Transcript Urban Influences on Climate Geography 1050 Outline • The urban heat island effect • Causes of altered climate due to urbanization • Controls on.

Urban Influences on Climate
Geography 1050
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
• Precipitation & cloud cover
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
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 automobiles)
Hypothetical Urban Heat Island
http://adaptation.nrcan.gc.ca/perspective/images/health_fig2_e.jpg
Actual (measured) urban heat island effect
London
2143 hours
28 July 2006
Atlanta, Georgia, 11 May 1997
Daytime air temperature 26.7o, Surface temperatures as high as 47.8o
New York City
14 August 2002
The cooling effect of urban vegetation
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? ….
Outline
• The urban heat island effect
• Causes of altered climate due to urbanization
• Controls on magnitude of climate changes due to
urbanization
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)?
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
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
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
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
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
Aerosal effects on insolation: temperature inversions
Aerosal effects on insolation: temperature inversions
http://leggnet.com/labels/sky.html
Salt Lake City, Utah
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%
Outline
• The urban heat island effect
• Causes of altered climate due to urbanization
• Controls on magnitude of climate changes due to
urbanization
Magnitude of urban influences on climate
depends on
• Size of population
• Nature and amount of energy consumption
• Surrounding topography
• Wind speed
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
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
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)?
Outline
• The urban heat island effect
• Causes of altered climate due to urbanization
• Controls on magnitude of climate changes due to
urbanization