四级练习2014.02.08 usually, depends, because

Rebecca949

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.

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longyuan

It is compose of tiny drops of water
It is became solid to be air temperature further than frozing
since water is 800 times denser than the air.