624842013031284450721

8/19/2019 624842013031284450721

http://slidepdf.com/reader/full/624842013031284450721 1/12

Nov. 2012, Volume 6, No. 11 (Serial No. 60), pp. 1492–1503

Journal of Civil Engineering and Architecture, ISSN 1934-7359, USA

The Effects of Building Form on Microclimate and

Outdoor Thermal Comfort in a Tropical City

Anisha Noori Kakon1 and Nobuo Mishima

2

1. Department of Urban and Regional Planning, Jahangirnagar University, Dhaka 1342, Bangladesh

2. Graduate School of Science and Engineering, Saga University, Saga 840-8502, Japan

Abstract: The urban microclimate has direct implications with regards to thermal comfort indoors as well as outdoors. In the tropics,

the outdoor thermal comfort conditions during daytime are often far above acceptable comfort standards due to intense solar radiation

and high solar elevations. This study aims to know effects of simple and fundamental building forms on microclimate and outdoor

thermal comfort in a high dense tropical city, focusing on Dhaka, Bangladesh as a study city. Investigations are carried out on existingarea and model areas with modified building forms (in respect of height and shape) on the microclimate as well as on outdoor thermal

comfort during daytime in summer. This study has demonstrated that the model using less ground coverage and higher buildings can

offer a better thermal climate than the models using maximum ground coverage in a high-density tropical city.

Key words: Building form, thermal comfort, tropical city, three-dimensional microclimate model, PET (physiological equivalent

temperature).

1. Introduction

Rapid urbanization in the tropics has brought in its

wake many hitherto changes to the physical

environment of the cities. Many of these changes are

well studied by researches in diverse fields such as

agriculture, medicine and engineering. However, the

integration of climate dimension in the planning and

design process has received little attention. The

environmental factors that characterize a climate such

as radiation, temperature, wind speed, relative

humidity, precipitation, and cloudiness are affected by

the three-dimensional characteristics of urban forms

and surface materials [1]. The important urban design

elements that affect the urban microclimate are the size

of the city, orientation and width of streets, density of

the built-up area, height of the buildings, and the

presence of parks and other green areas [2]. Therefore,

it is possible to modify the urban climate through the

physical restructuring of a city and appropriate urban

Corresponding author: Nobuo Mishima, PhD, associate

professor, research fields: urban design and architecture,

E-mail: [email protected].

policies. Building form (especially the shape and

height of buildings) is an independent design feature

that can affect urban climate in many ways. Studies in

Pune, India, showed that the unplanned increase in

building heights increases the discomfort level in the

city [3]. However, in the case of Colombo, Sri Lanka, it

was found that the wide streets with low-rise buildings

and no shade trees made the outdoor conditions worse,

and the most comfortable conditions were found in

narrow streets with tall buildings, especially if shade

trees were present [4]. In addition, it was found that a

very deep street canyon had considerably lower air

temperature than a shallow street canyon [5]. However,

most of these studies were conducted for the existing

cases only.

The urban microclimate has direct implications with

regards to thermal comfort indoors as well as outdoors.

Increased thermal stress leads to loss of efficiency and

productivity, and has been identified as one of the

causes of decrease in life expectancy in urban areas in

comparison to the rural areas [6]. In the tropics, the

outdoor thermal comfort conditions during daytime are

often far above acceptable comfort standards due to

DAVID PUBLISHING

D

8/19/2019 624842013031284450721


The Effects of Bui lding Form on Microclimate and Outdoor Thermal Comfort i n a Tropical City 1493

intense solar radiation and high solar elevations [7].

Thus, the ability to moderate outdoor thermal stress

conditions would certainly benefit the users of urban

outdoor space as well as indoor conditions.

This study aims to know effects of simple and

fundamental building forms on microclimate and

outdoor thermal comfort in a high dense tropical city.

2. Method of Study

2.1 Study Area

Present research is focusing on Dhaka, Bangladesh,

a location in South Asia within the tropical monsoon

climate zone, as a study city. Presently, in Dhaka city,

increasing the heights of buildings is commonly

adopted to cope with the rapid urbanization. Moreover,

heat stress in summer is a growing environmental

concern for Dhaka city. Thus, there is an urgent need to

evaluate the effects of different building forms on the

thermal climate in the city. Dhaka is a fast growing

mega city in the world. Approximately 12.3 million

people live in the metropolitan area, resulting in a high

density of about 23,029 persons per square kilometer[8]. To accommodate this large population, the city is

growing both horizontally and vertically. This study is

based on a locality namely Dhanmondi area, which is

one of the first planned residential communities in the

city. However, gradual invasion of non-residential uses

has drastically affected the quality and changed the

character of this area. The study area is shown in Fig. 1

and marked by the dotted line. The selected area is a

Fig. 1 Study area in Dhanmondi, Dhaka.

well-known commercial zone located along both the

sides of a busiest street of 22 m wide with pedestrian

ways. Up to 2006, the height of buildings was limited

to 6-storied in this area and thus most of the buildings

are within 6-storied. The orientation of this canyon is

60° NE-SW.

