If seasonal rains are important for crops to grow in a certain country and those rains don’t happen for two years in a row, what may happen to the people of that country2. If a drought
Trang 1Scott Foresman Science 5.8
Nonfi ction Draw Conclusions • Captions
• Sidebar
• Maps
• Glossary
Weather Patterns
ISBN 0-328-13939-4
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Scott Foresman Science 5.8
Nonfi ction Draw Conclusions • Captions
• Sidebar
• Maps
• Glossary
Weather Patterns
ISBN 0-328-13939-4
ì<(sk$m)=bdjdjc< +^-Ä-U-Ä-U
Trang 21 How can a region’s climate make it more
likely to suffer from droughts?
2 If seasonal rains are important for crops to
grow in a certain country and those rains don’t happen for two years in a row, what may happen to the people of that country?
3 Why do some scientists think that droughts
will happen more often in the future?
normally dry regions, causes drought in others, and causes extreme weather around the world Write to explain some of the effects of El Niño Use examples from the book to support your answers
5 Draw Conclusions Why should
water be conserved when a drought has already started?
What did you learn?
Extended Vocabulary
cover crops deforestation desertifi cation economic famine malnutrition overpopulation resources
terracing topsoil
Vocabulary
air mass
anemometer
barometer
climate
convection current
front
rain gauge
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3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 05
by Beth Parlikar
Trang 3Weather is made up of the conditions of the atmosphere
at a given moment, and it can change very quickly
Measurements of temperature, air pressure, wind speed and
direction, and precipitation are used to describe the weather
The atmosphere is made up of air, which is a mixture
of gases such as nitrogen and oxygen The atmosphere has
fi ve layers Air temperature changes as you go up through
the layers
Cool air is heavier than warm air because the gas
particles in cool air are closer together The differences
in air temperature cause air to move, and air movement
creates currents, which affect weather When cool air and
warm air come together, the cool air sinks and the warm air
rises This looping movement of air is called a convection
current These currents can create clouds, rain, and other
weather patterns
What You Already Know
2
When warm air
and cool air meet,
rain can occur.
An air mass is a huge body of air that has similar conditions throughout
Air masses move because
of winds and can be warm
or cold, wet or dry Fronts are boundaries between air masses Precipitation often happens when a front moves through an area
Meteorologists use patterns of weather changes from the past to predict future weather They use many instruments to measure weather conditions A barometer measures air pressure, an anemometer is used to determine wind speed, and a rain gauge measures how much rain has fallen
Climate is not the same as weather Climate is the average of weather conditions for an area over a number
of years
In this book you will read about a weather condition called drought You will learn how droughts affect environments and people
area affected by drought, Botswana
3
Trang 4What is drought? If you think it’s a period of time with
little or no rainfall, you’re partly right Droughts are long
periods of reduced precipitation that usually last for months
or even years Drought is actually a normal part of climate and
can occur anywhere on Earth, from deserts to rain forests
Depending upon its location, length, and severity, drought
can have many different effects on people and the environment
Agriculture is often affected during a drought; crops might
fail, and farm animals might die People have less food
to eat, and they might suffer from hunger,
malnutrition, or starvation
Introduction
Drought can cause crops to fail.
4
Drought can have many other effects Dry conditions can cause wildfi res and dust storms, and wild animals can die from lack of food and water as natural vegetation decreases
Water supplies are lower than usual during a drought, so people might be asked to conserve water If a drought lasts a long time and is severe, people may even abandon their homes and move to an area with more water Aid organizations provide food for drought sufferers.
5
Trang 5Why do droughts happen? Scientists have found that
droughts are usually a result of many factors Climate is
one thing that determines when and where droughts occur
Some parts of the world are more prone to droughts
because the weather varies a lot from year to year Some
regions get seasonal rains that are important for animals’
survival and for crops to grow If those rains don’t come,
the drought can kill animals and crops, and people may
even die Climate change can cause droughts to happen
more often and turn fertile land into desert
Global weather patterns can also help to cause droughts
Certain weather patterns happen periodically and can lead
to drought in different parts of the world
6
One such weather pattern that happens every few years is called El Niño This pattern is caused by a shift in the winds and ocean currents off the coast of South America The effects of this event can spread across the world, bringing droughts to normally wet regions and fl oods to dry ones
People can cause droughts to be more likely to happen
For example, poor farming practices, such as overgrazing and clear-cutting of trees for fi rewood, can cause the land
to lose moisture easily This can make a drought far worse than usual If many people move to an area that is prone to droughts, they may have problems The more people there are, the more water they need to survive If the rains don’t come, those people may run out of water quickly
Survival in a dry region can be very diffi cult.
7
Trang 6An area’s average weather conditions are called its
climate However, the weather at any one time can be very
different from the average For example, the summer in
California is usually hot and dry, but sometimes there are big
storms that bring rain Changes from normal conditions can
lead to droughts if precipitation is less than normal These
droughts can be devastating if a region needs seasonal rains
for crops to grow
Climates don’t always stay the same over time In fact,
they can change drastically and sometimes quickly For
example, sudden volcanic eruptions can block sunlight and
change the climate over a large area Other times, climate
changes slowly Sometimes the cause of slow climate change
is natural One natural cause of climate change is variation in
the strength of the Sun’s energy, which warms the Earth
People also cause climate change Most scientists believe
that a process called global warming is happening because
cars and factories make gases that warm up the atmosphere
Global warming is changing the patterns of the water cycle
Scientists think that land in the middle of continents will
have more droughts, and deserts will grow bigger
Climate
As an area becomes drier, some rivers may dry up entirely, as this former river in Argentina has.
Water is very scarce in some parts of India.
