四级练习2015.10.23
[audio]http://sound.yywz123.com/tingbbs/cet4/cet420151023.mp3[/audio]What makes it rain? Rain falls from clouds for the same reason anything falls to Earth. The Earth’s gravity pulls it. But every cloud is made of water droplets or ice crystals. (1)_________________________. The droplets of ice crystals in clouds are very small. The effect of gravity on them is minute. Air currents move and lift droplets so that net downward displacement is zero, even though the droplets are in constant motion。
(2)_________________________. To the casual observer, dust seems to act in a totally random fashion without fixed direction. But in fact dust particles are much larger than water droplets and they finally fall. The cloud droplet of average size is only 1/2500 inch in diameter. It is so small that it would take sixteen hours to fall a mile in perfectly still air, and it does not fall out of moving air at all. Only when the droplet grows to a diameter of 1/125 inch or larger can it fall from the clouds. The average raindrop contains a million times as much water as a tiny cloud droplet. (3)_________________________. This important growth process is called “coalescence。”
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