2.2 Investigation

Investigations are carried out on existing area and

model areas with modified building forms (in respect

of height and shape) on the microclimate as well as on

outdoor thermal comfort during daytime in summer.

This study was carried out for hot and dry summerconditions (March-May). During this season, the solar

radiation on horizontal surface is high with comparison

to the rest of the year, and the maximum occurs in April

(5.75 kWh/m2/day). The relative humidity in this

period is about 40%, and the near-ground wind speed is

about 2.0 m/s, with slight variation during the daytime

[9].

The microclimate was evaluated based on solar

radiation, air temperature T a, mean radiant temperature

T mrt , and wind velocity vs. To access the outdoor

thermal comfort, the PET (physiological equivalent

temperature) is considered. The thermal microclimatic

parameters and the outdoor thermal comfort are

compared between the existing and model areas.

3. Modification of Building Form: Model

Areas

From April 2006, Dhaka city had a new Building

rule — Dhaka Metropolitan Area BuildingConstruction Rules 2006 which was implemented after

Jan. 31, 2007 [10]. As a result, the previous rule

regarding height limitation has been withdrawn

allowing higher buildings in accordance to these rules.

According to this rule, the building height in number of

stories will be assigned by the FAR (floor-area-ratio)

corresponding to the plot size and land use. In this

study, the building forms are modified according to the

rules of site setback, ground coverage, FAR and so on.

8/19/2019 624842013031284450721


The Effects of Bui lding Form on Microclimate and Outdoor Thermal Comfort i n a Tropical City1494

The layout of the existing area is shown in Fig. 2. Apart

from the existing case, three models are considered

using three different building forms shown in Fig. 3.

However, the following things are considered while

modifying the building forms:

The building volume on each plot is same for the

three models;

The maximum allowable building volume on

each plot is estimated by the maximum FAR. FAR of

3.25, 3.50, 3.75, 4 and 5 are used for plot sizes of

150–300 m2, 300–500 m

2, 500–650 m

2, 650–1,300 m

2

and greater than 1,300 m2, respectively;

All buildings are of the same form in a particularmodel;

The maximum allowable ground coverage is

60% and 50% for plot sizes of 150–650 m2 and

650–1,300 m2, respectively;

All the models set aside the mandatory open

space on to the rear side;

m

Pedestrian way: Line 1 Pedestrian way: Line 2 Fig. 2 Layout of the existing area.

Fig. 3 Building forms of existing and model areas.

Buildings are considered on all the lots. Thus,

there is no vacant plot in the model areas.

The characteristic features of the modified building

forms of the three models are shown in Table 1.

Features of the existing area are shown for reference.

As the building setback is applied in Model 2, the

back part of the building became larger to keep the

volume fixed. However, in Model 3, the reduced

ground coverage increases the building height

compared to the other models.

Fig. 4 shows the SVF (sky view factors) which

represents the percentage of free sky at a specific

location at a height of 1.2 m above ground for theexisting area as well as for the three models.

4. Methodology

4.1 Numerical Modeling

In urban climatology, an extremely limited number

of field studies related to outdoor thermal comfort and

its dependence on urban growth are available due to the

large number of urban variables and processes

Table 1 Features of the modified building forms.

Models

Ground

Coverage

FAR Site Setback

Special

Features

ExistingAsexisting

As existing As existing ---

Model 1Allowablemaximum

Allowablemaximum

Allowable

minimum inthe front

and sides

---

Model 2Allowablemaximum

Allowablemaximum

Allowable

minimum inthe front

and sides

Building

setback onfront after

3-storied

Model 3

Reductionof 10% ofallowablemaximum

Allowablemaximum

Allowableminimum in

the frontand right

side

Increasedside distances between buildings

Existing Model 1 Model 2 Model 3SVF = 61.8% SVF = 34.6% SVF = 42.1% SVF = 35.6% Fig. 4 Sky view factors of different model areas.

