The water cycle describes how water is exchanged (cycled) through Earth's land, ocean, and atmosphere. Water always exists in all three phases, 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. Increasing amounts of greenhouse gases in the atmosphere also contribute to global warming.
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 is the case with evaporation and condensation, precipitation is 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.
Runoff, for instance, describes a variety of ways liquid water moves across land. Snowmelt, for example, is an important type of runoff produced as snow or glaciers melt and form streams or pools.
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
The water cycle also influences the physical geography of the Earth. Glacial melt and erosion caused by water are two of the ways the water cycle helps create Earth's physical features.
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.