Water is in the atmosphere, on the land, in the ocean, and underground. It moves from place to place through the water cycle. As it moves through the water cycle, water often changes from a liquid, to a solid (ice), to a gas (water vapor). Water in oceans and lakes is typically liquid; but it is solid ice in glaciers, and often water vapor in the atmosphere.
Temperature and pressure determine the phase of water (solid, liquid, or gas).
Water is essential for life on Earth. It is recycled through the water or hydrologic cycle, which involves the following processes:
- Evaporation, the changing of water from a liquid to a gas
- Condensation, the changing of water from a gas to a liquid
- Sublimation, the changing of water from a solid to a gas
- Precipitation, the process by which water molecules condense to form drops heavy enough to fall to the Earth's surface
- Transpiration, the process by which moisture is carried through plants from roots to leaves, where it changes to vapor and is released to the atmosphere
- Surface runoff, the flowing of water over the land from higher to lower ground
- Infiltration, the process of water filling the porous spaces of soil
- Percolation, groundwater moving in the saturated zone below the surface of the land
Water at the surface of the ocean, rivers, and lakes can become water vapor and move into the atmosphere with a little added energy from the Sun through a process called evaporation. Snow and ice can also become water vapor through a process called sublimation. Water vapor gets into the atmosphere from plants by a process called transpiration.
Because air is cooler at higher altitude in the troposphere, water vapor cools as it rises high in the atmosphere and transforms into water droplets by a process called condensation. The water droplets that form make up clouds. If the temperature is cold enough, ice crystals form instead of liquid water droplets. If they grow large enough the droplets or ice crystals eventually become too heavy to stay in the air, falling to the ground as rain, snow, and other types of precipitation.
Through these processes, the amount of water on Earth remains nearly constant and is continually recycled through time. Water molecules may remain in one form for a very long period of time (for example, water molecules can be locked in Antarctic ice for thousands of years) and in other forms for very short times (for example, water molecules in desert rainstorms spend mere minutes as surface water before evaporating into vapor again).