2. Materials and Methods
2.1. Measurement of the UHI Effects
2.1.1. Study Area and Measurement Points
2.1.2. Survey Time, Method, and Measuring Instruments
2.1.3. Correction of Time Synchronization
2.1.4. Background Weather Conditions
2.2. Analysis of the 100 m Scale Built-Environment Factors
2.2.1. D-Factors: Building Coverage Ratio (BCR) and Green Coverage Ratio (GCR)
- PBCR: BCR pixel number of the circular area with a 100 m radius
- PGCR: GCR pixel number of the circular area with a 100 m radius
- Ptol: total pixel number of the circular area with a 100 m radius
2.2.2. D-Factors: SVF and FAR
- FAR: Floor area ratio
- FAve: Average story number in the area
- HTot: Total story number in the area
- n: Building number in the area
- Fi: Story number of a single building
2.3. Correlation Analysis
3. Results and Discussion
3.1. UHI Survey Results
3.2. Analysis of the Canopy-Scale Built-Environment Factors
3.2.1. Green Coverage Ratio (GCR)
3.2.2. Building Coverage Ratio (BCR)
3.2.3. Floor Area Ratio (FAR)
3.2.4. Sky View Factor (SVF)
3.3. Correlation between the UHI and the Canyon-Scale Built-Environment Factors
3.3.1. Daytime Relationship
3.3.2. Midnight Relationship
3.3.3. Comparison to Previous Studies
- During the daytime, the wind speed was 1.84 m/s, and the radiation reached approximately 2.17 MJ/m2 on average. Under these conditions, the 2D and 3D built-environment factors attained similar coefficients of correlation with the air temperature. The coefficient of correlation attained a peak value of 0.4 at 13:00.
- The wind speed reached 1.84 m/s at midnight, and the daytime radiation was approximately 2.76 MJ/m2 on average. Under these conditions, the 2D and 3D built-environment factors exhibited the highest correlation with the air temperature at 00:30, and the coefficient was from 0.1 to 0.4.
- The 2D factors such as the GCR and BCR attained high correlation coefficients with the UHI. The trend was more significant in medium-sized cities than it was in large cities in the same country.
- In regard to the correlation between 2D factors such as the GCR and BCR and 3D factors such as the FAR, the present study produced similar observations as previous studies. The SVF, a 3D factor, revealed a positive or negative correlation with the air temperature. Similar results were recently reported [20,37,38]. The correlation between the SVF and air temperature requires further study, especially considering different urban geometric characteristics and SVF calculation methods.
- With buffer sizes of 100, 200, and 1000 m, built-environment factors were analyzed and correlated to the air temperature. The buffer size of 200 m generated the highest correlation, whereas the buffer size of 1000 m yielded the lowest correlation. As the number of relevant studies is small, it is recommended to further study the buffer size and scale effects in other regions worldwide.
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|28 July||30 July 2018||31 July 2018|
|Wind velocity (m/s)||1.1||2.0||3.1||2.6||1.1||0.7||0.9||0.4||0.3|
|Cloud cover (0~10)||7||7||7||8||--||--||--||--||--|
|Time||Daytime (28 July 2018)||Midnight (30–31 July 2018)|
|Northern Zone (45%, BCR)||GCR||−0.33||−0.22||−0.27|
|Southern Zone (40%, BCR)||GCR||−0.26||−0.39||−0.59|
|Western Zone (30%, BCR)||GCR||0.33||−0.48||−0.39|
|Northern Zone (45%, BCR)||GCR||−0.02||−0.21||−0.10|
|Southern Zone (40%, BCR)||GCR||0.25||−0.60||−0.44|
|Western Zone (30%, BCR)||GCR||−0.13||−0.41||−0.59|
|Chiayi City||Taipei |
|Present Study||Previous Study [23,24]|
|Survey and Analysis Methods||Survey Method||Locomotive-Mobile Observations||Locomotive-Mobile Observations||Locomotive-Mobile Observations|
|Buffer Size||100 m||1000 m||200 m|
|Artificial Coverage Ratio (ACR)|
|Coefficients of Correlation (r)||Daytime||Midnight|
|R = 200 m||2D||GCR||0.16||−0.37||−0.31||0.02||−0.44||−0.29|
|R = 1000 m||2D||GCR||0.02||−0.17||−0.43||−0.04||−0.29||−0.31|
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