| We experience different forms of the
sun's energy every day. The sun is the major source of energy for our planet. It
causes the evaporation(蒸发)of water from the oceans and lakes. Sunlight also
provides the energy used by green plants to make their own food. Much of the energy that comes from the sun never reaches the earth's surface. It is either reflected or absorbed by the gases in the upper atmosphere. Of the energy that reaches the lower atmosphere, 30% is reflected by clouds or the earth's surface. The remaining 70% warms the surface of the planet, causes water to evaporate, and provides energy for the water cycle and weather. Only a tiny part, approximately 0.023%, is actually used by green plants to produce food. Many gases found in the atmosphere actually reflect heat energy escaping from the earth's surface back to the earth. These gases act like the glass of a greenhouse in that they allow energy from the sun to enter but prevent energy form leaving. They are therefore called greenhouse gases. When sunlight strikes an object, some of the energy is absorbed and some is reflected. The amount reflected depends on the surface. Dark-colored surfaces, such as dark soil or forest, absorb more energy. |