The water cycle
By NASA.gov, adapted by Newsela staff on 12.15.16
Word Count 829
TOP: Water is the only common substance that can exist naturally as a gas, liquid or solid at the relatively small range of
temperatures and pressures found on the Earth’s surface. Sometimes, all three states are even present in the same time
and place, such as this wintertime eruption of a geyser in Yellowstone National Park. Photo: Photo courtesy Neal Herbert/
Yellowstone National Park.
Water covers as much as 75 percent of the Earth’s surface. Geologic evidence suggests
that large amounts of water have likely flowed on Earth for the past 3.8 billion years —
most of its existence. Water is a vital substance that sets the Earth apart from the other
planets in our solar system. In particular, it appears to be a necessary ingredient for the
development and nourishment of life.
Substances can be found in one of three states: gas, liquid or solid. The state depends on
the kind of substance, the temperature and the pressure at which it is found. Water is the
only known substance that can naturally exist in all three states within a relatively small
range of temperatures and pressures. For this reason, it is easier for water to change state.
On Earth there are about 1.39 billion cubic kilometers (331 million cubic miles) of water.
About 96.5 percent of it is constituted by global oceans. As for the rest, approximately 1.7
percent is stored in the polar ice caps, glaciers and permanent snow. Another 1.7 percent
is stored in groundwater, lakes, rivers, streams and soil. Only 0.001 percent of the water on
Earth exists as water vapor in the atmosphere.
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Water Vapor Has A Major Influence On The Planet
Despite its small amount, this water vapor has a huge influence on the planet. Water vapor
is a major driver of the Earth’s climate, since it travels and transports heat around the
globe. This heat is obtained when water switches from liquid or solid to vapor; the heat is
released when water condenses from vapor back to liquid or solid, creating cloud
droplets. This results in freshwater, which is important for agriculture and for drinking.
Freshwater accumulates in lakes, rivers, groundwater and is frozen as snow and ice.
The water cycle, also called hydrologic cycle, describes the movement of water as it
travels. This gigantic system, powered by energy from the sun, is a continuous exchange
of moisture between the oceans, the atmosphere and the land.
When water changes from a liquid to a gas, the process is called evaporation. Studies
have revealed that evaporation from oceans, seas and other bodies of water provides
nearly 90 percent of the moisture in our atmosphere. Most of the remaining 10 percent is
released by plants, through a process called transpiration. Plants take in water through
their roots, then release it through small pores on the underside of their leaves. This is a
significant process; for example, a cornfield 1 acre in size can generate as much as 4,000
gallons of water every day.
Water Vapor Travels In An Ongoing Cycle
It is also possible for water to change directly from solid to gas, a process known as
sublimation. Evaporation, transpiration and sublimation account for almost all the natural
water vapor in the atmosphere.
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After the water enters the lower atmosphere, rising air currents carry it up where the air is
cooler. In the cool air, water vapor is more likely to condense from a gas to a liquid to form.
This results in rain, snow, sleet, freezing rain or hail. As a result, the condensed water falls
back from the atmosphere to the Earth’s surface.
Once on the surface, water can take various paths. Some of it evaporates, returning to the
atmosphere; some seeps into the ground as soil moisture or groundwater, or runs off into
rivers and streams. Almost all of the water eventually flows into the oceans or other bodies
of water, where the cycle continues. At different stages of the cycle, some of the water is
intercepted by humans or other life forms for drinking, washing, irrigating and a large
variety of other uses.
Ocean Warming Has Caused Over-Replenishing
Water continually evaporates, condenses and falls back to the surface on a global basis;
in a year, it is as if the entire amount of water in the air were removed and replenished
nearly 40 times. Overall, the same amount that evaporates then falls back to the earth.
However, more water tends to fall over continents, whereas more tends to evaporate over
the oceans.
In the case of the oceans, the continual evaporation would eventually leave the oceans
empty if they were not being replenished. Ocean water is returned largely through runoff
from the land areas. Over the past 100 years, oceans have in fact been over-replenished,
leading to a rise in sea level around the globe by approximately 17 centimeters (6.7
inches). One reason is that the warming of the oceans has caused water to expand and
increase in volume. On top of this, more water has been entering the ocean due to melting
ice sheets and glaciers.
Throughout the hydrologic cycle, water can take an immense variety of routes that leads it
repeatedly through the three phases. Water that once fell 100 years ago as rain on your
great-grandparents’ farmhouse in Iowa might now be falling as snow on your driveway in
California.
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Quiz
1
Which section of the article BEST highlights the idea that the water we have on Earth today is
the same water that has always been part of the hydrologic cycle?
(A)
Introduction [paragraphs 1-3]
(B)
"Water Vapor Has A Major Influence On The Planet"
(C)
"Water Vapor Travels In An Ongoing Cycle"
(D)
"Ocean Warming Has Caused Over-Replenishing"
2
Select the paragraph from the section "Water Vapor Travels In An Ongoing Cycle" that explains
how the water cycle influences the weather.
3
Which answer choice accurately describes how warmer ocean temperatures have affected the
water cycle?
4
(A)
Warmer ocean temperatures cause water to evaporate at a more rapid rate,
possibly causing oceans to eventually become empty.
(B)
Warmer ocean temperatures cause more solid water that was stored as ice
to become liquid, leading to a rise in sea levels.
(C)
Warmer ocean temperatures lead to more evaporation over land, eventually
causing heavier rain to fall over oceans and fill them too quickly.
(D)
Warmer ocean temperatures lead to more solid water stored as snow and
ice at the polar ice caps, leading to a drop in sea levels.
What is the MOST likely reason why the author included the information about plant
transpiration in the section "Water Vapor Has A Major Influence On The Planet"?
(A)
to show that plants play a significant role in Earth's water cycle
(B)
to show that plants generate the majority of Earth's water
(C)
to explain that corn is a valuable crop because it creates water
(D)
to explain that more farmland should be created because it produces water
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