These particles around which water vapor condenses are called condensation nuclei. Eventually, each condensation nucleus collects enough water to become a tiny droplet. The droplets are so small that they float in the air. A large number of the droplets forms a cloud. If the air is cooled below 0'C , clouds will also form from tiny ice crystals, which are much larger than the droplets.
Water vapor condenses only on the surface.
Most clouds pass overhead without producing rain or snow. The water droplets or ice crystals of these clouds stay small enough to remain suspended by air currents. Sometimes water droplets are heavy enough to fall, but evaporate before they reach the ground. For a cloud to produce precipitation, it must contain a very large amount of moisture. Position a beaker on a stand over the spout of a boiling teakettle.