Rain clouds form when clusters of water droplets and crystallized, frozen water accumulate at high altitudes, and gravity causes this condensed water to fall as rain. The average size of one of these cloud droplets is miniscule, but if the droplets contain enough water, white clouds turn into rain clouds. The influence of gravity causes a droplet to turn into a raindrop and fall to earth.
Clouds form via the process of evaporation and condensation. When the temperature is warm, molecules of water are agitated, expanding the distances between them. Water molecules then switch form between liquid and vapor phases. When this happens, molecules leave a body of water, such as a lake or an ocean, and become water vapor, which rises into the air. The water vapor rises high up to form clouds, which are essentially large masses of tiny water droplets and crystallized water molecules. If the clouds get thick enough or high enough, the light from above does not make it through, leading to the gray or dark look of a rain cloud.
Snow, sleet, hail and lightning are also produced by rain clouds. The different shapes and sizes of clouds depend on the weather conditions under which they were formed.