Quantifying the Effect of Building Shadowing and Cloudiness on Mean Radiant Temperature in Singapore

: Improving the quality of life in urban areas has become a major concern in the last few decades. With a constantly increasing urban population and in a climate change context, detailed knowledge of the impact of urban elements on the outdoor thermal environment is relevant. In this work, we present the results of several climatic campaigns carried out in Singapore aiming to evaluate local urban climate variables. Sensors were deployed simultaneously in different sites. The effect of building shadowing in the diurnal cycle of mean radiant temperature (T mrt ) is evaluated in different seasons. Although during the Inter-Monsoon season, mean T mrt reduction due to building shadow is ≈ 19 ◦ C, during clear skies days, it can be reduced by ≈ 30 ◦ C. The T mrt difference between sites is analyzed based on the weather conditions, the sky view factor (SVF), and the type of surrounding urban elements. Under building shadow conditions, higher SVF showed higher T mrt values, although no correlation was found between T mrt and diffuse solar radiation (measured above the urban canopy). The results suggest a relevant contribution of other radiation components (e.g., longwave radiation). The quantitative analysis of the T mrt provided in this work is relevant for outdoor thermal comfort strategies in tropical areas such as Singapore.


Introduction
With the aim of improving outdoor thermal comfort (OTC) in urban areas, research and studies have been increasingly carried out during the last decades. One of the main drivers is the necessity of adapting urban areas to climate change where the population will continue to grow in the future [1]. Urban heat mitigation strategies can help minimize the increase of air temperature inside the urban fabric [2][3][4][5], but they do not in all cases improve OTC [6][7][8].
Outdoor thermal comfort in urban areas is highly governed by the urban morphology. Many studies have pointed to street orientation and street aspect ratio as the most relevant parameters to determine outdoor thermal comfort levels [9][10][11][12][13][14]. However, other urban characteristics can also play an important role. Urban materials [15,16], the presence of vegetation [17][18][19], and anthropogenic heat emissions [20,21] can modify significantly OTC levels inside the urban area.
Several meteorological parameters (air temperature, humidity, wind speed, and radiation) influence thermal comfort levels. Between these variables, radiation has a relevant impact in outdoor thermal comfort [22]. However, the heat exchange by radiation between the human being and the outdoor spaces is complex due to the existence of heterogeneous longwave and shortwave radiant flux densities [23], which are influenced by the surrounding landscape. Mean radiant temperature (T mrt ) is a measure of how human beings experience radiation. It is defined as "the uniform temperature of an imaginary enclosure

Materials and Methods
To evaluate the impact of urban planning and design on OTC in Singapore, several measurement campaigns were carried out in two areas during 2019 and 2020 [44]. In each area, 4-5 sites with different thermal characteristics were selected. In each site, a set of sensors were deployed to measure local climate variables. This paper is focused on the effect of building shadowing and SVF on mean radiant temperature (T mrt ) in different solar radiation conditions, so we will present only the measuring sites relevant for this purpose (Section 3.1). A complete description of the campaigns is presented in Acero et al. [44].

Measurement Campaigns
Two campaigns of several months were carried out in two different urban areas of Singapore. In each area, several sets of instruments were deployed to measure simultaneously the microclimate conditions at specific sites (see Section 3.2). Sensors operated continuously throughout the campaign. The following climate variables were measured: air temperature (T a ), relative humidity (RH), wind speed (WS), wind direction (WD), precipitation (R a ), and globe temperature (T g ). For the first five variables the Weather Transmitter WXT520 of Vaisala was used. Vapor pressure (e) was derived from T a , RH, and the saturation vapor pressure of water [45]. T g measurements were carried out with the black globe radiant temperature device (150 mm of diameter) of Scientific Campbell.
At each site, both sensors were mounted on a reinforced metallic arm and attached to a lamp post (Figure 1). The metallic arm extended around 1.2 m away from the lamp post to avoid wind obstructions. All sensors were between 2.2 and 2.4 m above the ground level to comply with governmental agencies' requirements.

Materials and Methods
To evaluate the impact of urban planning and design on OTC in Singapore, several measurement campaigns were carried out in two areas during 2019 and 2020 [44]. In each area, 4-5 sites with different thermal characteristics were selected. In each site, a set of sensors were deployed to measure local climate variables. This paper is focused on the effect of building shadowing and SVF on mean radiant temperature (Tmrt) in different solar radiation conditions, so we will present only the measuring sites relevant for this purpose (Section 3.1). A complete description of the campaigns is presented in Acero et al. [44].

