Urban Heat Island (UHI) 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

Urban Heat Island Madison County Growth 1984 – 13% Developed LEGEND Developed Land Undeveloped Land Change: 1984 – 1990 Change: 1990 - 2000

Spot the city

Salt Lake City 7/13/98

Population size v. heat island effect

New York City

Chicago is 101 degrees F, surrounding country is in the mid 70’s.

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

Urban Heat Island Atlanta Urban Heat Island • Atlanta Central Business District (CBD) - May 2007 Midtown Residential CBD

Urban Heat Island Atlanta Urban Heat Island (Con’t) • Atlanta cross-city temperature Midtown Residential CBD

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

Urban Heat Island 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 SO2, CO, NO, PM, CO2, accelerates the formation of harmful smog, higher O3 - Increases chemical weathering of building materials

Urban Heat Island 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 the thermal destruction of some hydrocarbon-nitrate molecules (peroxyacetyl- nitrate or PANs) that free NO2 > Increases the vegetative emission of VOCs and evaporation of VOCs

Urban Heat Island 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.

Changes over time? Night temperatures in Phoenix are 7 to 13 degrees F warmer than before 1970 Winds over cities decrease by 20-25%

Effects: Can raise temperatures over cities to more than 10 degrees 10C over that of surrounding areas. Montreal and Paris are 10-24 degrees C warmer. Can increase peak energy demand Air conditioning costs increase Air pollution increase Heat-related illness and mortality increase

As of 1990, 45% of people live in cities UN study says that by 2025, 80% will live in cities

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

Urban Heat Island 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. • 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.

Space-based rain radar on NASA satellites show precipitation downwind of cities is an average of 28%, and up to 51% greater than upwind

And max. rainfall rates are on average 48-116% greater