题型：语法填空（语篇） 题类：常考题 难易度：普通
高中英语人教版选修六Unit 4 Global warming同步练习 (3)
There are two main (reason) why the temperature of the air is always (change). One reason is that the air around you (move) in from somewhere else. Air usually moves in large bodies (call) air masses. Some air masses move in from the north. These air masses usually have cold air that causes the temperature (drop). Air masses also move in from the south. They usually have warmer air that causes the temperature to rise.
Temperature changes during the day or night can be great small. This depends (part) on how many clouds are in the sky. there are many clouds during the night, they act like covers on your bed. They blanket the earth and keep the air from (lose) much of its heat. The temperature usually does not drop much during such a night. When there are many clouds during day, some of the sun's energy will not reach the earth. The temperature usually does not rise much on such a day.
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