3. Simulation and analysis
3.2 Methodology for simulation
72
3. Simulation and analysis
73 cumulative score by developing a method for determining performance indicator. The results of these scores are then analyzed in a causal-comparative manner to find out the relationship between the input and output variables.
Figure 3.1: Methodology for Simulation and Analysis
74 3.2.1 Microclimate of Dhaka
The city of Dhaka lies between longitude 90º20’E and 90º30’E and between latitudes 23º40’N and 23º55’N. Dhaka experiences a hot, wet and humid tropical climate. Under the Koppen climate classification, Dhaka has a tropical savanna climate. The city has a distinct monsoonal season, with an annual average temperature of 27.5°C (81.5°F) and monthly means varying between 19.5°C (67°F) in January and 32°C (90°F) in April .Dhaka City has mainly three distinct seasons – the hot dry (March-May), the hot humid (June-November) and the cool dry season (December-February).
Generally the cool dry season is short while the summer is long and wet. April is the hottest month with average maximum temperature that varies from 30.3ºC to 34.8ºC and January is the coldest month with average temperature ranging from 9ºC to 15.2ºC.
Although overheating is the major problem of Dhaka City, it is due to some associated factors. For example, it is observed that from March to May there is high air temperature associated with high solar radiation (Joarder, 2007) (Figure 3.2).
Figure 3.2: Hourly solar radiation averaged by month for TRYs, Dhaka (source: U.S. Department of Energy, 2008).
From June to October, conditions with high humidity are associated with high air temperature. So from March to May, minimizing the impact of solar radiation can potentially moderate the overheated condition, whereas from June to October maximizing wind flow can contribute to minimize the over-heating situation. This is the overall climatic condition of Dhaka city related with basically thermal environment (Table 3.1). However, the luminous environment of the city is related to the duration of
75 sunshine hours and sky condition, two major climatic factors determining the quality and quantity of daylight. These two are discussed separately below.
Table 3.1: Climate data of Dhaka (Climate, 2018).
Climatic Period Cool-dry Hot-dry Warm -humid Warm -humid
Month
Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov
a .Maximum air
temperature 26.4 25.8 28.1 32.5 33.7 32.9 32.1 31.4 31.6 31.8 31.6 29.6
b. Minimum air
temperature 14.1 12.7 15.5 20.0 23.6 24.5 26.1 26.2 26.3 25.9 23.8 19.2
RH% 73 71 64 62 71 76 82 83 82 83 78 73
Rainfall
12.8 7.7 28.9 65.8 156.3 339.4 340.4 373.1 316.5 300.4 172.3 34.4
UV index 6 6 8 10 12 12 12 12 12 11 9 7
Daylight hours 11 11 11 12 13 13 14 13 13 12 12 11
Sunshine hours 9 9 8 7 6 5 3 2 2 3 6 8
Wind speed
.092 1.23 1.60 2.5 4.21 3.80 3.70 3.64 3.21 2.24 1.28 0.93
Wind Direction N,S-W S,S-E S,S-E,S-W S,S-E
3.2.2 Sunshine hours and sun path diagram
Daylight availability of any locations is influenced by latitude and weather patterns (A.G.S. 2000). In the cool period Dhaka has more than 8 hours of sunshine per day. But during monsoon months (warm-humid season) this comes down to 4 hours per day due to cloud cover. It is after June and July that this once again increases steadily (Joarder 2007). Figure 3.3 shows the monthly average sunshine hours of 2015 for Dhaka city.
).
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Figure 3.3: Average monthly hours of sunshine of Dhaka over the year (Source: World Weather Climate and Information data report, 2015
The atmospheric condition during the month of July to November period is cloudy. So, the diffused component of the daylight is considerably high. The variation in sunshine hours during July to November is wide. Figure 3.4 shows the sun path diagram of Dhaka, Bangladesh.
Figure 3.4: The sun path diagram of Dhaka, Bangladesh (Source: SUNTOOL - Solar Position Calculator, 1998)
77 3.2.3 Sky condition
Dhaka has a composite climate, where both clear sky, overcast and partly overcast skies are observed round the year (Figure 3.5), and designers have to face difficulties while considering both sky conditions (Ahmed, 1987).
Figure 3.5: Sky condition of Dhaka city (Ahmed, 1987).
Direct sunlight is intense and varies substantially as the sun's position changes throughout the day (up to 1, 00,000 lux). Daylight from a clear sky can be 10 to 25 % of the intensity of direct sunlight (10000–25000 lux). Daylight under partly cloudy conditions can vary largely; daylight under full overcast conditions can be 5% to 10%
of sunny conditions (5000-10000 lux).
Figure 3.6: Monthly average cloud cover and monthly average sunshine hours in Dhaka (Source:
Bangladesh Meteorological Department, Dhaka, 2005; cited from: Rahman, 2007).
In context of Dhaka, the sky remains clear and overcast in different parts of various seasons (Figure 3.6). During summer (Hot Dry), the sky remains both clear (sunny with sun) and overcast. During the warm-humid (March-November) period, the sky remains
78 overcast considerably. During monsoon (June-September) which is one third of the whole year the sky remains significantly overcast. During the winter (December- February) the sky mostly remains clear. While during the rest of the year, both clear and overcast conditions are observed (Joarder 2009a: 2009c). Table 3.2 shows sky condition of Dhaka city with respect to cloud cover for a year.
Table 3.2: Sky condition with respect to cloud cover over a year round. (Source: Climatic Division, Bangladesh metrological department, Dhaka, 2005).
Type of sky Pre-monsoon (March-May)
Monsoon (Jun-Sept)
Post- Monsoon (Oct-Nov)
Cool Dry (Dec-Feb)
Total (day)
Clear sky 62 38 39 77 215
Overcast sky 30 84 22 14 150
Total sky 92 122 61 90 365
3.2.4 Annual wind flow condition
In case of wind the majority cases in summer time in Dhaka city wind comes from south and southeast direction (Figure 3.7(a)), which is approximately 42% of all the wind coming from all directions. According to section 2.4.1 the wind flow coming from
Figure 3.7(a): Average wind direction of Dhaka city (January 2000 - December 2008) (Source:www.weatheronline.co.uk/weather]
southeast and southwest is mostly during monsoon and summer period respectively.
The wind frequency with the directions are shown in figure 3.7(b).
7%
8%
8%
20%
32%
9%
5%
10%
0% 5% 10% 15% 20% 25% 30% 35%
N NE E SE S SW W NW
Wind Direction and Percentage for Dhaka City
79 The data showed in figure 3.7(a) was collected at a height of 10m from ground. The data is referring to the wind pattern in Dhaka city. The maximum wind flow comes from south with the wind speed of 2.78 m/s for more than 632 hours and 568 hours of wind flow comes from south with the wind speed of 4.18 m/s. Sometimes the wind speed can reach up to 13.88 m/s speed from north, northeastern side, east, southeast and south. But the highest wind speed of 13.88 m/s mostly comes for 63 hours. Majority of the time the wind speed remains between 0 m/s to 5.56 m/s.
Figure 3.7(b): The duration of the prevailing wind of Dhaka city (cited from: Debnath, 2014)