Impact of Urbanization on the Thermal Comfort Conditions in the Hot Humid City of Chennai, India.

Impact of Urbanization Impact of Urbanization on the Thermal on the Thermal

Comfort Conditions in Comfort Conditions in the Hot Humid City of the Hot Humid City of

Chennai, India. Chennai, India.

A. Lilly RoseA. Lilly RoseAssistant Professor,

Department of Architecture,Sathyabama University- Chennai

Outline

1. Introduction

2. Area of Study

3. Methodology

4. Results & Discussions

5. Conclusions

1. Introduction

“Cities are increasingly expanding their

boundaries and populations, and from the

climatological point of view, human history is

defined as the history of urbanization”

M. Santamouris 2001

The urbanization effect

The most recognisable and unambiguous effect of humans on weather and climate is found in urban areas where nearly every meteorological parameter (radiation, precipitation, humidity, temperature, fog, etc.) is altered.

Causes

Urban areas create distinctive climates due to:

• Physical form and composition. The former alters the surface with which the atmosphere interacts.

• Urban activities that alter the nature of the atmosphere by emitting waste heat and materials.

• These are not exclusive. Altering the composition of the atmosphere affects the surface energy exchanges.

• These urban effects are usually represented as difference from ‘natural’ or background climate conditions.

• The study of urban climates is important in ensuring a healthy and comfortable environment for urban dwellers and to prevent the harmful effects of urbanization on larger scale climates.

Urban Climate

The urban effect is scale dependent. So evaluating its effect depends on how and where you measure.

Oke (2006)

SCALES OF URBAN CLIMATE

Urban Heat Island

Urban growth, resulting in the increase of buildings has led to the transformation of the natural landscape to impervious urban land. The growth of the urban fabric leads to warming, showing an increase in both regional and global temperatures.

Cities, being the hub of various activities and services, become sources of heat and pollution affecting the thermal structure of the atmosphere above them.

“The concentration of human activities in urban areas creates an "island" of heat surrounded by a "sea" of cooler rural areas, called the urban heat island”.

TYPES OF URBAN HEAT ISLAND

Source: Voogt (2007)

MEASUREMENT METHODS OF UHI

Source: Oke (1995)

Urban Heat Island Studies

Urbanization reduces the diurnal temperature range (difference between maximum and minimum temperatures) in cities and is a clear indication of the presence of the UHI (Landsberg 1981, Oke 1987).

Oke (1982) found that the maximum UHI occurs during the night, under calm and clear nights.

Urban Heat Island in Chennai

Chennai, a tropical city characterized by high temperatures and humidities, suffers extensively due to the urban heat island effect which affects the outdoor thermal comfort conditions significantly.

In developing countries, the outdoor comfort conditions are unnoticed, and the most affected are the urban poor who spend much of their time outdoors.

Therefore, this paper aims to analyze the impact of urbanization on thermal comfort conditions in the hot humid city of Chennai, India.

2. Area of StudyMaximum air temperatures (May and June): 38°C to 42°C

Minimum air temperatures (December and January) :18°C to 20°C.

Average monthly relative humidity : 63% (June) to 80% (November).

Mean daytime and night time temperatures in Chennai vary between 28°C and 37°C and 20°C and 28°C respectively, resulting in low diurnal temperature ranges, below 10°C.

3. MethodologyTo understand the impacts of urbanization on the thermal comfort conditions in Chennai Metropolitan Area, the time series data obtained from the two weather stations in Chennai were analyzed.

The air temperature and comfort trends of the CMA were studied, using historic climate records of Nungambakkam and Meenambakkam meteorological stations, located within the study area for 20 years from 1988 to 2008.

Being close to the CBD, the climatic data from the Nungambakkam meteorological station is assumed as urban. The data from the Meenambakkam meteorological station is assumed as rural as it is an airport station located outside the city limits.

Thermal Comfort Trends

The thermal comfort trends in Chennai Metropolitan Area were analyzed in terms of Temperature Humidity Index (THI) and Relative Strain Index (RSI).

The THI includes the effects of air temperature and relative humidity and

the RSI includes the effects of clothing and net radiation.

Temperature Humidity Index

THI = 0.8t + (RH t / 500) (Nieuwolt 1977)

t = air temperature in oC

RH = Relative humidity in %

21 ≤ THI ≤ 24 = 100% of the subjects felt comfortable.

24 < THI ≤ 26 = 50% of the subjects felt comfortable.

THI>26 = 100% of the subjects felt uncomfortably hot.

Relative Strain Index

RSI = (t-21) / (58-e)

(Unger 1999)

t = air temperature in oC

e = vapour pressure in hpa

0.1 – 100% unstressed (i.e. everyone comfortable)

0.2 – 75% unstressed

0.3 – 0% unstressed (i.e. upper limit of comfort)

0.4 – 75% distressed

0.5 – 100% distressed

4. Results and Discussions

Air Temperature Trends

Daytime Trends

Night time Trends

Diurnal Variation

Thermal Comfort Indices

Temperature Humidity Index

Relative Strain Index

Impact of Urbanization on the Thermal Comfort Trend in the CMA

Air Temperature Trends - Daytime TrendsDay time

y = 0.0598x + 32.644

R2 = 0.656

y = 0.0217x + 33.365

R2 = 0.2091

32

33

34

35

1988 1993 1996 1999 2002 2005 2008

Ma

x. T

em

pe

ratu

re in

o C

Nungambakkam MeenambakkamLinear (Nungambakkam) Linear (Meenambakkam)

Both the stations show a similar trend till 2000 beyond which there is a steep decline at the Meenambakkam station. The trend after 2000 is attributed to the urbanization at the airport station, tending it to shift from the rural to the suburban / city station character

The rate of increase in the daytime temperature is lesser in the rural station

(0.02oC/year) than in the city station

(0.06oC/year).

