Sustainable Sites Treatment | Urban Heat Island | Alison
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Design Issues: Urban Heat Island

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Good morning, welcome back to this new lecture for the ongoing course on SustainableArchitecture. And in the previous week, we have been talking about Sustainable Sitedevelopment as part of sustainable architecture. Now we have focused upon how todevelop the sites. So, previously we have been talking about the selection of site and theoverall development of site. While from today’s lecture, we will be talking more aboutthe design issues and how to handle them for development of sustainable sites.(Refer Slide Time: 00:57)So, in today’s lecture we are going to look at what urban heat island is, what causesurban heat island and through the development of site in a sustainable manner. What alldo we need to do, what all do we need to look at while designing the site, so that thisurban heat island can be mitigated or it is effect can be reduced. So, here we are lookingat the city plan of Bangalore city and if you look at how it has been progressing. We seethat more and more of the Bangalore has started getting constructed. And if we look atthis image for 2020, we see that almost the entire of Bangalore is a dense city with builtup area all over and this is not just the story of Bangalore; it is almost everywhere. So, with this growth which we can see happening in terms of the built mass coming up indifferent cities, this is what happens.(Refer Slide Time: 02:07)So, we are looking at temperatures and the NDVI values. So, here because of thisaddition of built mass which is replacing the vegetation, the greenery; we can see that thetemperatures have been on rise. We experience them on a day to day basis. But what weare majorly seeing here as a focus is the microclimate of the city within the cityboundary which is not physically defined, but this is our physical limitation. So, weexperience that the micro climatic conditions of the city which is the first 100 metersfrom the ground surface, the temperature for this micro climate has been on a rise; it hasbeen increasing. This is what urban heat island is. (Refer Slide Time: 03:00)So, what actually happens and what causes this urban heat island is answered, if we askthe question that what happens when all the sun’s energy which is coming uniformlyacross the surface of earth. I am not talking about the differences between tropics andpolar, but if we consider cities and areas within the same latitude, we see that the earth’ssurface receives the same amount of energy here.So, what happens if it falls say on a green patch like this which is an agricultural feed orthere may be a forest somewhere, where there are bigger trees and as compared to thatwithin the heart of the city. So, when the sun falls in this vegetated area agricultural fieldor a forest, this heat this solar radiation will be absorbed by the plants for their processeslike photosynthesis and transpiration and it also shades the ground.So, the sunlight is not reaching the ground for most part of the year. However, when weare looking at the city area, when we are looking at the city limit because of the buildingsgetting constructed, a lot of the solar radiation which is falling on the surfaces buildingsurfaces is getting absorbed and then released during the night time. A large part of thesolar radiation is also falling on the ground which also has been paved using differentmaterials. So, this is the major difference which brings in the concept of urban HeatIsland. (Refer Slide Time: 04:37)So, what happens is that the temperature because of a lot of heat which has beenabsorbed by these buildings and then reradiated during the night? So, the night timetemperature suddenly increases, it increases in comparison to the rural areas and also thesuburban areas which are not as dense as the downtown city. This phenomena where thetemperatures of the cities increases as compared to the surrounding rural areas. Thisphenomena is called heat island.(Refer Slide Time: 05:15) And this phenomena of heat island has been observed in all the cities across the worldirrespective of their geographical or climatic context. The intensity of it may vary, but itis present everywhere. So, we can see a lot of different these isotherms these temperaturemaps where we can see that the downtowns are the hottest areas wherever there is aconcentration of building activity. These are the hottest areas within the city and as wego towards the rural areas as we expand, these temperatures they fall down.(Refer Slide Time: 05:55)So what actually causes heat island? So, some of the most prominent causes of heatisland, I will discuss today. So, one is change of surface, heat production, changes in thecomposition of atmosphere and increased turbidity. Let us look at each one of them inlittle detail. (Refer Slide Time: 06:15)So, first is changes of surface. Now what is happening as I said in the very first slide thatwhen we cut down the forest area and the greenery and we construct the buildings andthe built mass comes up. So, this change in the surface is responsible for this heat islandfor multiple reasons. One, the heat which otherwise would be incident on the tree isutilized for different purposes like photosynthesis and transpiration as I said. Now thisheat if it falls on the buildings will not be utilized for anything, so since energy overallremains constant. This heat this energy solar radiation is absorbed by these buildings andthen