The Water Cycle

In general, we can say that water exists in three different places across the world: these are the land, the oceans and the atmosphere. The hydrological cycle describes how it travels between these places in various guises, like rivers, clouds and aquifers.

This is a continuous progression where evaporation occurs before travelling into the air, joining a cloud and then falling back to earth as rain, before once again evaporating. The journey is separated into three distinct phases, evaporation, condensation, and precipitation.

What is Evaporation?

During the process of evaporation, the surface part of a liquid is converted into a gaseous state. In the hydrological cycle, the surface of a river, lake or ocean evaporates to become vapour. This vapour is around us all the time and makes up a significant proportion of the air we breathe. It is also a greenhouse gas, and along with other gases, like carbon dioxide, it shields the earth, keeping it at just the right temperature for the flora and fauna that live here.

Evaporation is mainly pushed on by heat from the sun. As this touches the surface of a liquid, it is transformed into the vapour or gas phase. It’s not just the sun which can drive the process however, because evaporation is also affected by how dense the ocean or lake is, how warm or cold it is, and the wind speed.

Illustration of water cycle. Ground water discharge to freshwater storage.
U.S. Dept. of the Interior - U.S. Geological Survey

How Does Condensation Work?

When a gas changes into a liquid, we call it condensation. In terms of the hydrological cycle, it occurs when vapour in the atmosphere cools, condenses, and converts into a liquid. This can happen in a couple of ways, either at ground level or higher in the atmosphere. Clouds are created as vapour condenses, or becomes more tightly packed and dense. Vapour droplets take on a more concentrated form around tiny particles of dust or salt that exist inside a cloud, scientists call these cloud condensation nuclei or CCN. The same interaction can occur at ground level when clouds form as mist or fog patches.

As with evaporation, the temperature can have a major influence on the formation of condensation. When vapour begins to cool, it reaches a dew point, and we are all accustomed to seeing it settle as a liquid on our lawns or cars. Dew points differ between geographical areas, as they can be influenced by air pressure as well as how warm it is.

Forms of Precipitation

Precipitation is a product of condensation and evaporation, it is the term we use for any type of liquid or solid water that descends to earth, because of condensation. Hail, rain and snow are all forms of precipitation, but fog is not. Any droplets suspended in fog are not precipitating or liquefying or falling onto earth. Instead, we call mist and fog suspensions because the liquid they contain is held in the atmosphere.

Precipitation is one of many ways water is cycled from the atmosphere to the earth or ocean.

Other Processes in the Hydrological Cycle

Key to the movement of water around the world are evaporation, condensation, and precipitation, but there are other processes at work.

Melting Snow
A chief element in the hydrological cycle of colder climates, snowmelt runoff is found in rivers and streams. It starts high in the mountains, where snow is held in icy storage like a natural reservoir during the winter months. This is built up into huge snow packs as more precipitation lands, until the warm season arrives. This causes the ice and snow to melt, flowing into streams and rivers, and enriching the local water supply.

Only around 35% of precipitation finds its way to an ocean or sea, the remaining 65% become absorbed by soil and some evaporates. The precipitation which is not absorbed or evaporated is called runoff. This moves along the surface of the ground and collects in a variety of places, like ditches, storm drains, ponds and rivers where it eventually makes its way to the sea. A by-product of runoff is erosion, as it carries sediment and vegetation along with it, but also pollution, as more noxious substances can form part of a flow.

Moisture is passed through vegetation, right from the roots to the tiny openings on the bottom of leaves where it converts into vapour and evaporates. This is called transpiration. Around 10% of atmospheric moisture is a produced from plants as part of this process.

Illustration of water cycle. Evaporation from oceans, lakes and streams. Large blue arrows. White mountains.

Is the Climate Influenced by this Cycle?

The earth’s climate and its diverse ecosystems are all affected by hydrological processes.

When scientists examine the weather conditions within a particular geographical area over an extended period of time, they can form an opinion on its climate. Two of the major contributing factors which influence climate are temperature and humidity, both of which are shaped by the cycle.

Humidity refers to the level of vapour in the air. In some places, it is present in higher volumes, like on the coast and around islands, and in others, it is lower, like inland areas and deserts. This goes some way to producing the drastically varied climates found across the globe, but the temperature is also instrumental.

As part of evaporation, precipitation and condensation, exchanges of heat take place and temperatures change. For example, during evaporation, energy is absorbed into the liquid and this acts to cool the surrounding environment. Condensing has the opposite effect, as it discharges energy.

How Long has this Process Been Happening?
Scientists estimate that the hydrological cycle has been taking place on our planet for more than 4 billion years. That means the water you drank this morning has previously quenched the thirst of a dinosaur, formed part of an ice age glacier and been navigated by the earliest humans, it’s a brilliant example of natural recycling.

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These information were not written by the American Food and Drug Administration, or any other official authority. This site and its products do not promise to diagnose and cure diseases, or protect you from them. Always consult your doctor when you are sick.