The water cycle describes how water is exchanged (cycled) through Earth's land, ocean, and atmosphere. Water always exists in all three places, and in many forms—as lakes and rivers, glaciers and ice sheets, oceans and seas, underground aquifers, and vapor in the air and clouds.
Evaporation, Condensation, and Precipitation
The water cycle consists of three major processes: evaporation, condensation, and precipitation.
Evaporation is the process of a liquid's surface changing to a gas. In the water cycle, liquid water (in the ocean, lakes, or rivers) evaporates and becomes water vapor.
Water vapor surrounds us, as an important part of the air we breathe. Water vapor is also an important greenhouse gas. Greenhouse gases such as water vapor and carbon dioxide insulate the Earth and keep the planet warm enough to maintain life as we know it.
The water cycle's evaporation process is driven by the sun. As the sun interacts with liquid water on the surface of the ocean, the water becomes an invisible gas (water vapor). Evaporation is also influenced by wind, temperature, and the density of the body of water.
Condensation is the process of a gas changing to a liquid. In the water cycle, water vapor in the atmosphere condenses and becomes liquid.
Condensation can happen high in the atmosphere or at ground level. Clouds form as water vapor condenses, or becomes more concentrated (dense). Water vapor condenses around tiny particles called cloud condensation nuclei (CCN). CCN can be specks of dust, salt, or pollutants. Clouds at ground level are called fog or mist.
Like evaporation, condensation is also influenced by the sun. As water vapor cools, it reaches its saturation limit, or dew point. Air pressure is also an important influence on the dew point of an area.
As was the case with evaporation and condensation, precipitation is also a process. Precipitation describes any liquid or solid water that falls to Earth as a result of condensation in the atmosphere. Precipitation includes rain, snow, and hail.
Fog is not precipitation. The water in fog does not condense sufficiently to precipitate, or liquefy and fall to Earth. Fog and mist are a part of the water cycle called suspensions: They are liquid water suspended in the atmosphere.
Precipitation is one of many ways water is cycled from the atmosphere to the Earth or ocean.
Evaporation, condensation, and precipitation are important parts of the water cycle. However, they are not the only ones.
Transpiration is another important part of the water cycle. Transpiration is the process of water vapor being released from plants and soil. Plants release water vapor through microscopic pores called stomata. The opening of stomata is strongly influenced by light, and so is often associated with the sun and the process of evaporation. Evapotranspiration is the combined components of evaporation and transpiration, and is sometimes used to evaluate the movement of water in the atmosphere.
States of Water
Through the water cycle, water continually circulates through three states: solid, liquid, and vapor.
Ice is solid water. Most of Earth's freshwater is ice, locked in massive glaciers, ice sheets, and ice caps.
As ice melts, it turns to liquid. The ocean, lakes, rivers, and underground aquifers all hold liquid water.
Water vapor is an invisible gas. Water vapor is not evenly distributed across the atmosphere. Above the ocean, water vapor is much more abundant, making up as much as four percent of the air. Above isolated deserts, it can be less than one percent.
The Water Cycle and Climate
The water cycle has a dramatic influence on Earth's climate and ecosystems.
Climate is all the weather conditions of an area, evaluated over a period of time. Two weather conditions that contribute to climate include humidity and temperature. These weather conditions are influenced by the water cycle.
Humidity is simply the amount of water vapor in the air. As water vapor is not evenly distributed by the water cycle, some regions experience higher humidity than others. This contributes to radically different climates. Islands or coastal regions, where water vapor makes up more of the atmosphere, are usually much more humid than inland regions, where water vapor is scarcer.
A region's temperature also relies on the water cycle. Through the water cycle, heat is exchanged and temperatures fluctuate. As water evaporates, for example, it absorbs energy and cools the local environment. As water condenses, it releases energy and warms the local environment.
The Water Cycle and the Landscape
As glaciers slowly expand across a landscape, they can carve away entire valleys, create mountain peaks, and leave behind rubble as big as boulders. Yosemite Valley, part of Yosemite National Park in the U.S. state of California, is a glacial valley. The famous Matterhorn, a peak on the Alps between Switzerland and Italy, was carved as glaciers collided and squeezed up the earth between them. Canada's "Big Rock" is one of the world's largest "glacial erratics," boulders left behind as a glacier advances or retreats.