reradiated during the night creating this urban heat island which is what we see.Another is because of the moisture content which is not present in these built surfaces,these forests the vegetated cover also holds a lot of moisture and because of this theenvironment, the atmosphere also holds a lot of moisture. Because buildings cannot;these materials cannot hold moisture the city limits they become drier and the watercontent the moisture content further drops down in the downtown area in the heart of thecities where a lot of buildings have come up. That is further responsible for the heatisland, increasing the intensity of heat island.These surfaces hard surfaces have these thermal absorptance and emmittance to play arole, which is what when I talk about the green cover versus the built cover. And withinthe built cover, the different kinds of materials which are there will have differentthermal properties, the amount of heat that they absorb and then they release later, emit. So, on the basis of that the temperature variations would come into place and the heatisland intensity would vary. So, the thermal properties of these surfaces have a large roleto play and also the colour of the surface lighter. The surface more is the amount of heatthat it reflects and hence lesser amount of heat is absorbed and further reemitted. So,these are the properties of the surface which impact the urban heat island and it isintensity.(Refer Slide Time: 08:59)If you look at this image, we can very clearly see the difference between the rural areasand the urban areas. If you look at the total solar incident radiation which we assume thatis the same, since it is the same latitude and the place receives the same amount of solarradiation, we can see the difference coming in. So, we have a lot of solar radiationreflected in case it is a rural area while lesser amount of solar radiation is reflected backto the atmosphere which implies that a lot of heat is absorbed by the materials. And ifthere is more heat which is absorbed, we know that a lot of heat will be released later inthe form of infrared radiation. The long wave radiation which cannot escape theatmosphere and it will be contained within that. So, there will be more heat in theatmosphere causing the urban heat island.Then we are also looking at the latent heat because there is more moisture in because ofthis vegetation, because of the greenery. We know that the moisture present will be ableto take up more amount of heat from the atmosphere. While in case of urban areas, this latent heat decreases. Another worry is the storage heat because there is not much hardsurface in the rural areas and there is a lot of built surfaces hard surfaces in urban areas.This is the major cause of heat island effect coming into picture.Another thing which is absolutely not present in the rural scenario is this anthropogenicheat. Now this anthropogenic heat is what has been added to the environment because weconsume the fuel.For example we are running cars in urban areas now that is adding to the anthropogenicheat, we are adding a lot of air conditioners to our building that is adding to theanthropogenic heat. So, this is a major cause of worry. So, this is what overall causes ourcities to be warmer, hotter and that is what we are calling as heat island.(Refer Slide Time: 11:23)Another major cause of heat island which we have seen in the energy balance diagramjust now is the Heat Production. So, in cities we have industrial activities being carriedout, we have cooling and heating of buildings of our indoor environment which is addingto the anthropogenic heat outside and also there is a lot of concentration of people in thecities unlike the rural areas which are sparse which are not as dense. So, all of thattogether is adding to a lot of anthropogenic heat in the cities and the atmosphere in theenvironment. So, that is another major cause of heat island. (Refer Slide Time: 12:05)Then we also have the changes in composition of atmosphere. Now when we are addingwhen we have more and more of industries and when more and more of greenhousegases are being increasingly added to the atmosphere so, we very clearly know whatgreenhouse effect is and these gases they absorb these are suspended particles that therenot just these gases but the suspended particles. So, all of these they absorb a lot of heatwhich is there in the atmosphere, which is coming from the sun and it is also beingrereleased in form of this long wave radiation.When it is absorbed, it increases the temperature of the atmosphere. Greenhouse effect iswhat is helping us survive. If greenhouse effect was not there probably earth would notbe a habitable planet. But today what is happening because of the increasedconcentration of these greenhouse gases like carbon dioxide, carbon monoxide. Nowthese two are the commonly taken names, but these are the gases which have least globalwarming potential. There are gases which are much more, they have more capacity toabsorb the heat the nitrogen oxides and the sulphur oxides and many more gases like thatwhich absorb a lot of heat and are the major culprit towards the greenhouse effect.So, all these will impact and they will add up to the rising temperatures. Now since theconcentration of these gases is more in the cities because, there are more vehicles whichrelease these greenhouse gases or there are more industries which will release these greenhouse gases and all. So, we will have the heat island effect more pronounced in thecities.