Measurement Campaigns
Two campaigns of several months were carried out in two different urban areas of Singapore. In each area, several sets of instruments were deployed to measure simultaneously the microclimate conditions at specific sites (see Section 3.2). Sensors operated continuously throughout the campaign. The following climate variables were measured: air temperature (Ta), relative humidity (RH), wind speed (WS), wind direction (WD), precipitation (Ra), and globe temperature (Tg). For the first five variables the Weather Transmitter WXT520 of Vaisala was used. Vapor pressure (e) was derived from Ta, RH, and the saturation vapor pressure of water [45]. Tg measurements were carried out with the black globe radiant temperature device (150 mm of diameter) of Scientific Campbell.
At each site, both sensors were mounted on a reinforced metallic arm and attached to a lamp post (Figure 1). The metallic arm extended around 1.2 m away from the lamp post to avoid wind obstructions. All sensors were between 2.2 and 2.4 m above the ground level to comply with governmental agencies' requirements. A data logger (Scientific Campbell CR300) was used to store 1-min average values recorded by the sensors. Together with the necessary battery (18 Amph at 12 V) to power all the instruments at each site, the data logger was secured in a waterproof enclosure attached to the same lamp post.
The sensors provided the following accuracy for the measurements: ±0.3 °C for Ta and Tg, ±3% for RH, ±3% for WS, ±3° for WD, and <5% for rainfall. A data logger (Scientific Campbell CR300) was used to store 1-min average values recorded by the sensors. Together with the necessary battery (18 Amph at 12 V) to power all the instruments at each site, the data logger was secured in a waterproof enclosure attached to the same lamp post. The sensors provided the following accuracy for the measurements: ±0.3 • C for T a and T g , ±3% for RH, ±3% for WS, ±3 • for WD, and <5% for rainfall.
T mrt was derived from measurements using Formula (1) where a mean convection coefficient [24] is specifically defined for Singapore [46]: where T g is the globe temperature ( • C), WS is the wind speed (ms −1 ), T a is the air temperature ( • C), D is the globe diameter (m), and is the globe emissivity. In our case, = 0.95 (for black globe). Additionally, 10-min values of global solar irradiance measured in station 322 of the Solar Energy Research Institute in Singapore (SERIS) were used to evaluate the level of cloudiness during the measurement campaign. This station is located on a roof (above the urban canopy) close to the two areas (Section 3.2) where sensors were deployed. Although cloudiness in Singapore can vary significantly in short distances, it is considered that the measured global and diffuse solar radiation in station 322 is representative of the areas of study. The uncertainty of the irradiance sensors is 5% [47].

Areas and Sites of Measurements
Two different areas of Singapore (Duxton Park and the Commercial and Business District (CBD)) were selected to deploy several sensors simultaneously ( Figure 2). Specific measuring sites inside each area were selected to provide relevant and representative data to analyze the impact on T mrt of building shadowing and cloudiness in different types of urban developments. Tmrt was derived from measurements using Formula (1) where a mean convection coefficient [24] is specifically defined for Singapore [46]: where Tg is the globe temperature (°C), WS is the wind speed (ms −1 ), Ta is the air temperature (°C), D is the globe diameter (m), and ϵ is the globe emissivity. In our case, ϵ = 0.95 (for black globe). Additionally, 10-min values of global solar irradiance measured in station 322 of the Solar Energy Research Institute in Singapore (SERIS) were used to evaluate the level of cloudiness during the measurement campaign. This station is located on a roof (above the urban canopy) close to the two areas (Section 3.2) where sensors were deployed. Although cloudiness in Singapore can vary significantly in short distances, it is considered that the measured global and diffuse solar radiation in station 322 is representative of the areas of study. The uncertainty of the irradiance sensors is 5% [47].

Areas and Sites of Measurements
Two different areas of Singapore (Duxton Park and the Commercial and Business District (CBD)) were selected to deploy several sensors simultaneously ( Figure 2). Specific measuring sites inside each area were selected to provide relevant and representative data to analyze the impact on Tmrt of building shadowing and cloudiness in different types of urban developments.