Glacial melt can also create landforms. The Great Lakes, for example, are part of the landscape of the Midwest of the United States and Canada. The Great Lakes were created as an enormous ice sheet melted and retreated, leaving liquid pools.
The process of erosion and the movement of runoff also create varied landscapes across the Earth's surface. Erosion is the process by which earth is worn away by liquid water, wind, or ice.
Erosion can include the movement of runoff. The flow of water can help carve enormous canyons, for example. These canyons can be carved by rivers on high plateaus (such as the Grand Canyon, on the Colorado Plateau in the U.S. state of Arizona). They can also be carved by currents deep in the ocean (such as the Monterey Canyon, in the Pacific Ocean off the coast of the U.S. state of California).
Reservoirs and Residence Time
Reservoirs are simply where water exists at any point in the water cycle. An underground aquifer can store liquid water, for example. The ocean is a reservoir. Ice sheets are reservoirs. The atmosphere itself is a reservoir of water vapor.
Residence time is the amount of time a water molecule spends in one reservoir. For instance, the residence time of "fossil water," ancient groundwater reservoirs, can be thousands of years.
Residence time for water in the Antarctic ice sheet is about 17,000 years. That means that a molecule of water will stay as ice for about that amount of time.
The residence time for water in the ocean is much shorter—about 3,200 years.
The residence time of water in the atmosphere is the shortest of all—about nine days.
Calculating residence time can be an important tool for developers and engineers. Engineers may consult a reservoir's residence time when evaluating how quickly a pollutant will spread through the reservoir, for instance. Residence time may also influence how communities use an aquifer.
Breaking the Cycle
The water cycle can change. Glacial retreat is the process in which glaciers melt faster than their ice can be replaced by precipitation. Glacial retreat limits the amount of fresh water available on Earth. We are experiencing the fastest rate of glacial retreat in recorded history.
force pressed on an object by air or atmosphere.
an underground layer of rock or earth which holds groundwater.
(atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.
deep, narrow valley with steep sides.
all weather conditions for a given location over a period of time.
visible mass of tiny water droplets or ice crystals in Earth's atmosphere.
cloud condensation nuclei (CCN)
microscopic bits of clay, salt, or solid pollutant around which water vapor condenses in clouds to form raindrops.
process by which water vapor becomes liquid.
steady, predictable flow of fluid within a larger body of that fluid.
number of things of one kind in a given area.
temperature at which water in the air condenses to form water droplets on objects near the ground.
to spread out or scatter.
community and interactions of living and nonliving things in an area.
act in which earth is worn away, often by water, wind, or ice.
process by which liquid water becomes water vapor.
loss of water from the Earth's soil by evaporation into the atmosphere and transpiration by plants.
to constantly change back and forth.
clouds at ground level.
having to do with a habitat or ecosystem of a lake, river, or spring.
rock, deposited by a glacier, that differs from the geology and landscape in which it is found.
depression in the earth created by a moving glacier.
mass of ice that moves slowly over land.
largest freshwater bodies in the world, located in the United States and Canada. Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior make up the Great Lakes.
gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.
water found in an aquifer.
amount of water vapor in the air.
water in its solid form.
area of fewer than 50,000 square kilometers (19,000 square miles) covered by ice.
thick layer of glacial ice that covers a large area of land.
to cover with material to prevent the escape of energy (such as heat) or sound.
body of water surrounded by land.
specific natural feature on the Earth's surface.
the geographic features of a region.
area of the United States consisting of the following states: Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.
study of the natural features and processes of the Earth.
large region that is higher than the surrounding area and relatively flat.
chemical or other substance that harms a natural resource.
all forms in which water falls to Earth from the atmosphere.
amount of time a water molecule spends in one place in the water cycle.
large stream of flowing fresh water.
overflow of fluid from a farm or industrial factory.
water supplied by snow.
(singular: stoma) tiny openings on the surface of leaves that control the exchange of gases in a plant.
degree of hotness or coldness measured by a thermometer with a numerical scale.
evaporation of water from plants.
depression in the Earth between hills.
visible liquid suspended in the air, such as fog.
movement of water between atmosphere, land, and ocean.
state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.