Model 1Existing

Model 2 Model 3

3-storied 6 m

8/19/2019 624842013031284450721



involved. Thus, numerical modeling is more popular

than comprehensive field measurements, because

researchers have greater control over modeling, and it

is more economical with regard to time and resources

[11].

4.2 Modeling of Microclimate

The present microclimate is evaluated by using a

three-dimensional numerical model ENVI-met

(three-dimensional microclimate model) [12, 13] which

simulates the microclimatic changes within urban

environments in a high spatial and temporal resolution.

This model calculates all important meteorological parameters such as the solar radiation, air temperature,

relative humidity, wind speed, as well as mean radiant

temperature and so on. Mean radiant temperature is the

key variable in evaluating thermal sensation outdoors

during the daylight hours in summer. It has been

confirmed that human comfort indexes such as PMV

(predicted mean vote) and PET are strongly dependent

on T mrt [14]. The equations for the calculation of T mrt in

ENVI-met are given in Ref. [15]). The basic input

parameters for simulation are shown in Table 2.

Further, the results of air temperature in the present

existing area obtained from the field measurements and

numerical simulation show good agreement which was

shown in previous studies [15, 16].

4.3 Modeling of Thermal Comfort

The thermal comfort is accessed by the PET, which

is an up-to-date thermal index [17]. It is based on the

MEMI model (Munich energy balance model forindividuals) [18]. PET is defined as the air temperature

at which, in a typical indoor setting (air temperature =

mean radiant temperature, relative humidity = 50%,

wind speed = 0.1 m/s), the heat budget of the human

body is balanced with the same core and skin

temperature as those under complex outdoor

conditions. Thus, PET enables a person to compare the

integral effects of complex thermal conditions outside

with his or her own experience indoor [17]. PET has

Table 2 Basic input parameters for simulation.

LocationDhaka, Bangladesh, 23.24ºN,90.23ºE, 8.8 m asl

Simulation day Typical summer day, 18th AprilSimulation duration From 6:00 to 22:00 LST (16 h)

Spatial resolution 160 × 88 × 25, grid size 2 × 2 × 3

Wind speed and direction 2.0 m/sec from 180º (south)

Initial atmosphere

temperature20C (293 K)

Relative humidity 40%

Heat transmissionWalls: 1.7 W/m2K,Roofs: 2.2 W/m2K

Albedo Walls: 0.3, Roofs: 0.15

been employed as a bioclimatic index in several

studies of outdoor thermal comfort [19–21].

In the present study, PET is evaluated by the recent

model, RayMan [22] (developed in the Meteorological

Institute, University of Freiburg) due to its availability

and ease of applications. The validity and versatility of

the RayMan model are confirmed by several recent

researches [21–24]. For the calculation of PET, the

simulated values (from ENVI-met) of meteorological

parameters such as air temperature Ta, mean radiant

temperature T mrt , RH (relative humidity) and wind

velocity vs are used. In addition, the thermo- physiological parameters such as heat resistance of

clothing (clo units) of 0.5 clo and activity of humans

while walking of 200 watt are considered in present

RayMan model.

5. Results and Discussions

5.1 Microclimate Analysis

The microclimate is investigated mostly on the

pedestrian ways (Lines 1 and 2 as shown in Fig. 2)since the assessment of thermal comfort at these

positions is most important for the pedestrians. 20

points at a distance of 10 m are taken along the

pedestrian ways. Further, results will be shown at the

middle of the canyon at point “m” shown in Fig. 2.

Parameters are evaluated at a height of 1.2 m above

ground as this height is representative for the comfort

assessment for a standing person.

5.1.1 Solar Radiation

8/19/2019 624842013031284450721



The direct S and diffuse D components of the hourly

mean short-wave radiation at point “m” (shown in Fig.

2) are presented in Fig. 5 for the existing area together

with the three models. Since the global radiation (G) is

the sum of S and D, the value of G is mostly affected by

the direct solar radiation S because of the tropical

location and clear sky conditions. The relation between

solar radiation and SVF is previously analyzed and it

was found that the higher the SVF the larger solar

radiation can reach in urban canyon [25]. Therefore S

decreases as SVF decreases in the models. The average

S values are 657.3 W/m2, 496.9 W/m

2, 533.8 W/m

2 and

387.0 W/m2

in the existing area, Model 1, Model 2 andModel 3, respectively. It is noticeable that although the

