🌆3.2.3.4 - Urban Climate

Question 1

What aspects of the climate can urban areas modify?

Answer 1

Wind speeds
Temperature
Clouds and precipitation
Pollution

Question 2

In what ways can winds be modified by urban areas?

Answer 2

Lower speeds
Greater variability
Large-scale convection

Question 3

Why are wind speeds generally lower in urban areas?

Answer 3

The roughness of the land surface, consisting of buildings at a variety of heights, creates greater surface friction

Question 4

What is an urban canyon?

Answer 4

A street with tall buildings either side but a space in between

Question 5

What is the Venturi effect?

Answer 5

The effect of ‘squeezing’ the airflow, increased as buildings get taller towards the city centre

Question 6

Where is the highest air pressure?

Answer 6

In the upper part of the building with air flowing down the front and over the top

Question 7

How does convection occur over urban areas?

Answer 7

Convection and uplift over the area means that air pressure is lowered and air is drawn in from surrounding rural areas

Question 8

What is the urban heat island?

Answer 8

Urban areas are on average 1-2 degrees warmer than their surrounding rural areas

Question 9

What is albedo?

Answer 9

The degree of warming of a surface depends on the amount of insolation absorbed by the surface, which in turn depends on how much is reflected. The reflection is albedo.

Question 10

What is sky view factor?

Answer 10

The amount of sky you can see without your view being impeded by tall buildings

Question 11

What is specific heat capacity?

Answer 11

The amount of energy required to raise the temperature of 1kg of a material by 1 Kelvin

Question 12

What is the intensity of an urban heat island?

Answer 12

The maximum difference between the temperature of the rural area and the peak urban temperature

Question 13

How can weather conditions impact the intensity of the urban heat island?

Answer 13

High pressure systems with clear nights and sunny days increase the intensity

Question 14

What are the five main factors causing urban heat islands?

Answer 14

Anthropogenic heat
Height and arrangement of buildings
Nature of building materials
Presence of water
Pollutants

Question 15

What is sensible heat?

Answer 15

Heat energy which can be felt, as opposed to latent heat which must be measured

Question 16

How does cloud cover differ in urban areas?

Answer 16

It is greater

Question 17

Why is cloud cover greater in urban areas?

Answer 17

There are more condensation nuclei, so cloud formation is greater

Question 18

Why are there more thunder storms in cities?

Answer 18

Enhanced convectional uplift from the heat island can lead to instability in thermals, creating more storms

Question 19

How do precipitation levels differ in urban areas?

Answer 19

They are higher

Question 20

What is the speed effect?

Answer 20

Wind is slowed by friction from the building, friction with obstacles

Question 21

What is the downdraught effect?

Answer 21

Turbulence is created around the top of the building, downdraught forms at the front

Question 22

What is a downwind eddy?

Answer 22

A circular motion of wind behind the building and turbulence near the top of the building occurs as the wind is forced upwards

Question 23

What can photochemical pollution lead to?

Answer 23

Smog - consisiting of ozone and peroxyacetyl nitrate (PAN)

Question 24

When were London’s ‘Pea Soupers’?

Answer 24

December 1952

Question 25

How did the smog form in London?

Answer 25

Windless, cold climate and an anticyclone (little wind, high pressure, still air) nothing moved and the particulates were trapped

Question 26

What did the smog cause?

Answer 26

Pneumonia, TB, heart failure, bronchitis

Question 27

How many died in the London smog?

Answer 27

4000+

Question 28

When was the clean air act introduced?

Answer 28

1956

Question 29

What is urban greening?

Answer 29

Involves growing plants wherever possible in urban areas

Question 30

What are the benefits of urban greening?

Answer 30

Improved air cooling, making towns and cities more bearable Improving air quality by absorbing gases such as CO2 Improved respiratory health

Question 31

What is zoning of industry?

Answer 31

The process of dividing land into zones, and moving industry to the outskirts

Question 32

What does zoning of industry lead to?

Answer 32

Less particulates being trapped over the city in the cool, moist air above

Question 33

What are transport solutions?

