Download Notes - 7.8A

CATASTROPHIC EVENTS
reflect
Have you ever been through a severe storm? Even if you haven’t, you have probably
seen one on the news or in a movie. The effects of catastrophic weather events can be
devastating. (A catastrophe is an extremely destructive event.) Catastrophic weather events
can happen anywhere, but they are more likely to happen in some areas than in others.
Which areas of the country do you think are at highest risk for destructive weather events
such as floods, hurricanes, or tornadoes? Why do you think so?
Floods
Normally, when it rains or snow melts,
the ground absorbs the water or it runs
into reservoirs such as streams or ponds.
If more rain falls or snow melts than the
ground can absorb or streams and ponds
can hold, a flood can occur. This can
happen for several reasons. When the
ground is porous, or full of many small
holes, it has the ability to absorb a lot
of water. Loosely packed soil is more
porous than packed soil, so flooding is
more likely to occur where the ground has
been packed—for example, where heavy
machines have driven over the ground.
During a flood, water can rise to
dangerous levels. Flooding can damage
property and kill living things.
In addition, if it rains too hard, water will land on the ground faster than it can be absorbed.
This can result in flooding until the rain stops or the excess water runs into a reservoir or
evaporates. Flooding can also happen where the ground is not porous. For example, if the
ground is frozen or mostly made of rock, it will not absorb much water. Cities are also at risk
of flooding because much of the ground there is paved. The nonporous pavement blocks
water that would normally be absorbed by the ground.
During severe flooding events, the rush of water picks up a lot of sediment and debris. The
energy of the moving water and the layers of sediment carried by the water can do a lot of
damage. The moving water can uproot or bury plants, reshape the land, and destroy roads
and buildings. Even if floodwaters are not moving rapidly, they can rot wood over time and
spoil other materials. This can cause a lot of damage to homes and other buildings.
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CATASTROPHIC EVENTS
what do you think?
Look at the ground where you live. Is it mostly solid rock or soft soil? Is the soil loose or tightly
packed like clay? Is the ground mostly in its natural state, or is it mostly paved over? Does the
ground slope into a reservoir such as a pond or stream? After examining these factors, what
do you think is the risk of flooding during a period of intense rainfall in your area?
Which soil do you think will absorb more water? Which area
is at greater risk of flooding?
Hurricanes
Hurricanes are violent storms that form over warm ocean
waters. As some of the sea water evaporates, warm, wet
air rises into the atmosphere. (Convection is the process
by which warm fluids—including air—rise and cool fluids
sink.) This rising of warm, wet air removes some of the air
near the surface of the ocean water, creating an area of
low pressure. Warm air continues to rise as more and more
warm ocean water evaporates. This causes the atmospheric
pressure to become even lower beneath the rising air.
Meanwhile, new air rushes into the area beneath the rising
air. Because this area is warm, the new air warms and rises
too, continuing the cycle. As more air rushes in, the whole
system begins to spin.
When the rising air gets high enough in the atmosphere,
it cools down. The moisture in this air forms clouds as it
cools. The result is a massive, spinning cloud with intense
winds and rainfall. The center of this spinning storm is called
the eye.
In this photograph of a
hurricane, the eye is a
dark spot in the center of
the storm. Although winds
are calm in the eye, the
area surrounding the eye
is the most intense part
of a hurricane.
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CATASTROPHIC EVENTS
Hurricanes form over water but often move onto the land. On land, a hurricane is no longer
powered by evaporating sea water, so it slowly weakens. Before the hurricane weakens,
however, it may cause significant damage to the area over which it passes. The intense
winds and rainfall can damage homes and other buildings and knock over power lines
and trees. The winds and the low pressure can also affect sea water causing large waves,
called storm surges, to flood the coast. These surges can cause erosion—the carrying
away of sand from the beach to other locations. As the beach erodes, it becomes narrower
and more vulnerable to future hurricanes and surges.
Classifying Hurricanes
Scientists classify hurricanes by their wind speed and the amount of damage they cause.
This categorization system is called the Saffir-Simpson Hurricane Wind Scale. The SaffirSimpson scale breaks hurricanes into five categories:
Wind
Speeds
74–95 miles
per hour
Category
How Destructive?
Category 1 hurricanes can be very dangerous. They can
cause minor damage to buildings, knock down power lines,
and uproot small trees with shallow roots.
96–110 miles Category 2 hurricanes can be extremely dangerous. They
per hour
can cause moderate damage to buildings, knock down
power lines, and uproot larger trees.
111–129
Category 3 hurricanes can be devastating, causing major
miles per
damage to buildings. They can uproot many trees and cause
hour
lengthy power outages even after the storm passes.
130–156
Category 4 hurricanes can be catastrophic; even well-built
miles per
houses may be ripped apart, and most trees will be broken
hour
or uprooted. It may be months before people can return to
the area.
Greater than Category 5 hurricanes can be catastrophic, causing many
156 miles
buildings to completely collapse and breaking or uprooting
per hour
nearly all trees. It may be months before people can return
to the area.
1
2
3
4
5
look out!
Hurricanes generally do the most damage in tropical areas near where the storm forms.
However, hurricanes can damage areas far from where they form. After a hurricane forms,
it can move up or down the coast into cooler regions. Sometimes the hurricane will move
over land. This can cause flooding and wind damage to areas where people are not
generally prepared for hurricanes because they are not used to them.