SVF of Model 1 and Model 3 is very close, S is lower in

Model 3 than in Model 1. This is due to the duration of

sunshine hours at point “m” in Model 3 is lowest 4.5

hours (10:30-15:00 LST) while this duration is 6 hours

(9:30-15:30 LST) in Model 1. Since the day of

simulation is a clear sky day, the diffuse radiation D is

very low (< 95 W/m2) in all the cases. The maximum

value of D is found at 12:00 LST. D decreases in the

models than the existing case. However, D is highest in

Model 2 comparing the other models. Adding the S and

D components, it can be observed that global radiation,

G is highest at 12:00 LST in all the models because of

the effect of the position of the sun (75.9° for Dhaka),

which in this month is highest at this time. It is found

that in the existing case, the average global radiation G

is 729.9 W/m2. However, the average G values are

549.7 W/m2, 593.2 W/m

2and 436.9 W/m

2 in Models 1,

2 and 3.5.1.2 Air Temperature

Figs. 6–8 illustrate the distribution of T a at the points

on pedestrian way along Lines 1 and 2 at 9:00 LST,

12:00 LST and 15:00 LST, respectively. At 9:00 LST

and along Line 1 (Fig. 6a), Ta decreases in almost all

the points in Models 1, 2 and 3 comparing with the

existing case. The lowest T a at this time is found in

Model 3 and the highest difference in Ta is found about

1.16°C to that from the existing case. Furthermore, T a

distributes non-uniformly for all the cases. However,

all points average value shows that Ta decreases to

0.33°C, 0.15°C and 0.90°C for Models 1, 2 and 3,

respectively from the existing case. On the other side of

pedestrian way along Line 2 at 9:00 LST (Fig. 6b), Ta

increases all through the way in Model 2 than the

existing case. This is due to setback of building that

reduces the building height in the front resulting more

direct solar radiation in this model. However, T a

decreases almost all the points in Model 3 as the

buildings are the tallest from the other models. In this

case, the maximum decrease in T a at a particular point

is 0.9°C and the point average decrease is 0.6°C.Further, in Model 1, T a decreases to some extent in

some parts on the way while it increases in other parts.

At 12:00 LST (Fig. 7), T a is lower in all the models

than the existing case on both the sides of pedestrian

way. As in this time the sun is in its highest position

and there is no shading effect during this time, the

distribution pattern of T a is very similar along both the

sides of the pedestrian ways. Along Line 1 (Fig. 7a),

considering the point average value, T a decreases to

Fig. 5 Direct (S) and diffuse ( D) radiation fluxes received

at 1.2 m above ground.

600

650

700

750

800

850

8 9 10 11 12 13 14 15 16 17 18

ExistingModel 1Model 2Model 3

0

20

40

60

80

100

8 9 10 11 12 13 14 15 16 17 18

Existing

Model 1Model 2Model 3

8/19/2019 624842013031284450721



0.69°, 1.06° and 0.73°C in Models 1, 2 and 3,

respectively to that from existing case. On Line 2 (Fig.

7b), these reductions are 0.54°, 0.80° and 0.59°C in

Models 1, 2 and 3, respectively.

Similar to 12:00 LST, T a is lower in the models than

the existing case on both the sides of pedestrian way at

15:00 LST as shown in Fig. 8. At this time, on both the

sides Ta is lowest in Model 2. Along Line 1 (Fig. 8a),

the average decreases of T a are 0.88°, 1.49°, and

1.04°C in Models 1, 2 and 3, respectively to that from

the existing one. Along Line 2 (Fig. 8a), these

reductions are 0.87°, 1.54° and 0.97°C in Models 1, 2

and 3, respectively.5.1.3 Mean Radiant Temperature

Mean radiant temperature, T mrt is the most important

parameter governing human energy balance, especially

on hot sunny days. Figs. 9–11 show the distribution of

T mrt along Lines 1 and 2 at 9:00 LST, 12:00 LST and

15:00 LST, respectively. The effect of sun on buildings

is such that at a moment while the one side of

pedestrian way is under shaded condition, the other

side is sunny. It is important to mention that the taller

buildings along one side of the pedestrian way

sometimes create longer shadow which covers the

other side of the pedestrian way. The difference of T mrt

at a shaded and unshaded point is about 36°C. Fig. 9a

shows that at 9:00 LST along Line 1, T mrt is very high

(a) (b)Fig. 6 Distribution of air temperatureT a for the existing and model areas at 9:00 LST, along: (a) Line 1; (b) Line 2.

(a) (b)

Fig. 7 Distribution of air temperatureT a for the existing and model areas at 12:00 LST, along: (a) Line 1; (b) Line 2.

