18 1 Urban Changes Climate Urban Heat Island

18 -1 Urban Changes & Climate Urban Heat Island Urbanization Effects Science Concepts Surface Types Surface Energy Budget - Cities Ozone - Temperature Relationship Mitigation Strategies Climate and Global Change Notes

18 -2 Urban Heat Island How has urban growth changed the climate system over cities? • Atlanta Landsat 7 images from the 1970 s, 1980 s and 1990 s show trends in urbanization - Note changes in green, i. e. , forested area - For example, Atlanta, GA gobbled up 380, 000 acres of trees between 1973 and 1992 - averages about 55 acres of trees per day Climate and Global Change Notes

18 -3 Urban Heat Island Madison County Growth 1984 – 13% Developed 1990 – 21% Developed 2000 – 30% Developed 2020 – 50% Developed LEGEND Developed Land Undeveloped Land Change: 1984 – 1990 Change: 1990 - 2000 Climate and Global Change Notes

18 -4 Urban Heat Island What happens to the Sun’s energy at the Earth’s surface over a city? • An acre of forest or cropland receives as much sunlight as an acre of buildings • Green space will be cooler because of transpiration and shading of the ground • Urban surfaces get much hotter than vegetated surfaces during the day. • They release this energy at night, creating a dome of warmer air at night Climate and Global Change Notes

18 -5 Urban Heat Island What is “Urban Heat Island”? � • Urban areas are warmer, like an “island” of heat surrounded by cooler rural areas � • Large horizontal temperature gradient exist at the urban/rural boundary, could be as large as 4 C/km � • Urban areas appear like a “plateau” with weaker increasing gradient. This pattern is influenced by distinct intra-urban land uses: open areas, parks, lakes (cool), dense buildings, commercial areas or industrial areas (warm) • Intensity of the UHI depends on the urban-rural temperature difference Climate and Global Change Notes

18 -6 Urban Heat Island Atlanta Urban Heat Island • Atlanta Central Business District (CBD) - Midtown Residential May 1997 CBD Climate and Global Change Notes

18 -7 Urban Heat Island Atlanta Urban Heat Island (Con’t) • Midtown Residential Atlanta cross-city temperature CBD Climate and Global Change Notes

18 -8 Climate Elements Urban Heat Island Intensity • Factors below contribute to UHI, relative roles are not certain • Factors combine to make the urban area store sensible heat during the day, and inhibit this excess heat from dissipating rapidly at night. Altered energy budget terms Features of urbanization underlying energy budget change Increased absorption of short wave radiation Canyon geometry - increased surface area and multiple reflection, dark materials (e. g. , asphalt Increased long wave radiation for sky Air pollution - greater absorption and emission Decreased long wave radiation loss Canyon geometry -- reduction of sky view factor Anthropogenic heat source Waste heat from city buildings and traffic Increased sensible heat storage Construction materials - increased thermal admittance Decreased evaporative cooling Construction materials - increased water-proofing, less vegetation, shrubs, and other plants Decreased total turbulent heat transport Canyon geometry - reduction of wind speed Climate and Global Change Notes

18 -9 Urban Heat Island Effects • UHI has several direct and indirect effects; including biological, economic and meteorological effects - Increases demand for cooling energy - Increases electricity generation which leads to higher emissions of SO 2, CO, NO, PM, CO 2, accelerates the formation of harmful smog, higher O 3 - Increases chemical weathering of building materials Climate and Global Change Notes

18 -10 Urban Heat Island Effects (Con’t) • UHI has several direct and indirect effects; including biological, economic and meteorological effects (Con’t) - Temperature affects ozone amount > > Increases thermal destruction of some hydrocarbon-nitrate molecules (peroxyacetylnitrate or PANs) that free NO 2 Increases the vegetative emission of VOCs and evaporation of VOCs Climate and Global Change Notes

18 -11 Urban Heat Island Effects (Con’t) - Induces convergence over city - Initiates convective activity downwind of city - Precipitation and cloud cover enhancement occur downwind of city - Trends of global warming are higher in urban areas; most weather stations are located near urban areas, which could bias the trends of global warming. Climate and Global Change Notes

18 -12 Urban Heat Island What can be done to mitigate the effects of urbanization? • NASA has been flying aircraft over nine cities to determine their thermal characteristics that drive the development of the urban heat island • Working with city officials, urban planners, architects, schools and other groups to educate and affect change - Color of roofs More trees Better community planning Climate and Global Change Notes

18 -13 Urban Heat Island Summary • Urban areas are usually warmer than rural areas, especially at night. Under ideal conditions, UHI generates its own circulation, radially inward flow at street level, radially outward flow above. • First noticed ~170 years ago. Even with numerous studies since the 1960’s, it is still not well-understood. • Formation of UHI and its magnitude results from urban and geographical characteristics, city size (population, height-to-width ratio), meteorological conditions (wind speed, cloud cover). • UHI has distinct seasonal behavior, usually greatest in the summer, weakest in the winter. • UHI not only affects temperature, it affects cloudiness, precipitation and air quality. It also impacts global warming. Climate and Global Change Notes