Urban Climate

Question 1

Define urban heat island effect

Answer 1

The temperature difference between urban / suburban areas and the surrounding rural areas

Question 2

What is the average temperature difference between an urban area and its surroundings?

Answer 2

1-3°C warmer in a city.

Clear, calm night -> 12°C warmer.

Question 3

What are the factors that cause urban temperatures to vary?

Answer 3

Season
Weather conditions
Sun intensity
Ground cover

Question 4

Give reasons for when UHI varies

Answer 4

Largest in summer

Decreases as size of city decreases

Question 5

Give reasons why cities are warmer than rural areas

Answer 5

• Less reflective surfaces -> much lower albedo, absorb heat during day, slowly release heat at night.
• Air pollution from industry / vehicles -> increase cloud cover + ‘pollution domes’ absorb outgoing radiation.
• Disposal of water runoff -> changes urban moisture / heat budget.
• Heat emitted from industry / vehicles / buildings / people.

Question 6

Why is UHI a concern?

Answer 6

• Hot temps make indoors / transport uncomfortable.
• Heat stroke, asthma, organ damage, deaths -> esp babies / elderly.
• UHI caused by hot, still, anticyclonic weather conditions -> more air pollution.
• Hot temps -> strain on energy for cooling / air con.
• Strain on water resources -> higher human consumption (drinking), more evapotranspiration, plants extract more from soil.
• Earlier flowering times / prolonged growing season -> allergies.
• Higher reproduction / lifespan of animals, insect pests.
• More algae in water.
• More deterioration of historical monuments / buildings -> more temp-related chemical weathering.
• Climate change -> increase intensity of UHI.

Question 7

Name the policies for reducing pollution / managing UHI

Answer 7

Cool surfaces.
Green roofs.
Urban greening.
Sky view factor.
Cool cars.

Question 8

What are ‘cool surfaces’ as a policy to reduce pollution / manage UHI

Answer 8

Eg cool roofs, cool pavements

-> made of materials with high solar reflectance -> absorb / store less solar energy during day -> emit less at night

Question 9

What are ‘green roofs’ as a policy for reducing pollution / managing UHI?

Answer 9

Eg Plants on rooftops

• > lower roof temps by 20-40°C on a sunny day.
• > reduce rainwater runoff.
• > increase biodiversity.
• > insulators.

Question 10

What is ‘urban greening’ as a policy for reducing pollution / managing UHI?

Answer 10

Eg Planting trees / vegetation

• > provides shade.
• > reduce peak temps by 5-20°C.
• > trees reduce urban flooding.
• > trees filter pollutants.
• > trees = Carbon store.

Question 11

Define ‘sky view factor’

Answer 11

Relative openness between buildings

Question 12

How does ‘sky view factor’ affect UHI?

Answer 12

Restricted sky view…

• > reduce escape of heat from surfaces -> heat builds up in ‘street canyons’.
• > reduce ventilation in UHIs if streets at 90° to prevailing winds.

Question 13

What are ‘cool cars’ as a policy for reducing pollution / managing UHI?

Answer 13

Eg light colour car shells

• > reflect more sunlight -> reduce need for air conditioning.
• > cars are large contributors to UHI and pollution.

Question 14

Which parts of local climate do urban areas affect?

Answer 14

Wind speed
Temperature
Clouds / precipitation
Pollution

Question 15

What percentage of the world’s population is predicted to live in urban areas by 2025?

Answer 15

80%

Question 16

How can urban areas affect wind?

Answer 16

Lower speeds
Greater variability
Large scale convection

Question 17

Do urban areas have lower wind speed than outlying suburbs? Why?

Answer 17

Average 5% lower wind speed in city centre.

-> roughness of land surface -> increase surface friction

Question 18

Why does wind speed vary in cities?

Answer 18

Due to layout of buildings / building height.

Question 19

Where do ‘urban canyons’ form? How?

Answer 19

Straight streets lined with tall buildings -> urban canyons.

Buildings funnel winds -> Venturi effect -> high velocity

Question 20

How is wind turbulence created?

Answer 20

High rise buildings disrupt the flow of air -> turbulence.

Any air not flowing in the general direction will cause turbulence

Question 21

Give an advantage and disadvantage of wind turbulence / gustiness

Answer 21

Adv - turbulence ‘flushes out’ pollutants at street level.

Dis - unpleasant for pedestrians

Question 22

What does ‘large scale convection’ caused by UHI lead to?

Answer 22

-> lowering air pressure -> air is drawn in from surrounding rural areas

Question 23

Define ‘intensity’ of UHI

Answer 23

The maximum difference between the temperature of the rural area and peak urban temperature -> at night

Question 24

Name the factors that influence the intensity of UHI

Answer 24

• Weather conditions -> high pressure systems with clear nights, still sunny days -> increase intensity.
• Season

Question 25

Name the main factors that create UHI

Answer 25

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

Question 26

Define anthropogenic heat

Answer 26

Heat released by human activity | Eg vehicles, central heating (/air conditioning), industry

Question 27

Give the reasons for rainfall patterns in urban areas

Answer 27

* UHI creates convection : rapid evapotranspiration -> clouds. * High rise buildings / range of heights -> air turbulence + increased vertical motion. * Surface winds drawn in from surrounding areas due to low pressure / prevailing winds move over city -> friction from urban boundary acts similar to mountain barrier -> air travels downwind of high rise buildings -> converge and rise upward -> rain clouds. * City pollution -> pollutants are 'water attracting'. * Industry / power stations -> release water vapour -> clouds.

Question 28

What percentage more rainfall is there downwind from major areas, compared to upwind?

Answer 28

20% more rainfall downwind from major areas

Question 29

Why is there more rainfall downwind from major areas than upwind?

Answer 29

Heating of the surface / overlying air -> instability in atmosphere -> encourages air to rise. Cools as it rises -> water vapour condenses into rain -> falls downwind of the city.

Question 30

Why has the occurence of fog in urban areas increased?

Answer 30

Industrialisation Eg London early 1700s - 20 days of fog per year London late 1800s - over 50 days of fog per year

Question 31

Why is there more fog in urban areas than rural areas?

Answer 31

Greater average number of particles in the are in urban areas. Particles act as condensation nuclei -> encourage fog formation at night under high pressure conditions.

Question 32

What strategies have been used to reduce the amount of fog in urban areas?

Answer 32

Clean Air Acts of 1950s | -> reduced smoke production, fewer particulate emissions in U.K. -> fewer foggy days

Question 33

Give examples of places that experience high levels of fog. | What is the issue associated with this?

Answer 33

New Delhi, Beijing -> regular winter fogs. 'Airpocalypse' - fog traps pollutants -> toxic smog -> high death toll.

Question 35

How is thunder created?

Answer 35

Extreme temperature -> rapid expansion of air -> shockwave -> thunder.

Question 36

What is a reason thought to explain why there are more thunderstorms in urban areas?

Answer 36

Air pollution -> disturbs electrical charge of clouds -> lightning

Question 40

How is lightning created?

Answer 40

By conventional heating and rapid uplift of air -> instability in atmosphere. Uplifted air rapidly cools and condenses due to cold clouds -> releasing its heat -> further uplift. Positive electrical charges build up in clouds -> lightning