(a) (b)

Fig. 8 Distribution of air temperatureT a for the existing and model areas at 12:00 LST, along: (a) Line 1; (b) Line 2.

x

T

0 40 80 120 160 20035

35.5

36

36.5

37

( o C )

(m)

a

x

T

0 40 80 120 160 20034.5

35

35.5

36

36.5

37

( o C )

(m)

a

x

T

0 40 80 120 160 20031.5

32

32.5

33

33.5

( o C

)

(m)

a

x

T

0 40 80 120 160 20031

31.5

32

32.5

33

( o C )

(m)

a

:Existing , : M o d el 1 , : :Model 2 , Model 3

x

T

0 40 80 120 160 20037.5

38

38.5

39

39.5

40

40.5

( o C

)

(m)

a

x

T

0 40 80 120 160 20037.5

38

38.5

39

39.5

40

40.5

41

( o C

)

(m)

a

8/19/2019 624842013031284450721



(a) (b)

Fig. 9 Distirbution of mean radiant temperature T mrt for the existing and model areas at 9:00 LST; along (a) Line 1 and (b)

Line 2.

(a) (b) Fig. 10 Distirbution of mean radiant temperature T mrt for the existing and model areas at 12:00 LST; along (a) Line 1 and

(b) Line 2.

(a) (b) Fig. 11 Distirbution of mean radiant temperature T mrt for the existing and model areas at 15:00 LST; along (a) Line 1 and

(b) Line 2.

in the range of 64–67°C in all the models. Comparing

with the existing case, Tmrt increases to some extent

at most of the points in the Models 1 and 2. The

difference in T mrt is very small and among these

models the highest average increase is 1.02°C in

Model 2. In fact in the morning, Line 1 is under sunny

condition and the other side (Line 2) is under shade.

In Model 3, at some points (8 out of 20) along Line 1,

T mrt falls down to some extent due to shading from the

buildings of the other side. Along Line 2 (Fig. 9b), in

the existing area Tmrt is very high (64–66°C) at most

of the points, whereas in the models T mrt is very low

(28–30°C). Thus in Models 1, 2 and 3, T mrt decreases

by upto 36°C from the existing case in most of the

parts along Line 2.

Again at 12:00 LST (Fig. 10a) similar to the

previous case T mrt is high in all models and slightly

higher in the models than in the existing. In this case,

the maximum increase is 1.65°C at a particular point in

Model 2. Along Line 2 (Fig. 10b), Tmrt is very high

(61–63°C) all through the line in existing case and

Model 2. At most of the points of Model 1, T mrt values

are very close to the existing case. However, in Model

3, T mrt decreases variably by 24–30°C at most of the

points.

x

T

0 40 80 120 160 200

64

65

66

67

( o C

)

(m)

m r t

x

T

0 40 80 120 160 20020

30

40

50

60

70

( o C

)

(m)

m r t

:Existing , : Model 1 , : :Model 2 , Model 3

x

T

0 40 80 120 160 20061

62

63

( o C

)

(m)

m r t

x

T

0 40 80 120 160 20020

30

40

50

60

70

( o C )

(m)

m r t

T

0 40 80 120 160 20030

40

50

60

70

80

( o C

)

(m)

m

r t

x

T

0 40 80 120 160 20071

72

73

( o C

)

(m)

m

r t

8/19/2019 624842013031284450721



Line 1 at 15:00 LST, is under building shade, thus

Tmrt is lower in this side which is confirmed by Fig.

11a. In the existing case, the building height is lower.

Moreover, most of the buildings are erected

maintaining a larger distance from the road, thus there

is lack of shading resulting in high T mrt . Again in Model

2, T mrt is extremely high (the average is 71°C). This is

due to building setback which lowers the front height

of the building in this model and allows more solar

radiation. However, in Models 1 and 3, the values of

T mrt are very close and these are significantly lower

than the existing case. T mrt decreases by 34–37°C from

the existing case almost all the points in Models 1 and 3.Whereas along Line 2 at 15:00 LST (Fig. 11b), T mrt is

very high (about 71–73.2°C). Although in the models

T mrt is lower to some extent at most of the points than

the existing case, these differences are small. However,

the maximum reduction is found in Model 3.

5.1.4 Wind Velocity

The wind direction at this period of the year is not

favorable for this area due to the orientation of the

canyon. Fig. 12 shows the distribution of daytime

average wind velocity vs along Lines 1 and 2. The wind

is more along Line 1 (point average is 0.95 m/s) than

Line 2 (point average is 0.71 m/s). The wind velocity

decreases at most of the points in the models than the

existing case. Again in the existing case at the point in

front of the vacant space (no building), vs is higher.