(Refer Slide Time: 14:01)So, when I say greenhouse effect, no it is not global warming. Greenhouse effect is justthe phenomena, but yes when there is an increase in the presence of these greenhousegases beyond a limit, it will lead us to global warming which is actually because of thepresence of these greenhouse gases.(Refer Slide Time: 14:27) The last one of the most prominent causes for heat island is the increase in turbidity. Byincrease in turbidity we mean that the suspended particles, the when in very fine particleswhich are suspended which are release in the air and they remain suspended whichabsorb the heat, retain it and further heat up the atmosphere are more in cities. So, if wego from a city or an urban area to a rural area, we would see that the suspended particlessuddenly disappear. So, there is less of dust, there is less of other suspended particles,when we go from urban to rural and this increase turbidity presence of the suspendedparticles is a major cause for this urban heat island.(Refer Slide Time: 15:21)So, all these together lead to heating up of the environment atmosphere in the city area,so there is an increase temperature and we now know what urban heat island is. So, whatis the effect of urban heat island? Now urban heat island effects in multiple ways, so weare looking at the environmental impacts of urban heat island and not the social impactsand the economic impacts here. Now the urban heat island can actually change theweather pattern in a city. It may impact on the precipitation pattern how the rains arereceived within a city. So, if the city becomes much hotter, now depending upon thegeographic context, the city may not allow for the formation of clouds and hence theprecipitation or in some other cases it may that is totally dependent upon the geographiclocation and the context. In some other cases, it may happen the other way around, the city may attract and allowformation of more clouds leading to a flood like situation. So, this is an impact of urbanheat island. Another major impact is on how the winds are formed. So, the winds may bepronounced because there is low pressure which is created, so the winds may bepronounced in some of the cities.Again I repeat here it may be it is dependent upon the geographic location and thecontext of a city or place. So, these are some of the effects of urban heat island. Besidesthat as the temperature of the city increases, there is further requirement of energy tokeep our indoor environment cool and comfortable so we add more air conditioners.In rural areas, we do not need air conditioners because the outer environment iscomfortable and we can be comfortable at those temperatures. While in a city because ofthis urban heat island, we further need to cool down our indoors and that adds toanthropogenic heat and it is a cycle. So, that is another impact where the energyconsumption goes up, because of the urban heat island because people are notcomfortable.(Refer Slide Time: 17:47)So, when we are looking at impact of a heat island, it is also dependent upon the intensityor the magnitude of this heat island and that further depends upon a lot of these factorswhich are listed here. So heat island may be there, but the magnitude will vary from city to city and that I saidas I just mentioned is also based upon the geographic location. And some of theseparameters like the size of the cities; bigger cities have a tendency to have an intenseurban heat island. The local micro climatic conditions that is dependent upon theTopography and the Geography both. So, we have the local micro climatic conditionthen the diurnal and weekly variation. The seasonal variation which is also again ageographic condition resultant of a geographic condition, then the building density howdense the city is the smaller cities could also be very dense and then the distance fromcity centre.So, within a city, there would be variation in the magnitude of heat island dependingupon which is the denser part of the city and as we move away it may vary.(Refer Slide Time: 19:15)So, if we look at each of these parameters individually so, we have Topography. Sowherever we have cities which are closer to oceans, rivers, lakes, forests pastures and athigh elevation where the altitude is high, we would see that the urban heat island is lesspronounced. Because of the presence of water body and the winds that it creates and alsobecause the rural areas or the surrounding areas are flooded with vegetation this all thesetogether help to reduce the urban heat island. (Refer Slide Time: 19:59)The next is diurnal and weekly variation. So, the urban heat island is more pronouncedduring nights, because that is the time when these suburbs are cooler. When the suburbshave already cooled down, the rural areas have cooled down, that is when the differencebetween the rural areas and suburban area is more pronounced as compared to thedaytime when a solar radiation is present. So, it will be not that steep, there is seasonalvariation as well. So, UHI urban heat island is more pronounced during winters for thesame reason that the rural areas and suburban areas are cooler there are colder.