Air Temperature Trends – Night time Trends

There is a significant increasing trend at the rural station (R2 = 0.22) as against the

trend at the city station (R2 = 0.06) which is lesser. This is due to the fact that in the CMA, the scope for growth in the suburban limits is greater compared to that within the city limits

Night time

y = 0.0096x + 24.71

R2 = 0.0603

y = 0.0213x + 24.332

R2 = 0.2214

23

24

25

26

1988 1993 1996 1999 2002 2005 2008

Min

. T

empe

ratu

re in

o C


The existence of the UHI in the CMA is clearly evident through higher night time temperature at the city station when compared to that of the rural station.

Air Temperature Trends - Diurnal Variations

This is contrary to the general concept of the UHI, usually indicated by reduced diurnal temperatures

The diurnal temperature shows a significant increase in the city station

(0.05oC/year, R2 = 0.61).

y = 0.0502x + 7.9339

R2 = 0.607

y = 0.0004x + 9.0338

R2 = 0.0002

6

7

8

9

10

1988 1993 1996 1999 2002 2005 2008

Diu

rna

l Te

mp

era

ture

in o C


Thermal Comfort Indices –Daytime THI

The daytime THI of the city station shows a significant increasing trend of R2

= 0.55 exhibiting the thermal discomfort experienced in the city.

Daytime

y = 0.043x + 30.714

R2 = 0.5519

y = 0.0127x + 31.15

R2 = 0.1115

30

31

32

1988 1993 1996 1999 2002 2005 2008

TH

I in

o C


Thermal Comfort Indices – Night time THI

The night time comfort level of the airport station is better than that of the urban station at present, but the trend indicates that it would reverse in the next few years.

Night time

y = -0.0002x + 23.597

R2 = 3E-05

y = 0.0218x + 23.085

R2 = 0.231

22

23

24

25

1988 1993 1996 1999 2002 2005 2008

TH

I in

o C


Thermal Comfort Indices –Daytime RSI

The upper limit of comfort of the RSI is 0.3, whereas the RSI at both the city and the airport stations are around 0.4 indicating that 75% of the people would be distressed.

Daytime

y = 0.0009x + 0.3847

R2 = 0.0651

y = -0.0003x + 0.3911

R2 = 0.0139

0.30

0.40

0.50

1988 1993 1996 1999 2002 2005 2008

RS

I

Nungambakkam MeenambakkamLinear (Nungambakkam ) Linear (Meenambakkam)

Thermal Comfort Indices – Night time RSI

The night time RSI trends at both the stations are generally at 0.10 indicating that the nights are 100% comfortable.

Night time

y = 2E-05x + 0.0988

R2 = 0.0002

y = 0.0005x + 0.0853

R2 = 0.1569

0.00

0.10

0.20

1988 1993 1996 1999 2002 2005 2008

RS

I

Nungambakkam MeenambakkamLinear (Nungambakkam ) Linear (Meenambakkam)


The impact of urbanization on the thermal comfort conditions of the CMA is analyzed by comparing the THI trends of the typical climate (before urbanization) and the recent climate (during rapid urbanization).

The data from 1950-1980 is assumed as the typical climate

A significant transition is evident in the air temperature and the thermal comfort trends from 2003 and the data from 2003 to 2008 is considered as the recent climate.


The recent climate show an average increase of 1.1oC against the typical climate. . Even the lowest THI recorded in the recent climate during the winter months of January is 1.5oC higher than the typical climate and 2oC above the lower limit of 100% discomfort (26oC).

Daytime

24

26

28

30

32

34

36

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

TH

I in

oC

Typical Climate Recent Climate

Lower limit of 100% discomfort


The increase in the night time (0.7oC) comfort limits of the recent climate with respect to the typical climate, is not as significant as that of the daytime (1.1oC).

Night Time

18

20

22

24

26

28

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

TH

I in

oC

Typical Climate Recent Climate

Upper limit of 100% comfort

5. Conclusions

The existence of the UHI is evident through the higher night time temperatures at the city station when compared to the rural station.

Even though the city station shows increasing diurnal temperature contrary to the general concept of the UHI, the lesser diurnal temperature at the city station when compared to the airport station confirms the existence of the UHI.

ConclusionsDuring daytime, the thermal comforts at both the stations are above the upper comfort limit indicating the increasing discomfort. The percentage of people feeling uncomfortably hot during the day is 100% as per THI and 75% would feel distressed as per RSI.

During night time, the thermal comfort in the CMA is around 24oC (the upper limit of 100% comfort) according to THI and 0.10 (100% comfortable) according to RSI.

The recent climate shows an increase in THI values indicating the increasing discomfort throughout the year during daytime and during nights seven months in a year were not comfortable and are attributed to the impact of urbanization.

Conclusions

Urbanization, associated with the rapid transformation of the natural land cover to hard impervious surfaces, accelerates the thermal discomfort of a region.

It was found that strong and statistically significant trends exist between thermal comfort and urbanization.

The study found a statistically significant increasing trend in the discomfort experienced during daytime, with fairly comfortable nights, and also highlights the need to improve the daytime comfort conditions in the CMA, through a planned growth of urban areas.

Impact of Urbanization on the Thermal Comfort Conditions in the Hot Humid City of Chennai, India.

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