Download The demographic transition model

Fronts
Fronts occur where two different air masses meet.
1.
Warm fronts are formed when warm air
rises over a mass of cold air. As the air lifts
into regions of lower pressure, it expands, cools
and condenses the water vapour as wide, flat
sheets of cloud.
Warm fronts are shown on synoptic charts by a
solid line with semicircles pointing towards the
colder air and in the direction of movement. On
coloured weather maps, a warm front is drawn
with a solid red line with red semicircles.
2.
Cold fronts are usually associated with depressions. A cold front is the transition
zone where a cold air mass is replacing the warmer air mass. The cold air is following
the warm air and gradually moves underneath the warmer air. When the warm air is
pushed upwards it will rain heavily. Often more rain will fall in the few minutes the cold
front passes than it will during the whole passage of a warm front. As the cold front
passes, the clouds roll by and the air temperature is cooler.
Cold fronts are shown on synoptic charts by a solid line with triangles along the front
pointing towards the warmer air and in the direction of movement. On coloured weather
maps, a cold front is drawn with a solid blue line with blue triangles.
3.
Occluded fronts occur at the point where a cold front takes over a warm front or the
other way around. If a cold front undercuts a warm front it is known as a cold occlusion
and if the cold front rises over the warm front it is called a warm occlusion. Occluded
fronts bring changeable weather conditions.
On a synoptic chart occluded fronts are represented by semicircles and triangles
positioned next to each other. The triangles are in blue and the semicircles are in red, or
both are purple (mixing both red and blue colours together).
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Depressions
Depressions are areas of low atmospheric pressure which produce cloudy, rainy and windy
weather. These low-pressure systems often begin in the Atlantic, moving eastwards towards
the UK. They are responsible for the UK's changeable weather.
The diagram shows a depression with a leading warm front and a trailing cold front moving
from west to east across Britain.
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At the warm front, lighter, warmer air from the south (tropical maritime air)
meets cooler air from the north (polar maritime air) and rises gradually over it.
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As the warm air slowly rises it cools, its water content condenses and clouds form
(nimbostratus then altostratus). The result is steady rain, later giving way to drizzle and
finally clearer skies with high cirrus clouds.
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Behind the warm front is an area of warm, rising air and low pressure - the
centre of the low-pressure system. As this part of the depression passes over, there may
be a short period of clear, dry weather. However, at the trailing cold front, heavier,
cooler air meets the warm air at the centre of the depression, undercutting it and
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forcing it steeply upwards. Quickly moving air masses produce high winds and cooler
temperatures.
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As the rapidly rising warm air cools, its water condenses and clouds form
(cumulonimbus, then cumulus). The result is heavy rain or thunderstorms, giving way to
showers and finally to clear skies as the cold front moves away eastwards.
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Anticyclones
Anticyclones are the opposite of depressions - they are an area of high atmospheric pressure
where the air is sinking.
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As the air is sinking, not rising, no clouds or rain are formed. This is because as
the air sinks it warms, meaning it can hold more water.
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The absence of fronts means winds may be very light.
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Consequently, high-pressure areas are often associated with settled, dry and
bright conditions.
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In summer, anticyclones bring dry, hot weather. In winter, clear skies may bring
cold nights and frost.
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In cold conditions, anticyclones may also bring fog and mist. This is because the
cold forces moisture in the air to condense at low altitudes.
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Hurricanes
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Severe weather conditions - like hurricanes or drought - can have a
devastating effect on communities. It is impossible to prevent weather from
affecting a region but there are measures that can be taken to minimise the
impact.
The strongest tropical storms are called hurricanes, typhoons or tropical cyclones.
The different names all mean the same thing, but are used in different parts of the
world. If these huge storms start in the Atlantic, off the west coast of Africa, they are
called hurricanes.
In an average year, over a dozen hurricanes form over the Atlantic Ocean and head
westwards towards the Caribbean, the east coast of Central America and the southern
USA (Florida in particular). Hurricanes may last as long as a month and although they
travel very slowly - usually at about 24 km/h (15 mph) - wind speeds can reach over
120 mk/h (75 mph).
Effects of tropical storms
The intense winds of tropical storms can destroy whole communities, buildings and
communication networks. As well as their own destructive energy, the winds generate
abnormally high waves and tidal surges. Sometimes the most destructive elements of a
storm are the subsequent high seas and flooding.
MEDCs are better placed to reduce the effects of tropical storms because they have more
financial, educational and technological resources to help deal with them. They better
able to observe and predict storm behaviour and can invest in infrastructure to withstand
storms - as well as spending more money on repairing the damage caused.
How hurricanes form
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When this warm and wet air rises, it condenses to form towering clouds, heavy
rainfall. It also creates a low pressure zone near the surface of the water.
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Rising warm air causes the pressure to decrease at higher altitudes. Warm air is
under a higher pressure than cold air, so moves towards the ‘space’ occupied by the
colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm
surroundings, which then also rises. A continuous upflow of warm and wet air continues
to create clouds and rain.
Air that surrounds the low pressure zone at the centre flows in a spiral at very
high speeds - anti-clockwise in the northern hemisphere - at speeds of around 120 km/h
(75 mph).
Air is ejected at the top of the storm – which can be 15km high – and falls to the
outside of the storm, out and over the top, away from the eye of the storm. As this
happens, it reduces the mass of air over the ‘eye of the storm’ - causing the wind speed
to increase further. Some ejected air also cools and dries, and sinks through the eye of
the storm, adding to the low pressure at the centre.
The faster the winds blow, the lower the air pressure in the centre, and so the
cycle continues. The hurricane grows stronger and stronger.
Seen from above, hurricanes are huge circular bodies of thick cloud around 450
km (300 miles) wide. The cloud brings heavy rain, thunder and lightning.
