Download Catastrophic Events

Catastrophic Events (7.8A)
Student Expectation
The student is expected to predict and describe how different types of catastrophic events
impact ecosystems such as floods, hurricanes, or tornadoes.
Key Concepts
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Key Concept 1: Extreme weather events such as floods, hurricanes and
tornadoes, are classified by the extent and intensity of their impact on the
ecosystem.
Key Concept 2: Floods result when rainfall exceeds the holding capacity of a
drainage system. For example, water overflows the banks of a river and spreads
out across its floodplain. The energy of the moving water and the layers of
sediment carried by the water can uproot or bury plants, reshape the topography,
and destroy roads and buildings.
Key Concept 3: Hurricanes develop over warm ocean waters as an area of low
pressure in the atmosphere. Over time, as more ocean water evaporates into the
weather system, the air pressure in the hurricane’s eye decreases, and the intensity
of the wind increases. When hurricanes move onshore, they impact the ecosystem
in a variety of ways, including a storm surge of seawater along the coast. As it
moves across land, intense wind damage and excess flooding events occur all
along the path of the storm. Frequently, tornadoes form within hurricanes.
Key Concept 4: Tornadoes are violent, rotating columns of air extending from
the base of intense storm clouds. Although affecting an area much smaller in
width and distance than that covered by a hurricane, tornadoes have much
stronger wind speeds. Objects in the path of a tornado are totally destroyed,
leaving a scar of devastation to the ecosystem.
Key Concept 5: As a result of the careful study of atmospheric patterns
associated with catastrophic storms, meteorologists have been able to forecast the
probability, intensity, and paths of heavy rainstorms, flooding, hurricanes, and
tornadoes.
Teacher Background
The Sun’s radiation provides the energy for Earth. This energy from the Sun heats up
Earth’s surface and all things on it. Even though the Sun emits a constant amount of
energy, Earth does not receive equal amounts of energy during the year as it revolves
around the Sun. In addition, due to the tilt of Earth’s axis, not all areas on Earth receive
equal amounts of energy from the Sun. For example, the equator receives more direct
radiation than the poles. The process of convection redistributes this energy north or
south from the equator to the poles.
Convection is the process that transfers heat, either vapor or water, by mass movement.
When you boil a pot of water on the stove, the water on the bottom heats first since it is
closest to the heat source. As this water warms, it expands, becomes less dense, and rises
to the surface. Cooler (more dense) water sinks to the bottom to replace it and this
circulation pattern continues. The same occurs in the atmosphere. As Earth’s surface is
warmed by the Sun, the air mass closest to the surface is warmed, expands, rises, and is
replaced at the surface by cooler air from above. Air pressure and winds are critical to the
convection process in the atmosphere.
Air masses in the atmosphere are made up of millions of gas molecules that are being
pulled towards Earth’s surface by its gravity. The force that these air masses exert on
Earth’s surface is known as surface pressure. Air pressure decreases as you go up in
elevation because at higher elevations, there are fewer air molecules above you. In other
words, air pressure is the weight of the air above a given level. Air pressure varies with
temperature. High pressure systems, known as anticyclones, have winds moving away
from the high pressure center towards areas of lower pressure, in a clockwise direction in
the northern hemisphere. These systems are associated with cool, dry air masses and clear
skies. Low pressure systems, known as cyclones, have winds moving towards the low
pressure center in a counter-clockwise direction in the northern hemisphere. These
systems are associated with warm, moist air masses and stormy weather. Extreme
examples of this would be hurricanes.
When meteorologists talk about weather systems they talk in terms of high or low
pressure. Warmer air masses are typically associated with high pressure and cooler air
masses with low pressure. It’s called high or low pressure because a warmer, less dense
air mass will exert the same amount of pressure on the surface from a higher elevation, as
a colder, denser air mass will exert from a lower elevation. Meteorologists can measure
air pressure using an instrument called a barometer. A barometer consists of a tube with a
liquid in it (for example, water or mercury). This tube is set up vertically, closed at the
top and open at the bottom. The bottom of the tube sits in a tiny cup filled with the same
liquid in the tube. The liquid in the cup not covered by the tube is exposed to the
atmosphere, so air pressure pushes on this liquid’s surface. If air pressure is high, it
pushes this liquid up the tube to a higher level. When the tube is marked in units of
pressure (e.g., mbars) we can measure changes in pressure.
Wind is the result of differences in air pressure. Differences in air pressure create a force
(known as the pressure gradient force) that causes air to move. Air moves from areas of
high pressure to areas of low pressure. This movement of air masses caused by
differences in air pressure (or density) leads to the formation of fronts. Fronts occur at the
boundary between high and low pressure air masses. As fronts move into an area, they
change the weather conditions. A warm front occurs at the boundary between a mass of
warm, moist air and cold air, where the warm air mass is overtaking the cold air mass.
