Download Predicting the Weather

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identify that the distance between isobars on a weather map indicates the relative change of
atmospheric pressure in an area
explain why satellite photographs of cloud patterns
have improved the reliability of interpretations of
weather regularities and knowledge of global weather
patterns
Predicting the Weather
When Australia was settled in 1788, the colony nearly
starved due to a lack of food caused by drought. Soon the
variable nature of our weather and climate would be
immortalised in poetry. Dorothea Mackellar wrote:
I love a sunburnt country,
A land of sweeping plains, Of ragged mountain ranges,
Of droughts and flooding rains.
We have long known why the weather changes on a day-to-day basis. In recent years we have begun to
understand why we have these alternating periods of drought and flood.
Day-to-day Weather
One of the most basic measurements made by the Weather Bureau is air pressure. On weather maps,
lines joining places that have the same air pressure are called isobars. The pressure is marked on the
isobars. You will notice that some areas have relatively high pressure. The air pressure decreases as you
move away from such regions. Such regions are called highs or high pressure areas. Other areas on the
map have relatively low pressure. The air pressure increases as you move away from such regions. Such
regions are called lows or low pressure areas. Cold fronts are the other important feature of Australian
weather maps. These are masses of cold air that normally undercut hot air, forcing it higher into the
atmosphere.
The air associated with a high-pressure system sinks down from above. As it sinks, it becomes warmer
and is very stable. When compared with low-pressure systems, highs tend to cover a greater area, move
more slowly and have a longer life. When the high-pressure system is located over land the weather will
often be dry and free of cloud. A low-pressure system develops where relatively warm air rises from the
Earth's surface. If the rising air contains moisture, clouds will begin to form with associated rain
showers as the air cools. Winds arise as air moves between the sinking air of highs and the rising air of
lows. Notice the air spirals clockwise out from a
high-pressure area and spirals anticlockwise into
a low-pressure area. When the isobars are far
apart, it indicates a gradual change in pressure.
When the isobars are close together, it shows
the air pressure is rapidly changing. The closer
the isobars, the stronger the wind.
A cold front is the boundary between cold air
from polar regions moving towards the equator.
As the cold front moves north, it undercuts
warmer air moving towards the poles. If the
temperature difference across a cold front is
large, strong winds can result. If the warm air
also contains a lot of moisture, it becomes
unstable with the development of large clouds as
it is forced to rise. As a result, severe weather
Senior Science HSC Course
Weather maps Highs are due to sinking air; lows
are due to rising air. Cold fronts are masses of
cold air moving over an area (Bureau of
Meteorology).
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Disasters
such as thunderstorms and severe turbulence may
accompany these cold fronts.
Long-term Weather
Australia is a continent surrounded by oceans. Long
term weather changes in Australia are due to changes
in the temperatures of these oceans, particularly the
Pacific Ocean. In most years a strong current brings
relatively cold water northward along the west coast
of South America. Under the influence of trade
winds, the cold
water then flows westward along the equator and is
heated by the tropical sun. These normal conditions
make the western Pacific near Indonesia and
Australia about 3°C to 8°C warmer than the eastern
Pacific.
Along with the warm water, the trade winds bring
warm moist air towards the Indonesian region.
Here, moving over normally very warm seas,
moist air rises to high levels of the atmosphere.
The air then travels eastward before sinking
over the eastern Pacific Ocean. The rising air is
associated with a region of low air pressure, and
the towering clouds bring tropical rain. If this
process reaches an extreme, it can bring floods
to parts of Australia and is called a La Nina.
The El Nino is the opposite extreme. The trade
winds reverse and carry the warm water away
from Indonesia and Australia towards the
Americas. Instead of lows with rising moist air,
we have highs with sinking dry air over cooler
oceans. This produces little rain, and drought
and subsequent forest fires occur throughout
Australia and Indonesia. The warm water
accumulates in the middle of the Pacific Ocean.
Highs and lows Air can move from highs to lows
in a spiral. In the southern hemisphere, the
air moved anticlockwise around highs and
clockwise around lows (Bureau of Meteorology).
Cold fronts Cold air forces its way under warm air.
El Nino (a) During a La Nina, warm moist air rises over
Indonesia bringing rain. (b) During an El Nino warm moist
air rises over the Americas bringing rain.
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Disasters
To see what happens to
weather in New South Wales,
we can compare normal summer
weather maps with a summer
weather map in an El Nino year.
