3.2.3.4 Urban Climate

Question 1

Urban climates create their own climate and weather, or

Answer 1

Microclimate- this is sometimes called ‘climatic dome’ within which the weather is different from surrounding rural areas in terms of temperature,relative humidity, precipitation, air quality and wind speed

Question 2

There are 2 levels within the urban dome:

Answer 2

• below roof level there is an urban canopy where processes act in the spaces between buildings
• above this is the urban boundary layer

Question 3

What does albedo refer to?

Answer 3

the amount of energy that is reflected by a surface is determined by the reflectivity of that surface, called the albedo

Question 4

What does high albedo mean?

Answer 4

Means the surface reflects the majority of the radiation that hits it and absorbs the rest

Question 5

What does a low albedo mean?

Answer 5

A surface reflects a small amount of the incoming radiation and absorbs the rest

Question 6

Surfaces that are translucent or _____ have the highest level of albedo

Answer 6

White

Question 7

Explain how albedo is expressed on a scale of 0-1

Answer 7

• 0= total absorption e.g. dark surfaces

- 1= complete reflection of solar energy e.g. translucent or white surfaces

Question 8

The albedo of pavements and roofs has been identified as a possible reason for the urban area being warmer than the surrounding rural areas:

Answer 8

the urban heat island effect

Question 9

Urban environment albedos tend to be much lower than those of rural areas
Rural surface albedos may include:

Answer 9

deciduous forest, coniferous forest and sand

Question 10

Urban and suburban areas experience higher temperatures than surrounding rural areas and this difference constitutes UHI effect.
The annual mean air temperature of a city with one million or more people can be 1 – to 3 warmer than its surroundings. On a clear, calm night, this temperature difference can be as much as

Answer 10

12 degrees Celsius

Question 11

Smaller urban areas will produce heat islands but the effect tends to decrease as city size

Answer 11

decreases

Question 12

Temperatures are typically at their highest in the ___ _______ over the CBD

Answer 12

mid afternoon

Question 13

The temperature range from rural to city centre is often greatest at night due to

Answer 13

the high heat storage capacity of building materials compared to vegetation

Question 14

There is very little variation in surface temperatures over areas of water.
This is because

Answer 14

water maintains a fairly constant temperature over a 24 hour period due to its high heat capacity

Question 15

What are some causes of the UHI effect?

Answer 15

1- urban surfaces tend to be much less reflective than in rural areas- building materials such as concrete and tarmac have a much lower albedo, absorbing large quantities of solar energy during the day. Much of this heat is stored and slowly released at night

2- some urban surfaces, particularly buildings with large windows, have a high reflective capacity and multi-storey tend to concentrate the heating effect in surrounding areas by reflecting energy downwards

3- air pollution from industries and vehicles increases cloud cover and creates a ‘pollution dome’ which absorbs a large amount of the out-going radiation

4- heat comes from industries, buildings, and vehicles which all burn fuel. Although they regulate the temperature indoors, air conditioning units release hot air into the atmosphere- even people generate heat and cities contain large populations in a dense space

Question 16

What are some issues with London’s urban heat island

Answer 16

• As temperatures rise in the summer months conditional can become uncomfortable in buildings and on city transport. During extreme heat island events, the cases of heat stroke, asthma attack and even death increase- vulnerable groups such as babies and elderly are almost likely to be effected
• Hot and anticyclonic weather conditions that are responsible for intensive UHI events also produce higher air pollution levels. This is because the chemical reaction that produces O-Zone and smog of accelerated by high temperatures
• in warmer period the the added heat from the urban heat island can lead to increased water consumption by residents and businesses placing extra strain on water supply infrastructure and can lead to water use restrictions
• there is an increased risk of the deterioration of historical monuments and buildings through increased rates of temperature related chemical weathering

Question 17

What are some strategies for managing the urban heat island?

