Heat islands

Fig. 17-21. Urban heat island (light regional wind).

There have been some indications, although controversial, of increased precipitation downwind of major metropolitan areas. Urban addition of nuclei and moisture and urban enhancement of vertical motion due to increased roughness and the urban heat island effect have been suggested as possible causes. 

Surface topography can affect the local wind patterns one example is the onshore and offshore breeze, and another example is the heat island over large urban areas. Another manmade effect is the generation of mechanical turbulence caused by the nonuniform height of buildings in a city. We will discuss this effect in more depth later on. 

Heat island Relating to an area where the average air temperature is higher than the surroundings. 

Besides increasing systemwide cooling loads, summer heat islands increase smog production. Smog... 

Heat Island Group. (1999). . Berkeley, California Lawrence Berkeley National Laboracoi y. 

Pomerantz, M. Akbari, H. Chen, A. Taha, H. and Rosenfeld, A. H. (1997). Paving Materials for Heat Island Mitigation. Report LBNL-38074. Berkeley, CA Lawi eiice Berkeley National Laboratoiy. 

Rosenfeld, A. Romm,J. Akbari, H. and Pomerantz, M. (1998). Cool Coinmunitics SU atcgics for Heat Islands Mitigation and Smog Reduction. Energy and Buildings 28(l) 51-62. 

Tyndall related his radiation studies to minimum nighttime temperatures and die formation of dew, correcdy noting that dew and frost are caused by a loss of heat through radiative processes. He even considered London as a heat island, meaning he thought that die city was warmer than its surrounding areas. 

Contaminant Emissions in Urban Areas 195 6.8.2 Residential Density and Urban Heat Islands... 

The urban heat island effect has implications for chemical emissions in several ways. Elevated temperatures increase the consumption of energy for air conditioning and other cooling systems, with attendant increases in fossil fuel combustion-related contaminants (Cardelino et al., 2001 Adams, 1999). Increased ambient air temperatures also increase volatilization of POPs such as PCBs, PCNs and PBDEs from urban sources (Priemer and Diamond, 2002 Helm and Bidleman, 2003). 

There are many unanswered questions regarding chemical interactions and fate processes. What is the composition and impact of chemical interactions given the complex mixture of chemicals emitted and found in urban matrices (e.g. enhanced rates of chemical reaction and a wider range of transformation products) What are the roles of urban form and management practices in mediating transport dynamics (e.g. the implications of stormwater management systems on total chemical fate) Furthermore, what role do different types of urban development patterns and heat island effects play with respect to contaminant fate Does the simplified vegetative community that is dominated by exotic species influence the rate at which chemicals are processed or accumulated ... 

Stone, B., Rodgers, M.O. (2001) Urban form and thermal efficiency - How the design of cities influences the urban heat island effect. Journal of the American Planning Association, 67(2) 186-198. 

These do not account for specifically urban dynamics and energetics and their impact on the ABL characteristics (e.g. internal boundary layers, urban heat island, precipitation patterns). 

Another local effect, in this case due to surface cover, is the urban heat island. Rapid heating of urban pavements and buildings occurs during the daytime because of the high absorbance of constructed surfaces and the absence of cooling from evapotranspiration. At night, rural areas cool more effectively than urban areas because of the relatively unobstructed exposure of the land surface, in contrast to the impediment to heat radiation presented by tall, closely spaced buildings. As a result, daily minimum and maximum... 

Anthropogenic heat fluxes, including the so-called urban heat island ... 

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