water droplets or ice crystals floating
together in the air.
The air always contains water vapor—
water in the form of gas—which is invisible.
The amount of water vapor that air
can hold depends on the air’s temperature.
The cooler the air, the less water it
can hold. When air cools, some of the
water vapor condenses, or forms visible
water droplets. The droplets form
around tiny particles in the air, such as
dust or sea salt. Near the ground, the
condensed water vapor becomes fog. Up
in the sky, it forms clouds.
Air cools as it rises, so clouds form when
warm, moist air rises. Air is forced
upward when it blows against a moun-
Different types of clouds form at different heights.
BRITANNICA STUDENT ENCYCLOPEDIA Cloud 143
tainside. Air is also forced to rise when
the land gets very hot. Sometimes clouds
form when a cold air mass meets a
warm, moist air mass. The warm air rises
up over the cold air, causing the warm
air to cool and clouds to form. Once
clouds have formed, they will remain
until the air is warmed or rain falls.
Meteorologists (people who study
weather) classify clouds mainly by their
appearance. The three main types of
clouds are cirrus, cumulus, and stratus.
Cirrus clouds are high, thin clouds made
up of ice crystals. Cumulus clouds are
puffy clouds that are often piled up like
a mountain. Stratus clouds are layered.
Scientists combine these terms with
others—including “nimbus,” meaning
rain—to describe 10 different cloud
types. For example, cumulonimbus
clouds are huge piled-up clouds that
produce thunderstorms.
Coal
The major source of fuel throughout the
world is coal. Coal is a black or brown
rock that, when burned, releases energy
in the form of heat. One of coal’s main
uses is the production of electricity.
How Coal Formed
The coal used today began forming
about 300 million years ago. At that
time swamps with giant ferns and other
plants were common on Earth. When
these plants died, they sank in the water.
They did not completely decay, or break
down. Instead they formed a substance
called peat. Over time layers of rock and
other materials built up over the peat.
Heat and pressure hardened the peat
into coal.
Forms and Uses of Coal
Coal has several different forms. Anthracite
is hard coal. It produces the greatest
amount of heat and is used mainly for
heating homes. Bituminous coal is soft.
Power plants use it to produce electricity.
Bituminous coal is also used in steel
mills to make coke, another kind of fuel.
Lignite, the softest coal, is used to produce
gas and electricity.
Coal Mining
People get coal by mining. In one process,
called strip mining, miners take
coal from just below the surface. In deep
mining, miners dig long tunnels to get
to coal that is buried far underground.
Mining coal can be dangerous for the
miners. Coal miners can be injured or
Huge piles of coal wait to be
shipped from a port in Poland.
In the summer,
small cumulus
clouds that
appear in the
morning often
turn into dark
cumulonimbus
clouds during
the day.
144 Coal BRITANNICA STUDENT ENCYCLOPEDIA
killed when mine roofs collapse. Gases
trapped in coal sometimes cause fires or
explosions. Many miners have health
problems from breathing in coal dust
over the years.
Disadvantages of Coal
Compared to other energy resources,
coal is inexpensive and plentiful. However,
coal has several disadvantages.
Burning coal can create acid rain, a dangerous
form of pollution. Burning coal
also increases the temperature of Earth’s
atmosphere, which may be harmful to
living things. Finally, because coal takes
so long to form, it is known as a nonrenewable
resource. Eventually, Earth’s
supply of coal will run out.
#More to explore
Energy • Fossil Fuel • Mining
• Pollution
Cobra
Cobras are poisonous snakes that flatten
their necks into the shape of a hood
when they sense danger. There are several
species, or types, of cobra. Not all of
them are closely related.
Cobras live in the southern parts of Asia
and Africa. Some types dwell in grasslands,
forests, or deserts. Others live in
trees or along the shores of lakes.
Cobras are usually a single color or spotted.
They are medium to large in size.
The king cobra of Asia is the world’s
longest poisonous snake. It can reach a
length of about 18 feet (5.5 meters).
When a cobra feels threatened, it raises
up its head and extends the ribs in its
neck. This stretches the skin to create
the hood. Some types of cobra have
markings on the hood.
Cobras hunt in the evening and at
night. Their prey includes mammals,
birds, snakes, lizards, frogs, and even
fish. When cobras bite, their short front
fangs deliver venom, or poison, into the
prey. Some types of cobra can also spit
a stream of venom at the eyes of
intruders.
Some types of cobra lay eggs. Others
give birth to live young. The king cobra
builds a nest for its eggs. It also guards
the eggs until they hatch.
#More to explore
Poison • Snake
Cochise
Cochise was a chief of the Chiricahua
band of the Apache people. In the 1860s
he led a fierce fight to hold onto Apache
A black-necked cobra spreads its neck to
create a hood.
BRITANNICA STUDENT ENCYCLOPEDIA Cochise 145
