Impact of Urbanization on
the Thermal Comfort
Conditions in the Hot
Humid City of Chennai,
India.
A. Lilly Rose
Assistant 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.
Urban Climate
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.
SCALES OF URBAN CLIMATE
The urban effect is scale dependent. So evaluating its
effect depends on how and where you measure.
Oke (2006)
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 Study
Maximum 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. Methodology
To 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
RH
=
=
air temperature in oC
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 Trends
Day time
Max. Temperature in oC
35
34
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).
y = 0.0217x + 33.365
2
R = 0.2091
y = 0.0598x + 32.644
R2 = 0.656
33
32
1988
1993
1996
Nungambakkam
Linear (Nungambakkam)
1999
2002
2005
2008
Meenambakkam
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
Air Temperature Trends – Night time Trends
Night time
Min. Temperature in oC
26
y = 0.0096x + 24.71
25
2
R = 0.0603
y = 0.0213x + 24.332
2
R = 0.2214
24
23
1988
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.
1993
1996
Nungambakkam
Linear (Nungambakkam)
1999
2002
2005
2008
Meenambakkam
Linear (Meenambakkam)
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
Air Temperature Trends - Diurnal Variations
10
Diurnal Temperature in oC
y = 0.0004x + 9.0338
The diurnal
temperature
shows a
significant
increase in the
city station
(0.05oC/year, R2
= 0.61).
2
R = 0.0002
9
y = 0.0502x + 7.9339
2
R = 0.607
8
7
6
1988
1993
1996
1999
Nungambakkam
Linear (Nungambakkam)
2002
2005
2008
Meenambakkam
Linear (Meenambakkam)
This is contrary to the general concept of the UHI, usually indicated by reduced
diurnal temperatures
Thermal Comfort Indices –Daytime THI
Daytime
32
y = 0.0127x + 31.15
2
THI inoC
R = 0.1115
y = 0.043x + 30.714
R2 = 0.5519
31
30
1988
1993
1996
1999
Nungambakkam
Linear (Nungambakkam)
2002
2005
2008
Meenambakkam
Linear (Meenambakkam)
The daytime THI of the city station shows a significant increasing trend of R2
= 0.55 exhibiting the thermal discomfort experienced in the city.
Thermal Comfort Indices – Night time THI
Night time
25
24
THI inoC
y = -0.0002x + 23.597
R2 = 3E-05
y = 0.0218x + 23.085
R2 = 0.231
23
22
1988
1993
1996
Nungambakkam
Linear (Nungambakkam)
1999
2002
2005
2008
Meenambakkam
Linear (Meenambakkam)
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.
Thermal Comfort Indices –Daytime RSI
Daytime
0.50
RSI
y = 0.0009x + 0.3847
R2 = 0.0651
0.40
y = -0.0003x + 0.3911
R2 = 0.0139
0.30
1988
1993
1996
Nungambakkam
Linear (Nungambakkam )
1999
2002
2005
2008
Meenambakkam
Linear (Meenambakkam)
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.
Thermal Comfort Indices – Night time RSI
Night time
0.20
RSI
y = 2E-05x + 0.0988
R2 = 0.0002
0.10
y = 0.0005x + 0.0853
2
R = 0.1569
0.00
1988
1993
1996
Nungambakkam
Linear (Nungambakkam )
1999
2002
2005
2008
Meenambakkam
Linear (Meenambakkam)
The night time RSI trends at both the stations are generally at 0.10 indicating
that the nights are 100% comfortable.
Impact of Urbanization on the Thermal Comfort
Trend in the CMA
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.
Impact of Urbanization on the Thermal Comfort
Trend in the CMA
Daytime
36
34
THI in oC
32
30
28
Lower limit of 100% discomfort
26
24
JAN
FEB
MAR
APR
MAY
JUN
Typical Climate
JUL
AUG
SEP
OCT
NOV
DEC
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).
Impact of Urbanization on the Thermal Comfort
Trend in the CMA
Night Time
28
THI in oC
26
Upper limit of 100% comfort
24
22
20
18
JAN
FEB
MAR
APR
MAY
JUN
Typical Climate
JUL
AUG
SEP
OCT
NOV
DEC
Recent Climate
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).
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.
Conclusions
During 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.