天象描写:天空-Sky

天空
【内容提示】
以“Sky”为题写一篇描写文。内容要点如下：
①天气晴朗时仰望天空，地球好像被一个蓝色的大圆顶所覆盖。
②天空之所以呈蓝色，是因为阳光射入地球大气层时起了变化。
③阳光与微小的空气分子碰撞，便散射开来。散射开的光有多种颜色，这些颜色的组合便是蔚蓝色。
④有时日落时天空呈红色。这时的阳光在天空中以较低的角度射来。
⑤太阳在天空下落时，其光线穿过更多的空气，于是波长较长的橙色和红色也散射开来。日落时，波长较短的蓝色光不能穿透大气层，所以蓝光就看不见了。
⑥站在一块地势平展的地方极目四望，天空好像处处都与地球交接。交接处叫地平线。
⑦无论你往哪个方向看，地平线与你的距离总是等长。
⑧无论你在何地，地球表面总是向下弯的，弯向远方；那是人们目力不及之处，这便是地平线。
【作文示范】
Sky
When you look at the sky on a clear day, it seems as if the earth is covered by a blue dome①. The sky is blue because of what happens to sunlight as it enters the earth＇s atmosphere. Sunlight is made up of light rays of all lengths. The longer rays make up red light and the shorter rays make up blue light. As sunlight enters the earth＇s atmosphere, it strikes the tiny molecules ② of air and is scattered③. The scattered light is made up mostly of violet④, blue, a little green, and a little yellow and red. The combination of these colors is sky blue.
Sometimes at sunset the sky looks red. The sunlight is coming toward you from a lower angle in the sky. The lower the sun, the longer is the path that the light rays must travel through the air. The shorter blue rays are scattered immediately when striking the molecules of air. The sunlight that you see lacks the blue and violet rays that the air has scattered. As the sun gets lower in the sky, the light rays must pass through more air,and so the longer orange and red rays also become scattered. At sunset, the shorter blue rays never get through the atmosphere, so you do not see them.
If you stand at a place where the surface of the earth is flat, the sky seems to meet the earth in any direction you look. The point at which the sky and the earth seem to“meet”is called the horizon. The horizon is at the same distance from you, no matter in what direction you look. If you walk toward the horizon, you do not get closer to it. No matter where you are, the surface of the earth curves ⑤ downward and away from you. The distance at which you can no longer see the earth＇s surface----because it has curved down too far----is the horizon.
【词语解释】
①dome [d+um] n. 圆盖；穹窿
②molecule [＇m&likju:l] n. 分子
③scatter [＇sk$t+] v. 使消散；使分散
④violet [＇vai+lit] n. 紫色；紫罗蓝色
⑤curve [k+:v] v. 成曲形；弯曲
【写法指要】
这是一篇科技知识小品式的天象描写文。作者以从上到下的空间顺序安排结构，先写苍穹的蔚蓝，用一个带有明喻“as if”的句型开头，写出天空像一个蓝色的大圆盖，然后以光波长短来分析天空呈蓝色的道理。接着作者写日落时天空呈红色的情景：这时的阳光在天空中以较低的角度射来，太阳越低，光线穿过空气的路程就越长；波长短的蓝色碰撞空气分子后立即散射，因此人们看不见，人们看到的是波长较长的橙色和红色。最后，作者写人们站在平地上极目四望的情景：人们看到的是天地相接处的地平线。这样，作者通过上、中、下三个层面的描写，给人们描绘出了天空呈蓝色、红色以及地平线形成的科学道理。