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CUE 3234 (Urban Climates) Flashcards

1
Q

Outline the urban canopy layer.

A

Below roof level, directly above buildings. Where processes act in space between buildings.

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2
Q

Outline urban boundary layer.

A

Layer way above buildings and UCL.

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3
Q

What does a high albedo mean?

A

The surface reflects the majority of the radiation that hits it, and absorbs the rest (even though it’s only a little)

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4
Q

What does a low albedo mean?

A

A surface reflects a small amount of the incoming radiation and absorbs the rest (the majority).

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5
Q

Which surfaces have the highest albedo?

A

Surfaces that are translucent or white (eg ice and snow)

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6
Q

How is albedo expressed?

A

On a scale from 0-1.
0 = total absorption (dark colours)
1 = complete reflecting of solar energy (light colours)

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7
Q

Urban environment albedos tend to be much _____ than those of rural areas.

A

Lower

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8
Q

In the CBD, when are temperatures typically highest?

A

Mid afternoon

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9
Q

In the UHI, where do you see secondary peaks of high temperatures?

A

Over other built up areas such as suburban residential areas.

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10
Q

The temperature range from rural to city centre is often ________ at night.

A

Greatest

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11
Q

Why is the temperature range from rural to city centre greatest at night?

A

Because of the high heat storage capacity of building materials compared to vegetation.

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12
Q

How much variation in surface temperature in surface temperatures over areas of water? Why?

A

Very little. Because water maintains a constant temperature 24/7 due to its high heat capacity.

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13
Q

What are the 5 strategies for managing the UHI?

A
  • cool surfaces (eg cool roofs)
  • green roofs
  • urban greening
  • sky view factor
  • cool cars
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14
Q

What are the two levels within the urban dome?

A

Urban canopy and urban boundary layer

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15
Q

Cool surfaces

A

Cool roofs built from materials with a high albedo absorb and store less solar energy during the day, and do are not major emitters of heat into the urban atmosphere at night.

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16
Q

Green roofs

A

Green roofs can reduce rooftop tenure tire by 20 - 40* on a sunny day. They also reduce:

  • rainwater runoff
  • act as insulators
  • increase urban biodiversity by providing habitat space for birds and small animals.
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17
Q

Urban greening

A

Planting vegetation provides shade and can have a natural cooling effect as seen by lower temperatures in urban parks. They also:

  • act as carbon stores
  • reduce urban flooding
  • filter pollutants from the air (eg photosynthesis)
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18
Q

Sky view factor

A

This describes the relative openness between buildings in an urban area. A restricted sky view, as found for narrow streets and tall buildings will reduce the escape of hear from the street and building surfaces. This can contribute to the accumulation of heat within street canyons and lead to the increase in air temperatures.
In addition, if streets are angled perpendicular to the prevailing wind, during intense UHI events, this will reduce the change in ventilation and removal of heat and pollutants which accumulate between buildings.

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19
Q

Cool cars

A

A lighter coloured car shell reflects more sunlight than a traditional dark car. This cools the inside of the car and reduces the need for air con. Cars in cities contribute significantly to higher temperatures and pollution levels.

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20
Q

Rainfall is ______ over urban areas than rural areas.

A

Higher

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21
Q

Why is rainfall higher over urban area than a rural area?

A
  1. UHI generates convection currents so uplift causes cloud formation.
  2. Lower pressure over urban areas because of rising air.
  3. Buildings promote vertical uplift of air; encouraging rain.
  4. Pollution increases cloud formation and hygroscopic nuclei assist in raindrop formation.
  5. Lots of water vapour from industries and power stations.
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22
Q

Under what conditions are intensity, frequency and length of fogs greatest?

A

Anticyclonic conditions

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23
Q

In which areas are the likelihood of thunderstorms greatest?

A

Urban areas, particularly in hot, humid conditions eg late summer in late afternoon

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24
Q

Cloud cover is increased in _____ areas

A

Urban

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25
Q

What are convection currents generated by?

A

Higher temperatures of the urban microclimates and so there is a larger number of condensation nuclei which will therefore form more clouds.

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26
Q

In urban areas, why are the hydroscopic nuclei larger?

A

Because there’s a greater amount of pollution from dust from cars, fuel and industry which contribute to it.

27
Q

How are clouds formed?

A
  • convection currents

- the result of smog, as this causes smoke which will appear as low lying clouds.

28
Q

Outline relative humidity.

A

The amount of water vapour in the air, expressed as a percentage of the maximum amount that the air could hold at any given temperature.

29
Q

Is relative humidity lower or higher in urban areas?

A

Lower

30
Q

What’s the main reason why relative humidity is lower in urban areas?

A

The air is warmer, meaning it can hold more moisture - however, the lack of vegetation and surfaces limit evapotranspiration

31
Q

How is relative humidity affected seasonally?

A

Humidity can be 2% lower in the winter as the air is generally cooler so can hold less moisture.