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CATASTROPHIC EVENTS
Tornadoes
Tornadoes are spinning columns of air that are smaller than
hurricanes. However, tornadoes may be more violent. This
is because many tornadoes have stronger wind speeds than
hurricanes. Unlike hurricanes, which form over tropical seas,
tornadoes can form all over the world.
Scientists are still working to fully understand how tornadoes
form. Tornadoes generally start as thunderstorms. Winds along the ground are slower due
to friction along Earth’s surface. Winds in the clouds move faster due to less friction; they
are also moister. If slower winds along the ground move in the opposite direction of faster,
moister air in storm clouds, the air may begin to rotate when the winds meet. Scientists
think this can create a tornado—a long funnel of swirling air extending from the storm cloud
to the ground. Scientists are not sure why these conditions do not always create tornadoes.
They hope that continued research and advances in technology will lead to a better
understanding of how tornadoes form.
Classifying Tornadoes
Tornadoes are categorized by their wind speeds and the amount of damage they cause.
This categorization system is called the Fujita scale. The Fujita scale breaks tornadoes into
six categories.
Category
F0
F1
F2
F3
F4
F5
Wind
Speeds
40–72 miles
per hour
73–112 miles
per hour
113–157
miles per
hour
158–206
miles per
hour
207–260
miles per
hour
261–318
miles per
hour
How Destructive?
These relatively weak tornadoes cause minor damage to
homes, small trees, and road signs.
These moderate tornadoes may blow roofs off houses,
overturn mobile homes, and blow vehicles off the roads.
These significant tornadoes can destroy mobile homes,
knock over train cars, and uproot large trees. Winds may
shoot small objects through the air like bullets.
These severe tornadoes can rip apart even strongly built
houses and throw large vehicles through the air. Winds may
uproot entire forests.
These devastating tornadoes may destroy or blow away
even very strong buildings. Winds may pick up and shoot
large objects through the air like missiles.
These tornadoes cause incredible damage to anything in
their paths. Winds may rip houses from their foundations
and throw them far away. Winds may throw cars over
100 yards through the air and rip the bark off trees.
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CATASTROPHIC EVENTS
Career Corner: Storm Chaser
Scientists are continually trying to
understand more about catastrophic
storms. This allows them to better predict
major storms and provide people with
more advanced warnings, particularly in
high-risk areas. While many scientists use
computer modeling and lab simulations to
study storms, real-life observations provide
a unique perspective. Some scientists
regularly chase after tornadoes in order to
study them.
The NSSL’s Mobile Mesonet has
The National Oceanic and Atmospheric
instruments that can measure air
Administration (NOAA) has a National
temperature, humidity, and wind speed
Severe Storms Laboratory (NSSL) for
and direction during a storm.
studying catastrophic storms. Storm chasers
at the NSSL have several techniques
for observing storms in the field. One of these techniques involves a probe called a Mobile
Mesonet. The Mobile Mesonet is a minivan that is equipped with weather monitoring
instruments mounted to the roof. These instruments are connected to computers inside the
minivan, so the scientists can examine the data immediately as they drive the Mobile Mesonet
through a tornado or hurricane. Storm chasing is a dangerous job, but it allows scientists to
collect direct, real-time data of catastrophic weather events.
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CATASTROPHIC EVENTS
What do you know?
Read the characteristics of catastrophic events in the box below. Decide whether each
characteristic describes a flood, hurricane, or tornado. Write each characteristic in the
appropriate spot on the Venn diagram on the next page.
Characteristics of Catastrophic Events
• Can be caused by rainfall on nonporous
• Violent, rotating columns of air extending
ground
from the base of intense storm clouds
• Categorized using the Saffir-Simpson
scale
• Categorized using the Fujita Scale
• Develop over warm ocean waters only
• Categorized by wind speed and the
amount of damage caused
• May happen anywhere
• Can be caused by rapid rainfall
• Can result in damage to homes
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CATASTROPHIC EVENTS
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CATASTROPHIC EVENTS
connecting with your child
Preparing for Catastrophic Weather Events
To help your child learn more about
Here are some questions to discuss with
catastrophic weather, create an emergency
your child:
storm kit together. First, search newspapers
• How do catastrophic weather events
or the Internet for current events that relate
affect the areas they hit?
to catastrophic weather events that have
• What kinds of catastrophic weather
occurred where you live. Do you live in an
events could happen where we live?
area where floods, hurricanes, or tornadoes
• How can people prepare for catastrophic
have occurred in the past? Do you live in
weather events?
an area that receives heavy rain or snowfall
on a regular basis? Discuss the information
• What kinds of information do you think
together, focusing on how these weather
we should discuss in order to prepare for
events have affected people in the past.
catastrophic weather events?
Discuss things that people could have done
• What kinds of supplies do you think
to be better prepared for these events.
would be useful to have in case of an
emergency?
Then, come up with an emergency plan
for your household. Include information
such as:
• Where is the safest place to take shelter
in your home?
• What emergency phone numbers should
you call?
• How do you use a generator (if you have
one)?
Next, gather other supplies that may be
useful in an emergency (e.g., batteries,
flashlights, bottles of drinking water,
nonperishable food items, blankets,
toiletries, first aid supplies). A batterypowered radio is also important for listening
to emergency broadcasts in the absence
of electricity. Ask your child to explain the
importance of each item you include in your
emergency kit. Put all the items together,
along with your emergency plan, and store
them in an area where they will be readily
accessible during an emergency.
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