Solid - ice Liquid - water Gas - steam and humidity The greenhouse effect The water cycle

Water (H₂O) exists as all three states on Earth. It is solid in the polar ice caps and glaciers, liquid in the oceans, lakes, rivers, underground artesian basins and clouds, and as a gas in the atmosphere. The three states of water are named, unlike any other substance.

Solid - ice
At temperatures less than 0°C, water exists in solid form as ice. Ice is one of the very few solids which float on their liquid; most solids sink. Ice floats because it is less dense than the liquid water around itself.

Icebergs and polar ice sheets melt in summer and freeze in winter because they float on the surface instead of sinking to the ocean floor. They also absorb heat energy on melting and releasing that energy on freezing, thus helping to moderate the Earth's climate.

The weather patterns that make our world a place that we can live in are the product of the melting and freezing of this sea ice. If ice did not float, it would sink and the oceans would be solid ice with a layer of liquid water. The Earth would also be a much less windy and colder place.

Liquid - water
Most of the water on Earth exists in the liquid state as a salt solution in the oceans, with smaller quantities of fresh water in lakes, rivers and clouds. The temperature must be between 0°C and 100°C for water to exist as a liquid at atmospheric pressure.

The Earth is in a unique position within the Solar System being just far enough away from the Sun to allow liquid water to exist in significant quantities. If it were closer, all water would be gas; if it were further away, all water would be solid ice.

Clouds form when gaseous water vapour in the atmosphere is cooled and droplets of water form. It is these droplets of water that we see as fog and clouds.

Gas - steam and humidity
Gaseous water is a colorless and odourless gas. The vapours often seen above boiling water or as fog on a cold day are in fact a mist of liquid droplets. The real steam is the invisible gas just on top of boiling water and below the visible cloud.

Pure steam exists at temperatures above 100°C at sea level and 1 atmosphere air pressure. At lower atmospheric pressures - altitudes above sea level - water boils at less than 100°C, hence, steam can exist at temperatures less than 100°C at higher altitudes.

At lower temperatures, gaseous water exists as a mixture in air. Water evaporates from surface liquid water and mixes with the air; this is measured as humidity. Air humidity is another very important modifier of climate. The increased humidity of the air in the tropics keeps temperatures down in the 30's°C, unlike desert regions where the drier air allows temperatures of 40°C to 60°C to be reached.

The greenhouse effect
Water vapour is a powerful "greenhouse" gas. If water was not in the atmosphere, trapping the infra-red radiation escaping back into space, the Earth would be so cold that it would be uninhabitable. There is current debate about warming of the Earth's atmosphere due to increased carbon dioxide concentrations, creating an "enhanced greenhouse effect". This is covered in more detail in the topic The greenhouse effect.

The water cycle
The Earth has a natural recycling process of ice, water and vapour. From snow, glaciers, rivers, oceans, clouds and rain; water of Earth freely and quickly changes between the three states.

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