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Urban Heat Island Effect
Abhijit Banerjee | 13 Apr 2008

Urban areas both generate and trap heat. In urban areas trees are cut down to make room for commercial growth. On a hot summer day certain parts of urban areas can be warmer than its surrounding areas. These are areas where there are fewer trees, shrubs,

??????>?? <>????;???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????q` ??cbjbjqPqP 4,::c???????????????? ? ? ? 4? ?^?? :    ? ? ? ???????$ h?B?? ? ? ? ? ??  ?? ? ? ? ? ? ?? ? ?? ? :?,??% ? @Gz]H??? ? ? ? .0^?.? ??%%??9?? ? ? ? ? ? ? ? ? ? ? ? ^? ? ? ? ???$??????????????????? Urban Heat Island Effect Urban areas both generate and trap heat. In urban areas trees are cut down to make room for commercial growth. On a hot summer day certain parts of urban areas can be warmer than its surrounding areas. These are areas where there are fewer trees, shrubs, and other plants to shade buildings, intercept solar radiation, and cool the air. Here the materials used to make the buildings do not reflect but absorb the sun?s rays. Scientists have named such places ?Urban Heat Islands.? Satellite readings from NASA have shown average temperature in cities to be 5 to 10 degrees Fahrenheit hotter than surrounding areas. See  HYPERLINK "http://eetd.lbl.gov/HeatIsland/HighTemps/" http://eetd.lbl.gov/HeatIsland/HighTemps/ Sketch of an Urban Heat-Island Profile. Different from Global Warming This is, however, not to be confused with global warming, though it is not certain as to how much this additional warmth affects the (global) temperature record. Indirectly it may contribute to global warming because of increased demand for air conditioning, which results in additional power plant emissions of heat-trapping greenhouse gases. Major Causes Concrete and asphalt used in urban areas have significant heat capacity and thermal conductivity which bring about change in energy balance and raise the temperature compared to the rural areas. Multiple surfaces for the reflection and absorption of sunlight in many tall urban buildings also increase urban heating. This is called the "canyon effect". Tall buildings also block the wind and prevent cooling by convection. Extra heat is also generated from waste heat from air conditioning, industry, and other sources. High levels of pollution in urban areas is also responsible for urban heat island effect. The urban heat also creates a thermal circulation between the city and surrounding region, resulting in increased cloudiness and precipitation in cities. Growth in population centers too contribute to increase in temperature. Other Causes Presence of vegetation in the rural areas helps evaporate water following solar energy absorption. In this way while there is a net solar energy gain, this is compensated to some degree by  HYPERLINK "http://cimss.ssec.wisc.edu/wxwise/defintn.html" \l "evapcooling" evaporative cooling. But in cities the solar energy input is absorbed by the buildings, streets and sidewalks. As the pavements in cities are largely nonporous, evaporative cooling is less. This contributes to raising the air temperatures. Heat generated by cars and trains eventually makes its way into the atmosphere. This heat is often as much as one-third of that received from solar energy. Tar, asphalt, brick and concrete used in buildings are better conductors of heat Difference is More During Nights Though heat islands can occur any time of the year and during the day or night, the difference in temperature is greater in summer than in winter and during the night than day. This can be beneficial for some cities during the winter as warmer temperatures can reduce heating energy needs and may help melt ice and snow on roads. On the other hand these cities in summer will experience increased air pollution and greenhouse gas emissions and experience heat-related illness and mortality. Health Effects This increase in temperature, along with smog formations, damages the environment and harms human health. Smog is formed when nitrogen oxide (NOx) -- mainly coming from cars and power plants -- combine with high outside temperatures. Extreme heat is reported to kill around 1000 people every year in the US Offsetting Urban Heat Effect A rooftop garden is a way of offsetting the urban heat effect. According to a study carried out in Singapore, roof gardens help reduce temperature by as much as 4oC and heat transfer in rooms below is also considerably lessened. Another study in Tokyo has indicated a possible saving of S$1.6 million per day in electric bills if temperature can be reduced by even 0.8o Another way of reducing temperature in urban heat areas is using lighter colored asphalts and roof shingles that reflect light. Black surfaces in the sun can become up to 70?F (40?C) hotter than the most reflective white surfaces. A simple solution is to use lighter colors for roofs and pavement For more such articles visit:  HYPERLINK "http://copperwiki.org/index.php/Main_Page" http://copperwiki.org/index.php/Main_Page ?k l ? ? ? ? ? ? ? ? ? ? ? ?   ??????????x?xaxP?>?#hZ*h?6?5?CJOJQJ^JaJ hZ*h?CJOJQJ^JaJ-hZ*h?6?0JB*CJOJQJ^JaJph?)jhZ*h?6?CJOJQJU^JaJ hZ*h?B&CJOJQJ^JaJhZ*CJOJQJ^JaJh?\>CJOJQJ^JaJ hZ*h?6?CJOJQJ^JaJ hZ*h`?CJOJQJ^JaJ hZ*huCJOJQJ^JaJ#hZ*h?y5?CJOJQJ^JaJl  1 ? ? Z ???f???????&C?????????????????????d??d?d[$\$gdE? & Fd??d?d[$\$gd?1?d??d?d[$\$gdh% $??^??a$gd?yc?  1 A B ? ?  ? ? ? ? ? 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