Consider a single molecule of water. It might be in your glass right now. It might have been, last month, in a cloud; last year, in a river; long ago, in the depths of the ocean or locked in ancient ice. Water molecules are not created and destroyed in everyday processes—they travel, endlessly, around the planet. That grand journey is the water cycle.

A Cycle Powered by the Sun

The water cycle is, at its heart, a solar-powered system. The energy that drives the whole loop comes from the Sun, with gravity doing much of the rest. Together they keep water moving perpetually among the ocean, the air, the land, and living things.

The cycle has no true beginning or end, but it is easiest to follow it stage by stage.

Rising: Evaporation

The journey upward begins with evaporation. The Sun's energy heats water at the surface of oceans, lakes, and rivers, and some of that water turns from liquid into water vapor, an invisible gas that rises into the air.

Plants contribute too. Through a process called transpiration, plants release water vapor from their leaves. The combined rise of water vapor from surfaces and from plants lifts an enormous quantity of water into the atmosphere.

Forming Clouds: Condensation

As water vapor rises, it encounters cooler air. Cooling causes the vapor to condense—to turn back from gas into tiny liquid droplets.

Vast numbers of these tiny droplets, suspended in the air, are what we see as clouds. Condensation is the stage that gathers dispersed, invisible vapor back into visible, concentrated form.

Returning: Precipitation

When the droplets in a cloud grow large and heavy enough, they can no longer remain aloft. They fall back to the surface as precipitation—rain, or in cold conditions, snow.

This is the water's return to the Earth's surface, completing the airborne portion of its journey.

Across the Land: Collection and Flow

Once water has fallen, it travels again. Some flows over the surface as runoff, gathering into streams and rivers that journey back toward the sea. Some soaks down into the ground, becoming groundwater that moves slowly through the soil and rock, and some is taken up by plants and living things.

Eventually, much of this water finds its way back to the oceans and lakes—where the Sun can lift it once more, and the cycle begins again.

Storage on Many Timescales

A key insight is that water does not move at a single pace. It can spend a few days in the atmosphere, a season in a river, years in the soil, or vast spans of time locked in ice or deep underground. The water cycle is a system of flows and stores, and a given molecule's journey may be swift or extraordinarily slow.

The Same Water, Forever

The water cycle holds a quietly astonishing truth: the water on Earth is, essentially, the same water, used and reused without end. The molecule in your glass has completed this circuit countless times, through clouds and rivers and oceans and living things, across a span of time almost beyond imagining. Understanding the water cycle is to understand one of the great unifying patterns of nature—a piece of science that connects every drop of water to every other, across the whole history of the planet.