Explore how water droplets form and fall from the sky in part three of the water cycle series.
Water vapor drifting above the oceans is carried over land by winds, and eventually falls to the surface in the form of rain and snow. As evaporated water rises in the atmosphere, it expands and cools. In the presence of dust, ice or salt, water vapor in the saturated air condenses around these particles into tiny droplets or ice crystals, forming clouds. Around half of our planet is covered with clouds at any one time. Since clouds reflect sunlight away from the Earth, they play a vital role in the Earth's climate and energy balance. As these droplets and ice crystals accumulate more water, they become heavier and are pulled from the sky by gravity as rain and snow. In this way, water is returned to land in a form that plants and animals can use. About 100 trillion tons of water falls on land each year, compared to 400 trillion tons over the oceans. Watch how water vapor moves through the atmosphere and returns to Earth as rain and snow in the visualizations below—first on a globe and then on a map of the entire world.
Intense rainfall caused flooding and landslides in China in May 2010, as seen by NASA's TRMM satellite instrument.