The Water Cycle: What It Is and How It Works, Stage by Stage

A journey with no starting point

The water in your glass may have passed through an ocean, a cloud over the Alps and the root of a pine tree. The water cycle (textbooks also call it the hydrologic cycle) is this never-ending journey between the planet’s three great reservoirs: the oceans, the atmosphere and the land. Along the way, water changes state — liquid, vapor, ice — but it is never created or destroyed: it only changes place.

Two forces keep everything moving. The heat of the sun lifts water by making it evaporate; gravity brings it back down and pushes it, drop by drop, toward the sea.

The way up: from water to sky

The sun warms oceans, lakes and rivers, and some of the molecules at the surface break free and become water vapor, an invisible gas that rises into the air: this is evaporation. According to UCAR, about 86% of the water that evaporates worldwide each year comes from the oceans.

Plants take part too. Through transpiration they release vapor from their leaves, out of the same stomata that let in carbon dioxide for photosynthesis. Snow and ice, finally, can skip a stop: through sublimation they pass directly from solid to vapor.

Practical example: laundry on the line dries even on a cold, windy day. The water does not “vanish”: it evaporates molecule by molecule, and the wind carries the vapor away before the air around the clothes gets saturated.

Up in the sky: condensation and clouds

As it rises, the air cools and the vapor condenses: it turns back into microscopic liquid droplets, or freezes into tiny crystals, that stay suspended in the air. These are clouds. When droplets and crystals merge and grow too heavy, gravity wins: they fall as precipitation — rain, snow or hail, depending on the temperatures they meet on the way down.

Practical example: the fogged-up bathroom mirror after a shower is condensation in miniature. Warm vapor meets a cold surface and turns back into liquid. Clouds are born the same way, just a few kilometers higher up.

The return: rivers, soil and aquifers

Back on land, water takes one of two roads. Part of it flows over the surface (runoff), collecting in streams and rivers until it reaches the sea again. Another part soaks into the ground (infiltration) and fills the pores and cracks of rocks: these are the aquifers, which according to the USGS store 30.1% of all the planet’s freshwater. From there, water resurfaces in springs and wells, or stays underground for a long time.

A small share travels through us: we drink it, and every cell in our body works bathed in water. Then we hand it back to the cycle, partly through breath and sweat, the rest down the drain.

How long each stop lasts

The journey has no single rhythm: some stops last days, others millennia. The average times estimated by UCAR:

Where Earth’s water is

Almost all the planet’s water — 96.5%, according to the USGS — is salty and sits in the oceans. Freshwater is just 2.5% of the total, and almost all of it is hard to reach: 68.6% is frozen in glaciers and polar ice caps, and 30.1% lies in underground aquifers. Rivers carry just 0.006% of all freshwater, lakes 0.26%.

Why the cycle matters to us

The cycle decides where and when freshwater is available. As the climate warms, it is changing pace: more heat means more evaporation and, according to UCAR, heavier precipitation in some regions and stronger droughts in others. That is why NASA follows the journey from above with its satellite missions: the data they gather helps predict floods and droughts further in advance.