Answer 17

As many of the effects are negative, there has been a considerable focus on urban planning strategies to reduce UHI effect:
1- cool surfaces= cool roofs built from materials with high solar reflectance (high albedo) absorb and store less solar energy during the day and so they’re not major emitters of heat into urban atmosphere at night

2- cool cars= lighter coloured shell reflects more sunlight than a traditional dark car shell, cooling the inside of the car and reduces the need for air conditioning. On average, silver coloured shells have a cabin air temperature of 5-6 degrees lower than otherwise identical black shells. Cars in cities contribute significantly to higher temperatures and pollution levels, so use of cool cars would benefit cities and car drivers significantly

Question 18

In urban areas, there are fewer rainfall days than rural areas, but when it does rain they get more rainfall- number of reasons why:

Answer 18

1- UHI generates convection currents so uplift causes cloud formation
2- lower pressure over urban areas due to 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

Question 19

Intensity, frequency and length of fogs are much greater particularly under __________ conditions

Answer 19

Anticyclonic

Question 20

The further away from the urban areas of a city, towards rural areas, fog density ________

Answer 20

Decreases

Question 21

Higher temperatures of urban areas mean that the likelihood of___________ is increased by 25%, particularly in hot humid conditions e.g. late summer in the late afternoons

Answer 21

thunderstorms

Question 22

The higher temperatures also turn snow coming from rural areas into sleet, meaning the number of days with snow laying on the ground is decreased by

Answer 22

15%

Question 23

Cloud cover is also increased in urban areas.

There tends to be more cloud cover over urban areas, compared to rural areas- couple of reasons why:

Answer 23

1- Convection currents are generated by the higher temperatures of the urban microclimates and so there is a larger number of condensation nuclei which will therefore form more clouds. The hydroscopic nuclei are also larger and this is all a result from the greater amounts of pollution in the urban area, with more dust from cars, fuel, and industry which contribute to the hydroscopic nuclei making them larger

2- The result of smog, a mixture of fog and smoke, as this causes smoke which will appear as low lying clouds. This only happens in these particular humid conditions. An example of this is in London which has a very dense population with lots of pollution and smoke from cars and industries

Question 24

The increase or decrease in amount of cloud cover can directly impact the ________ ______ in urban areas

Answer 24

precipitation levels

Question 25

Relative humidity is the amount of water vapour in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature
Relative humidity can be up to 6% lower in an urban area- number of reasons why:

Answer 25

• The air is warmer meaning it can hold more moisture, however the lack of vegetation and surfaces limit evapotranspiration. In London the city centre is warmer than the suburbs. So, the air has the ability to hold more moisture. However, the lack of open water in the centre of London means there is less evapotranspiration so humidity is lower
• The humidity is also affected seasonally with the humidity being about lower in the winter as the air is generally cooler so the air can hold less moisture

Question 26

Buildings can exert a powerful frictional drag on the air moving around them and cause changes in wind speed and direction.
3 types of effects:

Answer 26

1- Buildings create turbulence, giving rapid and abrupt changes in wind direction and speed. Average wind speeds are lower in cities than in the surrounding areas and also lower in city centres than in suburbs

2- High rise buildings may slow air movement but they also channel air into the canyons between them. Winds in such places can be so powerful that they make buildings sway and know pedestrians off their feet

3- On calm, clear nights, when the urban heat island is at its greatest, convectional processes can draw in strong localised winds from cooler surrounding areas.

Question 27

what are the 4 main pollutants?

Answer 27

• carbon monoxide (CO)
• nitrogen dioxide
• particulate matter (PM)
• sulphur dioxide

Question 28

what is the main cause of carbon monoxide?

Answer 28

produced by incomplete combustion of fuel- road transport is responsible for 90% of all CO emissions in UK

Question 29

what is the main cause of nitrogen dioxide?

Answer 29

road transport responsible for 50% of nitrogen dioxide emissions

Question 30

what is the main cause of PM?

Answer 30

particles originate mainly from power stations and vehicle exhausts

Question 31

what is the main cause of sulphur dioxide?

Answer 31

in the UK, the major contributors are coal and oil burning by industry e.g. power stations

Question 32

what are the most significant impacts of CO?

Answer 32

affects the transport of oxygen around the body- breathing in low levels can result in headaches, nausea and fatigue

Question 33

what are the most significant impacts of nitrogen dioxide?

Answer 33

• can inflame the linings of the lungs (impacts worse in people with asthma)
• oxides of nitrogen can cause accelerated weathering of buildings and acid rain

Question 34

what are the most significant impacts of PM?

