Source: The Water Cycle (adapted from the USGS website, http://ga.water.usgs.gov/edu/watercycle.html
)
The water cycle describes the existence and movement of water on, in, and above the Earth. Earth's water is always moving: it is always changing states, from liquid to vapor to ice, and back again. The water cycle has been working for billions of years; all life on Earth depends on it continuing to work.
The water cycle is all about water movement. However, much more water is "in storage" for long periods of time than is actually moving.
There are three main types of water storage:
 The oceans are the major storehouse for all the water on Earth. The pie chart, on the right, shows that 96.5% of Earth's water is found in the oceans. A single water molecule might be stored in the oceans for thousands of years before it re-enters the water cycle.
 Water is stored in polar ice caps and land glaciers. Water can be stored in the form of frozen ice for very long periods of time. Glacier ice at the base of some very large glaciers as been determined to be over 100,000 years old! Core ice from Antarctica is over 500,000 years old. This long-term storage has helped climate scientists determine the conditions of Earth’s atmosphere and climate from long before any “written” record.
 Lastly, water is stored in underground aquifers, also known as groundwater. A single water molecule might take decades to move into and out of groundwater storage. In some aquifers, water can be stored for hundreds or thousands of years.
There is always water in the atmosphere. Clouds are the most visible form of atmospheric water, but even clear air contains water particles that are too small to be seen. One estimate is that the volume of water in the atmosphere at any one time is about 0.001 percent of Earth's total water volume. Importantly, water is moved around the globe by the atmosphere, which acts like a superhighway for water transport.
Even though clouds are absent in a clear blue sky, water vapor particles that are too small to be seen are still present. Depending on conditions, water particles can combine with tiny particles of dust, salt, and smoke in the air causing condensation. Condensation is the process in which water vapor in the air is changed from a gas to a liquid. Condensation results in the formation of cloud droplets, which grow and develop into clouds.
Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle.
Condensation in the atmosphere (clouds) occurs higher in the sky where the air is cooler. Water droplets combine (also known as coalescence) with each other, until clouds develop. Strong winds move the clouds around the world until water returns as precipitation to the surface of the Earth.

Clouds form in the atmosphere because air containing water vapor rises and cools. Air near the Earth's surface is warmed by solar radiation. But, why does the atmosphere cool above the Earth's surface? The reason has to do with air pressure. At sea level, the weight of a column of air pressing down on your head is greater than the weight of air pressing on you at higher altitudes. This is because there are fewer air molecules at higher altitudes in a similar volume as compared to the number of air molecules at sea level. Therefore, the air at higher altitudes is less dense; the air particles are spread further apart (also known as “expansion of air”). Since fewer air molecules exist in a certain volume of air, there are fewer molecules colliding with each other, and as a result, less heat is produced. This means cooler air. Cooler air allows for condensation to occur, causing water vapor to change to liquid cloud droplets.
Precipitation is water falling from the base of a cloud in either its liquid or solid state. Precipitation can occur in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that delivers the water in the atmosphere back to the Earth’s surface. Most precipitation falls as rain.
The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as
precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud, you can see some parts disappearing (evaporating) while other parts are growing (condensation). For precipitation to happen, water droplets in a cloud must grow when the added water particles collide. If enough collisions occur to produce a big enough droplet, then the droplet will fall out of the cloud as precipitation. This is not a small task since millions of cloud droplets are required to produce a single raindrop.
Many people probably have an overly simplified idea that when precipitation falls on the land, it flows over the land and into rivers (runoff), which then empty into the oceans. That is
"overly simplified" because some of that water seeps into the ground. Still, it is true that most of the water in rivers comes directly from runoff from the land surface, which is defined as
surface runoff.
Surface runoff is easy to see as it flows down a concrete driveway to the curb and then into a storm sewer. In a natural

setting, more of the rainfall penetrates the ground to become part of the groundwater. When the ground becomes saturated, the excess rainfall becomes surface runoff. It begins to flow downhill along channels as it moves into larger creeks, streams, and rivers, beginning its journey to the ocean.
As with all parts of the water cycle, surface runoff varies according to geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors (such as amount of precipitation) and the physical geology of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other twothirds is evaporated or soaks into groundwater.
You see water all around you every day as lakes, rivers, ice, rain and snow. There are also vast amounts of water that are unseen, in and under the ground. Even though groundwater is unseen, it is moving below your feet right now. As part of the water cycle, groundwater is a major contributor to many streams and rivers and has a strong influence on river and wetlands. People have been using groundwater for thousands of years and continue to use it today, largely for drinking water and irrigation.
Life on Earth depends on groundwater just as it does on surface water.
Anywhere in the world, a portion of the water that falls onto the land as rain and snow moves into (infiltrates) the subsurface soil and rock. Some water that infiltrates into the ground will remain in the shallow soil layer, where it will gradually move through the soil and subsurface material.
Eventually it might enter a stream by seepage into the stream bank. Some of the water may infiltrate deeper, recharging ground-water aquifers, the underground layers where water is stored. If the aquifers are shallow or porous enough to allow water to move freely through it, people can drill wells into the aquifer and use the water for their purposes. Water may travel long distances or remain in ground-water storage for long periods before returning to the surface or seeping into other water bodies, such as streams and the oceans.
Evaporation is the process by which water changes from a liquid to a gas (water vapor).
Evaporation is the primary pathway that moves water from the liquid state back into the water cycle as atmospheric water vapor. Heat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that keep water molecules together in a liquid state. The process of evaporation removes heat from the environment, which is why water evaporating from your skin cools you.
Evaporation from the oceans is the primary source for the surface-to-atmosphere portion of the water cycle. After all, the large surface area of the oceans (over 70 percent of the Earth's surface is covered by the oceans) provides the opportunity for such large-scale evaporation to occur. On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This varies geographically, though. Evaporation is more common over the oceans than precipitation; while over the land, precipitation often exceeds evaporation. Once evaporated, a water molecule spends about 10 days in the air.

Most of the water that evaporates from the oceans falls back into the oceans as precipitation. About 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation.
Studies have shown that the oceans, seas, lakes, and rivers provide nearly
90 percent of the moisture in our atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration.
Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves. Plants put down roots into the soil to draw water and nutrients up into the stems and leaves. Plant transpiration is pretty much an invisible process, since the water is evaporating from the leaf surfaces. Just because you can't see the water doesn't mean it is not being put into the air, though. During a growing season, a leaf will transpire many times more water than its own weight; a large oak tree can transpire 40,000 gallons (151,000 liters) per year into the atmosphere.