(Refer Slide Time: 20:45) Another one is building density. So, the denser the city is so if we see that the urban heatisland is more pronounced in the downed town, because there is more of the hard surfacethe built mass is more in the city centre in the core of the city. And so all the reasons thatwe have just seen there are more hard surfaces, there is less of surfaces which allowwater to percolate down and allow the moisture to be added up, there is less of vegetationand all these factors together. So, wherever there is higher building density, we will havegreater magnitude of UHI and that is reflected in the distance from city centre.So, usually the city centre is the area where you find higher building density, becauseeverybody wants to be in the city centre close to the city centre because that is theeconomic hub and that is the activity hub. So, it implies it directly you know affects thebuilding density. So, closer to the city centre usually implies higher building density andthen higher magnitude of urban heat island.(Refer Slide Time: 21:59)So, if we look at these factors which affect the magnitude of heat island, we can see thatthe size of cities the building density and the cloud cover are directly proportional to themagnitude of heat island. However, if we look at the factor of wind speed, so greater isthe wind speed lesser is the magnitude of heat island and it is inversely proportional. So,as the wind speed increases, the magnitude of heat island decreases; the same as withdistance from city centre. So, as the distance from the city centre increases the magnitude of heat island decreases.But if we look at the distance, so as we are coming far from the city centre we can seethat initially it is a very steep curve and then it gradually becomes flat. So, if we gofurther away, it will not have much impact and we would have the same amount of heatisland present there, so that is what it implies. So, we know which factor to considermore and which factor will be affecting more.(Refer Slide Time: 23:09)Another is temperature distribution within the city. So, people have conducted a lot ofstudies across the world and they have identified that the intersection of the streets arethe coldest spots within the city. So, overall if we see this is the curve that the cityfollows for a UHI, but within that there might be some variations. So, these variationsthis could be a street intersection, where there is a heavily travelled main road it could befurther higher. Wherever there is a stoplight, wherever there are stoplights on the streetswhere vehicles stop for a longer duration, so because of the addition of anthropogenicheat again it may go up. So, within the city there is this distribution of urban heat island. (Refer Slide Time: 24:03)Now, once we have established that these are the factors which lead to urban heat island,we will now look at mitigation strategies and we are looking at these mitigationstrategies from architectural point of view. So, how do we develop our sites if you wantto mitigate urban heat island?Now this is critical factor while we develop our design our site for sustainable sitedevelopment. We are looking at some of the mitigation strategies here; one veryimportant one is the green roof or green wall. Here we also have the tree plantation andpreservation which is connected to creating more city parks, when we are looking at anurban level when we looking at a larger development level. So, creating more spaceswith vegetation leaving more area as green is another, but most important mitigationstrategy.The next one is where we select the materials carefully. So, we have to use the materialswhich are light coloured materials and which have a higher SRI value, I will come towhat SRI value is in subsequent slides. But the choice of materials, so we talking aboutvegetation as the prime strategy so where we can leave more area on the ground foradding vegetation trees, plants just green cover or if you do not have enough areas to beleft on the ground, then adding greenery to the roof and wall so that the hard surfaces arereduced. If we cannot even do that then select the materials which are light coloured and whichhave higher SRI values. So, that less and less amount of heat is absorbed and convertedinto long wave radiation. Other strategies also have the presence of water bodies and lowimpact development techniques; let us go over couple of these in subsequent slides.(Refer Slide Time: 26:13)So, if this is what Teri published so, Teri is the energy research institute which workslargely on the in the energy domain and when they were discussing about this urban heatisland effect, they talked about some of these strategies, which are common strategiesand are also common sense strategies.For example parking lots should have a tree canopy and this is such a simple strategy,but it is a very effective strategy because most of the parking lots are paved with hardmaterials. Another very interesting strategy is to have height of the buildings to exceedwidth of the road. Here we are talking about the canyon ratios. So, we if we look at thetraditional settlements we would find that the streets are quite narrow us compared to thebuildings which are quite tall on their sides. Now that helps in keeping the streets shadedthroughout the day almost throughout the day. So, streets which are essentially hardpaved, they require hard surfaces. (Refer Slide Time: 27:23)So, when we reduce the amount of direct solar radiation falling on them, we are reducingthe amount of heat which will be absorbed and further reradiated. So, some of thesedesign strategies will be employed while we are talking about design of site forsustainability. So, we are choosing we are talking about the materials. So, always a lightcoloured surface is better and should be preferred over a dark coloured surface andpavement. Lighter surfaces they reflect more of the incident sun while dark surfaces theyreflect less.If the surface is reflecting more, there is lesser