语法填空
阅读下面材料，在空白处填入适当的内容（不多于3个单词）或括号内单词的正确形式。
There are two main reasons        the temperature of the air is always        (change). One reason is        the air around you moves 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 partly       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.