The water filling a glass today may once have fallen as rain on a forest thousands of years ago, or drifted as vapour above an ocean on the other side of the world. This article explains how the water cycle moves the same finite supply of water endlessly through the planet, and where all that water actually sits.
Earth doesn't get new water — the planet has essentially the same amount of water today as it did billions of years ago. What keeps that water usable, and what fills rivers, fuels rain, and refills lakes, is a continuous loop called the water cycle, powered entirely by energy from the Sun. This article follows a single drop of water through that loop, then looks at where Earth's water actually sits.
The water cycle is the continuous movement of water between the oceans, the atmosphere, and the land. It has no real starting point — it's a loop — but it's usually described in four main stages: evaporation, condensation, precipitation, and collection. The entire cycle is driven by heat from the Sun, which provides the energy needed to turn liquid water into vapour.
A water molecule that falls as rain today might have evaporated from an ocean thousands of kilometres away just days earlier — and the same molecule could be evaporating and falling again as rain somewhere else within weeks.
Evaporation happens when heat from the Sun causes liquid water at the surface — mostly the oceans — to turn into water vapour and rise into the atmosphere. Oceans account for the large majority of the water that evaporates into the atmosphere, since they cover about 71% of Earth's surface.
Plants add to this process through transpiration: water absorbed by roots travels up through the plant and evaporates from tiny pores in its leaves. Because evaporation and transpiration happen together across a landscape, scientists often refer to the combined process as evapotranspiration.
As water vapour rises higher into the atmosphere, it cools. Cooler air can hold less water vapour, so the vapour condenses — changes back into tiny liquid droplets — around microscopic particles of dust, salt, or pollen floating in the air. Billions of these tiny droplets clustered together form what we see as a cloud.
Condensation isn't limited to the sky — the same process explains why a cold glass of water "sweats" on a humid day. Warm, moist air touching the cold glass cools rapidly, and the water vapour in it condenses into visible droplets on the outside of the glass.
Inside a cloud, water droplets constantly collide and merge with each other, gradually growing larger. Once a droplet becomes too heavy for rising air currents to keep it suspended, it falls to Earth as precipitation — rain, snow, sleet, or hail, depending on the temperature of the air it falls through.
Once precipitation reaches the ground, it doesn't just sit still — it takes one of several paths, together known as collection or runoff:
An area of land where all the runoff drains toward the same river, lake, or ocean outlet is called a watershed, or drainage basin. From here, most water eventually reaches the ocean, ready to evaporate and begin the cycle again.
Despite Earth being called the "Blue Planet," most of that water isn't available for drinking or farming. The vast majority is saltwater, and even the small freshwater share is mostly locked away in a form we can't easily use.
| Water Source | Share of Earth's Total Water |
|---|---|
| Oceans (saltwater) | About 97% |
| Freshwater — total | About 3% |
| ↳ Glaciers & ice caps | Roughly 68% of all freshwater |
| ↳ Groundwater | Roughly 30% of all freshwater |
| ↳ Rivers, lakes & atmosphere | Less than 1% of all freshwater |
That means the rivers and lakes people rely on most directly for daily water make up only a tiny sliver of Earth's total water supply — which is part of why freshwater conservation matters so much, even on a planet that looks mostly blue from space.
The water cycle is one of the few truly closed loops on Earth — the same water has been evaporating, condensing, falling, and collecting for billions of years, powered entirely by the Sun. Understanding that loop, and how little of Earth's water is actually usable freshwater, is the foundation for understanding rivers, droughts, and why water conservation is a genuinely global concern.
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