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  3. During the early period of the earth’ s history, the weather on earth apparently fluctuated between bright sun and showers. In r

During the early period of the earth’ s history, the weather on earth apparently fluctuated between bright sun and showers. In r

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问题    During the early period of the earth’ s history, the weather on earth apparently fluctuated between bright sun and showers. In recent times, however, the mountains and valleys on land and under water have had a great influence on the earth’ s weather. They effect the movements of the thin film of water we call the sea and the thin layer of air around the earth. These movements produce changes in the temperature and humidity prevailing at the surface.
   Although the sun is approximately 93 million miles away, its rays warm the earth. The areas that receive the sun’ s rays vertically are, naturally, warmer than those areas that receive only slanting raya. 【B1】 Vertical rays do not need to make as long a journey through the atmosphere before reaching the surface of the earth. Therefore, less of their heat is lost in the air and more of their heat is saved to warm the earth. Furthermore, slanting rays are spread over available at a given point.
   The sun’ s rays do not heat the vast stretches of space between the sun and the earth. 【B2】 If they did, we should expect to be warmer on top of a high mountain or riding in an airplane than we are on the ground. Actually, however, we feel cooler at the top of a mountain; and, if we fly a plane high enough, we encounter freezing temperatures, even over the equator.
   This is true because the sun’ s rays are like the waves used in radio broadcasting. Music sent out by a radio transmitter does not fill the air with sounds for all to hear. Instead, the music travels in the form of electromagnetic waves, which must pass through a receiver and an amplifier to be heard. 【B3】 Similarly, the sun’ s rays cross our atmosphere and are absorbed by the surface of the earth. These rays warm the surface, which sends back some of this heat into the air.
   When the air near the earth’ s surface has been warmed, it rises and is replaced by cooler air which comes down to take its place. This cool air, in turn, is warmed and rises to be replaced by more cool air. As air rises it becomes cooler because it expands and is less concentrated, the air is suddenly let out of a tire, the air expands rapidly and becomes much cooler. On the other hand, when air is put under greater pressure, it becomes warmer. This is what happens to the cold air that comes down to take the place of rising warm air.
   The earth is much cooler than the sun, and the wave length of the earth’ s radiations is much longer than that of sun rays. 【B4】 These longer heat waves cannot pass through the atmosphere as easily as the short waves coming from the sun. 【B5】 Similarly, radio stations that transmit programs to distant countries broadcast in short waves, which can travel all round the world. Stations that broadcast local programs send out longer waves, which can be received more easily, but only in a limited area.
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答案这些较长热波，就不如太阳发出的短波那样容易穿过大气层了。
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