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J3.8 URBAN HEAT ISLANDS AND ENVIRONMENTAL IMPACT

Suryadevara S. Devi*

Andhra University, Visakhapatnam, India

ABSTRACT

The rapid urbanization and industrialization havebrought about microclimatic changes particularlywith regard to its thermal structure. The welldocumented climatic modification of the city is urbanheat island. The present paper discusses the natureand intensity of heat islands at Visakhapatnam, thetropical coastal city of South India. A detailed studywas carried out with regard to urban heat islandsfor the last ten years. The study reveals that theintensity of heat island varies from 20C to 40C and

intensity is high during winter season compared tosummer and monsoon seasons. At Visakhapatnamthe formation of heat island is controlled bytopography and urban morphology. The land andsea breeze circulation also interacts with the heatisland. It has been found that cooling at night timeis less inside the builtup area than the suburban.Urban cooling rates are compared with the sub-urbanand rural environment. The urban heat island helpsin setting up of the recirculation of pollutants thusmaking the pollution problems more serious. Heatisland coupled with heat wave conditions during

summer season causes human discomfort andhigher death rates. At Visakhapatnam, summermonths of April, May and June with maximumtemperatures of 350C to 400C are uncomfortable withoppressive heat. There is a record of 94 heat wavesduring 1951-2000. Prevalence of heat waveconditions and heat island deteriorate the situationfurther and residents experience thermal stress andheat deaths. Thermal comfort can be improved bydeveloping green belts which control temperatureand reduce heat island effect. Cities must beplanned with climate input to make the environmentsmore pleasant and healthier places and to reduce

undesirable effects.

1. INTRODUCTION

An understanding of the urban heat island isimportant for a variety of reason. The radiationabsorbed warms the ambient air increasing the lowlevel stability and consequently preventing the

pollution dispersal which will result in an increasein pollution concentration. The urban heat islandadds to the development and self sustenance of adust-dome and a hazehood of contaminatingparticles. It also helps in setting up of therecirculation of pollutants thus making the pollutionproblems more serious. With the increasingemphasis on planning for healthier ad comfortablephysical environments in cities, the need torecognise the role of cities in causing and meetingthe challenges posed by climate change hasbecome greater. The very presence of a city affects

the local climate and as the city changes, so does,its climate. The modified climate adds to the cityresidents discomfort and even ill-health. The welldocumented climatic modification of the city is urbanheat island. The higher temperatures are bestdeveloped over city cetres at night under stableconditions and the situation is often referred to asthe urban heat island. It is not an instantaneousphenomenon but progressively develops followingsunset. The magnitude or intensity of heat islanddepends on various factors such as, heat storageby buildngs and pavements; heat generated by

industrial processes, transportation, air conditioningand also as a result of reduced evaporational coolingfrom vegetation Their combined effect, however,results in the formation of the heat island whichvaries in intensity from city to city and from seasonto season.

Of the many meteorological parameters that areaffected by urbanization, temperature anomalies arewidely reported (Landsberg, 1970; Chandler, 1970)and Lowry (1977). Most of the reports were basedon temperature observations from mid latitude cities.Oke (1974, 1979, 1984 & 1999) has given a

comprehensive review of urban heat island studies.Philip, Daniel and Krishnamurthy (1974) and Murthy(1979 & 1999) have analysed the formation of heatislands in some Indian cities during winter months.There are many reports about heat islandphenomena from all over the world. Number oftechnical papers were presented during theinternational conference on urban climate organisedby WMO during Nov99 at Sydney. Internationalorganizations such as WMO, WHO, UNEP andInternational Geographical Unions are activelyinvolved in the urban climatic studies.

The present study focuses on the nature andintensity of urban heat islands in the city of

* Corresponding address :Prof. S. Sachi Devi,

Department of Geography, Andhra University,Visakhapatnam - 530 003, India; email :ssachidevi@yahoo.com

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Visakhapatnam, multifunctional port city of SouthIndia. It is one of the fast growing cities of India withmore than 1.5 million population. The city is boundedby hill ranges in north and south and Bay of Bengalforms the eastern boundary. Being a coastal stationthe city enjoys moderate climate with mean annualtemperature ranges from 220C to 310C.Visakhapatnam is presently witnessing rapidindustrialization and urbanization and consequentlanduse changes which affect local climate to large

extent.

2. METHODOLOGY

Field surveys were carried out in and aroundVisakhapatnam and surface temperature data werecollected using thermometers at a number ofpredetermined points. The observational points wereso chosen as to ensure adequate representationof all parts of the city. The observation period wasnear the minimum temperature epoch, when the heatisland effects are known to be most prominent.Days with clear skies and calm winds were chosen

as far as possible. Thermographs were also installedat represetative locations to know the temperaturetrend. The temperatures were then plotted on amap of city and isotherms were drawn takingtopography into consideration and maps wereinterpreted.

