How do clouds defy gravity?

To say clouds defy gravity would be misleading, as they do not defy gravity. However rising air currents keep the clouds in the air. The air currents are stronger than the rate at which gravity makes the rain fall (which is not that fast). Clouds are so light, that even the smallest upward air current can make a clouds stay up. Rain falling is basically when the mass of the drops exceed the air currents.