An Urban Heat Island is a metropolitan area that is significantly warmer than its surrounding areas. The phenomenon was first described in the 1810s.
“The main cause of the Urban Heat Island is modification of the land surface by urban development which uses materials that retain heat. Buildings absorb (solar) heat that is sent out at night. Waste heat generated by energy usage is a secondary contributor”.
The biggest temperature differences are at night, due to the slow release of heat from urban infrastructure and buildings. When the weather is calm the difference can be about 3 to 5 degrees centigrade, but in the middle of a heat wave it can increase to 8 or even 12 degrees.
In winter there are some benefits, like less icy roads, but Urban Heat Islands increase summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and water quality. People can’t cool down which could cause ‘heat stress’ that can lead to aggression. In the U.S. heat typically kills more people each year than tornadoes, hurricanes, floods and lightning put together.
There are several mitigation measures. Planting helps reduce the effects through shade and through ‘evapotranspiration’. Plants absorb water through their roots and emit it through their leaves. This movement of water is called “transpiration.” Evaporation, the conversion of water from a liquid to a gas, also occurs from the soil around vegetation and from trees and vegetation as they intercept rainfall on leaves and other surfaces. Together, these processes are referred to as evapotranspiration, which lowers temperatures by using heat from the air to evaporate water.
Green roofs are successful: they compensate for the vegetation that had to make place for the building. A green roof not only prevents the roof from absorbing heat, but cools the air around it, offsetting the UHI effect.
And since it is the dark surfaces heat the area a logical thing to do is to increase your albedo. Albedo is the extent to which an object diffusely reflects light from the Sun. The word is derived from Latin albedo “whiteness”. The range of possible values is from 0 (dark) to 1 (bright). Whiteness is an answer to the Urban Heat Island effect.
Finally the White Dream of Adolf Loos’ seems to come through. In Ornament und Verbrechen (1908) he proclaims: “Soon the streets of the town will glisten like white walls. Like Zion, the holy city, the metropolis of heaven. Then we shall have fulfillment”.
And maybe more literally than Adolf could have imagined, because now there are the so-called Cool Pavements. The term refers to paving materials that reflect more solar energy, enhance water evaporation, or have been otherwise modified to remain cooler than conventional pavements. Conventional paving materials can reach peak summertime temperatures of 120–150°F (48–67°C), transferring excess heat to the air above them and heating stormwater as it runs off the pavement into local waterways. Due to the large area covered by pavements in urban areas (nearly 30–45% of land cover based on an analysis of four geographically diverse cities), they are an important element to consider in heat island mitigation. What about White Tarmac?
But if you don’t want to go snow-blind, Low-Reflectivity Coatings offer an alternative. They come in non-white colors. These coatings reflect invisible radiation without reflecting all light. So, they keep an object cool without sacrificing its dark color. Certain high-reflectivity coatings can also be applied to asphalt. Asphalt chip seals and emulsion sealcoats are two such examples that treat asphalt to make its surface more reflective.
Cheonggyechon, or “pristine stream” is a sensational instrument to reduce UHI effects in the city of Seoul. For decades the river disappeared under urban substance. But now it is Back.
In order to accommodate the expansion of Seoul – the population of Seoul exploded between 1950 and 1975 from 1million to over 7 million. It’s growth rate was 7.6, the biggest on the planet- the river was sacrificed: the stream was covered by ‘city’. An elevated highway on top of it was completed in 1976.
In July 2003, Seoul Mayor, now President Lee Myung-bak removed the highway and restored the stream. Years of neglect and development had left the stream nearly totally dry, so 120,000 tons of water have to be pumped in daily. The stream was opened to the public in September 2005 and features nozzles that cover part of the river in refreshing artificial fog.
The river Cheonggyechon is just 8 meter wide and less than one meter deep and 6 kilometer long but it helps to cool down the temperature on the nearby areas by 3.6 degrees centigrade on average relative to other parts of Seoul.
This BLOG contains samples from United States Environmental Protection Acency (EPA), UN.org, Wiki P. and How Stuff Works.
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