why aren't clouds down here?

[shine]

Water being heavier than air, and clouds being cold, so not warm enough to be lighter than air......what are they doing up there? Why aren't they down here? I don't get it.

[Isaacr]

Basically the water and ice particles in clouds are too small to come under the influence of gravity (i.e they have very small fall velocities) and so the particles continue to float with the surrounding air. To illustrate, a cubic kilometre of cloud with 1 gram of water per cubic metre would have a mass of one million kilograms. But a cubic kilometre of air would have a mass of one BILLION kilograms. Additionally, clouds generally form, survive and grow in air that is moving upward. The updrafts offset the already miniscule fall velocities of the water and ice particles in clouds.