In a normal summer, the highs
move in a relatively low path
across the continent bringing
moist onshore winds. When the
moist winds reach the coastal
ranges, rain is produced along
the coasts. As well, the highs
are not very strong and cold
fronts reach the coastal areas
bringing cooler air and rain. In
an El Nino year, the highs are
very much larger in size and
move in a path that is further
north. As a result, there is
little wind when the high
covers the state or the winds
are hot and dry from the
northern inland. As well, the
highs are so large and strong
that cold fronts that might
bring cool weather and rain are
forced to the south.
Senior Science HSC Course
Surface temperatures (a) During a La Nina, warm water moves
towards Indonesia. (b) During an El Nino, warm water moves towards
the Americas.
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Disasters
Summer weather map (a) Normal years are
moist in a New South Wales summer. (b) In
an El Nino year the summers are dry.
Winter weather map (a) Normal years are dry
in a New South Wales winter. (b) In an El
Nino year
the winters are also dry..
What happens in winter months? In a normal winter the highs are large and slow moving across the
middle of the continent.This means that New South Wales is generally dry and sunny. However, with the
highs further north it allows cold fronts to regularly move across the state. Since northern Australia is
not as hot, winds from that region are not as hot. In an El Nino year this general pattern persists, except
the slow moving highs often cover the whole continent so that the dry stable conditions persist for
longer periods. Cloudless nights bring many frosts. These very strong and slow moving highs force cold
fronts far to the south.
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Disasters
Questions:
1 What name is used for the lines on a weather map?
(A) Isobar.
(B) High.
(C) Low.
(D) Pressure.
2 The air pressure over a city is relatively low and gradually increases in all directions. What type of weather system
is over the city?
(A) High.
(B) Low.
(C) Cold front.
(D) Isobar.
3 What is indicated when the lines on a weather map joining points of equal air pressure are widely separated?
(A) Strong winds will be blowing.
(B) There is a gradual change in air pressure.
C) There is a rapid change in air pressure.
(D) There is a low-pressure area.
4 What is happening to the air to produce a high-pressure area over a city?
(A) The air is stationary.
(B) The air is rising.
(C) The air is sinking.
(D) The air is spiraling inwards.
5 What name is given to a large mass of air moving from Antarctica over the southern states?
(A) Low pressure area.
(B) Warm front.
(C) High pressure area.
(D) Cold front.
6 Explain why satellite photographs of cloud patterns have improved knowledge of global weather patterns such as
the El Nino effect.
7 Describe the movement of air currents between areas of high and low pressure.
8 Explain why isobars that are close together on a weather map indicate strong winds.
9 Describe why a low pressure area will produce rain if the air contains moisture.
10 The air pressure in a coastal New South Wales town changes from low to high during a normal winter. Identify any
changes in weather patterns that are likely and explain their cause.
11 During a normal summer, the water temperature around northern Australia is quite high.
(a) Explain the effect this will have on air pressure.
(b) Explain how this causes extensive rainfall.
12 During an El Nino summer, the water temperature in northern Australia is relatively low.
(a) Explain the effect this will have on air pressure.
(b) Explain how this causes extensive rainfall.
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Disasters
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describe the relative pressures involved in the formation of tropical cyclones and tornadoes
Cyclones and Tornadoes
Tropical cyclones are one of the most destructive types of
weather to affect Australia. The winds in cyclones are very
damaging with gusts approaching 300 kilometres per hour. Rainfall
can be as high as one metre within 24 hours. Some of the greatest
loss of life has been from storm surges and extremely rough seas
that result from cyclones. Cyclones weaken as they move south
into colder oceans or move over the inland, but strong winds can
still cause disruption while flooding can occur inland. But not
everything about cyclones is bad - tropical cyclones are the main
source of rainfall in parts of northern Australia. Serious El Nino
events sometimes end with a cyclone passing across the continent
from the Indian Ocean.
Tropical cyclone Satellite photo of
a tropical cyclone (Bureau of
Meteorology).
Cyclone Mahina caused the worst ever disaster in Australia's cyclone history when in March 1899 more
than 400 people died on Cape York. Some were victims of a 14-metre storm surge - water swept across
lowlying coastal land and carried about 100 people back out to sea. More than 300 died when about 100
pearling vessels sank in the rough ocean. Tonnes of fish and some dolphins were found several kilometres
inland.
Cyclones
A tropical cyclone is a tropical low-pressure
system that produces continuous gale force winds
of at least 63 kilometres per hour. A severe
tropical cyclone produces continuous winds of at
least 118 kilometres per hour. Cyclones are
classified according to their strongest wind gusts.
