Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft of sunlight. To the casual observer,dust seems to act in a totally random fashion,moving about chaotically without fixed direction. But in fact dust particles are much larger than water droplets and they finally fall. The cloud droplet of average size is only 1/2500 inch diameter. It is so small that it would take sixteen hours to fall half a mile in perfectly still air,and it does not fall out of moving air at all. Only when the droplet grows to a diameter of 1/25 inch or larger it can fall from the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size large enough to fall out is cause of rain and other forms of precipitation. This important growth process is called"coalescence.