The water we drink, swim in and use for various other reasons has been around in different forms for hundreds of millions of years. Whether as a liquid in the vast oceans or beneath the ground, as vapour, forming the clouds in the sky, or as ice, forming part of the great polar ice caps. The water that exists today in these forms is the same water that has existed throughout time. It has undergone many different changes in form as part of the water cycle, also known as the hydrologic cycle.

For as long as water has existed, it has changed forms due to various external factors such as heat and humidity. In high temperatures, water begins to evaporate. Animals may release water in the form of sweat while plants will transpire to cool down and allow more water to be taken in through the roots. Warm weather turns the liquid water into vapour. If this vapour touches a cold surface such as a window for example, it will return to its liquid form and become condensation. The lower temperatures cause the particles to move slower as they have less energy, and the vapour becomes water again. This is an example of part of the water cycle.

Condensation occurs when high temperatures turn water into vapour, but then the vapour comes into contact with colder surfaces and returns to its liquid state. If the water vapour does not come into contact with a cold surface, it will continue to rise and collect in the air. This is the part of the water cycle that creates clouds. Clouds are simply large collections of water vapour in the air, formed as a result of evaporation. The clouds move in the sky and when they become too dense, the vapour becomes a liquid once again and falls to Earth. This is what causes rain or snow. Colder temperatures combined with dense clouds cause the water to fall back down to the ground. Snow is created when the temperatures are so low that the drops of water freeze before they hit the ground. This is a very important part of the water cycle, and the part that brings the whole process back to the start. It is called precipitation.

When water reaches the ground, it can be collected in bodies of water such as lakes or oceans, or it can seep into the ground and collect beneath the surface. This creates ground water. Ground water is collections of water that are often very pure, and can range in size. Many pumps that are created to extract drinking water from the ground are taking from collections of ground water, as it is often uncontaminated.

There is no clear point where the water cycle starts or finishes. It is a continuous process, one that has been going on for a huge length of time. There are of course many other aspects of the cycle that can often be overlooked, but still contribute. Rivers and streams would not flow continuously without a source that constantly replenishes itself. These moving bodies of water usually start high up in mountains. Water is stored in these environments as ice and snow, and has to flow down the mountain when it melts. These rivers may end up in lakes or the sea, where the water is stored and contained until it evaporates and the water cycle continues.

Ground water can also interact with the sea and environments that store freshwater such as lakes. The water can sometimes filter up through the ground and well up on the surface. This is what happens in the case of springs. The water is often very pure, as it has been stored below the ground in an area free of pollution and contaminates. This water then evaporates to continue the cycle.

Ground water can also run into the sea, where it is stored until it also becomes part of the water cycle once again.
It is interesting to study the average residence times of molecules of water. The average residence times show how long a molecule may stay in a certain state or location before it takes on another form and continues being a part of the water cycle. This time ranges from water vapour in the atmosphere staying for just a few days, to water residing deep below the ground that may remain there for over ten thousand years before it infiltrates lakes or oceans for example and rejoins the water cycle.

Many factors affect the water cycle, most commonly temperature. Though temperature can cause the cycle to run normally without causing any problems, it can often fluctuate and result in floods or droughts. Any single decrease or increase at any point in the water cycle can have devastating consequences, therefore it is important to attempt to monitor the cycle as much as possible.