Duxton Park Area
The Duxton Park area is of particular interest because of its location and nature. Located close to the CDB, the high-rise developed area of Singapore, it is mostly used by residents. It is a short, narrow, and green corridor that offers an alternative route compared to the more heavy trafficked nearby streets. In addition, the area combines different urban typologies (Figure 3). Different street canyon aspect ratios (the ratio between the street width and the aside averaged building height) and building height asymmetry (e.g., Pinnacle high-rise building versus low-rise shop-houses) can be found.
Three sites (Pinnacle1, Pinnacle2, and Duxton3) have been selected to analyse the impact of urban typologies, building shadowing and cloud cover on Tmrt. In these sites, simultaneous measurements were carried out between 4 July and 23 October 2019 (3.5 months), including the SW-Monsoon season and part of the wet Inter-Monsoon season (see Section 2). A description of the specific sites where the sensors were located is presented in Table 1.

Duxton Park Area
The Duxton Park area is of particular interest because of its location and nature. Located close to the CDB, the high-rise developed area of Singapore, it is mostly used by residents. It is a short, narrow, and green corridor that offers an alternative route compared to the more heavy trafficked nearby streets. In addition, the area combines different urban typologies (Figure 3). Different street canyon aspect ratios (the ratio between the street width and the aside averaged building height) and building height asymmetry (e.g., Pinnacle high-rise building versus low-rise shop-houses) can be found.   innacle2 Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise build- Three sites (Pinnacle1, Pinnacle2, and Duxton3) have been selected to analyse the impact of urban typologies, building shadowing and cloud cover on T mrt . In these sites, simultaneous measurements were carried out between 4 July and 23 October 2019 (3.5 months), including the SW-Monsoon season and part of the wet Inter-Monsoon season (see Section 2). A description of the specific sites where the sensors were located is presented in Table 1.

CBD Area
The high-rise CBD of Singapore is different to Duxton Park (Section 3.2.1) not only because of the type of urban development but also the activities carried out. It is the heart of Singapore's financial activities and is currently mostly used by office worker commuters. The area where the sensors were deployed is less than 0.3 km 2 (Figure 4), around Robinson Road, and between Boon Tat Street and McCallum Street. It includes streets with different aspect ratios, street orientations (NE-SW and NW-SE), and building height asymmetry (e.g., BoonTat711 site), as described in Table 2.
At four outdoor sites (BoonTat711, BoonTatSt, RobinsonRd, and McCallumSt), measurements were carried out simultaneously between 15 November 2019 and 13 January 2020 (2 months), which includes the last part of the wet Inter-Monsoon season and the wet NE-Monsoon season (see Section 2). A description of the specific sites is presented in Table 2.

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon and trees in the morning.

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of highrise building in the afternoon and trees in the morning.

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor. SVF = 0.75 (1) Calculations were carried out with Rayman software [48,49].

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of highrise building in the afternoon and trees in the morning.

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor. SVF = 0.75 (1) Calculations were carried out with Rayman software [48,49].

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of highrise building in the afternoon and trees in the morning.

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor. SVF = 0.75 (1) Calculations were carried out with Rayman software [48,49].

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of highrise building in the afternoon and trees in the morning.

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor. SVF = 0.75 (1) Calculations were carried out with Rayman software [48,49].

Duxton3
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor.

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of highrise building in the afternoon and trees in the morning.

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor. SVF = 0.75 (1) Calculations were carried out with Rayman software [48,49].

Pinnacle1
Asymmetrical street canyon (high-rise buildings on one side and low-rise development on the other). NW street orientation. Shadow of highrise building in the afternoon and trees in the morning.

Pinnacle2
Asymmetrical street canyon (high-rise towers on one side and low-rise development on the other). NW street orientation. Shadow of high-rise building in the afternoon. Influence of nearby low height trees and bushes.
Low-rise development area. Buildings on both sides of the canyon are not higher than 10 m. Ground mostly covered with grass. NNW street orientation. Most relevant shadow provided by some tall palm trees (high transmissivity) before noon. Site with highest sky view factor. SVF = 0.75 (1) Calculations were carried out with Rayman software [48,49].
(1) Calculations were carried out with Rayman software [48,49]. Table 2. Description of the sites in the CBD area were measurements were carried out.

BoonTat711
Asymmetrical street canyon (high-rise buildings on one side and Telok Ayer Market (low-rise development) on the other). Street with NW-SE orientation. Shadow in the afternoon due to high-rise building. Site with unusual high SVF due to openness and low-rise development in one side.
Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 19 Table 2. Description of the sites in the CBD area were measurements were carried out.

