1、Urban Heat Islands On a hot, sunny day if we wear dark clothes, we feel warm. Dark colorsabsorb more heat than light colors. We exp erie nee ris ing heated air whe n we walk across a p aved parking lot on a sunny, summer day. The heat stored in the p aveme nt warms the air and sends out en ergy in t

2、he form of heat to our bodies. On the other hand, whe n we sit un der a tree, we feel cool. The moisture in trees and the shade they pr ovide cools the area arou nd them. What is an urba n heat isla nd? Temp eratures are n ot the same across a city area. The amou nt of vegetati on (such as trees and

3、 grass) and colors of buildi ng materials affect temp eratures.Gen erally the darker and den ser the buildi ng materials, the higher the temp eratures. Asp halt in parking lots and on rooftops, in particular, absorb heat from the sun s light. This heat is then released as infrared light (invisible t

4、o our eyes) into the surrounding air. Color is a very imp orta nt factor in heat gain. Dark colored materials such as asp halt used for building roads and for roofing tilesabsorb more heat from the sun than materials with lighter colors such as white roofi ng shi ngles or ceme nt. Lighter colorsrefl

5、ect the rays of the sun back into the atmos phere, and less heat is built up. F hotter tha n its surrou nding areas. Scie ntists When heat builds up in a city, it creates a hot spot comp ared to the n earby suburba n and rural areas. This hot spot is called anurba n heat isla nd On warm summer days,

6、 the air in urba n areas can be 6-8 call these cities urba n heat isla nds. Sketch of an Urban Heat-Island Profile 35 Rural Suburban Commercial Downtown Urban ResidentialResidential P ark Suburban Rural Residential Farmland 7 Cop yright ? 2010 Environmental Literacy and Inquiry Working Group at Lehi

7、gh University. Mod ified from US EPA, US Dept, of Energy, and NASA Mission Geogra phy materials. materials absorb and The temp erature differe nces show n in the image above are related to how release heat . Asp halt absorbs heat from the sun and quickly releases it as heat radiati on. For exa mp le

8、, temperatures in the Madison Square Mall parking lot during the summer are as high as 48.9C (120 F) during the day. Tree islands in the lot are only 31.7C (89 ) a differenee of 17.2 C (31 F)! Nearby wooded areas are as low as 29.4C(85 F). 5 Cop yright ? 2010 Environmental Literacy and Inquiry Worki

9、ng Group at Lehigh University. Mod ified from US EPA, US Dept. of Energy, and NASA Mission Geogra phy materials. Reduc ing Urba n Heat Isla nd Effects There are thi ngs that com mun ities can do to reduce the imp acts of urba n heat isla nds. These reducti on strategiesin clude: heat isla nd * Plan

10、ti ng trees and vegetati on; * In stalli ng cool or vegetated gree n roofs; and * Switch ing to cool paving materials. Reduc ing Heat Isla nd Effects with Trees and Vegetati on In creas ing the cover of trees and vegetati on in a city is a simple and effective way to reduce the urba n heat isla nd e

11、ffect. Trees, shrubs, and other plants shade build in gs, in terce pt solar radiati on, and cool the air by evapotranspiration Evap otra nspi rati onoccurs whe n plan ts secrete or transpire water through po res in their leaves. In a way, plants sweat like people do. Trees have a large water content

12、 and release water into the atmos phere to kee p cool. A mature tree wi th a treet op of 30 feet can eva pofFanspire ” up to 40 gall ons of water a day. This is similar to remov ing all the heat p roduced in four hours by a small electric sp ace heater. Comp ared to other surfaces found in cities, t

13、rees absorb a lot of the incoming heat and release it over a lon ger p eriod of time. Further, trees also ben efit the en viro nment in a n umber of other ways as described in the image below. LEAVESt TWIGSt branches absorb sound and blockBranches, leaves ERoeiDNCAUsiNa rajnfall 克口如口 smadc and fte&u

14、cc PE ED Leaves LTEH OANQEHQUS LLUTANTS FROM TmC ALfl Routs, leaveSp trunks PROVIDE HABITAT FDR SIROB, ANIMALSf AND INBECTS i Evapdtranspiratidn FROM LEAVES. C口 L5 5.URRDUNDINQ Ain ROOTS BTAHILIZC SOIL, PHEVENT ERDBIQN Reduc ing Heat Isla nd Effects with Roofs On a hot, sunny, summer day, traditi on

15、al roofi ng materials may reach summertime p eak temp eratures of up to 190 F(88 ). By comparison,cool roofs only reach peak temperatures of 120 F (49 ). cool roof is used to describe roofing material that has high solar reflectanee. These reflect a large portion of the suns energy. Cool roofs also

16、may have ahigh thermal emittanee The term materials and release a large p erce ntage of absorbed heat. This kee ps the material cooler and helps to reduce the heat isla nd effect. 一M 1 t- i- . W . * - VphBlbMi J適 Ano ther alter native to traditi onal roofi ng materials is a rooft op garde n or gree

17、n roof . Gree n roofs rep lace heat-absorb ing surfaces with plan ts, shrubs, and small trees that cool the air through eva po tra nsp irati on Plan ted rooft ops remai n sig nifica ntly cooler tha n a rooft op con structed from traditi onal heat-absorb ing materials. Further,gree n roofsreduce summ

18、ertime air con diti oning dema nd by loweri ngheat gain (the amou nt of heat in troduced to a sp ace from all heat p roduc ing sources) to a buildi ng. A M Reduc ing Heat Isla nd Effects with Paving Materials Cool paving materials surrou ndin gs. reduce the abso rp ti on of solar heat and later tra nsfer of this heat to the There are two types of cool paving materials: lighter-colored materials and po rous materials and Lighter-colored materials have higher solar reflectanee , so they absorb less of the suns energy stay cooler. Lighter-colored materials come in shades