Condensation can not occur in air that is throughly free of all impurities. Water molecules must have particles of some sort on which to condense (condensation nuclei). Such things as dust, smoke, soot, industrial pollution (smog), sea salt from ocean spray (most common hygroscopic material in the air) are often utilized by water particles. Large particles of water are formed by the addition of other small particles. This is produced by strong vertical motion of the air. Under these conditions ice crystals form very quickly. As they fall, they melt, thereby forming a rain drop. Saturation is produced by cool or cold air being brought over a warm surface. This brings about saturation very quickly, producing visible steaming (fog over sweet Lakes, Arctic "smoke", etc.). Saturation also is produced when cooling air with a given water content is cooled to a low enough temp. that saturation vapor pressure is reached. This is the more important of these 2 processes. Condensation occurs on a solid surface that is colder than the dew point of the air to which it is exposed. Example: dew forms on a cold night, but if the temp. is below the freezing point, frost occurs. There are four processes by which air is cooled below dew point: 1) Cooling by conduction of heat to a cold surface 2) Direct radiational cooling of air 3) Mixing of warm and cold masses of saturated air 4) Cooling by adiabatic expansion In hilly areas, cold air drains to low spots in the valleys at night, producing inversions in such places.