Earth's water is always on the move, traveling endlessly around our planet in a process known as the water cycle. Without water, life on Earth would not be possible.
Earth's water is stored in many forms, including oceans, rivers, lakes, glaciers, and groundwater. This water moves around constantly. The water cycle begins when the Sun's heat makes water evaporate into the atmosphere, where it becomes clouds, dew, or fog. This water falls back down to Earth's surface as rain or snow, then streams and rivers carry it to lakes or the sea, where it eventually evaporates and the whole cycle begins again.
The presence of water on the Earth is what gives our planet its wet and warm atmosphere. The atmosphere protects the Earth from the Sun’s radiation, which has allowed the evolution and survival of life on Earth. Earth is thought to be the only planet in the Universe to support life.
All water on Earth is included in the constant circulation of the water cycle. Even the snow on mountain peaks or in the ice sheets of the Antarctic is a part of the cycle - eventually, it will melt and be on the move again. Underground water is also involved - it flows in a similar way to rivers, despite being hidden out of sight.
Seawater is salty because it contains dissolved minerals.
Evaporation from sea
The Sun heats the surface of the ocean, so that some of the water evaporates and rises into the air as water vapor.
Heat from the Sun draws moisture from the ground into the air.
Forests and plants
Plants help soil retain moisture but also release water into the atmosphere.
Plants release moisture into the air.
Heated, rising water vapor cools and condenses, forming clouds.
Winds blow moisture-laden clouds inland. Clouds are moved by wind and heat energy.
Snow on mountains
Where there are high mountains, moisture in the air falls as snow.
Cold air freezes any moisture it carries, forming snowflakes.
When moisture-carrying clouds cool, they release water as rain.
When air temperatures rise, snow melts, releasing fresh water.
Water collects in hollows in the ground.
Underground water flow
Water can flow downhill underground as well as above ground.
Rivers steadily transport water to the oceans.
Lakes are formed when water fills hollows in the landscape. Most lakes contain fresh water, though they can also be salty. They range in size from ponds to large lakes and even inland seas. The depression lakes are formed in various ways and may be millions of years old or newly man-made.
Lakes are not permanent - they can gradually disappear as layers of sediment build up in them.
Movement of tectonic plates can create long hollows, which fill with water.
A circular lake is created when rainwater fills the hole left in a volcano’s summit after an eruption.
A steep-sided circular lake, formed when an underground block of ice melts.
People make lakes to generate hydroelectricity and create reservoirs of clean water.
When rain falls, the water drains from high ground to lower ground. Small channels of water join up, forming streams and rivers that flow into the sea or fill dips in the landscape to create lakes. The shape and character of a river varies - they are fast and narrow at their source and get steadily wider and slower toward the mouth.
Where rivers begin they are very fast-flowing. The water is full of sand and pebbles, which erode and deepen the stream channel.
On lower, flatter ground, rivers begin to slow down. They develop bends called meanders, and there is an increased risk of flooding.
As they reach lower ground, rivers widen and slow, then flow into lakes or the ocean. Sediment carried by the water is left behind as the river slows.