Apart from the existing case, comparing the models, vs

is higher to some extent in Model 3 along Line 1 at

most parts of the pedestrian way (Fig. 12a). Along this

line the point average vs are 0.80 m/s, 0.70 m/s and 0.85

m/s in Models 1, 2 and 3, respectively. Along Line 2

(Fig. 12b), vs is increased at some points in Model 3

than the existing case. However, the point average vs

are same in Models 1, 2 and 3 and the value is 0.56 m/s.

5.2 Thermal Comfort Analysis

The distribution of physiological equivalent

temperature, PET along Lines 1 and 2 is shown in Figs.

13–15 at 9:00 LST, 12:00 LST and 15:00 LST,

respectively.

PET schemes are basically influenced by T mrt in

summer days under sunny conditions. Therefore, the

patterns of the diurnal courses of PET and T mrt are

similar. This is not surprising as T a, relative humidity

(a) (b) Fig. 12 Distribution of average wind velocity, vs in the existing and model areas; along (a) Line 1 and (b) Line 2.

(a) (b) Fig. 13 Distribution of PET for the existing and model areas at 9:00 LST; along (a) Line 1 and (b) Line 2.

x

P E T

0 40 80 120 160 20046

47

48

49

50

51

52

( o C

)

(m) x

P E T

0 40 80 120 160 20025

30

35

40

45

50

55

( o C

)

(m)

:Existing , : M o de l 1 , : :Model 2 , Model 3

x

v

0 40 80 120 160 2000

0.5

1

1.5

( m / s )

(m)

s

x

v

0 40 80 120 160 2000

0.5

1

1.5

( m / s )

(m)

s

8/19/2019 624842013031284450721



(a) (b)

Fig. 14 Distribution of PET for the existing and model areas at 12:00 LST; along (a) Line 1 and (b) Line 2.

(a) (b) Fig. 15 Distribution of PET for the Existing and model areas at 15:00 LST; along (a) Line 1 and (b) Line 2.

and vs vary comparatively much less. In this research,

the thermal sensation and PET ranges are considered

as that of the case of Sun Moon Lake, Taiwan [26].

That region is also characterized by the tropical

climate which closely matches with the present studyarea. Table 3 shows the thermal sensation and

corresponding PET ranges.

At 9:00 LST and along Line 1 (Fig. 13a), PET is

high (47.0–51.8°C) showing “very hot” thermal

sensation. PET is higher in Models 1 and 2 at most of

the points compare to the existing case. In Model 3 at

most of the points, PET is lower than the existing case

with the maximum difference of 3.7°C. Alternatively,

at this time along Line 2 (Fig. 15a), PET decrease

significantly by 16–21°C than the existing case.

Table 3 Thermal sensation and PET Classes.

Thermal sensation PET (C)

Slightly cool 22 – 26

Comfortable 26 – 30

Slightly warm 30 – 34

Warm 34 – 38

Hot 38 – 42

Very hot > 42

Similarly, at 12:00 LST along Line 1 (Fig. 14a),

PET is high (48.5–51.5°C). Comparing to the existing

case, PET increased to some extent at half portion of

the pedestrian way while at other half, it decreases in

Models 1 and 2.In Model 3, PET is lower at most of the points (14

out of 20) than the existing case with the maximum

difference of 2.4°C. Alternatively along Line 2 at

12:00 LST (Fig. 14b) PET varies within a large range

of 35–51°C. Here in existing case, PET is high

(>48°C) indicating “very hot” condition all through

the pedestrian way. At this time, PET decreases at

most of the points in Models 1 and 3 than the existing

case. It differs notably by 10–17°C in most parts of

the way in Model 3 and in some parts of Model 1. For

the case of Model 2, PET is high and the values are

close to the existing case.

Moreover at 15:00 LST along Line 1 (Fig. 15a),

PET is high (> 57°C, “very hot”) at most of the points

in the existing case and also in Model 2. In Models 1

and 3, PET values are within 38–40°C (“hot”).

The daytime highest PET values are found along

Line 2 at 15:00 LST (Fig. 15b), where the range of

PET is 56.8–59.7°C for all models which corresponds

x

P E

T

0 40 80 120 160 20048

49

50

51

52

( o C )

(m)

P E

T

0 40 80 120 160 20030

35

40

45

50

55

( o C )

(m)

P E T

0 40 80 120 160 20035

40

45

50

55

60

65

( o C )

(m)

P E T

0 40 80 120 160 20056

57

58

59

60

( o C )

(m)

8/19/2019 624842013031284450721



7 9 11 13 15 17 19

Fig. 16 Diurnal distribution of thermal sensation in the existing and model areas.

to “very hot” sensation. In this case, PET decreases

slightly in the models than the existing case although

the point average values are very close for all the

models. Thus it is found here that PET decreases

largely in Model 3 than the existing case along one

side either on Line 1 or Line 2 throughout the day.

