However, not all water on Earth can support life. Ninety-seven percent of the water on Earth is salty. Most organisms cannot drink saltwater. Most living things depend on freshwater for survival. Only three percent of all Earth's water is freshwater. Sixty-seven percent of the world's freshwater is frozen in polar icecaps and glaciers. That makes it unavailable for use by most species. This leaves only thirty-three percent of the world' s freshwater--or approximately 1% of all the water on Earth--available to sustain life.
Water in the hydrosphere can exist as a solid (ice), a liquid (water), or a gas (water vapor). It is constantly changing among these three forms, or phases. Water moves around Earth as it changes form. The movement of water occurs in a group of repeating events. The group of repeating events is called the water cycle or hydrologic cycle. It is diagramed in the figure below. Glass image © 1999 -www.arttoday.com
Heat from the sun causes water in oceans, lakes, swamps, rivers, plants, and even our own bodies to evaporate into water vapor. Water vapor in the atmosphere forms clouds as it cools and condenses. Clouds are collections of millions of tiny droplets of water. When these tiny droplets of water cool, they form larger drops of water. When the drops of water become too heavy to stay in the cloud, they fall to the Earth as precipitation. Rain, snow, sleet, and hail are all forms of precipitation. The form of precipitation that falls depends on the temperature of the air. During warm temperatures, precipitation falls in the form of water (rain). During cold temperatures, precipitation falls in the form of ice (snow). Water from precipitation flows across the surface of the land into lakes, streams, and oceans. Some of the water enters the ground. Plants absorb water from the ground through their roots. The water eventually evaporates. And so the water cycle continues.
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