The Water Cycle.

Dear Diary,

I'm still covering water in Topic 1, Part 1 - What is Water of my S111 module from the Open University, who knew there was so much to think about when it comes to something everyone takes for granted. 

So how does water exist and move on earth?

Water covers about 71 % of the earths surface, and 96.5 % of that is saltwater contained in earths oceans that means freshwater, which is what pretty much every animal and plant needs to thrive, only constitutes 3.5 % of the water available on earth. 

That 3.5 % of freshwater is made up from all the rivers, lakes, and streams, the water locked up in ice, glaciers and polar ice caps, the groundwater that is deep beneath our feet, as well as inside not only plants and animals but our own human bodies (we are around 50-65 % water)

The Water Cycle

We use the water cycle to describes how water is exchanged through Earth's land, ocean, and atmosphere. This was probably taught to you at junior school if you recall, (told you they were starting with the very basics).

Water always exists in all three places, and in many forms at the same time, as I mentioned above, it exits as lakes and rivers, glaciers and ice sheets, oceans and seas, underground aquifers (groundwater reservoirs), and vapour in the air and clouds.

The water cycle consists of three major processes which keeps things turning, they are; 

Evaporation the process of a liquid changing to a gas. There is increased evaporation if the surface area of the liquid is increased and if there is air flowing over it. 

Water vapour is an important part of the air we breathe, it is all around us and an important greenhouse gas. Greenhouse gases such as water vapour and carbon dioxide insulate the Earth and keep the planet warm enough to maintain life as we know it. However too many greenhouse gases can have a detrimental affect on our planet. (not covered in this post)

The evaporation process taking place within the water cycle is driven by the sun, as the sun interacts with liquid water on the surface of the ocean (remember, the larger the surface area the easier it is to evaporate) the water becomes a gas called water vapour. Wind, temperature and the density of the body of water can all also affect the process of evaporation.

Condensation the process of a gas changing to a liquid. In the water cycle, water vapour in the atmosphere condenses and becomes liquid. This can happen either high in the atmosphere or at ground level. 

To form clouds water vapour, that invisible gas which has risen from bodies of water on the ground into the air, condenses into visible water droplets or ice crystals. These droplets or crystals then 'stick' to tiny particles of salt or dust which are also in the atmosphere (called cloud condensation nuclei CCN) and eventually the created water droplets start sticking to each other, creating clouds. Clouds that form at ground level are called fog or mist.

Like evaporation, condensation is also influenced by the sun. As water vapour cools, it reaches its saturation limit, or dew point, that is the point when the air cannot hold anymore water and the cloud forming process begins. 

Precipitation which is not a process unlike evaporation and condensation, it describes any liquid or solid water that falls to Earth as a result of condensation in the atmosphere, that includes rain, snow, and hail.

Fog is not classed as precipitation because the water in fog does not actually precipitate, or liquefy and fall to Earth, fog and mist are a part of the water cycle called suspensions. They are liquid water suspended in the atmosphere (and a general pain to have to walk through). Precipitation is one of many ways water is cycled from the atmosphere to the Earth or ocean.

Other Processes

Evaporation, condensation, and precipitation are important parts of the water cycle, however, they are not the only ones.

Runoff, describes a variety of ways liquid water moves across land. Snowmelt, for example, is an important type of runoff produced as snow or glaciers melt and form streams or pools.

Transpiration is another important part of the water cycle. Transpiration is the process of water vapour being released from plants and soil. Plants release water vapour through microscopic pores called stomata, the opening of stomata is strongly influenced by light, and so is often associated with the sun and the process of evaporation.

States of Water

Water is a truly interesting compound as it can exist in so many different forms, it continually circulates through three states: solid, liquid, and vapour.

Ice is solid water, most of Earth's freshwater is ice, locked in massive glaciers, ice sheets, and ice caps. Ice can float on the liquid form of itself, because once it reaches 4°C or lower it actually becomes less dense, it also expands by about 9 % hence why cans of coke explode if you freeze them. (if you try that please don't blame me for it)

As ice melts, it turns to liquid, the ocean, lakes, rivers, and underground aquifers all hold liquid water.

Water vapour is an invisible gas, it is not evenly distributed across the atmosphere, it is much more abundant above the ocean making up as much as 4% of the air, however above isolated deserts, it can be less than 1%.

Water is the driving force of all nature. - Leonardo da Vinci.

I could probably write reams more about water on earth in this post but I am trying to keep it straightforward and I hope I've managed to do that here. If you've any pointers about how to improve my science writing, or any topics you'd like to hear about drop me a note below. I'd love to hear from you.