Trang 7Ethiopia is a country in
East Africa with a population
of sixty-seven million people
Of these, 90 percent are
subsistence farmers, meaning
that they make a living by
growing just enough food to eat
The soil is good, but there isn’t
much rainfall most of the year
The rains usually come between
June and September and are very
important for farming and for
the people’s survival
Case Study: Ethiopia 1981–1985
Most of Ethiopia, a country
in East Africa, suffered from drought for several years
in a row.
Ethiopia
Area worst affected by drought
AFRICA
Ethiopia
10
Beginning in 1981, the seasonal rains that are so important for survival in Ethiopia did not come In most parts of the country, all or almost all of the crops died, and people did not get enough to eat This continued through the years of 1982, 1983, and 1984, until the drought was so severe that eight million people were in danger of starving
to death
Finally the people of the world came to help, bringing food, medicines, and clothing But it was already too late for many: almost one million people starved to death
Thousands of others left Ethiopia and went to nearby countries in search of food and shelter
Since the terrible famine of 1981–1985, Ethiopia has suffered from two more major droughts Today the country
is trying to conserve water more carefully and educate its people about health, nutrition, and good farming practices
Millions of dollars were spent on food and medical aid for Ethiopians during the drought of 1981–1985.
11
Trang 8El Niño means “the child,” and it gained its name
because it arises around Christmas It is a weather pattern
that causes disruptive events every few years El Niño starts
out in the southern Pacifi c Ocean, where the winds usually
blow from east to west Warm water normally collects along
the eastern coast of Australia Meanwhile, currents of cold
water fl ow along the western coast of South America
During El Niño, the warm water current suddenly rushes
across the ocean from west to east The warm, moist air that
usually brings the monsoon rains to Southeast Asia is no
longer there, so the monsoons don’t come on time Across
the ocean, the desert along the coast of Peru receives heavy
rains The effects don’t stop there, though El Niño can affect
weather patterns from Florida to Africa!
Scientists aren’t sure what makes El Niño happen,
but they do know something about its effects The sudden
presence of warm, moist air over South America causes
convection currents in the atmosphere to shift These huge
currents affect weather throughout the world
El Niño
El Niño often causes drought
in the Amazon River region in Brazil, increasing the risk of forest fi res.
12
El Niño
Under normal conditions
in the South Pacifi c, warm water collects in the west, and cold water in the east
This makes the air warm and moist in Southeast Asia, while coastal Peru is
a desert.
During El Niño, a mass of warm water moves across the ocean from Australia and along the equator
to the coast of South America This brings rain to normally dry Peru and can affect the weather around the world.
Mexico
Mexico
Equator
NORTH AMERICA
SOUTH AMERICA
CENTRAL AMERICA
Pacifi c Ocean
SOUTHEAST ASIA
NORTH AMERICA
SOUTH AMERICA
CENTRAL AMERICA
Pacifi c Ocean
SOUTHEAST ASIA
Australia
warm currents
13
N E S W
N E S W
Trang 9The strongest El Niño of
the past century struck in 1997
It brought extreme weather that
devastated the countries of Central
America Droughts occurred
throughout the region, causing
many crops to fail Unusual
rains caused fl oods that also hurt
food production and endangered
people’s lives
There were outbreaks of
tropical diseases, which made
thousands of people very sick
The worst forest fi res on record
destroyed rain forests and
polluted the air as far away as
the eastern United States Water
levels in rivers and lakes fell so
low that the country of Honduras
declared a state of emergency
Case Study: Central America 1997–1998
El Niño caused drought,
fl ooding, and forest fi res, which overwhelmed Central American countries in 1997–1998.
NORTH AMERICA
SOUTH AMERICA CENTRAL AMERICA
Many farmers in Central America
lost their crops due to drought
caused by El Niño.
Gulf of Mexico
Pacifi c Ocean
14
Wildlife suffered from the severe weather as well
Fewer sea turtles hatched on beaches in Central America and Mexico because of high tides and drought Many coral reefs, which are made up
of tiny animals called coral polyps, died or were damaged because the water grew too hot Fishermen also reported that there were very few fi sh
to be found in the oceans
As the terrible effects of El Niño continued, thousands of people found themselves without enough food to eat Aid organizations from around the world stepped in to help, bringing food and other supplies for the people Unfortunately, relief for the people of Central America did not last long, as Hurricane Mitch devastated the region in 1998
Another drought struck in 2001, and this time three-quarters of a million people were in need of food aid The people of Central America suffered a great deal from these natural disasters When terrible events like these happen, it is important for people to work together to help everyone recover
International aid workers gave food
to thousands of hungry people.
15
Trang 10People’s actions can affect how bad droughts are and
how often they happen For instance, if too many people
decide to live in a region that is prone to droughts, they can
put a strain on land and water resources People need water
for cooking, washing, and growing food The more people
that live in an area, the more water they need The increasing
number of people in the world is also a problem The more
people there are on Earth, the more water is required to
meet their needs Overpopulation is therefore one factor
that can worsen droughts
Poor farming practices can also increase the risk of
drought In some areas farmers clear land so that they can
grow more crops, even if the land is not good for farming
In order to clear land, they burn or cut down forests This
makes the soil dry up easily and blow away or be washed
away by storms This soil erosion makes it very hard to
grow crops
Overpopulation and
Poor Farming Practices
The soil in deforested areas
often becomes hard and dry,
making it drought-prone and
diffi cult to farm.
16
Deforestation is also bad for drought prevention because trees help to control global warming by capturing some of the gases that cars and factories release
Animal grazing can also make the land dry If animals such as cows and goats are allowed to eat too much of the grass in an area, this can have the same effect as burning down forests Soil erosion and overgrazing can cause areas that used to have plant life to become deserts This process
is called desertifi cation
17