Answer 34

particles smaller than 10 micrometres called PM10 can settle in the air ways and lungs, causing health problems

Question 35

what are the most significant impacts of sulphur dioxide?

Answer 35

• short-term exposure can cause respiratory issues

- can also produce acid rain, damage to plants and building corrosion

Question 36

why is air quality in urban areas often poorer than in rural areas?

Answer 36

due to a combination of dust, soot and gases produced from vehicles and industrial processes = negative impact on human health in many cities worldwide

Question 37

explain how air pollution varied with the time of the year and with air pressure?

Answer 37

concentration of pollutants may increase 5 or 6 times in Winter because temperature inversions trap them over the city

Question 38

explain the occurrence of temperature inversions in urban areas

Answer 38

• this occurs where a layer of warm air is formed on top of a layer of cold air, unlike in normal atmospheric conditions

Question 39

the dome of smog seen above the city also links to the concept of UHI which is a result of the increase in the amount of

Answer 39

pollutants released into the area- can be seen in Los Angeles

Question 40

the mixture of ___ and _____ produces smog

Answer 40

fog and smoke

Question 41

why was smog common in European cities in the 19th and first half of 20th century?

Answer 41

because of high incidences of coal burning- particularly on domestic fires

Question 42

Britain suffered particularly badly, many of the smogs being so thick they were called

Answer 42

‘pea soupers’

Question 43

60 years ago, thick fog descended on London contributing to the death of ____ people- visibility was so poor that buses and taxis ground to a halt, so tube was the only way of transport

Answer 43

4000

Question 44

there has been an increase in photochemical smog- when does this occur?

Answer 44

Photochemical smog is a type of smog produced when ultraviolet light from the sun reacts with nitrogen oxides in the atmosphere

Question 45

the action of sunlight on nitrogen oxides and hydrocarbons in vehicle exhaust gases causes a chemical reaction resulting in the production of

Answer 45

the ozone

Question 46

Los Angeles has had a serious problem with ___________ ___ for decades because of its high density of vehicles, frequent sunshine and basin topography that traps photo-oxidant gases- linked to famous Los Angeles ‘eye sting’

Answer 46

photochemical smog

Question 47

what strategies have been attempted to combat Los Angeles’ photochemical smog issue?

Answer 47

• attempts have been made to improve public transport but LA is a large sprawling city, so it’s easier to get around by car
• recent strategies include city vehicle emission standards and Clean Air Action plan for the port of LA

Question 48

why is photochemial smog particularly hazardous during anticylonic conditions?

Answer 48

once the air has descended, it is relatively static owing to the absence of wind

Question 49

why does photochemical smog build up in LA?

Answer 49

• high pressure over LA creates dense, cold, still air at the surface ; warmer air above
• this is an example of temperature inversion
• cold, dense air sits in the urban basin and traps pollution above LA
• the stable conditions allow photochemical smog to build up

Question 50

reducing air pollution in cities has become increasingly important for national and local governments = different strategies been adopted:

Answer 50

1- clean air acts
2- zoning of industry
3- vehicle control and public transport

Question 51

what is involved in clean air acts?

Answer 51

After London’s smog of 1952, British government introduced legislation to prevent smoke entering the atmosphere- The Clean Air Act of 1956 introduced smoke-free zones into urban areas

Question 52

has The Clean Air Act of 1956 worked?

Answer 52

air quality standards have improved but still high- measures to clean up construction sites have been introduced e.g. using dust suppressants at industrial sites

Question 53

what is involved in zoning of industry?

Answer 53

industry is located downwind in cities if at all possible and planning legislation has forced companies to build higher factory chimneys to emit pollutants above the inversion layer

Question 54

what is involved in vehicle control and public transport?

Answer 54

in London, Ultra-Low Emission Zone is being introduced from 2020- exhaust emission standards are set and a daily non-compliance charge introduced to encourage cleaner vehicles to drive in London. All double decker buses will be hybrid and all single deck buses will be zero emission

Question 55

state any additional vehicle and transport strategies?

Answer 55

• greater use of waterways for transport e.g. park and glide, Thames, London
• park and ride
• banning cars on certain days and alternating number plates with even and odd numbers e.g. Mexico city