molecule found in your glass of water today has probably been on the Earth for billions of
years. It may have been in a glacier or far below the ground. It may have been high up in
the atmosphere and deep in the belly of a dinosaur. Who knows where it will end up today,
when you’re done with it!
Figure 13.6: Water on Earth is constantly in motion. ( 22 )Let’s studyFigure13.6for a moment. The Sun, many millions of kilometers away, provides
the energy which drives the water cycle. Since the ocean holds most of the Earth’s water,
let’s begin there. As you can see in the illustration, water in the ocean evaporates as water
vapor into the air. The salt in the ocean does not evaporate with the water, however, so the
water vapor is fresh. Some of the invisible water vapor in the aircondensesto form liquid
droplets in clouds. The clouds are blown about the globe by wind. As the water particles
in the clouds collide and grow, they fall from the sky asprecipitation. Precipitation can
occur in forms such as rain, sleet, hail, and snow. Sometimes precipitation falls right back
into the ocean. Other times, however, it falls onto the solid earth as freshwater.
That freshwater, now on the Earth, may be found in a solid form as snow or ice. Some