In summer, it can be 10% lower as the air is much warmer which is increased by the land surfaces within the urban area that absorbs heat due to the dark colour of the buildings and road surfaces.

32
Q

Generally, winds ________ in urban areas compared to the suburbs / rural areas.

A

Decrease

33
Q

How do buildings affect winds?

A

Due to their shape, size and spacing.

34
Q

Why is the speed of wind lower in altitudes in built up areas?

A

Because buildings are deflected over high rise buildings.

35
Q

What are the 4 pollutants that lead to air pollution?

A

Carbon monoxide, nitrogen dioxide, particulates and sulphur dioxide.

36
Q

How is carbon monoxide (CO) formed?

A

By the incomplete combustion of fuel

37
Q

How is nitrogen dioxide formed?

A

When material or fuel containing nitrogen is burned.

38
Q

How does nitrogen dioxide form particles?

A

The nitrogen dioxide reacts with hydrocarbons with sunlight to create ozone - and contributes to the formation of pollutants.

39
Q

What is the cause of carbon monoxide?

A

Road transport is responsible for 90% of CO emissions in the UK

40
Q

What is the cause of nitrogen dioxide?

A

Road transport is responsible for 50%

41
Q

What is the cause of particles?

A

They originate mainly from power stations and vehicle exhausts. Also come from ash, dust, pollens, soil particles, engine wear.

42
Q

What are the accuses of nitrogen dioxide?

A

Coal and oil burning by industries (eg power stations / refineries).

43
Q

Impacts of CO?

A
  • affects transport of oxygen around the body

Breathing in low levels results on headaches, nausea, fatigue.

44
Q

Impacts of nitrogen dioxide?

A
  • inflames lining of the lung
  • worse for asthmatics
  • oxides of nitrogen can cause accelerated weathering of buildings and acid rain 🌧
45
Q

Impacts of particles?

A

Particles smaller than 10 micrometers can settle in airways and lungs.

46
Q

Impacts of sulphur dioxide?

A

Short term exposure can cause coughing, tightening of the chest, narrowing of the airways.
Can also produce haze, acid ran, damage to buildings and building corrosion.

47
Q

Why might the concentration of pollutants increase in winter?

A

Because temperature inversions trap them in the city.

48
Q

Name the three main pollution reduction policies.

A
  1. Clean air acts
  2. Zoning of industries
  3. Vehicle control and public transport
49
Q

Why was the Clean Air Act introduced?

A

Because the 1952 London smog left the city unable to see anything, so 4 years later, we introduced smoke free zones into urban areas.

50
Q

Outline zoning of industry.

A

This is where industry of cities is located downwind in cities if possible, and planning legislation has forced companies to build higher chimneys to emit pollutants above the inversion layer.

51
Q

Outline vehicle control and public transport.

A

In London, an ULEZ is being introduced in September 2020. In this, exhaust emissions standards are set and a daily non-compliance charge introduced to encourage cleaner vehicles to drive in central London.
All double decker buses in the centre will en hybrid and all single deck will be 0 emission. ⛽️

52
Q

How could river be used for reducing pollution?

A

Park and glide - eg Thames, London

53
Q

How could you encourage fitness and reduce pollution?

A

Greater provisions for cyclists. Eg Snake bridge in Copenhagen. Encouraging Active Travel.

54
Q

Define albedo.

A

The reflectivity of a surface. It is the ratio between the amount of incoming insolation and then amount of energy reflected back into the atmosphere.
Lighter surfaces reflect more than dark surfaces so have a greater albedo.

55
Q

Define microclimate.

A

The small scale variations in temperature, precipitation, humidity, wind speed and evaporation that occur in a particular environment such as an urban area.

56
Q

What is the urban dome?

A

Th urban microclimate is sometimes known as an urban dome. This is the area within which the weather is different of that from surrounding rural areas.
Two levels of the dome: urban canopy and urban boundary layer.

57
Q

Define photochemical pollution.

A

A form of air pollution that occurs mainly in cities and can be dangerous to health.
Exhaust fumes become trapped by temperature inversions, and in the presence of sunlight, low level ozone forms.
Associated with high pressure weather systems.

58
Q

What is a temperature inversion?

A

An atmospheric condition in which temperature, unusually, increases with height. As inversions are extremely stable conditions and do not allow convection, they trap pollutants in the lower level of the atmosphere.

59
Q

What is the Urban Heat Island?

A

The zone around and above an urban area which gas higher temperatures than the surrounding areas.

60
Q

Define isotherm.

A

A line joining places with equal temperatures.

61
Q

Define channelling.

A

Wind redirected down long, straight, canyon like streets where there is less friction. (Sometimes called urban canyons)

62
Q

What is the Venturi Effect?

A

The squeezing of wind into an increasingly narrow gap, resulting in a pressure decrease and velocity increase of winds.

63
Q

What is prevailing wind?

A

A wind from the direction that is predominant / most usual at a particular place or season.

g: cue (14 decks)

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