In other parts of the world, tropical cyclones are
called hurricanes or typhoons. In the average
cyclone season, ten tropical cyclones develop over
Australian waters, of which six cross the coast
mostly over north-western Australia and over
north-east Queensland. The number of cyclones
varies from year to year, from none to as many as
16 in 1963. Tropical cyclones in Australian waters
are less common in El Nino summers and more
common in La Nina summers. This is due to
changes in wind patterns and cooler water
temperatures in El Nino summers and warmer
water temperatures in La Nina summers.
Cyclone classification
A tropical cyclone will only develop from an intense tropical low when the surface of the ocean is warm
enough. The intense tropical low provides the ascending air. There must be a 50-metres depth of water
at 27°C or more in order to supply enough evaporating moisture, which provides the initial energy source
of the cyclone. The life cycle of a tropical cyclone may be divided up into four stages:
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Disasters
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In the formative stage,
clouds start developing over the
ocean close to the equator. Cyclones
never move across the equator.
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If the conditions are
favourable, in the immature stage
clouds collect and move with the
winds that start rotating clockwise
around a low-pressure area. Winds
start to increase in strength.
•
In the mature stage the
cyclonic winds reach their greatest
speed and destructive power.
•
The power of the cyclone
Where tropical cyclones form Map showing average annual
frequency of tropical cyclones in the Australian region (Bureau
decreases during the decay stage
of Meteorology).
when it moves over land or south
over colder waters. In this decaying
stage the winds often decrease rapidly and
the cyclone eye and cloud patterns disappear.
The eye of a cyclone is in the centre where
conditions are almost calm and clear. At the
ocean's surface in the southern hemisphere,
winds spiral inwards in a clockwise direction
towards the eye. Huge bands of cloud spiral in
towards the eye and then begin to spiral
upwards. As the air ascends, massive
thunderstorms form around the eye and in
the spiral bands. High in the atmosphere the
air spirals outwards from the cyclone.
Tornadoes
Structure of a tropical cyclone A cyclone in
A tornado is a violently rotating column of air
cross-section.
in contact with the ground and appearing to
hang from a thunderstorm cloud. Tornadoes
are much smaller than cyclones, often no more than 150 metres wide.
Nor do they last for long, often only a few minutes. Tornadoes can
have wind speeds up to nearly 500 kilometres per hour, which is
higher than a cyclone. With wind speeds like this, tornadoes are very
destructive. In the United States they can totally destroy homes
along a narrow path. Tornadoes are much rarer in Australia, with
perhaps 12 a year. They have much lower wind speeds, and as they
occur in isolated areas, they cause little damage. However, at least
one has caused deaths by lifting a car off the ground and dropping it
100 m away.
No one is totally sure how a tornado forms. However, the worst
tornadoes are produced in a very large thunderstorm called a
supercell . Supercells are giant long-lasting thunderstorms that are
responsible for producing large hail and dangerous winds. Normal
thunderstorms have a fairly flat top, while the top of a supercell is
dome shaped. The air in a supercell rises at very high speeds and is
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Tornado A violently rotating
column of air.
Disasters
turning as it rises, which can give parts of the clouds a corkscrew appearance. As the updraught
strengthens, the spinning increases, until the updraught becomes a narrow, rotating column. Tornadoes
often drop from the supercell near the edge of the
rain-free area under the storm.
Less intense tornadoes can occur near the fronts of
approaching storm fronts, beneath rapidly swelling
thunderstorm clouds, and within hurricanes. These
non-supercell tornadoes, or land spouts, form when
updraughts in a rapidly developing thunderstorm
cloud draw in low-level air that is already slowly
rotating. This pre-existing slow rotation of the lowlevel air can be caused by airflow around mountains
or ridges or the convergence of sea breezes or gust
fronts. The rotation of many non-supercell
tornadoes begins near the ground and grows
upwards.
Comparing Cyclones and Tornadoes
Although cyclones and tornadoes are both rotating
masses of air, they differ dramatically in size,
pressure and wind speed. The graph for a
cyclone starts at a normal air pressure of
around 1010 hPa (hectopascals) and starts
dropping as we move towards the eye of the
cyclone. It drops down to around 950 hPa then
increases as we move out of the eye. The wind
speed also drops from about 150 kilometres per
hour to near zero in the eye and then increases
again as we move out from the eye. Cyclones are
typically 500 kilometres across.
Thunderstorms and tornadoes Tornadoes often
form around supercell thunderstorms.
