The Earth s gravity pulls it.But every cloud is made of water droplets or ice crystals. Why
doesn t rain or snow fall constantly from all clouds? The droplets or ice crystals in clouds are exceedingly small.The effect or gravity on them is minute. Air currents move and lift droplets so that the net downward displacement is zero, even though the droplets are in constant motion.
Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft of sunlight. To the casual observer, dust seems to act in a totally random fashion, moving about
chaotically without fixed direction. But in fact dust particles are much larger than water droplets and they finally fall. The average size of a cloud droplet is only 0.0004 inch in diameter. It is so small that it would take sixteen hours to fall half a mile in perfectly still air,and it does not fall out of moving air at all. Only when the droplet grows to diameter of 0.008 inch or larger can it fall from the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size large enough to fall out is the cause of rain and other forms of precipitation. This important growth process is called "coalescence.
23. What is the main topic of the passage?
(A) The mechanics of rain
(B) The weather patterns of North America
(C) How Earth s gravity affects agriculture
(D) Types of clouds
24.The word "minute in line 4 is closest in meaning to which of the following?
(A) second
(B) tiny
(C) slow
(D) steady
25 .Thc word "motion in line 5 is closest in meaning to
(A) wind
(B) change
(C) movement
(D) humidity
26.Ice crystals do NOT immediately fall to Earth because
(A) they are kept aloft by air currents.
(B) they combine with other chemicals in the atmosphere
(C) most of them evaporate
下一篇：超完美攻略--托福考试满分备考计划表(中)
2．2备考中期
对托福考试进行分类突破，本阶段，对2005年以前的真题，进行分类练习。题目做法：从2005年的真题开始，采取倒着做的策略。不用全做，感觉自己在听力、阅读、语法、写作方面，有所提高，就可以停止。
2．2．1词汇突击：坚持背单词
2．2．2听力突击
1、本阶段采取精听原则，切实按照考试的节奏，先做一遍，然后，回过头来，切实听懂每一个句子。弄清...[查看详情]