In the centre is the eye of the hurricane, about 45 km across (30 miles) across.
Often there will be no clouds in the eye. Seen from below it will seem calmer, with a
circle of blue sky above. The eye is formed because this is the only part of the hurricane
where cold air is descending.
In the northern hemisphere, the prevailing easterly tropical winds tend to steer
hurricanes toward land - although their course is unpredictable. As hurricanes move
inshore, their power gradually reduces because their energy comes from sucking up
moist sea air.
Preparation and prediction
Preparation and prediction techniques to help cope with hurricanes can be very different
in MEDCs and LEDCs.
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MEDCs
MEDCs have the resources and technology, such as satellites and specially equipped
aircraft, to predict and monitor the occurrence of storms. They are also equipped to train
the emergency services appropriately and to educate people about necessary
precautions.
Storm warnings can be issued to enable the population to evacuate or prepare
themselves for the storm. People can prepare by storing food and water or boarding up
their windows.
LEDCs
LEDCs are often less prepared. They may rely
on aid (sometimes reluctantly) from MEDCs for the
rescue and recovery process, as was the case with
Cyclone Sidr in Bangladesh, November 2007.
Hurricane Katrina
Impacts
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Katrina was a category 4 storm.
Storm surges reached over 6 metres in height.
New Orleans was one of the worst affected areas because it is below sea level
and protected by levees. These protect it from the Mississippi River and Lake
Ponchartrain. The levee defences were unable to cope with the strength of Katrina, and
water flooded into the city.
Despite an evacuation order, many of the poorest people remained in the city.
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People sought refuge in the Superdome stadium. Conditions were unhygienic and
there was a shortage of food and water. Looting was commonplace throughout the city.
Tension was high and many felt vulnerable and unsafe.
1 million people were made homeless and about 1,200 people drowned in the
floods.
Oil facilities were damaged and as a result petrol prices rose in the UK and USA.
Responses
Flooding caused by Hurricane Katrina in New Orleans 2005
There was much criticism of the authorities for their handling of the disaster. Although
many people were evacuated, it was a slow process and the poorest and most vulnerable
were left behind.
$50 billion in aid was given by the government.
The UK government sent food aid during the early stages of the recovery process.
The National Guard was mobilised to restore and maintain law and order in what became
a hostile and unsafe living environment.
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The demographic transition model
The demographic transition model shows population change over time. It studies how
birth rate and death rate affect the total population of a country.
The five stages of the demographic transition model
1.
Total population is low but it is balanced due to high birth rates and high death
rates.
2.
Total population rises as death rates fall due to improvements in health care and
sanitation. Birth rates remain high.
3.
Total population is still rising rapidly. The gap between birth and death rates
narrows due to the availability of contraception and fewer children being needed to
work - due to the mechanisation of farming. The natural increaseis high.
4.
Total population is high, but it is balanced by a low birth rate and a low death
rate. Birth control is widely available and there is a desire for smaller families.
5.
Total population is high but going into decline due to an ageing population. There
is a continued desire for smaller families, with people opting to have children later in
life.
As a country passes through the demographic transition model, the total population
rises. Most LEDCs are at stage 2 or 3 (with a growing population and a high natural
increase). Most MEDCs are now at stage 4 of the model and some such as Germany
have entered stage 5.
As populations move through the stages of the model, the gap between birth rate and
death rate first widens, then narrows. In stage 1 the two rates are balanced. In stage 2
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they diverge, as the death rate falls relative to the birth rate. In stage 3
they converge again, as the birth rate falls relative to the death rate. Finally in stage 4
the death and birth rates are balanced again but at a much lower level.
Limitations of the model
1.
The model was developed after studying the experiences of countries in Western
Europe and North America. Conditions might be different for LEDCs in different parts
of the world.
2.
The original model doesn't take into account the fact that some countries now
have a declining population and a 5th stage. Most texts will now show this stage as
it is relevant to an increasing number of MEDCs in the 21st century.
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Italy – changing population
The change results from two processes:
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older people are living longer
fewer babies are being born
Older people are living longer because of:
developments in medical science
improved health care provision
Fewer babies are being born because:
more women go out to work
educated women may choose to develop a career rather than raise a large family
the age of marriage has increased
contraceptives are more easily available
there are changing lifestyles, where families prefer to have fewer children
in the past children contributed to the family's income - nowadays they're more
likely to be economic burdens
better health care means that people don't have to have large families to ensure
the survival of a few
These factors have led to a decrease in the EU fertility rate from 2.75 children to each
woman in the appropriate age group in the mid-1960s, to a current rate of 1.43 children
per female. To maintain a stable population, a level of 2.1 is required. All across Europe
fertility rates, and therefore populations, are decreasing.
We've learned that across Europe there has been a drop in fertility, leading to an ageing
population. This can cause problems, and these problems can arise at both ends of the
age structure.
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The increased population of older people means that:
there is an increased demand for health and social care
it becomes increasingly difficult for governments to provide satisfactory pensions,
which are ultimately funded by the working population
The reduced numbers of babies and children means that:
there is less need for schools and school teachers
industries which provide products for children - for example pram manufacturers
- may suffer
in the longer term, companies may have difficulty recruiting youthful workers
Possible solutions
These are some of the issues, but are there any solutions? The statement from the MSP
that 'Scots should be paid to breed' suggests that the use of financial incentives is one
possible solution. The Singapore government has launched a pro-natalist campaign, and
hopes to increase the birth rate by:
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giving financial inducements for third children
giving paid maternity leave
providing state-funded child care centres
Whether these incentives would make any difference in Western Europe is debatable.
The alternative option, tried in a number of countries experiencing a labour shortage, is
to encourage immigration.
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