A、Why fog freezes at low temperatures. B、How very small droplets can form. C、Why the density of fog varies. D、How water droplets

You may not 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 icy crystals that form an ice fog. lce fogs usually occur only in extremely cold climates, because water droplets are so tiny they don’t solidify until the air temperature is far below freezing, generally thirty degrees below zero Celsius or lower. These droplets are nearly spherical. The transparency of fog depends mainly on the concentration of droplets. The more droplets, the denser the fog is. Since water is eight hundred 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 water vapor in the air.