hydrosphere makes up about 70% of the Earth’s surface. However,
that water does not just sit around, inert. The location and
state of water on the earth is constantly changing in a fairly regular
pattern: this is called the water cycle.
*The sun drives the majority of the water cycle. It does so by
heating exposed water (in oceans, lakes, or rivers) so that it turns
into a gas. This is called evaporation. As water
evaporates, the vapour gathers in the air.
*Depending on the condition of the air, the amount of water it can hold
varies. Warm air can hold more molecules of water vapour than
cold air, which is why the summer is (or can be) so much more humid
than the winter.
*Air eventually cools, either by rising too high, by circulating to
latitudes farther from the equator, or simply by experiencing the
cooling effect of darkness at night. As the air grows cooler, it
approaches the dew point, the point at which a given volume of air
becomes too cold to contain all the water vapour it holds.
*As moist air approaches the dew point, the water molecules begin to
turn from vapour into liquid form. This is known as
condensation. This forms clouds or may become visible as fog
(which is why fog is seen most often in cool, damp areas).
*When moist air reaches its dew point, it may deposit some of its
moisture as dew, or, if it falls far below the dew point, the water
that condenses as clouds will be released as precipitation—the name for
water in any form that falls to the ground as a liquid. Depending
on the temperature, this may be rain, snow, or sleet.
*When air that is rich in moisture passes over mountains (due to wind
or to the rotation of the earth), it cools off as it rises over
them. As the air gets cooler, clouds form, and eventually rain
falls on the mountains. This means that mountains typically see
more rain on the windward side (usually the west side) than the leeward
side. The side that gets less rain is said to be in a rain
shadow. This is also known as the orographic effect.
*In some cases, particularly with very high mountains, the orographic
effect creates a desert in the rain shadow area and particularly wet
and (usually) fertile areas on the rainy side of the mountains.
In the Appalachian Mountains this is not a big deal, but in other parts
of the world it significantly affects local climates.
*Eventually, the water that returns to the Earth through precipitation
evaporates again, and the water cycle continues. The amount of
water in the global cycle stays more or less constant, although it can
be changed somewhat as water becomes locked up in glaciers and ice
caps, or released from them.
*Water is also transferred from the Earth to the atmosphere through
transpiration, the release of water vapour from trees and plants.
In some cases, this carries a significant amount of water inland across
vast jungles, particularly the rain forests.
*97% of Earth’s water is in our four oceans: the Pacific, the
Atlantic, the Indian, and the Arctic. The Pacific Ocean (largest
of the four) covers more of the Earth’s surface than all the continents
*Oceans tend to be subdivided into smaller regions, particularly seas
(areas largely, but not entirely, separated from the main body of the
ocean by land), and by gulfs and bays (areas partly surrounded by land).
*About 3% of the hydrosphere is fresh water, but most of that is locked
up in glaciers; over ½ of 1% of our fresh water is
underground. Less than ½ of 1% of the Earth’s potable
water is available in lakes, streams, and rivers.
*Because so little fresh water exists, many places with limited access
to fresh water, especially in the Middle East, have experimented with
desalination of ocean water. One desalination treatment boils the
water to separate the water from the salt, and then condenses the water
fresh. Overall, this is expensive and inefficient, but it is used in
some particularly dry area.
*Streams and rivers begin in glacial run-off or in springs. They
may flow together to form larger rivers, but they almost always flow
into the ocean. A few flow into inland lakes, however. Any
place a river flows into another body of water is called the river’s
mouth (although if a river simply flows into another river, it may be
called a confluence).
*In any given part of a continent, water will flow towards one major
body of water. This is because continents are divided by the
highest points, and water flows away from these points. For
example, the United States has its Eastern Continental Divide at the
crest of the Appalachian Mountains. Water on the east of the
Appalachians flows into the Atlantic Ocean. Water on the west
flows into the Gulf of Mexico. The Rocky Mountains form the
western Continental Divide.
*All the rivers that flow into a particular river are known as its watershed.
*Rivers can be useful for drinking water, for irrigation, for fishing, for drawing boundaries, and, if large enough, for trade.
*People also get useful water form groundwater. This is water
that lies below the Earth’s surface. Most comes from rain and
melted snow that soaks deep into the Earth until it hits a layer of
rock it cannot seep through. A layer of more porous rock that can
hold a lot of groundwater is called an aquifer. Some of these
contain water that has accumulated over centuries of millennia.
*Groundwater and aquifers are very important sources of water when they
are tapped for wells, especially in areas that do not have a great deal
rainfall today. However, some aquifers, particularly in the
American west, are being drained by man faster than they are being
replenished by nature.