The water cycle starts with EVAPORATION. It is a process where water absorbs heat energy from the sun and turns into vapors. Water bodies like the oceans, the seas, the lakes and the river bodies are the main source of evaporation.
As H2O vaporizes into water vapor, it rises up in the atmosphere. The water vapors changes into very tiny particles of hydrogen and oxygen droplets because the temperature at high altitudes is low. This is CONDENSATION. These particles come together and form clouds and fog in the sky.
Apart from evaporation, SUBLIMATION also contributes to more hydrogen and oxygen in the air. Sublimation is a process where ice directly converts into water vapors without converting into liquid water. The main sources of water from sublimation are the ice sheets of the North Pole and the South Pole and the ice caps on the mountains. Sublimation is a rather slower process than evaporation.
The clouds pour down as PRECIPITATION. This occurs because the hydrogen and oxygen droplets combine to make bigger droplets. Also when the air cannot hold any more water, it precipitates. These droplets fall down as rain.
As the water precipitates, some of it is absorbed by the soil. This water enters into the process of TRANSPIRATION. The roots of the plants absorb the water and push it toward leaves where it is used for photosynthesis. The extra water is moved out of leaves through stomata (very tiny openings on leaves) as water vapor.
As the water pours down, it leads to runoff. RUNOFF is the process where water runs over the surface of earth. When the snow melts into water it also leads to runoff, this runoff combines to form channels and then rivers and ends up into lakes, seas and oceans. Here a surplus of hydrogen and oxygen atoms enters hydrosphere.