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Urban Influences on Climate

Urban Influences on Climate. Outline. The urban heat island effect Causes of altered climate due to urbanization Controls on magnitude of climate changes due to urbanization. Impacts of Urbanization on Climate. Temperatures (Urban Heat Island) Insolation Wind regimes

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Urban Influences on Climate

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  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

  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

  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)

  6. Hypothetical Urban Heat Island http://adaptation.nrcan.gc.ca/perspective/images/health_fig2_e.jpg

  7. Actual (measured) urban heat island effect

  8. London 2143 hours 28 July 2006

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

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

  11. 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? ….

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

  13. 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)?

  14. 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

  15. Magnitude of albedo effects • Summer temps over asphalt 10-25 C° higher than air temps vs. bare soil, 1-5 C° higher • Diurnal cooling greater for soil & vegetation than for asphalt

  16. 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

  17. 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

  18. 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

  19. Aerosal effects on insolation: temperature inversions

  20. Aerosal effects on insolation: temperature inversions http://leggnet.com/labels/sky.html Salt Lake City, Utah

  21. 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%

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

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

  24. 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

  25. 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

  26. 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 600-800 litres of CH4 per day • Equivalent to 1.6 kg CH4 per day • Each methane molecule is 20-25 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)?

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