The Water Cycle is the movement of water on, in and away from the Earth. The amount of water
in the water cycle is always the same – so you could be drinking the same water that the
dinosaurs drank!
This diagram shows the three main ways that water travels from one part of the earth to
another. Though there is no real beginning to the Water Cycle, let’s start by examining the water
in the oceans and lakes on Earth’s surface. The sun heats up the surface of this water and some
of the liquid evaporates into water vapor. This water vapor is then carried by air currents into
the earth’s atmosphere. Here, because of the cooler temperatures so high up, the water vapor
condenses into clouds. Air currents move these clouds all around the globe and the water inside
them returns back to earth in various forms of precipitation. Two of the most common forms of
precipitation are rain and snow. The rain or snow can do one of two things when it reaches
Earth. It can come to rest as a liquid and return to the lakes and oceans, or it can come to rest as
a solid, in the form of ice. Ice is often not permanent and will end up melting into lakes or
oceans as well. And here we are back where we started!
Evaporation is the process by which the sun heats up liquid water enough to transform it into
water gas or vapor. The heat must break molecular bonds within the water particles in order to
change the water’s state from liquid to gas. 90 percent of evaporation occurs from the lakes and
oceans on earth. (The other 10 percent occurs by transpiration which is evaporation of water
from plants and earth.) Over 70 percent of the Earth’s surface is covered by the oceans. Thus,
these oceans are the largest contributors to evaporation.
Condensation is the opposite process of evaporation. After water vapor has risen into the
atmosphere the cooler temperature transforms the gaseous form into a liquid form once again.
Condensation forms the clouds that we see every day.
Why is it colder higher up? The air near the surface of the earth is warmed by solar radiation
(heat from the sun). The air in the atmosphere is less dense, or has less particles within it. The
less particles there are, the less that they will collide with one another. These collisions are what
cause heat. Therefore if there are fewer collisions, it will be less hot up there.
Precipitation is the release of the liquid water stored in clouds back to the Earth’s surface. There
are many forms of precipitation including: rain, snow, sleet, hail, and freezing rain. The most
common forms of precipitation are rain and snow.
The amounts of precipitation vary greatly across the globe. This is why in some areas there are
deserts and in other areas there are rain forests.
The water cycle is responsible for much of the weather that we experience. It is even
responsible for the temperature of the Earth! 86 percent of evaporation occurs from oceans.
After evaporation occurs the temperature of the water’s surface is reduced by a process called
evaporative cooling. Evaporative cooling is what keeps the surface temperature of the Earth at
a comfortable level.
Sometime the Water Cycle can have negative effects as well. When the distribution of water is
uneven or if precipitation occurs in extreme and excessive levels, there are often disastrous
results. We often call these results natural disasters. Below are a few of the most recent and
common natural disasters:
Hurricane Katrina: In 2005, Hurricane Katrina hit the gulf coast of the United States. A hurricane
is a storm system which has a low pressure center (called the eye of the hurricane) and a
collection of thunderstorms in a collection around that center. Hurricanes gain their strength
from the heat released as moist air rises, which results in the condensation of water vapor. One
of the five deadliest hurricanes in the history of America, Hurricane Katrina devastated the gulf
region for months, even years, after its arrival. Almost all of the city of New Orleans, LA was
underwater as a result.
Desertification: Man’s actions can often affect the water cycle. In many dry areas of the globe,
because men over-graze, over-cultivate or redirect water for irrigation purposes they cause
drastic deterioration of the landscape. This deterioration is called desertification because
previously rich areas of land are slowly becoming deserts. Climate change, which can be
considered another result of man’s actions, also contributes to desertification.
Polar Ice Cap Melt: Another result of climate change is the melting of the polar ice caps. At some
point in the distant past water precipitated onto the Earth’s surface and froze on the poles of
our planet. Now as the global temperature is changing, the poles are warming up and much of
these huge ice caps are melting into the oceans. If enough of these caps melt ocean levels will
rise drastically and may even cause coastal cities and towns to be abandoned and relocated
further inland. Even though the amount of water remains the same, the transformation of it
from one state to another (in this case from solid to liquid) can significantly affect the world we