The water cycle consists of three stages. The first is evaporation, as the sun's heat converts liquid water from rivers, lakes and oceans into water vapor. The second stage is condensation. As water vapor rises into the atmosphere, it cools and condenses into droplets of liquid water. Wind currents carry this water as clouds. The third stage is precipitation, in which water falls back to Earth as rain or snow.
Following the precipitation stage, most rain water and snowfall finds its way back to lakes or oceans, where the cycle repeats. Water that falls in polar regions or on high mountain peaks may become trapped in glacial ice. Some water soaks deep into the ground where it enters large underground pools called aquifers. Water in aquifers may remain undisturbed for centuries (although more and more of the world's aquifers are being tapped to irrigate farmland). Most of the remainder enters the biosphere. Animals, plants and most other organisms utterly depend on water for their survival.
Although the dynamics of the water cycle are fairly well understood, weather patterns are notoriously difficult to predict accurately beyond a few days. Even with the advent of supercomputers, the complex interplay of wind and ocean currents makes weather forecasting almost as much an art as a science.