Clouds are composed of liquid water droplets or ice crystals. So why don't these cloud particles fall? They do! Actually, they fall slowly, and the air in which they exist is also moving. The water droplets and ice crystals are almost microscopic and fall slowly in still air. A typical size cloud droplet falls at a speed of about 0.003 meters per second (0.007 miles per hour) in motionless air. This droplet would take days to fall from normal cloud heights! Added to this hurdle is the fact that the droplet has formed because the air is rising (and cooling adiabatically) so that the droplet is falling against the flow. If the droplet ever fell below cloud base into air that often has relative humidities less than 100%, the droplet would begin to evaporate. As a result, clouds are forced to stay up there.
If the cloud droplet were favored to grow to raindrop size however, the resulting drop would contain the water content of a million cloud droplets. This size drop falls through still air at a rate of about 4 meters per second (9 mph). These fall speeds will overcome the rising flow of air and reach the Earth's surface -- rain happens.
So when you see a cloud, you know the air in that region is rising. And the cloud is falling relative to the air flow, yet remains in the sky. In this way the atmosphere transports million of tons of water from the Earth's surface, first as vapor, then as water droplets or ice crystals in clouds, and finally as the never-ending resupply of precipitation.
For more information describing various aids that you can use in identifying clouds, you may consult the optional material in Tuesday's optional electronic Supplemental Summary Information .
To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 7 Chapter Progress Response Form, under section B. Daily Summary.