3. RESULTS AND DISCUSSION

The study reveals that urban heat islands arecomplicated zones with multiple corescorresponding to the urban morphology. It wasobserved that heat island was noticed over thickly

built up area (Fig. 1). A concentric pattern ofisotherms with a warm core at the centre,corresponding to the business district of

Visakhapatnam could be noticed. Temperaturevaries from 180C in the north along the foot hill zoneto 220C over city centre with the intensity of 40C.Warm pocket was also identified over the industrialarea in the south. Having seen the general patternof temperature distribution over the city and havingidentified the general areas susceptible to theformation of heat island, some more surveys wereconducted during the winter period for the last 10years. Intensity of heat island varies from 20C to

40

C. Temperature surveys were conducted duringsummer period also to observe the intensity of heatislands during summer season. It was observedthat the temperature difference between the citycentre and suburb was 2.50C. In generaltemperatures were high all over the city, varyingfrom 310C to 340C as it was summer season. Here,the highest temperature of 340C was observed overinterior places (residential colonies) unlike previouscases. Along the coast temperatue varies from 310Cto 320C due to maritime influence. The reason tohave selected the summer season for survey wasto find out the impact of heat island on humancomfort as Visakhapatnam is a tropical coastal cityand summer is oppressive and uncomfortable. Thediscomfort during the summer is not only due tohigh temperature but also due to high humidity andit varies from 72% along the coast to 60% towardsinterior places. The growth and intensity of heatisland depends upon the cooling rates of urban andrural environments. Because of the markedlydifferent surfaces, the rates of cooling of urban/ruralenvirons differ widely and the growth of the heatisland intensity varies with the time of the night.Fig. 2 shows urban and rural cooling rates during

Feb 2004. It was observed that the rate of decreaseof temperature during winter nights was 0.30C/hr inurban area whereas it was 0.50C/hr in rural area.

Fig.1 Surface temperature (0

C) distribution (11th February 2004)

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Temperature surveys were conducted in andaround Visakhapatnam during different periods,under varying meteorological conditions. It wasobserved that the heat islands can be formed under

all meteorological conditions during all seasons. Butheat islands of higher intensities can be developedonly during winter season under calm conditions.At Visakhapatnam, the formation of heat island iscontrolled by topography, urban morphology andproximity to a large water body, i.e. the Bay ofBengal. The land and sea breeze circulation alsointeracts with the heat island. Hence, intensity ofheat island is 20C to 40C at Visakhapatnam unlikeother Indian cities, where the intensities are morethan 40C.

Heat island increases the discomfort both

outdoors and indoors. The stress imposed by hightemperatures may lead to sickness. Duringsummer season demand for electricity increasesin tropical cities and urban heat island magnifiesthis demand and more energy is used for indoorcooling. As energy consumption is a major sourceof greenhouse gases, the concentration ofgreenhouse gases increases with intense use ofenergy which leads to climate change. Bydesigning, building and operating urban areas inan energy efficient way, significant decreases ingreenhouse gas emissions can be achieved.

It is clear that climatic discomfort is stronglyincreased by urban heat islands. Heat waveconditions coupled with heat island during summerseason causes human discomfort and higher deathrates. Visakhapatnam being a tropical coastalstation exhibits extreme temperatures and higherhumidities, which cause distress conditions. Thesummer months of April, May and June withmaximum temperatures of 350C to 400C areuncomfortable with oppressive heat. The analysisof summer maximum temperatures shows that thecity experienced ninety four heat waves, out of which

sixty three are moderate and thirty one are severeheat waves over a period of fifty years i.e. from 1951

to 2000. During heat waves, urban heat islandcauses extra thermal stress resulting in increasedurban death rates. There is evidence that aggressivebehaviour and heat deaths are increased in tropicalcities during hot weather.

Indoor discomfort also increases with hightemperatures and it can be reduced by makingmodifications in the design and orientation ofbuilding. Careful setting of vegetation around thebuilding has been recognised as a means of cooling.A direct consequence of landscaping would be lowertemperature effect on building surfaces as it reducesthe thermal load on building and create comfortableenvironment. In an investigation, the surfacetemperature of a common brick wall were comparedwith those of a brick wall covered with vegetation. Itwas noticed that the surface temperatures at theinner side of the covered wall were about 20C lowerthan those at the uncovered wall throughout thewhole day. Vegetation in and around buildings aswell as on the roof tops and hanging creepers canbe effectively used to achieve better comfortconditions.

It can be possible to achieve indoor comfort byappropriate constructional measures. It is not somuch the urban heat island itself which is causingan uncomfortable climate but in some cases it isthe unfavourable urban building properties.

4. REFERENCES

Chandler, T.J., 1970 : Urban climatology. Symp.WMO Tech. Note No. 108. WMO No. 254, Geneva.

Landsberg, H.E., 1970 : Climates and urbanplanning. WMO Tech. Note No. 108.

Lowry, W.P., 1977 : Empirical estimation of urbaneffects on climate - A problem analysis.____ J. Appl. Met.,16, 129-135.

Murthy, B.P., 1979 : Isothermal and isotherms inPune on clear winter nights. A Met. Study.

____ Mausam, 30, 473-478.

Murthy, B.P., 1999 : Hot cities in hot climate. WMOInt. Conf. on Urban Climatology, Sydney.

Oke, T.R., 1974 : Review of urban climatology 1968-73. WMO Tech.Note 134, WMO No. 383, Geneva.

Oke, T.R., 1979 : Review of urban climatology 1973-76. WMO Tech. Note 169, WMO No. 539, Geneva.

Oke, T.R., 1984 : Methods in urban climatology.ETH. Geog. Inst., Zurich.

Oke, T.R., 1999 : Multitemporal remote sensing ofan urban heat island. WMO Int. Conf. on urbanclimatology, Sydney.

Fig.2 Urban and Rural cooling rates

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Philip, Daniel and Murthy, K., 1974 : Seasonalvariation of surface temperature distribution overBombay. Proc. Symp. Environ. Pollution, Nagpur.

WMO, 1989 : Proc. of Int. Conf. on Urban Climate,Planning and Building, Kyoto, Japan.

WMO, 1999 : Proc. of Int. Conf. on UrbanClimatology, Sydney, Australia.