听力题 You may realize that fog is simply a cloud that touches the ground. Like any cloud, it is composed of tiny droplets of water, or in rare cases, of ice crystals that form an ice fog. Ice fog usually occurs only in extremely cold climates. Because wa?ter droplets are so tiny, they don’t solidify until the air temperature is far below freezing, generally thirty 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 the droplets are, the denser the fog is. Since water is 800 times denser than air, 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 that 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.