amount of radiation which will beabsorbed, as compared to the other one where larger amount of radiation will beabsorbed because lesser has been reflected. Now that is first property and we should tryto reflect as much heat as we can. Now there will always be a design sense which shouldprevail. We cannot have the surface of the building painted all white and create glare tothe building which is right opposite it. So, we have to be judicious enough in selectingthe colours. However, all the roof surfaces can definitely become white colouredsurfaces and that has been a very commonly practiced strategy in traditional architectureas well. (Refer Slide Time: 28:59)So, if we look at the scientific data, if we look at the values we would see the differencebetween the black surfaces and the white surfaces. And we see the amount of heat whichflows into the building from a dark coloured roof surface is much more than what iscoming through a white coloured surface. Because larger percentage of this heat isreflected back in a white surface.(Refer Slide Time: 29:27)So, here we are talking about reflectance when we are selecting the colours. The anotherproperty which we have here is the Emissivity. (Refer Slide Time: 29:37)So, once we have reflected the amount of heat which we can or the amount of radiationwhich is incident on the surface, the rest which is absorbed after reflecting part of it willbe emitted. So, if we look at say a surface now there is 100 percent of the radiationwhich is incident. If we reflect around say 25 percent which is for a typically weatheredgrey concrete around 20 percent of it will be reflected back through the material surfaceitself. The rest 75 percent of it has been absorbed by the material. Now of this which isabsorbed not all will be transmitted inside.During the time which is night time when the outside temperature falls down, the amountof heat which is absorbed will be reradiated to outside. When the temperatures fall, whenthe temperature of this surface is higher than the atmospheric temperature, then a part ofthis heat will be released outside. And if assuming the indoor temperature is also at thesame temperature as the outdoor one, then it will also release equally to the inside.Now this depends upon the temperature difference between the surface and the outdoorair. So, it will be re radiating or emitting this heat which is absorbed. Now what we arelooking here is to emit all that heat which is absorbed, so all of it is released, it is reradiated. If that happens then and if we are able to reduce the amount of heat which istransfer to inside by providing some sort of insulation or a barrier, most of the heat whichwill be absorbed will be re rereleased reradiated and there will be less of heat which willbe transferred inside. So, of this entire heat which is incident only 5 percent or 10 percent for a material withhigh SRI value will be transferred inside, which is what the intent is. So, we should beselecting now this SRI value which I was discussing which is the solar reflectance indexis a combination of these two properties immittance, emissivity and reflectance andhigher the SRI value higher is the amount of heat which is reflected back which isreleased.(Refer Slide Time: 32:19)The other strategy as we have just seen is Green Roof. So, if we now roof is the part ofthe building which absorbs which receives maximum amount of solar radiation. So, if weturn our roofs surfaces into green roofs where the vegetation is, it will reducesubstantially the amount of heat which is received by the building the hard surface. (Refer Slide Time: 32:47)So, this is the thermal imagery which shows the green roof versus a conventional roof.Now these darker temperatures are cooler. So, we can see this part of the roof which isgreen roof and we can see this part of the roof which is actually a dark coloured surfaceand we can see the stack difference between the two surfaces.(Refer Slide Time: 33:09).The same strategy when applied to the walls will be called as green walls. So, here wewill have vegetated walls green walls. Now there may be multiple strategies, where wemay have added attachments placed on the walls where the greenery could be added. It could be that the wall itself is green where it supports vegetation or in multiple ways. Butthis is the concept of green wall.(Refer Slide Time: 33:35)Now, when we are looking at the green building rating systems, so all of these that wehave discussed these strategies can be used in green buildings and they will be evaluatedpositively. So, we have two compliance criteria’s, where one we are talking about thenon roof components when we are talking about UHI and we will be talking about theroof components when we talking about UHI.So, these two criteria and these strategies which are used which are to be used are theones which we have just discussed as part of mitigation measures. So, these mitigationmeasures can be employed in our buildings and they will help us in achieving credits,while we are going ahead with green building compliance green building ratings.If you look at the non roof criteria, we have to ensure that the hard surfaces thehardscape on the site at least 50 percent of that follows the strategies which we have justdiscussed. So, at least 50 percent of the entire site hardscape has to use a combination ofthese strategies and we are talking about in terms of the SRI numbers, then at least anSRI of 29 has to be achieved for at least 50 percent of the site hardscape. 

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