas molecules.
spectrum A broad range of related waves.
Incoming radiation. Our atmosphere and clouds reflect about 40 percent of the incoming solar radiation back out into space. The remaining 60 percent is responsible for warming the Earth and atmosphere. About 20 percent of the total incoming radiation is absorbed directly by the atmosphere. The ozonosphere and ionosphere are responsible for most of this absorption of radiation. The remaining 40 percent of the total solar radiation reaches the surface of the Earth. As the solar radiation penetrates the lower layers where air is dense, some scattering occurs. Scattering is the bending of light rays in all directions by gas molecules. Some light rays are lost back to space. Since blue light is most affected by scattering, the sky appears blue.
Energy released by a very hot object like the sun is in the form of short-wave radiation. Land and water can absorb this form of radiation far better than can the atmosphere. As a result, the Earth's surface is warmed. Since the Earth is a warm object, it too releases energy. Ground radiation is of the long-wave form because the Earth emits longer, infrared radiation. Figure 5 above shows the abrupt change of the incoming solar radiation to longer wave length heat when the sunlight strikes the ground. Figure 6 indicates that about one half of the sun's energy is available to heat the surface of the Earth.
Greenhouse effect. Long-wave radiation given off by the warm Earth is absorbed by water vapor, clouds, and carbon dioxide in the atmosphere. The trapping of ground radiation (heat) by the Earth's atmosphere is called the greenhouse effect. In a greenhouse, glass takes the place of the water vapor and carbon dioxide. The sun's short-wave rays pass through the glass roof, but the glass will not permit the long waves to escape. As an example of the atmosphere's greenhouse effect, the temperature does not drop as much on a cloudy night as it does on a clear night.
Radiation balance. Scientists have made many measurements of the radiation reaching the Earth's surface. The exact amount of heat absorbed by the atmosphere and the Earth varies from place to place and from time to time. The data gathered over many years shows that the Earth as a whole is neither gaining nor losing heat. This means that the amount of radiation absorbed by the Earth and the amount of heat lost to space are equal. If the energy balance were not equal, the Earth would either become extremely hot like the inner planets or would cool off to frigid temperatures like the outer planets. Do you think that all the numerous factors producing an energy balance which are favorable to sustaining life have occurred by random accident, or by creative design? Loss of heat energy is greater at night than during the day. At the equator, more energy is gained than lost. In contrast, at the poles, more energy is lost than gained. Therefore, excess heat generated at the equatorial regions is transferred to the poles by global air circulation patterns.
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INFLUENCES ON LIFE
God created an atmosphere exactly suited for many forms of life. Our atmosphere is necessary to sustain life. The qualities of our atmosphere cannot be appreciated until changes take place that threaten life. The atmosphere plays a vital role in the processes of respiration and photosynthesis, in regulating temperature, and in protecting us from radiation and meteorites.
Respiration. All living things need energy to stay alive. Energy is obtained from food when it is combined chemically with oxygen. The oxygen necessary for this process is readily available in the atmosphere. A process in which living things obtain energy from food is called respiration. The waste products that result are carbon dioxide and water. Respiration can be represented by the following word equation:
Photosynthesis. Although carbon dioxide is a waste product of respiration, plants use it to make food. Plants take in carbon dioxide through their leaves and water through their roots. With the help of sunlight and a green substance called chlorophyll, plants convert carbon dioxide and water into food. This process by which plants make food is called photosynthesis. A by-product of photosynthesis is oxygen. Photosynthesis can be represented in this way:
