Transfers that change the size of the Earth's water stores

Whilst the overall volume of water contained in the stores does not change that significantly on the global scale, transfers and flows will affect the amount of water over time that remains in a store.

A simple example of this is how levels of evaporation will increase in summer time which will lead to higher atmospheric storage and a slight drop in ocean storage. However, these are not significant changes in volume and the ocean will always remain the primary store in the water cycle.

Evaporation

This is the process whereby liquid water changes into a gas (water vapour) when it absorbs heat energy.

Approx. 90% of the atmospheric water store is from evaporation from the oceans and seas. The remaining 10% comes from plant transpiration.

High levels of evaporation can occur in some tropical and desert areas. These are regional scale changes. Increased global changes are likely to occur due to climate change and increased atmospheric temperature.

The level of evaporation on global scale ocean and sea level is minimal, but within a drainage basin evaporation can have a large regional scale impact. Lakes or rivers in an area with a high maximum summer temperature will experience high levels of evaporation and hydrospheric storage will reduce significantly in that area.

Condensation

This is the process whereby the gaseous water vapour changes back into liquid water within the atmospheric water store.

This occurs as temperature falls and humidity increases. Tiny, microscopic water molecules that develop around dust and smoke particles (known as aerosols) will be carried invisibly in the air. Where they combine into larger molecules of liquid water or ice they may be seen as mist, fog or clouds. This occurs on a global scale but could alter as climate change increases global atmospheric temperature.

Cloud formation

Clouds form when water molecules aggregate (join up together).

They are frequently noticed at the altitude where air temperature has fallen to a point where condensation of invisible water vapour occurs (condensation – or dew point), or where the humidity content has risen such that water vapour cannot remain in that state and condenses. As the molecules grow, clouds form with the tiny water or ice particles kept aloft by rising air currents (thermals).

Precipitation

Generally, precipitation occurs when the water molecules within a cloud combine and become too big. This process is called coalescence.

If the droplet’s fall velocity is greater than the cloud’s updraft velocity, then precipitation will occur. Simply put, if the droplet falls faster than the cloud moves it upwards, rain, snow or hail will fall from the cloud.