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The water cycle
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 courtesy Neal Herbert/
Yellowstone National Park.
Viewed from space, one of the most striking features of our home planet is the water. In
both liquid and frozen forms, it covers approximately 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 rest of the planets in our solar system. In particular, water appears to be a
necessary ingredient for the development and nourishment of life.
Substances can be found in three states, either as gas, liquid or solid. The state depends
on the properties of the 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 the
relatively small range of air temperatures and pressures that exist at the Earth’s surface.
In all, the Earth’s water content is about 1.39 billion cubic kilometers (331 million cubic
miles), with the bulk of it, about 96.5 percent, being in the 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 a
thousandth of 1 percent of the water on Earth exists as water vapor in the atmosphere.
By NASA.gov, adapted by Newsela staff on 12.15.16
Word Count 965
This article is available at 5 reading levels at https://newsela.com. 1
Water Vapor Drives The Earth's Weather And Climate
Despite its small amount, this water vapor has a huge influence on the planet. Water vapor
is a major driver of the Earth’s weather and climate as it travels around the globe,
transporting heat with it. The heat is obtained by water molecules as they switch from
liquid or solid to vapor; the heat is released when the molecules condense from vapor
back to liquid or solid form, creating cloud droplets and various forms of precipitation.
For human needs, the amount of freshwater that exists in lakes, rivers, groundwater and
frozen as snow and ice is particularly important for drinking and agriculture.
The water, or hydrologic, cycle describes the movement of water as water molecules make
their way from the Earth’s surface to the atmosphere and back again. This gigantic system,
powered by energy from the sun, is a continuous exchange of moisture between the
oceans, the atmosphere and the land.
Evaporation Provides Most Of Our Atmosphere's Moisture
Studies have revealed that evaporation — the process by which water changes from a
liquid to a gas — from oceans, seas and other bodies of water (lakes, rivers, streams)
provides nearly 90 percent of the moisture in our atmosphere. Most of the remaining 10
percent found in the atmosphere 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.
This article is available at 5 reading levels at https://newsela.com. 2
In addition, a very small portion of water vapor enters the atmosphere through sublimation,
the process by which water changes directly from a solid (ice or snow) to a gas. The
gradual shrinking of snow banks in cases when the temperature remains below freezing
results from sublimation. Evaporation, transpiration and sublimation, plus volcanic
emissions, account for almost all the natural water vapor in the atmosphere.
After the water enters the lower atmosphere, rising air currents carry it upward where the
air is cooler. In the cool air, water vapor is more likely to condense from a gas to a liquid to
form as cloud droplets. Cloud droplets can grow and become rain, snow, sleet, freezing
rain or hail. Precipitation is the primary mechanism for transporting water from the
atmosphere back to the Earth’s surface.
When water falls back to the land surface, it can then follow various paths. Some of it
evaporates, returning to the atmosphere, while 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 Water Is Returned Through Runoff
Water continually evaporates, condenses and precipitates, and 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, evaporation equals precipitation, which means that the total amount of
water vapor in the atmosphere remains about the same over time. However, over the
continents, precipitation routinely exceeds evaporation, and conversely, over the oceans,
evaporation exceeds precipitation.
In the case of the oceans, the continual excess of evaporation versus precipitation 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) over one century. 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 than the amount evaporating from it. A
primary cause for this increase is the melting of ice sheets and glaciers.
Throughout the hydrologic cycle, water molecules can take an immense variety of routes
and branching trails that lead them repeatedly through the three phases. For instance, the
water molecules 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.
This article is available at 5 reading levels at https://newsela.com. 3
1 Read the paragraph from the introduction [paragraphs 1-3].
Substances can be found in three states, gas, liquid or solid. The state
depends on the properties of the 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 the relatively small range of
air temperatures and pressures that exist at the Earth’s surface.
Which idea is BEST supported by the information in this paragraph?
(A) Water's phases cannot exist anywhere but within the range of temperatures
at Earth's surface.
(B) Water's properties make it unique among the substances found on Earth's
(C) Water has adapted to Earth's mild temperature and pressure systems over
billions of years.
(D) Water is the only substance that has three different phases: gas, liquid and
2 Read the selection from the section "Ocean Water Is Returned Through Runoff."
Overall, evaporation equals precipitation, which means that the total
amount of water vapor in the atmosphere remains about the same over
time. However, over the continents, precipitation routinely exceeds
evaporation, and conversely, over the oceans, evaporation exceeds
Which of the following can be inferred from this selection?
(A) Different phases of the water cycle usually balance the flow and
replenishment of water.
(B) There is less water on Earth now because evaporation frequently exceeds
(C) Precipitation and evaporation occur in equal amounts over oceans and
(D) Condensation occurs in lesser amounts than evaporation and precipitation.
This article is available at 5 reading levels at https://newsela.com. 4
3 How does the article develop the idea that the water cycle is essential to Earth as we know it?
(A) by detailing how much of Earth is affected by and composed of water in its
(B) by depicting the specific impact of global warming on each phase of the
(C) by discussing how humans can conserve and purify water in its liquid form
(D) by describing the relationship between high amounts of rainfall and good
4 Which of the following statements BEST describes the relationship between water vapor and
(A) As water molecules vaporize, they gradually lower the temperature in the air;
this is the primary way cold air is transported from the atmosphere back to
(B) As water molecules vaporize, they rise to warmer air pockets; when
molecules reach warmer air, they condense into rain, snow, wind or hail.
(C) As water molecules vaporize, they release and lose heat; this causes them
to drop into the lower atmosphere, where they work to gain new heat for
(D) As water molecules vaporize, they gain and transport heat; when molecules
condense to liquid or solid form, they release heat and return to Earth as
This article is available at 5 reading levels at https://newsela.com. 5