While in Model 1, PET decreases considerably in the

morning (along Line 2) and afternoon (along Line 1).

PET values of Model 2 are very close to the existing

case in most of the hours of the day. The most

uncomfortable locations are those exposed to the sun.

Thermal comfort was also assessed at point “m”

(shown in Fig. 2) and the results are presented in

respect of thermal sensation in Fig. 16. In the existing

case, the duration of “very hot” (PET > 42°C) thermal

sensation is long (8:00–16:30 LST) with a total

duration of nine hours. This duration decreased in the

models and these are six hours in Model 1, seven

hours in Model 2, and the lowest 5 hours in Model 3.

Thus comparing all the models the duration of “very

hot” condition is shortest in Model 3. The average

PET in the “very hot” condition is also shown in this

figure. A maximum average decrease of 1.9°C is

found in case of Model 3 than the existing case.

Furthermore, the durations of comfortable and

slightly warm thermal sensation are only 1 hour in the

existing case, whereas these are 3.5 hours in Model 1, 3

hours in Model 2 and 4.5 hours in Model 3. Therefore

the condition of thermal comfort is best in Model 3

followed by Model 1. Comfort condition is also

improved in Model 2 than the existing situation.

As in the models the wind velocity, vs is not

improved well, thus the improved thermal comfort

condition in these models is mainly due to the shading

effect which causes lower T a and T mrt .

6. Conclusions

The important findings of the present study are that:

The global radiation decreases largely in the

models of modified building forms than the existing

case;

Air temperature T a decreases in the model areas

to that of existing area. However, the decreases are not

uniform throughout the day. At 9:00 LST, Ta

decreases by 0.33°C, 0.15°C and 0.90°C on an

average for cases of Models 1, 2 and 3, respectively.

At 12:00 LST, T a decreases by 0.69°C, 1.06°C and

0.73°C for cases of Models 1, 2 and 3, respectively.

Further, T a decreases by 0.87°C, 1.54°C and 0.97°C

for cases of Models 1, 2 and 3, respectively at 15:00

LST;

Unlike air temperature, mean radiant temperature

(T mrt ) is higher in the model with building setback

(Model 2) as it receives more solar radiation. In the

other two models (Models 1 and 3), mean radiant

temperature decreases considerably at most parts of

the pedestrian ways either one side or the other.

However, the maximum difference is found in the

model with highest building height (Model 3). In this

8/19/2019 624842013031284450721



model, T mrt decreases variably by 24–30°C at most

parts of the pedestrian way along one side at 12:00

LST. The increased building height of this model

causes the mean radiant temperature to decrease

largely, due to the shading effect. Nevertheless, in a

particular model the difference of mean radiant

temperature at a shaded and unshaded point is up to

36°C;

Similar to mean radiant temperature, PET values

also decreased largely in the model with highest

building height (Model 3) either one side or the other

along the pedestrian way all-over the day. In addition,

in Model 1, PET decreases considerably in themorning (along Line 2) and afternoon (along Line 1)

hours. PET values of Model 2 are very high and close

to the existing case in most hours of the day;

Furthermore, all models of modified building

forms considerably reduce the duration of worst

thermal sensation (“very hot” condition) to that of

existing case. This worst duration is 9 hours in the

existing case while in Model 3 this duration is found

to be 5 hours which is the minimum among the

models.

This study has demonstrated that the model using

less ground coverage and higher buildings can offer a

better thermal climate than the models using

maximum ground coverage in a high-density tropical

city. Further, the results obtained from this research

can contribute to improve outdoor thermal comfort in

urban environment.

Acknowledgments

We sincerely appreciate the support from Sasagawa

Kagaku Kenkyu Josei in 2009. This manuscript is a

slightly revised version of a paper presented in 11th

International Congress of Asian Planning Schools

Association at Tokyo University from the September

19th–21th, 2011.

References

[1] K. S. Ahmed, Approaches to bioclimatic urban design for

the tropics with special reference to Dhaka, Bangladesh.

Ph.D. Thesis, Environment and Energy Studies Program,

Architectural Association School of Architecture, London,

UK, 1995.

[2] D. Watson, A. Plattus and R. Shibly, Time-Saver

Standards for Urban Design, McGraw-Hill Professional,

New York, 2003.

