The Water Cycle
The Atmosphere and Weather are Essential to the Water Cycle
By
Tiffany Means
You've
likely heard of the hydrologic (water) cycle before and know that it describes
how Earth's water journeys from the land to the sky, and back again.
But
what you may not know is why this process is so essential.
Of
the world's total water supply, 97% is salt water found in our oceans.
That
means that less than 3% of available water is freshwater and acceptable for our
use.
Think
that's a small amount?
Consider
that of that three percent, over 68% is frozen in ice and glaciers and 30% is
underground.
This
means that under 2% of freshwater is readily available to quench the needs of
everyone on Earth!
Are you beginning to see why the water cycle is so essential? Let's explore the steps.
All
Water Is Recycled Water
Here's
some food (or drink) for thought: every drop of rain that falls from the sky
isn't brand new, nor is every glass of water you drink.
They
have always been here on Earth, they've just been recycled and re-purposed,
thanks to the water cycle which includes 5 main processes:
o Evaporation
(including sublimation, transpiration)
o Condensation
o Precipitation
o Surface runoff
(including snowmelt and streamflow)
o Infiltration
(groundwater storage and eventual discharge)
Evaporation,
Transpiration, Sublimation Move Water Into the Air
Evaporation
is considered to be the first step of the water cycle.
In
it, water that's stored in our oceans, lakes, rivers, and streams absorbs heat
energy from the sun which turns it from a liquid into a gas called water vapor
(or steam).
Of
course, evaporation doesn't just happen over bodies of water -- it happens on
land too.
When
the sun heats the ground, water is evaporated from the top layer of soil -- a
process known as evapotranspiration.
Likewise,
any extra water that isn't used by plants and trees during photosynthesis is
evaporated from its leaves in a process called transpiration.
A
similar process happens when water that's frozen in glaciers, ice, and snow
converts directly into water vapor (without first turning into a liquid).
Called
sublimation, this happens when the air temperature is extremely low or when
high pressure is applied.
Condensation
Makes Clouds
Now
that water has vaporized, it is free to rise up into the atmosphere. The higher
it rises, the more heat it loses and the more it cools off.
Eventually,
the water vapor particles cool so much that they condense and turn back into
liquid water droplets. When enough of these droplets collect, they form clouds.
Precipitation
Moves Water From the Air to Land
As
winds move clouds around, clouds collide with other clouds and grow.
Once
they grow big enough, they fall out of the sky as precipitation (rain if the
atmosphere's temperatures are warm, or snow if its temperatures are 32° F or
colder).
From
here, precipitating water can take one of several paths:
If
it falls into the oceans and other bodies of water, its cycle has ended and it
is ready to begin again by evaporating yet again.
On
the other hand, if it falls on land, it continues on the water cycle journey
and must find its way back to the oceans.
So
that we can continue exploring the complete water cycle, let's assume option #2
-- that the water has fallen over land areas.
Ice
and Snow Move Water Very Slowly Along in the Water Cycle
The
precipitation that falls as snow over land accumulates, forming seasonal
snowpack (layers upon layers of snow that continually accumulates and becomes
packed down).
As
spring arrives and temperatures warm, these large amounts of snow thaw and
melt, leading to runoff and streamflow.
(Water
also stays frozen and stored in ice caps and glaciers for thousands of years!)
Runoff
and Streamflow Moves Water Downhill, Towards Oceans
Both
the water that melts from snow and that which falls on the land as rain flows
over the surface of the earth and downhill, due to gravity's pull.
This
process is known as runoff. (Runoff is hard to visualize, but you've probably
noticed it during heavy rain or a flash flood, as water flows hurriedly down
your driveway and into storm drains.)
Runoff
works like this: As water runs over the landscape, it displaces the ground's
top-most layer of soil.
This
displaced soil forms channels which the water then follows and feeds into the
nearest creeks, streams, and rivers. Because this water flows directly into
rivers and streams it is sometimes referred to as streamflow.
The
runoff and streamflow steps of the water cycle play a key part in making sure
water gets back into the oceans to keep the water cycle going.
How
so? Well, unless rivers are diverted or dammed up, all of them eventually empty
into the ocean!
Infiltration
Not
all of the water that precipitates ends up as runoff. Some of it soaks into the
ground -- a water cycle process known as infiltration.
At
this stage, the water is pure and drinkable.
Some
of the water that infiltrates the ground fills aquifers and other underground
stores.
Some
of this groundwater finds openings in the land surface and re-emerges as
freshwater springs.
And
still, some of it is absorbed by plant roots and ends up evapostranspiring from
leaves.
Those
amounts that stay close to the land surface, seep back into surface bodies of
water (lakes, oceans) where the cycle starts all over again.
Additional
Water Cycle Resources for Kids and Students
Thirsty
for more water cycle visualizations? Check out this student-friendly water
cycle diagram, courtesy of the U.S. Geological Survey.
And
don't miss this USGS interactive diagram available in three versions: beginner,
intermediate, and advanced.
Activities
for each of the water cycle's main processes can be found at the National
Weather Service's Jetstream School for Weather Hydrologic Cycle page.
The
USGS Water Science School has two great resources: The Water Cycle Summary and
Where is Earth's Water?
Tiffany Means
Meteorology Expert
Education
B.S., Atmospheric Sciences and Meteorology,
University of North Carolina
Introduction
Studied atmospheric sciences and meteorology
at the University of North Carolina
Former administrative assistant for the
National Oceanic and Atmospheric Administration
Member of the American Meteorological Society
Experience
Tiffany Means is a former writer for
ThoughtCo who contributed articles about weather for five years. She has
interned with the domestic and international weather departments at CNN,
written monthly climate reports for NOAA’s National Centers for Environmental
Prediction, and participated in a number of science outreach events, including
the Science Olympiad Competition. Means has personally experienced such weather
greats as the Blizzard of 1993 and the floods of Hurricane Francis (2004) and
Ivan (2004).
Education
Bachelor's degree in atmospheric sciences and
meteorology from the University of North Carolina at Asheville
ThoughtCo and Dotdash
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https://www.thoughtco.com/the-water-cycle-4049926
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