Earth is called the blue planet due to the abundance of water. About 70% of the surface of the Earth is covered by water, and it is the only natural substance that can be found on Earth in all three states – liquid, solid and gas. Due to this unique property, water can be found just about everywhere.

Liquid, solid and gas

Rivers, groundwater, lakes, the world’s five oceans and rain represent the liquid phase of water.

In its gaseous state, water vapour is evaporated by the Sun’s solar radiation from the surface of water bodies like oceans or lakes, and from the surface of plants and the land. Water vapour can also evaporate directly from its frozen state.

Snow and ice represent the solid form of water and can be found in the Earth’s polar icecaps and on top of high mountains. Some of the snow and ice melts and turns into liquid water. In the polar regions, ice can stay frozen for thousands of years.

Only a small amount of the total amount of water (about 0.3%) is directly useable for human consumption.

Water’s journey

If you leave some water on a saucer by a window, it will eventually evaporate. This happens only if there is enough thermal (heat) energy available for the water molecule to vibrate so vigorously that the molecules ‘break’ out of their liquid structure and turn into a gas.

But why do the oceans not dry up? In fact, most of the evaporation occurs from ocean water. Much of the evaporated water rains back into the oceans again. Some falls on the land surface and might spend some time on there as ice, snow, groundwater or in streams, or it may be stored in lakes before it returns back to sea.

This journey is called the hydrological cycle. It describes the exchange of water in every form between the Earth’s systems and is part of what makes the Earth so unique.

Meet the scientists

David Hamilton, Louis Schipper, Dave Campbell and Keith Hunter are New Zealand scientists who each study one aspect of the hydrological cycle. In their studies, they need to consider the Earth as a whole, dynamic and interacting system.

Take up the challenge

The Hub has a number of activities that model aspects of the water cycle.

Other teaching resources include:

Key terms

For explanations of key concepts, see H2O on the go – key terms.

Published 28 May 2009