Heat Islands and Soil.

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Presentation transcript:

Heat Islands and Soil

Anthropogenic Caused or influenced by humans. Anthropegenic carbon carbon dioxide is that portion of carbon dioxide in the the atmosphere that is produced directly by human activities, such as the burning of fossil fuels, rather than by such processes as respiration  and decay. 

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

Urban Heat Island How has urban Atlanta growth changed the climate system over cities? • Landsat 7 images from the 1970s, 1980s and 1990s 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 Atlanta

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

Impact on Soils The main cause of the urban heat island effect is from the modification of land surfaces known as SOIL SEALING . https://www.youtube.com/watch?v=YU6jz061kTs The effect occurs because urban surfaces, such as roads and buildings, tend to absorb heat. There are also far fewer trees and other plants to provide cooling through the plant process known as transpiration.

Soil Sealing & Drainage

Solution---Green Roofs Among the biggest heat absorbers in cities are rooftops. Rooftops typically make up 5 to 35% of the urban landscape. The U.S. Environmental Protection Agency (EPA) reports that over 90% of the rooftops in the United States are dark in color. They also reflect very little sunlight – meaning they absorb all the incoming heat. As a result, city roof surfaces can reach temperatures of 150°-190°F (66°-88°C) during the summer. Yes, that’s hot enough to cook an egg! https://www.youtube.com/watch?v=FlJoBhLnqko

Green Roof Tech Green roof technology can help reduce urban heat islands and conserve energy. Green roofs have been shown to keep rooftops cooler than conventional roofs. They also have a cooling effect on a city’s surface temperatures. A study in the Baltimore-Washington metropolitan area showed that green roofs could cool the surface by nearly 2oF (1oC)! The project studied green roofs that covered at least 30% of the roof surface.

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

Green roofs can also reduce energy use across different climates Green roofs can also reduce energy use across different climates. The amount of cooling from the green roof depends on the material used for the roof. Green roofs are typically covered with a growing media that is different from the soil on the ground. It is specially-made mixtures of soil and inorganic materials like crushed clay and perlite. This keeps the material lightweight and provides good water drainage.

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.

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. • 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.