Fortunately there is a limited amount of moisture available in the atmosphere. The Earth's reservoirs are the global oceans, glaciers, lakes, rivers, and streams. They provide the principal sources of water evaporation that escapes into the Earth's atmosphere to form clouds.
Precipitation occurs in clouds when rapid condensation takes place.
The fallen moisture returns to the oceans, rivers and streams as runoff and ground water where it evaporates again. This recycling of moisture is the hydrologic cycle. The transition of atmospheric moisture from a gaseous state into a liquid state is condensation. Water vapor is the invisible source of clouds and rain and is also a form of heat transfer. Clouds develop when water vapor attaches itself to microscopic matter called nuclei.