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 Islandhttp://adaptation.nrcan.gc.ca/perspective/im

ages/health_fig2_e.jpg

Actual (measured) urban heat island effect

London2143 hours

28 July 2006

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

New York City14 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

Salt Lake City, Utah

http://leggnet.com/labels/

sky.html

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