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Solar energy evaporates exposed water from seas, lakes, rivers, and wet soil; the majority of this evaporation takes place over the seas. Water is also released into the atmosphere by the plants through photosynthesis. During this process, known as evapotranspiration, water vapour rises into the atmosphere.
Clouds form when air becomes saturated with water vapour. The two major types of cloud formation are stratified or layered grey clouds called stratus and billowing white or dark grey cloud called cumulus clouds.
Precipitation as rain, or hail ensures that water returns to Earth’s surface in a fresh form. Some of this rain, however, falls into the seas and is not accessible to humans. When rain falls, it either washes down hill slopes or seeps underground; when snow and hail melts, this water may also sink into the ground.
Rain fall also replenishes river water supplies, as does underground water. Snow fall may consolidate into glaciers and ice sheets which, when they melt, release their water into the ground, into stream or into the seas.