AQA A2 Geography
2 Weather and climate and
associated hazards
Section A
1 (a) Although the earth–atmosphere system as a whole has a balanced heat budget,
there are regional variations. The heat budget is positive in the tropics and subtropics (0–40 latitude). In middle and high latitudes there is a negative budget.
The result is a surplus of energy in the tropics and sub-tropics and a deficit
elsewhere. This imbalance drives the general circulation which evens out
geographical disparities in the heat budget. In low latitudes, warm tropical air is
exported polewards to higher latitudes via the Hadley Cells and tropical cyclones.
In middle latitudes, warm air from the tropics and sub-tropics meets colder air,
creating pressure differences. These differences give rise to the jet stream and
strong westerly air flows. Low-pressure areas or depressions, which develop within
the jet stream, transfer warm tropical air polewards.
(b) Global winds and warm ocean currents transfer surplus energy to middle and
high latitudes from the tropics and sub-tropics. These agents balance the energy
budget for all parts of the planet. Warm air around the equator is transferred by
large convective (or Hadley) cells. In middle and high latitudes the jet stream, with
its depressions performs a similar role. Warm surface ocean currents are also
important to the general circulation. Huge amounts of heat energy are evaporated
from ocean surfaces in the Atlantic and Pacific; transported polewards by air
masses this latent heat is released during condensation. The warming affect
of the North Atlantic current in western Europe demonstrates the effectiveness
of the coupling between the oceans and the atmosphere. Meanwhile, ocean
temperatures also influence atmospheric pressure which is key to the development
of the jet stream and its associated depressions.
(c) Between 19 and 20 November 2009, northern England was struck by a major
storm event. After several weeks of torrential rain, a cold front remained stationary
over northwest England, bringing record amounts of rain. On 19 November,
Seathwaite in the Lake District experienced the heaviest rainfall ever recorded in
the UK during a 24-hour period. This exceptional event produced extreme river
flows. The River Derwent in Cumbria peaked at 561 cumecs on 20 November.
The storm’s main impact was to cause widespread flooding across northwest
England, especially in Cumbria. There, 2,200 properties were flooded, including
over 900 in the small market town of Cockermouth. Around 100 farms were also
flooded, with losses of livestock. Extensive sheets of gravel and silt, left behind
when the floods receded, were deposited on fields. Three bridges were completely
destroyed and 20 others were damaged and closed temporarily. At Workington, a
police officer drowned when a bridge across the Derwent collapsed. Several major
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roads became impassable due to flooding and land slips. The port of Workington
at the mouth of the River Derwent was closed due to river erosion and
sedimentation of the harbour. The final cost of the floods was more than £275
million. Damage to commercial and residential properties accounted for nearly
80% of the costs while the repair bill for damaged infrastructure was around £35
million.
2 (a) The geography of the British Isles (i.e. a group of islands situated on the extreme
northwest margin of the Eurasian landmass) means that it is frequently exposed
to both oceanic and continental air masses that originate in tropical and polar
regions. This, together with the proximity of the polar front jet stream, largely
explains the variability of its weather and climate. Maritime air masses approach
the British Isles from a southwesterly to northwesterly direction. They bring
average to above average temperatures, variable amounts of cloud, and some
rain. Sudden changes in weather occur with a switch in wind direction. Northerly,
easterly and southerly air streams often bring extreme weather. Polar continental
and arctic maritime air masses are associated with severe winter weather (i.e. frost
and snow), while heat waves and drought are linked to tropical continental air
masses.
(b) Depressions are travelling low-pressure storms which dominate the weather and
climate in middle latitudes. They are associated with a rapid and predictable
sequence of weather changes. Most of these changes revolve around active
frontal zones where tropical and polar air masses meet. At the warm front, tropical
maritime air rises above a wedge of colder, denser polar air. Thus as the warm
front approaches, high cirrus cloud gradually thickens to form rain-bearing
nimbostratus and stratocumulus. Meanwhile pressure falls, as the wedge of cold
dense air narrows. In the vicinity of the warm front, precipitation is steady and
prolonged but as the warm front passes temperatures rise, pressure stabilises, the
wind veers, precipitation ceases and clouds may clear. These conditions in the
warm sector may last for a few hours (depending on the speed of the depression)
before the cold front approaches and clouds thicken as warm air is undercut by
colder air and forced aloft. This motion is more rapid and the ascent steeper than
at the warm front. As a result precipitation (often from cumulonimbus clouds) is
more intense, though less prolonged. At the passage of the cold front there is an
abrupt rise in pressure, the wind veers, there is a drop in temperature and the sky
clears; continuous precipitation is replaced by showers.
(c) Tropical cyclones are closely monitored by agencies such as the US National
Weather Service and the National Hurricane Center. Measurements of
temperature, humidity and wind speed are made, and storm paths are tracked
using satellites, aircraft, ships and buoys. National weather services issue
warnings of approaching storms. In the USA, two categories of warning are given:
(1) Hurricane Watch: announcements that hurricane conditions are possible within
the next 36 hours; and (2) Hurricane Warning: announcements that sustained
winds of 119 km h–1 and above are expected within the next 24 hours.
Early warnings provide a crucial window for evacuation from low-lying areas at
risk from flooding. Mass evacuations in Bangladesh in 2007 (Cyclone Sidr) and
Houston in 2008 (Hurricane Ike) saved thousands of lives. In 2005, 350,000
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residents in New Orleans were evacuated ahead of Hurricane Katrina. Those
unable to leave the city were accommodated in temporary refuges such as the
city’s superdome sports arena. Even so, not everyone heeded the warnings,
contributing to a death toll of more than 1,400.
