Water cycle - Credo Reference

Definition: hydrological cycle (water cycle) from The Penguin Dictionary of Science

The circulation of water around the surface of the Earth by evaporation, condensation, precipitation and the flow of liquid water on and under the surface.

Summary Article: The Water Cycle

from A Student Guide to Climate and Weather

The water cycle, frequently referred to as the hydrologic cycle, is a continuous movement of water on, above, and below the surface of the earth. It has no beginning or end. Water can be found in different states: water vapor, liquid, dew, ice, or cloud. The total amount of water on earth has remained constant over time. More than 97 percent of it is in the earth's oceans, more than 2 percent is stored in ice sheets and glaciers, and most of the remaining 1 percent is in lakes and rivers. The whole water cycle involves only a tiny fraction of earth's water, not more than 1 percent of the whole, yet the total volume of water in that 1 percent is huge: it has been estimated at more than 20 times the total amount of water in all the Great Lakes. A major factor in water's influence over weather and climate relates to the energy exchanges that occur when it changes state. When water changes to water vapor, for example, it extracts energy from its environment, making the environment cooler while water vapor gets warmer. When water vapor changes back to water, this energy, known as latent energy, is released back into the environment, making it warmer. Similar exchanges of latent energy occur in other transformations.

Water Ice and Water Vapor

Water can exist in three different states: liquid as water, solid as ice, and gaseous as water vapor. Each has its own properties. Furthermore, changes from one state to another require the transfer of heat energy. One of the simplest examples of differences in properties can be seen in the density of water compared with ice. The former is denser than ice and hence, once a pond freezes and ice is formed, the ice, because it is less dense, floats on the surface.The terms used to describe the changes that occur are as follows: ice melts to form water, water freezes to form ice, water evaporates to form water vapor, and water vapor, when it cools, condenses into water. The terms used for direct changes from solid to vapor and vice versa are less familiar: sublimation is the term used when ice changes directly into water vapor without first becoming water, and deposition is used to describe changes from water vapor to solid ice crystals without first becoming water.

Latent heat is often described as “hidden heat,” and it comes from the motions of molecules as a substance changes its state. It is an extremely important aspect of our environment because it operates at different scales, from evaporating a cup of water into water vapor to moving thousands of tons of water from equatorial regions to polar ones. Warm winds blowing across an ocean can evaporate huge volumes of water into water vapor and, in the process, draw heat from the ocean and store it as latent heat in the water vapor. Later, by condensation, the water vapor may be released in colder places, warming them by releasing its latent heat.

Figure 4-1 The global water cycle. (ABC-CLIO)

In fact, any change of state with water involves energy exchanges. Water can go from solid ice directly to water vapor and also from water vapor directly to ice or ice pellets or overnight frost. Latent heat is transferred in all of these. From solid to liquid, liquid to water vapor, or solid to water vapor, heat is withdrawn from the surrounding environment and stored as latent heat in the molecules of the new state. When changes go the other way, this heat is released. The water cycle is often seen as having two parts. It starts as the transfer of water from the earth into the atmosphere as water vapor. With its added warmth, this water vapor moves into the atmosphere and is carried globally by wind systems, thereby distributing water vapor around the world. The second part begins with cooling as the humid air reaches its dew point and precipitation begins. Rivers and underground flows of ground-water are the agencies of transportation to carry the water back to its main source, the oceans.

THE WATER CYCLE BEGINS

PRECIPITATION STORAGE AND RUNOFF

INTERFERENCE WITH THE WATER CYCLE

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