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Earth – THE WATER CYCLE – Background
From ice crystals to raindrops, water in its many forms is a remarkable substance that is essential
to all living things. Most organisms are made largely of water, many also live in or on the water,
and all require water to survive. The earth’s water is a finite resource but one that is constantly
on the move. As it cycles from the earth’s surface to the atmosphere and back, it changes form.
Water evaporates into vapor, condenses into droplets, and precipitates as rain or snow. Much of
the water that falls on land is transported back to the air by plants, a process that is only possible
because of water’s unique properties.
The earth’s water was formed billions of years ago as the planet was forming, though there is
still much to learn about how this came about. Water is an ancient resource and the water of
today is the same water in which the first living cells evolved, the same water that rained on
ancient tree ferns and dinosaurs, the same water that encased the land during the ice ages. Earth’s
water doesn’t increase or decrease, but it continually moves from the land to the oceans to the
atmosphere and back, in a continuous cycle powered by the sun.
The earth’s oceans cover three-quarters of the globe and contain 97% of all our water. A small
fraction of earth’s water, the remaining 3%, is fresh water. Of this, nearly two-thirds is frozen in
glaciers and icecaps mostly over Greenland and Antarctica. The remaining fresh water is found
in lakes and streams, in the soil or in the rocks beneath the soil, in plants and animals, and in the
atmosphere. Clouds are made of tiny water drops or ice crystals floating in the air. Water is also
present in the air around us as water vapor, an invisible gas.
What makes water move and change states? Heat from the sun is the energy source that powers
the water cycle. The sun warms the oceans and the land, causing winds to blow, water to
evaporate, and plants to grow. Heating water causes it to evaporate - water molecules escape
from the water’s surface to form gaseous water vapor in the air.
When warm, moisture-laden air meets colder air above the earth, the water vapor is cooled and it
condenses onto particles of dust, salt or pollen, to form water droplets or snow crystals
depending on the temperature. When water drops or snow crystals become too heavy to float in
the air they fall to earth as precipitation. Rainwater and snowmelt eventually find their way into
rivers and the ocean, or into the ground or perhaps into an animal or a plant. It may take a few
minutes or a thousand years before this water ends up in the air again.
Wind, which results from the sun’s unequal heating of the earth, is an aid to evaporation. As
anyone knows who uses a laundry line, a nice breeze will speed the process of drying as the air
picks up moisture from the clothes and carries it away. Plants are also an important part of the
water cycle, moving large quantities of water from the ground to the air.
An important property of water that helps it move through plants is the attraction of water
molecules for each other. Each molecule of water acts like a magnet with a slightly positive and
slightly negative end. This attraction is called cohesion and it is stronger in water than in many
other substances.
A result of cohesion is that water has a strong surface tension. Water seems to
have a thin skin over its surface. Picture a drop of water on a smooth
surface. Why is it rounded? Inside the drop, where all the water molecules
are surrounded by other water molecules, they feel roughly the same pull
in all directions (see diagram at right). However, at the surface, the
molecules feel a stronger pull from other water molecules inside the drop
than from outside the drop, where there is only air. The force of this internal
pull gives water a spherical or donut shape as it falls through air. On a surface,
water will ‘bead up’ and form a dome-shape, best seen when viewed from the side. When adding
water to a drop, it will hold together until the moment when the force of attraction between
molecules is overcome by the force of gravity pulling the water downwards. At that point, the
surface tension breaks and the water spreads out.
Because of water’s strong surface tension, anything that tries to go into the water must first push
apart the water molecules at the surface. Some things that would normally sink in water, like a
needle or a paper clip, can be made to float on the water if they are placed carefully on the
surface. The surface tension is holding them up. Similarly, some insects like water striders (and
even a few lizards and small mammals) can run across the surface of water. Soap or detergents
dramatically decrease the surface tension of water; that is how they help get things clean. If one
drops a little dish detergent into water where a paper clip is floating, the paper clip will
immediately sink. There isn’t enough surface tension anymore to hold it up.
In addition to the cohesion between water molecules, water can show an attraction for other
substances. This attractive force between unlike molecules is called ‘adhesion’ (think of
adhesive tape). Water shows a certain amount of adhesion for glass. When water is in contact
with glass, the water molecules will creep up the glass surface. If you look near the side of a
glass of water, you might see that the water right at the edge forms a curve upward toward the
glass. The water creeping up the edge of the glass by adhesion is dragging along some other
water molecules by cohesion. If the water is in a narrow tube (a ‘capillary’), the water will
actually rise up in the tube until the force of adhesion pulling the molecules up matches the force
of gravity pulling them down. This is called capillary action.
Capillary action is easy to see in absorbent materials like paper towels or cotton knits. These
materials are water-friendly and have tiny spaces between the fibers. Water molecules adhere to
them and then pull other molecules along by cohesion, moving upwards through the small
spaces. A similar effect explains the ability of plants to pull water up through their roots, into
their stems and out to their leaves. Plant roots, stems, and leaves contain cells that are like tiny
tubes. As water evaporates from the pores in the leaves, more water is drawn up through the
plant’s roots, a process called transpiration. In the warmer months of the year, trees move
hundreds of gallons of water from the soil to the air every day.
A unique characteristic of water is its change in density with temperature. In most other
substances, the solid form is heavier and denser than the liquid or gas. In water the situation is
different. Hot water is lighter than cold water, as expected. But frozen water, ice, is lighter than
water and floats at its surface. In fact, water is densest at 39°F. As a result, the oceans never
freeze completely, and many lakes freeze only partially. Aquatic creatures can continue to live
near the bottom through the winter. Were the reverse to be true, if ice were heavier than water, it
is thought that the oceans would fill with ice and life could not exist.
Though earth’s water is a limited resource, it is constantly in motion as it travels from place to
place in the water cycle. Heat from the sun, wind and weather, and transpiration by plants keeps
the water in circulation. A remarkable substance, water is vital to living things, fascinating to
study, and in its many forms, a source of great pleasure and fun.
Suggested Reading
Ball, Philip, Life's Matrix: A Biography of Water, University of California Press, 2001.
Berner, Elizabeth Kay and Robert A. Berner. Global Environment: Water, Air, and Geochemical
Cycles 2nd Ed. California-Princeton Fulfillment Services, 2012.
Browning, K.A., and R. J. Gurney (Editors), Global Energy and Water Cycles, Cambridge
University Press, 1999.
Skinner, Brian J., Stephen C. Porter and Jeffrey Park. The Dynamic Earth: An Introduction to
Physical Geology. New York: John Wiley and Sons, 2003.
http://www.whoi.edu/news-release/OriginEarthWater
http://water.usgs.gov/edu/watercycle-kids-adv.html
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