For a tornado the pressure drops to a much
lower value than for a cyclone, as low as 880
hPa . In theory there is an eye of a cyclone, but
in practice there is none as it moves in such an
erratic manner. The wind speed can be as high
as 500 kilometres per hour, but is often about
300 kilometres per hour, which is again higher
than for a cyclone. Tornadoes are much smaller
than cyclones, typically 100 to 600 metres
across.
Pressure and wind speed (a) Notice how both air
pressure and wind speed drop in the eye of
the cyclone. (b) In a tornado the central pressure is
lower. As the tornado is part of a thunderstorm
the velocity outside is still high. Values are only
approximate as accurate readings are hard to
take.
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Disasters
Questions: Cyclones and Tornadoes
1 Which of the following weather phenomena is the most devastating to humans?
(A) Cyclones.
(B) Typhoons.
(C) Hurricanes.
(D) All the above.
2 Why don't cyclones cross the equator?
(A) They would no longer be called cyclones.
(B) Cyclones form too far from the equator to travel such a long distance.
(C) The temperature is too low for a cyclone to exist.
(D) They would have to reverse the direction of rotation.
3 In which direction does a cyclone rotate in the southern hemisphere?
(A) The same direction as a high-pressure area.
(B) Anticlockwise like a low-pressure area.
(C) Clockwise like a low-pressure area.
(D) Anticlockwise like a high-pressure area.
4 How does a tornado compare to a cyclone?
(A) Tornadoes are bigger and have slower winds than cyclones.
(B) Tornadoes are smaller and have faster winds than cyclones.
(C) Tornadoes are bigger and have faster winds than cyclones.
(D) Tornadoes are smaller and have slower winds than cyclones.
5 What is it like in the eye of a cyclone?
(A) Little wind and no rain.
(C) Winds are very strong and there is torrential rain.
(B) Winds are very weak and there is torrential rain.
(D) Winds are very strong and there is no rain.
6 Describe the relative pressures involved in the formation of tropical cyclones and tornadoes.
7 Describe the formation of a tornado in a supercell thunderstorm.
8 Explain why satellite photographs of cloud patterns have improved readings of weather maps.
9 Explain why satellite photographs of cloud patterns have improved knowledge of global weather patterns such as
the origin of cyclones.
10 Use the data in theTable to answer the questions
(a) Plot the movement of tropical cyclone Steve onto the
Figure below.
(b) Explain why the intensity of the cyclone changed as it
moved over the Northern Territory.
(c) Explain why the intensity of the cyclone changed as it
moved into the Indian Ocean.
(d) Identify two areas that may have experienced
flooding.
Tropical cyclone Steve (Bureau of below.Meteorology)
(e) What was unusual about the cyclone when in the
Great Australian Bight? Justify your answer.
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Disasters
11 Use the Figure to answer these questions.
(a) Compare the lowest pressures found in cyclones and tornadoes.
(b) Compare the diameters of cyclones and tornadoes.
(c) Compare the maximum speeds of cyclones and tornadoes.
(d) Which type of storm is harder to measure? Justify your answer.
Path of a cyclone A grid to plot the path of a cyclone.
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Disasters
HSC Style Questions:
2005 HSC Many natural disasters have occurred in Australia since 1970. During your course you created a database to
present information on these disasters.
List TWO examples of Australian disasters you recorded, and for each describe ONE piece of information you collected
for the database.
2005 HSC The following information was published in the Sydney Morning Herald on Tuesday, 7 December 2004.
(i) What does the distance between the isobars on the weather maps indicate?
(ii) Explain the reasoning behind the forecasts for Sydney’s four-day outlook.
(iii) Explain how information about cloud cover is gathered, and how it has improved our ability to monitor and predict
weather patterns.
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Disasters
2002 HSC
(i) Name the lines around the letters ‘L’ and ‘H’ on the above weather map.
(ii) Describe the movement of air currents from one air pressure system to another.
2001 HSC (i) Identify ONE Australian disaster where nature and human activity have combined to produce the disaster.
(ii) Describe how the human activity contributed to this disaster.
(i) What is the lowest air pressure indicated by an isobar on this map?
(ii) The Bureau of Meteorology predicted the weather in Melbourne for Tuesday morning as ‘Cooler conditions with
strong south-westerly winds and rain’.
Using the information from the map, suggest why this prediction was made.
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Disasters
Answers
Natural Disasters
1 (D). 2 (A). 3 (B). 4 (C). 5 (B).