Hard engineering, through the construction of flood embankments or levees, is
an important (but not always effective) response to storm surges (i.e. the principal
natural hazard caused by tropical cyclones). New Orleans suffered catastrophic
flooding during the Katrina storm (80% of the city was indundated), largely
because the storm surge breached the levees. The effectiveness of flood defences
depends on their maintenance and their height. Those protecting New Orleans
were overwhelmed because they were not designed for a category 5 hurricane
like Katrina. In developing countries such as Bangladesh and India, storm shelters
provide vital refuge from storm surges and are a cheaper alternative to levees.
In the past 20 years they have helped to significantly reduce the number of
fatalities from storm surges in the Brahmaputra–Ganges delta.
The importance of early warnings, levees and storm shelters was evident in the
Cyclone Nargis disaster in 2008, when storm surges killed an estimated 140,000
people in the Irrawaddy delta in Burma. The government provided no advance
warning of the storm. There were no evacuation plans and no levées or storm
shelters to protect against storm surges. The destruction of coastal mangrove
forests (a natural barrier) also increased the vulnerability of populations living in
the delta.
Responses to tropical cyclones also occur in the aftermath of a disaster. By 2006,
the US government had responded to the Katrina disaster by allocating nearly
US$17 billion of federal funds for rebuilding damaged properties and strengthening
flood defences. The Federal Emergency Management Agency coordinated the
response to the disaster.
In contrast, the response to the Nargis disaster was hampered by poverty and the
Burmese government which did not welcome foreign intervention and initially
refused to acknowledge to the scale of the disaster. Reluctantly, the government
eventually agreed to admit foreign aid agencies and workers to the stricken areas.
Priority was given to humanitarian aid to meet the immediate needs of survivors.
Yet despite the massive international response, 12 months after the disaster,
130,000 people were still living in temporary shelters and jobs remained scarce.
Section C
3 Tropical revolving storms are also known as hurricanes, typhoons and cyclones. They
occur in the tropics and sub-tropics, forming over the oceans where sea surface
temperatures are 27ºC and above.
The impact of tropical revolving storms can be devastating. Examples of major natural
disasters caused by these storms are Hurricane Katrina in 2005, and Tropical Cyclone
Nargis in 2008. The main hazards associated with tropical revolving storms are
hurricane force winds, torrential rainfall (leading to river flooding), and storm surges
flooding low-lying coasts. Of these hazards, storm surges are by far the most deadly.
Those generated by Hurricane Katrina killed over 1,100 people in New Orleans and
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the Gulf of Mexico coast. A similar storm surge caused by Tropical Cyclone Nargis
in 2008 in Burma’s Irrawaddy delta is estimated to have killed more than 140,000
people. Powerful tropical revolving storms also cause huge economic damage.
Hurricane Katrina was the costliest natural hazard in US history. In the wake of
Tropical Cyclone Nargis, damage to farmland, infrastructure and food crops was in
the region of US$10 billion. As with other types of natural hazard, the overall impact
of tropical revolving storms depends on the preparedness and vulnerability of society.
For this reason these storms have their greatest impact in the developing world.
Modern technology can greatly reduce the impact of tropical revolving storms. In the
USA and other MEDCs, monitoring by satellite and aircraft permits accurate tracking
and forecasting. This allows governments to evacuate populations at risk. Storm
surges can be countered by the construction of levees and surge barriers, and
residents in areas exposed to revolving storms can be educated to minimise the risks
to themselves and their families. Because revolving storms often develop over
extended periods (1 or 2 weeks), unlike other natural disasters (e.g. earthquakes,
volcanic eruptions) responses by government agencies can be carefully planned to
mitigate their impact. However, these responses are often less effective in LEDCs and
the impact of tropical revolving storms (in terms of loss of life) is proportionately much
greater.
4 International efforts to tackle global climate change have had only limited success.
The Kyoto Protocol, the only international treaty on climate change to date, expires in
2012. No replacement treaty has been agreed, despite lengthy talks in Copenhagen,
Cancún and Durban. Kyoto was only a partial success because (a) many countries did
not meet their carbon dioxide emissions targets; (b) the USA, the biggest carbon
dioxide emitter (up to 2010) did not sign the treaty; and (c) LEDCs, including major
polluters such as China and India were exempt. As a result, carbon dioxide and other
greenhouse gas (GHG) emissions will continue to rise. Already, atmospheric
concentrations of GHGs are at record levels. For the forseeable future, any controls
by countries such as the USA, China and India will be voluntary. These and other
countries will continue to place economic growth and politics ahead of the
environment. Only EU countries (responsible for around 25% of annual GHG
emissions) are likely to sanction legally binding controls.
There is more enthusiasm for unilateral controls on GHG emissions, i.e. by individual
countries. Environmental concerns help to drive efforts by governments to expand
alternative, sustainable energy resources (e.g. HEP, wind and solar power) and
nuclear power. Many governments also implement policies to improve energy
conservation and favour less polluting fossil fuels like gas, ahead of coal. China’s
attempts to reduce air pollution (e.g. switch to cleaner fuels, legislation on emissions)
are mainly a response to localised pollution problems in highly urbanised and
industrialised regions. Action is prompted because this pollution is injurious to health,
rather than out of considerations of global climate change. Carbon trading has had
some success providing financial incentives to reduce emissions. Carbon capture and
storage remains an unproven technology. Despite some success at the national scale,
so far it has been insufficient to halt the inexorable global rise of atmospheric carbon
dioxide and other GHGs.
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