1. Urban Influences on Climate

2. Outline The urban heat island effect
Causes of altered climate due to urbanization
Controls on magnitude of climate changes due to urbanization

3. Impacts of Urbanization on Climate
Temperatures (Urban Heat Island)
Insolation
Wind regimes
Precipitation & cloud cover
stockholm

4. Recognition of Urban ‘Heat Island’ effects
Before & after temperature measurements for new suburbs / ‘satellite’ cities
Comparison of changing temperatures as cities grow in population
Changing temperatures as automobile usage increases
World War II – urban heat islands disappeared when energy use reduced by bombing in German & Japanese cities
Las vegas

5. Examples of Heat Islands
Toronto: present effect 3 to 4 Cº compared to surrounding rural area (automobiles, light winds)
St. John’s: ≤1 Cº (lesser population, stronger winds)
Beijing: 4 Cº (greater population than Toronto, but fewer cars)
St john’s

6. Hypothetical Urban Heat Island

7. Actual (measured) urban heat island effect

9. London
2143 hours
28 July 2006

10. Daytime air temperature 26.7o, Surface temperatures as high as 47.8o
Atlanta, Georgia, 11 May 1997
Daytime air temperature 26.7o, Surface temperatures as high as 47.8o

11. The cooling effect of urban vegetation
New York City
14 August 2002
The cooling effect of urban vegetation

12. Altered Urban Climate
For a ‘typical’ North American city in comparison to an adjacent rural area:
Particulate matter increased by 10 x
Total insolation decreased by 10 %
Precipitation increased by 10 %
Wind speed decreased by 25 %
Snowfall in inner core decreased 10 %
Snowfall in suburbs increased by 10%
Why? ….
toronto

13. Outline The urban heat island effect
Causes of altered climate due to urbanization
Controls on magnitude of climate changes due to urbanization

14. Why do cities change climates?
Changes in albedo, which would do what (ink shed, 2 mins)?
Changes in particulate concentrations (aerosols), which would do what (ink shed, 2 mins)?
Changes in gas composition (e.g., production of CO2, CH4, SO2, NOx) of atmosphere, which would do what (ink shed, 2 mins)?
Changes in winds & air flow, which would do what (ink shed, 2 mins)?
london

15. Albedo effects Differs between urban & rural
Very dependent on style and density of urban development
In North America, typical 4-6% decrease in overall albedo in urban areas vs. rural

16. Magnitude of albedo effects
Summer temps over asphalt C° higher than air temps vs. bare soil, 1-5 C° higher
Diurnal cooling greater for soil & vegetation than for asphalt

17. Aerosol effects on thermal IR
Interference with infrared wavelengths
Increased heating above urban area
Increased downward heat flux
Turbid layer thus contributes to atmospheric warming
Hong kong

18. Aerosol effects on UV Loss of UV, especially during winter
Overall, UV reduced by ≥ 75 %
Lesser reductions in Visible & near IR
Little reduction in thermal IR
Consequence: urban areas have lower UV, but higher temperatures
edmonton

19. Aerosal effects on insolation loss
Linked to style of industrial activity and energy consumption
Obstruction by dust ~ 5-20%
Loss greatest at lowest solar angles
Increased scattering and diffusion
Low-level inversions in winter
tokyo

20. Aerosal effects on insolation: temperature inversions

21. Aerosal effects on insolation: temperature inversions
Salt Lake City, Utah

22. Changes in wind flow: “Urban Canyon” effect
Not only do buildings alter insolation and radiation they also interfere with wind
Typical net decrease in wind speed 15-30%

23. Outline The urban heat island effect
Causes of altered climate due to urbanization
Controls on magnitude of climate changes due to urbanization

24. Magnitude of urban influences on climate depends on
Size of population
Nature and amount of energy consumption
Surrounding topography
Wind speed
bergen

25. Socio-economic differences
Low per capita income cities (<$100) dominantly produce particulate matter e.g., wood burning
Mid per capita income cities (<$4000) dominantly produce SO2 e.g., coal burning
High per capita income cities (>$10,000) dominantly produce CO2 e.g., vehicles
Thus, effects on climate will vary with city wealth
Sao paulo

26. Population, socio-economic difference, and urban climate
The relationships between population, socio-economic difference, and urban climate are highly complex and demand a geographical analysis that connects physical and social sciences
An example: cars and cows

27. Population, socio-economic difference, and urban climate
Typical car produces 2.2 kg CO2 / litre of gasoline
10 litres consumption per day = 22 kg CO2 equivalent per day
Typical cow produces litres of CH4 per day
Equivalent to 1.6 kg CH4 per day
Each methane molecule is times more effective at trapping thermal infrared radiation than each CO2 molecule
Cow wins! 1 cow = 4 cars!
Ink shed: where is/are the cause(s) of urban climate change (5 mins)?