[3] V. Deosathali, Assessment of impact of urbanization on

climate: An application of bio-climatic index,

Atmospheric Environment 33 (24-25) (1999) 4125–4133.

[4] E. Johansson and R. Emmanuel, The influence of urban

design on outdoor thermal comfort in the hot, humid city

of Colombo, Sri Lanka, International Journal of

Biometeorology 51 (2) (2006) 119–133.

[5] E. Johansson, Influence of urban geometry on outdoor

thermal comfort in a hot dry climate: A study in Fez,

Morocco, Building and Environment 41 (10) (2006)

1326–1338.

[6] B. Givoni, Climate Considerations in Building and Urban

Design, John Willey and Sons., New York, 1998.

[7] A. F. Toudart and H. Mayer, Effects of asymmetry,

galleries, overhanging façades and vegetation on thermal

comfort in urban street canyons, Solar Energy 81 (2007)

742–754.

[8]

Bangladesh Bureau of Statistics, Statistical Pocket Book,

Government of People’s Republic of Bangladesh, 2007.

[9] Meteorological Department, Climate Division Climate

Data, Dhaka, Government of People’s Republic of

Bangladesh, 2007.

[10] GOB (Government of Bangladesh), Dhaka Metropolitan

Building Construction Rules 2006, Ministry of Housing

and Public Works, Dhaka, Bangladesh, 2007.

[11] A. J. Arnfield, Two decades of urban climate research: A

review of turbulence, exchange of energy and water, and

the urban heat island, International Journal of

Climatology 23 (1) (2003) 1–26.

[12] M. Bruse and H. Fleer, Simulating surface-plant-air

interactions inside urban environments with a

three-dimensional numerical model, Environmental

Modelling Software 13 (4) (1998) 373–384.

[13] M. Bruse, ENVI-met Homepage, available online at:

http://www.envi-met.com/ (accessed Sep. 11, 2008)

[14] A. Matzarakis, H. Mayer and F. Rutz, Radiation and

thermal comfort, in: Proceedings of 6th Hellenic

Conference in Meteorology, Climatology and

Atmospheric Physics, Greece, 2002, pp. 738–744.

[15] A. N. Kakon and N. Mishima, An evaluation of

increasing building height in respect of thermal climate in

a high density city in South Asia using numerical

modeling, Journal of Asian Architecture and Building

Engineering 8 (2) (2009) 401–406.

[16] A. N. Kakon, N. Mishima and S. Kojima, Simulation of

the urban thermal comfort in a high density tropical city:

8/19/2019 624842013031284450721



Analysis of the proposed urban construction rules for

Dhaka, Bangladesh, Building Simulation: An

International Journal 2 (4) (2009) 291–305.

[17] P. Höppe, The physiological equivalent temperature — A

universal index for the biometeorological assessment of

the thermal environment, International Journal of

Biometeorology 43 (1999) 71–75.

[18] P. Höppe, Heat balance modeling, Cellular and Molecular

Life Sciences 49 (1993) 741–746.

[19] S. Thorsson, T. Honjo, F. Lindberg, I. Eliasson and E. M.

Lim, Thermal comfort and outdoor activity in Japanese

urban public places, Environment and Behavior 39 (2007)

660–684.

[20] S. Oliveira and H. Andrade, An initial assessment of the

bioclimatic comfort in an outdoor public space in Lisbon,

Journal of Biometeorology 52 (2007) 69–84.

[21]

T. P. Lin, A. Matzarakis and R. L. Hwang, Shading effect

on long-term outdoor thermal comfort, Building and

Environment 45 (2010) 213–221.

[22] RayManLink, available online at:

http://www.mif.uni-freiburg.de/rayman/intro.htm.

[23] A. Matzarakis, F. Rutz and H. Mayer, Modelling

radiation fluxes in simple and complex environments:

Basics of the RayMan model, International Journal of

Biometeorology 54 (2010) 131–139.

[24] H. Farajzadeh and A. Matzarakis, Climate potential for

tourism in northwest of Iran, Meteorological Applications

16 (2009) 545–555.

[25] A. N. Kakon and N. Mishima, The sky view factor effect

on the microclimate of a city environment: A case study

of Dhaka city, in: Proceedings of the 7th International

Conference on Urban Climate, Yokohama, Japan, 2009.

(CD-ROM)

[26]

T. P. Lin and A. Matzarakis, Tourism climate and thermal

comfort in Sun Moon Lake, Taiwan, International Journal

of Biometeorology 52 (2008) 281–290.

624842013031284450721

Documents