6 A disaster is an event associated with large-scale environmental or structural damage and/or loss of life.
7 Three of the following:
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Meteorological, for example, Cyclone Tracy destroys Darwin, Western Sydney Flood.
•
Geological, for example, Newcastle earthquake, Tsunamis of 1960.
•
Extraterrestrial, for example, Wolf Creek Crater in Western Australia
•
Biological, for example, Influenza epidemic of 1918, AIDS epidemic.
8 (a) People and businesses insure against those financial losses that they cannot reasonably afford to suffer, such as the
replacement of homes or cars.
(b) A defined events policy is usually cheaper and sets out a list of what is and is not covered by the insurance policy. For example,
if water enters a house through a hole in the roof due to storm damage, the damage is covered. But if the water enters the house
under the door as a result of floodwaters from local creeks, the damage is not covered. To get coverage for a flood, extra premium
must be paid.
An accidental damage policy is more expensive. This is a much broader coverage, and
only lists those things that are not covered. Thus, if you spill a can of paint on the
carpet it is covered by an accidental damage policy unless it is specifically listed as not
covered.
9 Database form (Figure) - many designs are possible.
10 Cyclone Tracy. (a) 1974 (b) Darwin, Northern Territory.
(c) 65 people died and about 650 were injured. Most buildings were destroyed or
severely damaged; much of the population was relocated to southern states.
(d) Building codes were changed to reduce damage in future cyclones. Strong
attachments must now be used to attach roofs to walls and walls to the ground.
(e) A cyclone watch has been established using satellite imagery to track the movement
of cyclones and issue alerts to populated areas in Northern Australia.
Disasters and Human Activity
1 (C). 2 (B). 3 (B). 4 (D). 5 (A).
6 Three of the following.
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Terrorist bombings (for example, Bali).
•
Transport accidents (for example, Granville train crash).
•
Bridge collapse (for example, Westgate Bridge, Melbourne).
•
Essential services disrupted (for example, Longford gas plant explosion).
7 Humans allowing overgrazing on farms combined with drought produced the 2002 dust storms.
8 Storms and errors of navigation caused the Cataraqui to be wrecked on King Island in Bass Strait, with the loss of more than 400
lives.
9 Heavy rain combined with poor design of rail embankments led to the Coledale landslide that killed a mother and her child.
10 In this course we define a disaster as events associated with large-scale environmental or structural damage and/or loss of life.
The large numbers of lives lost allows road deaths to be classified as a disaster. However, since they occur a few at a time and not
in one significant event, they are not normally called a disaster.
Studying the Weather
1 (C). 2 (B). 3 (C). 4 (D). 5 (A).
6 Atmospheric pressure is the force per unit area due to the air around us. In studying the weather, the units used to measure
atmospheric pressure are hectopascals where 1 pascal is 1 Newton per square metre.
7 Two of the following.
•
Weather radar has allowed the movement of storms to be tracked. This has not only improved our ability to make
forecasts of severe weather, but has also allowed us to better understand its origin. The wind movements within storms are
studied to better enable us to understand the dynamics of thunderstorms, cyclones and tornadoes.
•
Satellite imagery has allowed us to better predict the path of severe weather such as cyclones. It has allowed us to track
the movement of weather patterns across the continent, and to determine the presence of strong winds. This has helped us to
better understand the large-scale movements of the atmosphere.
•
Weather radar and satellite imagery both depend on the presence of rain and/or clouds to track movements. They are not
able to measure air movements in clear air. The development of ground or satellite-based lidar will help solve this problem. An
infra¬red laser is used to track the movements of tiny dust and aerosol particles in the air. This will help us to better understand
both the small-scale and the large-scale movements of the atmosphere.
8 (a) A microwave pulse is sent from the radar dish. If it strikes rain, hail or snow it will be scattered. Some of the pulse will be
reflected back to the radar dish.
(b) From the time taken for the pulse to return the location of the rain, hail or snow can be determined. Since we know that
microwaves travel at the speed of light, we can combine the speed and time to calculate the distance. A computer is used to
perform the calculations.
(c) As the radar beam scans across the sky, a computer is used to perform the calculations and combine together the many
reflections for display onto a map. If the reflections are weak, it indicates light rain. If the reflections are intense, then it may
indicate heavy rain or even hail. In each case, the map is colour coded accordingly.
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Disasters
9 Satellites take regular images of the Earth's cloud cover using both infra-red and visible light. Successive images provide a
picture of cloud movements across Australia and nearby regions
and helps in weather forecasting. The movements of clouds also allow wind speeds to be calculated. Regular satellite images are also
used to track the movements of cyclones.
