A Loop With No Start or End
The water cycle, also called the hydrologic cycle, is the continuous movement of water between the oceans, the atmosphere, the land and living things. It has no real starting point. Water that fell as rain this morning may have been part of a sea a few days earlier, and some of it may have spent a very long time frozen in a glacier or sitting in rock underground.
Two forces drive almost all of it. Energy from the Sun warms water and lifts it into the air, and gravity pulls it back down as rain, snow and flowing water. The total amount of water on Earth stays roughly the same over human timescales. What changes is where the water is and what form it takes.
Where Earth’s Water Is
Most of Earth’s water, about 97 percent, is in the oceans and is salty. Dissolved salts, a subject tied to ocean salinity, mean that seawater is not directly usable for drinking or most farming. Of the small share that is fresh, a large portion is locked up as ice in glaciers and ice sheets. The rest is mostly groundwater, with only a tiny fraction in lakes, rivers, soil and the air at any moment.
That tiny amount in the air matters far more than its size suggests, because it is the link that moves water from one place to another.
Step 1: Evaporation
Evaporation is the change of liquid water into water vapor, a gas. Sunlight warms the surface of oceans, lakes, rivers and wet soil, and some water molecules gain enough energy to escape into the air. This is one of the everyday examples of the states of matter changing: liquid becomes gas without the water needing to boil.
Evaporation happens faster when the air is warm, dry and windy. It also happens from snow and ice directly into vapor in a process called sublimation, though this is usually slower.
Plants add to the process. They take up water through their roots and release vapor through tiny pores in their leaves. This is called transpiration. Scientists often combine the two and call the total “evapotranspiration,” because it is hard to separate them over a forest or a field. Transpiration is also tied to how plant photosynthesis works, since leaves open their pores to take in carbon dioxide and lose water vapor as a result.
Step 2: Condensation
Water vapor is invisible. As moist air rises, it expands and cools. Cooler air cannot hold as much vapor, so at some point the vapor turns back into tiny liquid droplets, or into ice crystals if it is cold enough. This is condensation.
Droplets usually form on microscopic particles in the air, such as dust, sea salt or smoke. Huge numbers of these droplets together make a cloud. Fog is essentially a cloud that forms at ground level. Dew is condensation on cool surfaces, such as grass overnight.
Air rises for several reasons: it is warmed by the ground, it is forced up over mountains, or it is pushed up where two air masses meet. These are the same processes behind everyday weather systems, which is why clouds and weather are so closely linked.
Step 3: Precipitation
Cloud droplets are very small, and they stay aloft because they are so light. Precipitation happens when droplets or ice crystals grow large enough, by colliding and merging or by ice crystals growing at the expense of nearby droplets, that the air can no longer hold them up.
What reaches the ground depends on the temperature along the way:
- Rain: liquid drops, falling through air above freezing.
- Snow: ice crystals that stay frozen all the way down.
- Sleet: snow that partly melts and refreezes before landing.
- Hail: lumps of ice that grow in strong rising air currents inside storms.
The strongest rainfall can come from organized storms, and tropical storms gather huge amounts of moisture from warm oceans, which is central to hurricane formation.
Step 4: Runoff, Infiltration and Storage
Once water reaches the ground, it follows several paths.
Runoff. Water that cannot soak in flows downhill over the surface into streams, rivers, lakes and eventually the sea. Runoff is greater when the ground is steep, frozen, saturated or covered with pavement and roofs, which do not absorb water.
Infiltration. Some water soaks into soil. Plants use part of it, and some is held in the soil. The rest moves deeper and joins groundwater.
Groundwater. Water fills the spaces between soil grains and cracks in rock. The upper boundary of fully saturated ground is called the water table. Groundwater moves slowly, often much more slowly than a river, and it may emerge at springs or seep into streams. Its movement depends on the rock type, as the rock cycle shapes what the ground is made of.
Snow and ice storage. In cold climates, precipitation can be stored for months as snowpack or for much longer in glaciers. When it melts, it feeds rivers, which is why mountain snow is an important water supply in many regions.
Step 5: Back to the Start
Rivers carry water to the ocean, where evaporation begins again. Along the way, water may be held in a lake, absorbed by plants, drunk by animals or taken from a river or well by people. Eventually most of it returns to the air or the sea.
The time water spends in each store varies enormously. A water molecule in the atmosphere typically stays for days. In a river, it may be there for days to weeks. Deep groundwater and large ice sheets can hold water for far longer, from many years to thousands of years or more.
Why the Water Cycle Matters
The cycle shapes weather, supplies fresh water, carves valleys, carries nutrients and sediment, and sustains every ecosystem. It also moves heat around the planet: evaporation takes up energy from the surface, and condensation releases it into the atmosphere.
Human activity affects the cycle in several ways. Cities and paved areas increase runoff. Drawing groundwater faster than it refills can lower the water table. Clearing or planting forests changes how much moisture goes into the air. Climate scientists also expect a warmer atmosphere to hold more water vapor, which can change where and how heavily it rains, though the effects differ from region to region and are still being studied in detail.
Water used in homes and industry also takes part. It is pumped, used, treated and returned to rivers or the sea, so the cycle includes human systems as well as natural ones.
A Quick Summary of the Loop
- The Sun warms water, and it evaporates from oceans, lakes and land, and transpires from plants.
- Rising vapor cools and condenses into cloud droplets or ice crystals.
- Droplets grow and fall as rain, snow, sleet or hail.
- Water runs off the surface, soaks into the ground or is stored as ice.
- Rivers and groundwater return water to the sea, and the cycle repeats.
Seen this way, a glass of water is part of a very old loop. The water in it has almost certainly been rain, river, ice and cloud many times before.