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.