Define the term megacity.

Explain two processes responsible for population growth in megacities.

Using only an annotated diagram, explain the operation of a sustainable urban system.

Examine the effects of human activity on the climate of urban areas.

A city with a population of more than 10 million people [1 mark].

Award [1 mark] for each of two valid processes such as natural increase, in-migration, and boundary redefinition (urban sprawl) and [1 mark] for the explanation of the process. Natural increase occurs when crude birth rates exceed crude death rates where a youthful population structure exists. In-migration can be from rural or urban areas, or from other countries. Boundary redefinition may cause the megacity to expand and engulf surrounding settlements.

Sustainable urban systems have a circular system where inputs (energy, water, people, materials, products, food) are reduced and outputs (solid, atmospheric and liquid waste) are recycled.

Explanations without a diagram can achieve a maximum of [2 marks].

Explanations that separate the diagram from the text can achieve a maximum of [3 marks].

The examination could compare different types of urban area or address a range of climatic effects.

The effects of human activity include the creation of an urban heat island, reduced visibility, air quality such as increased incidence of smog, increased thunder storm activity and rainfall, reduced likelihood of snow and frost, increased gustiness, reduced average wind speed, changes to humidity.

Urban heat islands are formed under high pressure conditions, especially in winter and are most clearly evident at the end of night. Sources of heating include energy generation, industry, transport, buildings, appliances and people. Because of higher temperatures, relative humidity will be lower.

Reduced visibility and air quality are caused by an increase in atmospheric pollutants in urban areas (dust, aerosols, and NO_x and SO₂, O₃, particulate matter). This leads to a greater incidence of fog and smog (polluted fog).

Thunderstorms are most likely due to the additional heat found in urban areas, resulting in more convectional activity. For the same reason, snow and frost are less likely in urban areas.

Rainfall is sometimes higher because there are more hygroscopic nuclei. However, the extra heat means that the air can hold more moisture before dew point is reached and condensation occurs.

Winds may be channelled along “canyons” causing an increase in “gusty” conditions. Buildings create more turbulence. Overall wind speeds are likely to be reduced due to the increased friction with buildings in urban area compared with open spaces in rural locations.

To achieve band D a link should be made between human activity and the climate of urban areas. At least two effects should be explained.

Bands E and F require a structured examination of either a range of climatic effects or different urban environments.

Marks should be allocated according to the markbands.

Almost all correctly indicated 10 million. Most candidates used relevant processes, but not all developed their answers. Some used gentrification as a process.

Almost all correctly indicated 10 million. Most candidates used relevant processes, but not all developed their answers. Some used gentrification as a process.

Many candidates found difficulty with drawing an annotated diagram, and many diagrams were poorly drawn and very simplistic. Some candidates drew pictures rather than systems diagrams. Others seemed to have never heard of a sustainable urban system.

There were some excellent answers on urban climates, with detailed case studies. However, they did tend to be descriptive rather than examining the effects of human activity. Many candidates did not get beyond the urban heat island, and the nature of smog related pollution was often poorly understood. A large proportion of candidates wrote about how urban life can impact on climate change.