The abundance of water on Earth is a unique feature that clearly distinguishes our "Blue Planet" from others in the solar system.

1. The hydrosphere consists of all the earth's water, on the surface, in the ground, and in the air.
Water can be found in the four different  states of matter: solid (ice), liquid (water), gas (water vapor) and plasma (hydrogen nuclei and electrons).
2. About three-fourths of the Earth is covered with water, most of it in the ocean. About 70% of the Earth is covered with water (97% of that is part of the salty oceans). Only a small portion of the Earth's water is freshwater. This includes such things as rivers, lakes, and groundwater. Freshwater is needed for drinking, farming, and washing. There is even water in the form of ice at the poles.
3. The atmosphere contains water in the form of vapor.
4. The hydrosphere helps regulate the Earth's temperature and climate.
5. The ocean absorbs heat from the sun and distributes it around the earth in currents.
6. Evaporation from the ocean and other bodies of water produces rain as well as other forms of precipitation that sustain life on land.
7. The Earth's water is always in circulation. It has been recycled for the last 3 billion years. This process is called the water cycle.

The Hydrologic Cycle: The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 TheRegents of the University of Michigan; ©2000-01 University Corporation for Atmospheric Research