Page 251 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 5 Cold water systems

Water moves constantly in what is scientifically called the hydrological cycle.
We know it by its more common name: the rainwater cycle.

Simply explained, the rainwater cycle is a natural process where water is
continually exchanged between the atmosphere, surface water, groundwater,
soil water and plants. It can be divided into three main transfer processes:

1 evaporation from oceans and other water bodies into the air
2 transpiration from land plants and animals into the air
3 precipitation from water vapour condensing from the air and falling back to
Earth or into the ocean.

Water saturated cloud

Water vapour

Rain and snow
Heat from the Sun

Evaporation

River running back to the sea
to restart the process

p Figure 5.2 The rainwater cycle

As the Sun warms the Earth, water on its surface evaporates. This vapour rises
with the air and is carried by the prevailing winds. If the vapour passes over land,
some of it condenses to form clouds and, as more water vapour is attracted or
the ground rises (hills or mountains), the cloud becomes saturated to the point
where it can no longer hold the moisture and the vapour is released in the form
of rain, sleet, snow or hail.
On reaching the ground, there are many paths it may follow. Some of it may
be re-evaporated back into the atmosphere; it may be absorbed by the ground,
where it will travel towards the water table or aquifer; or it may remain on
the surface, where it will eventually find its way into rivers, streams, lakes or
the oceans. Here, the process begins again, an example of the Earth’s natural
recycling process.

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