10 Several answers are possible, depending on the experiments you have performed.
We observed the effects of changing the air pressure inside an empty drink can. We one¬quarter filled the drink can with water
and heated the water until it boiled. The steam forced some of the air from the can. Using tongs, we quickly inverted the hot can
over a bucket of cold water. When inverted in cold water, the steam condensed and reduced the air pressure inside the can. The air
pressure outside remained constant, and as it was much larger than the pressure inside, it collapsed the can.
Predicting the Weather
1 (A). 2 (B). 3 (B). 4 (C). 5 (D).
6 In a normal year, the warm water conditions to the north of Australia cause considerable evaporation and the formation of
clouds. In an El Niflo year, the warm water is found further east closer to the Americas. This means the extensive cloud formation
is in the central Pacific. This movement of the location of clouds can be tracked using satellite imagery. Satellites can also use
modem technology to measure the temperature of the ocean surface.
7 In the southern hemisphere, air spirals anticlockwise out from a high pressure area, and after it crosses the boundary, spirals
clockwise into a low pressure area.
8 Isobars that are close together show that air pressure is changing rapidly. Since wind is a result of changes in air pressure, the
bigger the difference in pressure over a short distance, the stronger will be the winds.
9 In a low-pressure area, the air is rising into the colder upper atmosphere. As moist air rises, the moisture cools and condenses to
form clouds which in turn will produce rain.
10 High pressure areas indicate sinking warm stable air. Thus the weather can be expected to be fine and clear.
11 Warm water will warm the atmosphere and cause it to rise to produce a low-pressure area. Since the water is warm, the rate of
evaporation will be high producing moisture-laden air.
12 When moisture-laden air rises into the atmosphere, it cools and clouds form. These clouds are the source of the extensive rain
in northern Australia.
Cyclones and Tornadoes
1 (D). 2 (D). 3 (C). 4 (B). 5 (A).
6 Both cyclones and tornadoes are formed in areas of low pressure. Cyclones form in tropical lows over warm oceans where the
central pressure can drop as low as 950 hPa. Tornadoes form in thunderstorms where there is low pressure due to rapidly rising air.
The pressure inside a tornado is lower than that inside a cyclone, and can drop to 880 hPa.
7 The air in a supercell rises at very high speeds and is turning as it rises. As the updraught strengthens, the spinning increases,
until the updraught becomes a narrow, rotating column. Tornadoes often drop from the supercell near the edge of the rain-free
area under the storm.
8 Satellite photographs allow the tracking of movements of major weather systems and storms. Cloud is often associated with lows
and cold fronts. The tracking of large weather systems such as lows and cold fronts allow better everyday predictions of weather.
This is particularly true of places such as Western Australia where most weather comes from the oceans to the west. Since there
are few weather stations in the ocean, images from weather satellites improve the accuracy of predictions.
9 Satellite photographs allow the tracking of movements of weather systems such as cyclones. Since cyclones have extensive cloud,
their origin can be monitored more readily from space as there are few weather stations in the oceans where they form. Cyclones
are also unpredictable in their movements. Tracking the movements of cyclones allows people in their path to be warned thus
helping reduce damage and loss of life.
10 (a) The movements of cyclone Steve (Figure ). (b) Cyclones obtain their energy from the evaporation of warm water. When a
cyclone moves over land, the energy supply ceases and the intensity of the cyclone usually
decreases. In this case it became an intense low.
(c) As the cyclone moved out over the ocean, its supply of energy from warm ocean waters increased. As a result the intensity of
the cyclone increased, reaching category 2.
(d) Intense lows formed over Cape York, Northern Territory and
Western Australia. All would have produced intense rain and result
in flooding.
(e) Cyclone Steve remained a category 1 cyclone over Western
Australia and into the Great Australian Bight. Normally the lack of
warm water over land, and the presence of colder water in the
Bight, would have reduced the cyclone to an intense low.
11 (a) The lowest pressure in the cyclone is around 950 hPa, and
around 875 hPa in a tornado.
(b) The cyclone is more than 600 kilometres in diameter, and the
tornado is more than 300 m in diameter.
(c) The highest speed of the cyclone is nearly 175 kilometres per
hour, while the highest speed of the tornado is close to 320
kilometres per hour.
(d) The small size and destructive power of the tornado makes it
vary hard to study. A plane can be flown through the much larger
cyclones to take measurements.
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Disasters