Answer 33

Bus only lanes introduced to encourage the use of public transport in cities, congestion charges introduced

Question 34

How does temperature vary in cities?

Answer 34

Pockets of cool air above parks and water bodies (temperature sinks) Highest temperature in industrial areas and densely populated areas

Question 35

Causes of Urban Island Effect

Answer 35

Absorption of heat by urban surfaces Air pollution Heat from human activity Less evapotranspiration

Question 36

Absorption of heat by urban surfaces

Answer 36

. Urban surfaces have low albedoes- absorb heat in day, release it at night . Windows reflect heat

Question 37

Air pollution

Answer 37

. Increased cloud cover . ‘Pollution dome’ . Reflect outgoing heat into the city

Question 38

Heat from human activity

Answer 38

Air conditioning Central heating Cars

Question 39

Less evapotranspiration

Answer 39

. Little vegetation- less heat energy used, increased temp . Water quickly removed through drainage systems, less to evaporate

Question 40

Issues with urban heat island effect

Answer 40

. Heat stroke . Pressure on energy supplies (air con) . Increased water consumption in warmer weather . Increased evapotranspiration rates . Plants flower longer + earlier (discomfort for allergy sufferers) . Increases rates of temperature related chemical weathering

Question 41

Strategies to reduce urban heat island effect

Answer 41

Green roofs Urban greening Light cars Discourage urban canyons

Question 42

How do green roofs reduce urban heat island effect?

Answer 42

Decreased roof temp 20-40C Increased urban biodiversity

Question 43

How does urban greening reduce urban heat island effect?

Answer 43

Shade = cooling

Question 44

How do light cars reduce urban heat island effect?

Answer 44

Reflect heat Cooler inside of car Less air con needed

Question 45

How does discouraging urban canyons reduce urban heat island effect?

Answer 45

. Urban canyons decrease ventilation if perpendicular to prevailing winds . Slows removal of heat

Question 46

Why are wind speeds overall slower in urban areas?

Answer 46

Tall buildings = friction

Question 47

Venturi effect

Answer 47

. Air squeezed between streets of tall buildings . Creates high velocity winds

Question 48

How does urban heat island effect worsen urban winds?

Answer 48

. Urban heat island effect greatest at night . Air drawn in from cooler surrounding areas . Creates strong, localised win

Question 49

How can urban wind be reduced?

Answer 49

Buildings on stilts Decreases pressure difference

Question 50

Causes of increased rain, fog, and thunderstorms in urban areas?

Answer 50

. Pollutants- condensation nuclei . Urban heat island

Question 51

How does urban heat island worsen urban rain, fog, and thunderstorms?

Answer 51

. Warm moist air holds more water . Rises (conventional uplift) . Condenses, rain (conventional rain)

Question 52

Why is there less snow and frost in urban areas?

Answer 52

. Increased temperatures . Melts quickly

Question 53

Causes of particulate pollution

Answer 53

. Vehicle exhausts (80%) . Burning (cigarettes and refuse) . Construction, mining, quarrying (cement dust) . Plants, moulds (pollen, mould spores)

Question 54

Causes of photochemical pollution

Answer 54

Burning fossil fuels

Question 55

How can air pollution be reduced?

Answer 55

Transport Clean Air Acts Industrial Zoning Urban Greening

Question 56

Ways to reduce air pollution: transport

Answer 56

Congestion charges Pedestrianisation Public transport Alternative fuels

Question 57

When was the Clean Air Act introduced in London?

Answer 57

1956

Question 58

Ways to reduce air pollution: industrial zoning

Answer 58

. Too late in HIC’s . LIC cities designed to have industry downwind . Taller chimneys . Force factories to reduce gas and particulate emissions (filters)

Question 59

Ways to reduce air pollution: Urban Greening

Answer 59

. Plants filter pollutants from air . Plants in urban canyons- decreased NO2 by 40% and particulate matter by 60%

Question 60

Walkie Talkie

Answer 60

. Its curved face is channeling gusts strong enough to knock people over . Reflects light intense enough to melt cars . This is the downdraught effect when wind is forced downwards - most forceful when facing prevailing wind . To solve problem protective fins have been added