Contemporary Urban Enviroments - Section 4 Urban Climate

Question 1

What is the urban heat island effect?

Answer 1

Urban areas are warmer than rural areas
Urban areas with higher air temperatures than the surrounding rural areas are called urban beat islands

Highest temps found in industrial areas and most densely built up areas eg) central business district

Question 2

What is albedo?

Answer 2

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

Question 3

What is a high albedo?

Answer 3

The surface reflects the majority of radiation that hits it and absorbs the rest.
Lighter colours reflect light

Question 4

What is a low albedo?

Answer 4

A surface reflects a small amount of the incoming radiation and absorbs the rest
Darker colours tend to absorb light

Question 5

What is the difference in albedo between urban and rural areas?

Answer 5

Urban albedos tend to be much lower than rural albedos

Question 6

What are the causes of the UHI effect?

Answer 6

1. Urban surfaces - tend to be much less reflective than rural areas, building materials eg) concrete, bricks, and tarmac have lower albedo absorbing large amounts of radiation and releasing heat as long wave radiation.
2. Air pollution - from industries and vehicles increase cloud cover creating a ‘pollution dome’
3. Heat from human activity - heat comes back from industries, buildings, vehicles.
4. Less evapotranspiration- in urban areas, water falling on surface is disposed of as quickly as possible in drainage systems so there is little surface water to evaporate. Little vegetation and so little transpiration. Evapotranspiration uses heat energy this changes the urban moisture and heat budget - reduced evapotranspiration means more energy is available to heat the atmosphere.

Question 7

How does the UHI vary diurnally (day and night)?

Answer 7

The UHI effect is stronger at night.
Urban day time temps are on average 0.6° warmer than surrounding rural areas, but by night temps can be 3-4° warmer as rural areas cool at night but urban areas don’t as much as the surfaces continue to release heat they have absorbed during the day.

Question 8

How does the UHI effect vary seasonally?

Answer 8

Stronger in summer - more solar radiation in summer so urban areas absorb more heat
Stronger when there is an anticyclone - these cause clear skies and low winds. If there are no clouds, more solar radiation reaches and heats the ground, low winds mean warm air is not blown away.

Question 9

Why does it rain more in urban areas?

Answer 9

UHI effect means air in urban areas is warm and warm air can hold more water. Warm moist air rises (convectional uplift). As it rises it cools, water vapour condenses and it rains (convectional rainfall)
Lower pressure over urban areas because of rising air
Buildings promote vertical uplift of air promoting rain
Lots of dust and pollution over urban areas - act as condensation nuclei encouraging clouds to form.
Lots of water vapour from industries and power stations

Question 10

What is fog like in urban areas?

Answer 10

Frequency of fog is greater in urban areas because of higher concentration of condensation nuclei
Intensity and lengths of fog is greater

Question 11

What are thunderstorms like in urban areas?

Answer 11

Higher temps of urban areas increase likelihood of thunderstorms
Convectional uplift and rising heat, water vapour and condensation nuclei from industry and vehicles creates intense precipitation and thunderstorms.
During condensation, latent heat is released further fuelling the convectional uplift. Raindrops are split in the uplift creating a positive electrical charge forming lightning.

Question 12

What is frost and snow like in urban areas?

Answer 12

Does not snow very often in urban areas but when it does the snow melts faster because of UHI effect.
Also have fewer days of frost due to UHI effect

Question 13

What are winds like in urban areas?

Answer 13

Winds are effected by buildings
Average wind speed usually lower in urban areas than rural areas because tall buildings create friction that slows moving air.
There are areas where wind speed is zero, some areas are totally sheltered by buildings
Turbulence occurs around buildings - when wind hits face of building some is deflected down some around sides and some over top - create vortices when these winds hit ground / other buildings
Powerful gusts - when wind is channelled down the street known as the canyon effect

Question 14

What are some examples of windy cities?

Answer 14

Chicago - grid system of buildings create wind tunnels where wind can pick up speed
Leeds, Bridgwater place - high rise building, a man was killed when lorry overturned due to strong winds and streets around have been closed at times due to strong winds,

Question 15

What are the types, causes and impacts of air pollution?

Answer 15

Particles or particulate matter - vehicle exhausts, burning fossil fuels, can cause health problems
Carbon monoxide - road transport, impacts transport of co2 around body
Nitrogen dioxide - road transport, causes accelerated weathering of buildings, acid rain , inflame lung lining
Sulphur dioxide - oil burning by industry and coal, causes acid rain, damage to plants, tightening of chest

Question 16

What are temperature inversions and photochemical smog?

Answer 16

pollutants come into contact with sunlight, UV light causes them to break down into harmful chemicals e.g. ozone which form photochemical smog.
Photochemical smog is a problem in many cities, It is more common in places with hot and sunny climates because there is more sunlight.
These often have a temperature inversion which keeps the pollutants at ground level
Photochemical smog is linked to health problems such as breathing difficulties and can also damage plants.

Question 17

What is a temperature inversion in urban areas?

Answer 17

occurs where a layer of warm air is formed on top of a layer of cold air, unlike in normal atmospheric conditions, also called thermal inversion.
The dome of smog seen above the city also links to the concept of the Urban Heat Island which is a result of the increase in the amount of pollutants released into the area

Question 18

What is a place example for temperature inversion / photochemical smog?

Answer 18

Los Angeles
High pressure over LA creates dense, cold, still air at surface, warmer air above - temp inversion

Cold dense air sits in urban basin and traps pollutants above - stable conditions allow photochemical smog to build up.
Problem due to high density of vehicles and frequent sunshine.
Attempt have been made to improve public transport but LA is a large sprawling city and easier to get round by car.

Question 19

What is the congestion charging strategy pollution reduction policy?

Answer 19

People are charged if they use their vehicle in certain places at certain times
Reduce pollution by reducing road traffic

In central London it reduced traffic and emissions
Hard to enforce
Some travel around edge of zones to avoid being charged, increasing traffic in these areas

Question 20

What is the pedestrianisation strategy pollution reduction policy?

Answer 20

Vehicles are prevented from entering certain places at certain times

Can lead to shops receiving fewer customers as people can only get to them by foot

Question 21

What is the public transport strategy pollution reduction policy?

Answer 21

Encouraging people to use public transport instead of cars to reduce pollution
Improved bus services to make journeys cheaper, faster, more efficient
Park and ride schemes
Trams and light railway services

Public transport improvements are often expensive
Park and ride schemes shift problems to rural areas
New developments can cause problems

Question 22

What are larger ways of tackling urban air pollution?

Answer 22

1. Legislation laws - setting air quality standards
   UK clean air acts of 1956 and 1968 reduced domestic pollution
2. Alternative fuels
   Petrol and diesel replaced with cleaner fuels that pollute less
3. Zoning of industry
   Industry located downwind of cities if possible
   Planning legislation forced companies to build higher factory chimneys to emit pollutants above inversion layer