四级听力练习2011.05.02（1）
[audio]http://sound.yywz123.com/tingbbs/cet4/cet420110502.MP3[/audio]You may not realize that fog is simply a cloud that touches the ground.Like any cloud (1)_______________,or in rare cases,of ice crystals that form an ice fog.Ice fogs usually occur only in extremely cold climates,because water droplets are so tiny (2)_______________,generally 30 degrees below zero Celsius or lower.The droplets of fogs are nearly spherical.The transparency of fog depends mainly on the concentration of droplets.The more droplets,the denser the fog is.(3)________________,investigators were puzzled for a long time as to why the water particles in fog didn't simply fall to the ground,making the fog disappear.It turns out the droplets do fall at a predictable rate,but in fog_creating conditions,they are either supported by rising air currents or continually replaced by new droplets condensing from the water vapor in the air.
**** Hidden Message ***** it can be composed of tiny drop of water
it can not be dolided until to air temperature farther low freezing
since water is 800 times dense than air it so hard ~~ is composed of any kind of water.
since water is 800 times denty than air.
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