Since the warm air is less dense than the air it is replacing, it gently gets pushed up over
the colder air mass. Warm fronts have wide areas of cloud cover along the front and
gentle precipitation. They are associated with high pressure and nice weather once they
have past. A cold front occurs when a cold, dry air mass overtakes a warm air mass. This
cold air mass is denser than the warm air mass already in the location. As the cold air
mass moves in, it pushes underneath the warm air already there, pushing this warm air up
quickly, leading to intense precipitation and sometimes, thunderstorms. After the cold
front passes, the area experiences colder temperatures.
Atmospheric circulation caused by differences in solar energy absorption on Earth’s
surface causes the weather we see day to day. Atmospheric circulation leads to changes
in atmospheric conditions, such as wind speed and direction, temperature, humidity, and
precipitation, which can be tracked over time. Weather stations exist all over the world
and measure these conditions daily, hourly, or even every minute. By knowing the
atmospheric conditions today and for the past week, month, year, or decade,
meteorologists can use this information to predict what the weather will do tomorrow and
next week.
Meteorologists use weather maps to keep track of atmospheric conditions. This allows
them to understand the large-scale patterns of atmospheric circulation, such as fronts or
storms. These weather maps can be used to forecast or predict the weather at smaller
scales, like for an individual city, such as Houston. Weather maps are created by plotting
data such as temperature, wind direction and speed, and atmospheric pressure for weather
stations at different locations on the map. By observing changes in these conditions at
weather stations over time or differences between weather stations, we can track weather
systems, such as storms and fronts, and predict what they will do next. For example,
since we know a cold front occurs when a cold, dry air mass pushes out a warm air mass,
we know higher temperatures occur ahead of the front, and lower temperatures behind.
The cold air pushing out the warm air leads to storms, so stations where there is
precipitation can indicate where the cold front is located. Meteorologists represent cold
fronts on weather maps with lines of blue triangles. Similar indicators (temperature, wind
speed and direction, pressure, and precipitation changes) are used by meteorologists to
determine where warm fronts occur, and they represent warm fronts on weather maps
with lines of red circles.
Wind not only redistributes energy in the atmosphere, but also influences ocean
circulation. As wind blows along the surface of the ocean, the surface water moves along
with it. These surface ocean currents transport energy from the tropics to the poles. For
example, wind is responsible for the Gulf Stream that moves warm, less dense surface
water from the tropics, north along the eastern coast of the U.S., all the way to Europe.
Though the major surface ocean currents are driven by wind, temperature and salinity are
also drivers of ocean currents, particularly deep ocean currents. This is because
temperature and salinity affect water density. Warm water is less dense than cold water
and fresh water is less dense than salty water. Water temperature and salinity vary with
location. The average global sea surface temperature is around 17 °C, but it ranges from
around freezing at the poles to 35 °C in the tropics. In addition, on average, 3.5% of the
global oceans is salt, but it can range from 1.0% to 4.1% depending on location and
depth. The thermohaline circulation is the major, deep ocean current that transports
energy from the tropics to the poles. The Gulf Stream is a component of this circulation,
transporting warm, tropical surface waters north. As water in the Gulf Stream moves
north, the heat energy in the water is transferred to the air and thereby the water becomes
cooler, denser, and sinks. This new cold, dense water then flows back towards the equator
deep in the ocean.
Since ocean currents transport energy (temperature) and moisture around the globe, they
play an important role in global weather patterns. As ocean currents move warm water
into an area, evaporation increases, moving moisture and energy to the atmosphere, hence
increasing the temperature and humidity in the area. If cold water is moved into an area
by ocean currents, it can lower surface air temperatures, as well as reduce evaporation
rates, leading to colder, relatively drier conditions in the areas nearby. Because oceans
play a key role in supplying energy and moisture to the atmosphere, they also influence
the formation of weather systems, such as thunderstorms, blizzards, and hurricanes.
Along the Texas coast, often thunderstorms develop because winds shift direction and
blow warm, moist air from the Gulf of Mexico onto the coast.
Hurricanes are more complex weather systems, which generate thunderstorms, rains, high
winds, hail, and even tornadoes. Hurricanes have sustained winds of over 75 mph and
form in warm, tropical waters in the Atlantic and Pacific Oceans, the Caribbean Sea, and
the Gulf of Mexico. They are fueled by warm, moist air. As water evaporates from warm
ocean waters this warm, moist air (less dense) rises in the atmosphere, leaving less air
near the surface, and forming a low pressure area. Since winds move from high to low
pressure areas, surrounding air moves into the low pressure area, is warmed, and rises.
