Urban systems and urban planning practice questions.

Night-time air temperatures in a dense city centre are typically 3-5 degrees Celsius higher than in the surrounding countryside. The main reasons for this urban heat island are

• higher surface albedo together with greater evapotranspiration from parks and gardens
• greater cloud cover over the centre and stronger night-time winds
• increased infiltration of rainwater and lower rates of surface runoff
• low surface albedo, high thermal mass in concrete and little evapotranspiration

Answer: low surface albedo, high thermal mass in concrete and little evapotranspiration

Dark asphalt and concrete have a low albedo, so they absorb more shortwave radiation by day and release it slowly as longwave radiation at night. Sealed surfaces also leave almost no water to evaporate, so the latent heat cooling that vegetated land provides is missing.

In Curitiba, Brazil, bus rapid transit runs along a small number of structural axes, and the zoning rules permit the highest building densities only along those same axes. The environmental value of linking transport and zoning in this way is that

• residents live within walking distance of high-capacity transit, so car use per person falls
• the city can keep expanding outwards indefinitely without any increase at all in its ecological footprint
• bus emissions are removed at source, because articulated buses burn no fossil fuel at all
• land values fall along the axes, which prevents further development in the city centre

Answer: residents live within walking distance of high-capacity transit, so car use per person falls

Tying residential density to transit capacity shortens journeys and shifts them from private cars to buses, which cuts the energy input and the air-pollutant output per person. The same buses would achieve far less if the population were dispersed across low-density suburbs.

A family leaves a rural district after three successive harvest failures and moves to the capital, where relatives have already found factory work. In an analysis of rural-to-urban migration the harvest failures are

• a pull factor acting at the destination
• a push factor acting at the origin
• an intervening obstacle between the two places
• a counter-urbanisation flow from the city to the countryside

Answer: a push factor acting at the origin

Push factors are negative conditions at the place of origin that make staying untenable, while the relatives' job contacts are the pull factor at the destination. Most real migration decisions combine the two rather than resting on either alone.

A city replaces several hectares of asphalt car park with permeable paving, rain gardens and street trees. Within the urban water system this alters the balance of flows by

• increasing surface runoff while reducing infiltration into the soil storage
• removing the groundwater storage from the urban system altogether
• increasing infiltration and evapotranspiration while reducing surface runoff
• converting the urban water system from an open system into a closed system with no outputs

Answer: increasing infiltration and evapotranspiration while reducing surface runoff

Sealed surfaces short-circuit the water cycle by turning nearly all rainfall into rapid surface runoff. Permeable surfaces and vegetation restore infiltration to the soil and groundwater storages and return water to the atmosphere by transpiration, which lowers and delays the flood peak downstream.

A large share of the urban population in low-income countries lives in informal settlements. The environmental condition that most directly threatens health there is

• the shortage of green space within walking distance
• the high level of night-time light from the city centre
• the noise generated by nearby industrial activity
• the absence of a safe water supply and of sewerage

Answer: the absence of a safe water supply and of sewerage

Settlements built without legal title are usually built without services, so water is bought from vendors or drawn from contaminated sources and sewage runs in open channels. The result is a diarrhoeal disease burden that falls hardest on young children, which is why water and sanitation are the first infrastructure priority in upgrading schemes.

A city designates a green belt around its edge in which new building is prohibited. A side effect commonly observed afterwards is that

• the population inside the city falls sharply within a decade
• new development leapfrogs the belt, lengthening commuting trips
• agricultural land inside the belt is converted to woodland
• the urban heat island effect intensifies within the green belt itself

Answer: new development leapfrogs the belt, lengthening commuting trips

Restricting supply at the edge does not remove the demand for housing, so development appears in towns beyond the belt and the journeys between home and work grow longer. A green belt therefore protects land while potentially raising transport energy use, which is why it is usually paired with densification inside the city.