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).
Several people have asked me for the full essay for this question, so here it is!
The diagram below shows the water cycle, which is the continuous movement of water on, above and below the surface of the Earth.
The picture illustrates the way in which water passes from ocean to air to land during the natural process known as the water cycle.
Three main stages are shown on the diagram. Ocean water evaporates, falls as rain, and eventually runs back into the oceans again.
Beginning at the evaporation stage, we can see that 80% of water vapour in the air comes from the oceans. Heat from the sun causes water to evaporate, and water vapour condenses to form clouds. At the second stage, labelled ‘precipitation’ on the diagram, water falls as rain or snow.
At the third stage in the cycle, rainwater may take various paths. Some of it may fall into lakes or return to the oceans via ‘surface runoff’. Otherwise, rainwater may filter through the ground, reaching the impervious layer of the earth. Salt water intrusion is shown to take place just before groundwater passes into the oceans to complete the cycle.
(156 words, band 9)