The rising, warm air cools, eventually reaching an elevation where it condenses and
forms clouds. At the top of the storm, winds flow outwards, allowing the warm air from
below to keep rising. In the northern hemisphere, hurricanes spin counterclockwise, due
to the rotation of the Earth (the Coriolis force). In the northern hemisphere the Coriolis
force defects air to the right. So as surface winds flow toward the center of the storm,
they deflect to the right, which creates a counterclockwise rotation. You can see this if
you draw a circle and draw arrows that curve to the right (as they reach the circle) all
around the circles edge. When you are done you can see that the arrowheads point around
the circle in a counterclockwise direction. As more ocean water evaporates and fuels the
hurricane, the low pressure at the surface will get stronger and it will spin faster, leading
to higher sustained wind speeds. Hurricanes need the right conditions in the oceans,
warm waters and light winds, in order to form.
Extreme events such as hurricanes, tornadoes, floods, droughts, earthquakes, volcanoes,
and fires are classified based on their intensity and extent of their impact on ecosystems.
Some catastrophic events have measurable scales. Hurricanes are categorized based on
strength, measured by their sustained wind speed and using the Saffir-Simpson Scale;
tornadoes are classified based on damage measured by Fujita Scale; earthquakes are
classified by their intensity as measured by the Richter scale. Impacts on ecosystems have
no formal scale. Weather-related, catastrophic events can be forecasted by meteorologists
by tracking atmospheric patterns and understanding what atmospheric conditions lead to
these events. They can forecast the probability, intensity, and path of hurricanes, and
some tornadoes, as well as flooding from heavy rainstorms. This can help people protect
their property and lives. The impacts catastrophic events can have on ecosystems can be
visible and last for decades.
When hurricanes make landfall they can impact ecosystems and people in a variety of
ways. Hurricanes are unique in that during a hurricane, you can have damage from other
catastrophic events, such as flooding and tornadoes. Storm surge associated with
hurricanes can lead to erosion of shorelines and destruction of coastal habitats, such as
wetlands. High winds and tornadoes can knock over trees in forests used for shelter by
animals, as well as houses and buildings used by people. Flooding further inland can
happen due to intense rainfall and put entire ecosystems underwater. Hurricane Katrina,
which hit New Orleans in 2005, was only a category 3 storm, but was the most expensive
(in terms of damage) and third most deadly (~1500 deaths) storm in recorded history. The
Hurricane of 1900 was a category 4 storm that hit Galveston Island and was the most
deadly storm, with estimates of over 8000 deaths. Hurricane strength measured by wind
speed does not always directly measure how much damage a storm will do. Storm
duration, flooding caused by rainfall and storm surge can also cause damage to
ecosystems and society.
Tornadoes are unique extreme weather events that can develop at the base of severe
thunderstorms or hurricane storm clouds. They are intense, rotating columns of air that
extend down from a storm cloud in the shape of a funnel and their rotation is evident at
the ground. Tornadoes have extremely strong wind speeds, stronger than those of
hurricanes. Luckily tornadoes do not usually impact wide areas and do not typically stay
on the ground for long distances. Anything found in the path of a tornado, including
houses, cars, forests, and other ecosystems, is often completely destroyed. It can take
ecosystems years to decades to recover from a tornado.
Floods occur when the ground, creeks, streams, and rivers can no longer absorb and hold
the amount of water being produced by a rain event. When this happens, water will
overflow the banks of a river and spread out across the land or floodplain. Flooding can
also occur in cities when sewers and runoff catchments cannot keep up with the amount
of rain. Often streets are the first to flood in this situation and waters will continue to rise
until the rains stop and the water is given time to drain. Buildings with basements,
tunnels, or houses at low elevations are susceptible to flooding. For example, the tunnel
system in downtown Houston, many highways, streets, houses, and buildings flooded
during Tropical Storm Allison in 2001. Often floods consist of moving water, and it can
be hard to tell how fast that water is moving. People sometimes try to drive their cars
through flooded areas in a road thinking that the water is shallow enough or that it is not
flowing, only to have to abandon their car and watch it float away down the road. This
moving water erodes sediment and can uproot plants and trees, destroy roads and destroy
buildings. After the waters subside, all this sediment can deposit, leading to a reshaping
of the area’s topography.
Droughts are the most subtle extreme weather event and occur when an ecosystem has
not received rain for days, weeks, or months, which the area needs in order to recharge its
groundwater. Droughts can cause plants to get sick and die if the drought lasts long
enough. Droughts can also lead to wildlife dying if they cannot get the water they need to
survive, or if all the plants die, the wildlife will no longer have a food source.
When catastrophic events occur, often we focus on the impact they have on people and
society. These events can often impact ecosystems. Extinctions in an area can occur due
to loss of habitat or food supplies. Ecosystem type can change after an event: for
example, if a tornado destroys a forest, a grassland will likely grow in its place. It is
important to understand how different ecosystems are impacted by and how to respond to
different catastrophic events.