BoonTat711
Asymmetrical street canyon (high-rise buildings on one side and Telok Ayer Market (low-rise development) on the other). Street with NW-SE orientation. Shadow in the afternoon due to high-rise building. Site with unusual high SVF due to openness and low-rise development in one side. SVF = 0.38

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon.

BoonTat711
Asymmetrical street canyon (high-rise buildings on one side and Telok Ayer Market (low-rise development) on the other). Street with NW-SE orientation. Shadow in the afternoon due to high-rise building. Site with unusual high SVF due to openness and low-rise development in one side. SVF = 0.38

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon.

BoonTat711
Asymmetrical street canyon (high-rise buildings on one side and Telok Ayer Market (low-rise development) on the other). Street with NW-SE orientation. Shadow in the afternoon due to high-rise building. Site with unusual high SVF due to openness and low-rise development in one side. SVF = 0.38

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon.

BoonTat711
Asymmetrical street canyon (high-rise buildings on one side and Telok Ayer Market (low-rise development) on the other). Street with NW-SE orientation. Shadow in the afternoon due to high-rise building. Site with unusual high SVF due to openness and low-rise development in one side. SVF = 0.38

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon.

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon.
Farthest site from the shore line.

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon. Farthest site from the shore line.

McCallumSt
High-rise development with street aspect ratio ≈2.5 (low sky view factor), and NW-SE orientation. Shadow of high-rise building in the afternoon.

Analysis of Climate Variables
At each site, 10-min mean climatic values were calculated from 1-min records stored in the data logger (see Section 3.1).
With the aim of analyzing the impact of high-rise buildings' shadow and cloud cover on Tmrt, diurnal cycles for different time periods (e.g., climatic seasons) were calculated. In addition, average values during the midday/afternoon period (11:10 to 16:00) were extracted to compare the daily highest Tmrt values between different sites. The analysis was carried out for different climatic seasons as well as for specific days.
In Pinnacle1 and Pinnacle2 (Duxton Park area), two days (13 July and 11 October 2019) with clear skies were selected. At each site, WS, Ta and e mean afternoon values were similar during both days, although Pinnacle1 always registered higher WS and lower Ta RobinsonRd High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon. Farthest site from the shore line.

McCallumSt
High-rise development with street aspect ratio ≈2.5 (low sky view factor), and NW-SE orientation. Shadow of high-rise building in the afternoon.

Analysis of Climate Variables
At each site, 10-min mean climatic values were calculated from 1-min records stored in the data logger (see Section 3.1).
With the aim of analyzing the impact of high-rise buildings' shadow and cloud cover on Tmrt, diurnal cycles for different time periods (e.g., climatic seasons) were calculated. In addition, average values during the midday/afternoon period (11:10 to 16:00) were extracted to compare the daily highest Tmrt values between different sites. The analysis was carried out for different climatic seasons as well as for specific days.
In Pinnacle1 and Pinnacle2 (Duxton Park area), two days (13 July and 11 October 2019) with clear skies were selected. At each site, WS, Ta and e mean afternoon values were similar during both days, although Pinnacle1 always registered higher WS and lower Ta

McCallumSt
High-rise development with street aspect ratio ≈2.5 (low sky view factor), and NW-SE orientation. Shadow of high-rise building in the afternoon.

RobinsonRd
High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon. Farthest site from the shore line.

McCallumSt
High-rise development with street aspect ratio ≈2.5 (low sky view factor), and NW-SE orientation. Shadow of high-rise building in the afternoon.

Analysis of Climate Variables
At each site, 10-min mean climatic values were calculated from 1-min records stored in the data logger (see Section 3.1).
With the aim of analyzing the impact of high-rise buildings' shadow and cloud cover on Tmrt, diurnal cycles for different time periods (e.g., climatic seasons) were calculated. In addition, average values during the midday/afternoon period (11:10 to 16:00) were extracted to compare the daily highest Tmrt values between different sites. The analysis was carried out for different climatic seasons as well as for specific days.
In Pinnacle1 and Pinnacle2 (Duxton Park area), two days (13 July and 11 October 2019) with clear skies were selected. At each site, WS, Ta and e mean afternoon values were similar during both days, although Pinnacle1 always registered higher WS and lower Ta RobinsonRd High-rise development with street aspect ratio ≈2.5 (low sky view factor) and NE-SW orientation. Only a short period (≈2 h) of direct solar radiation around midday/afternoon. SVF = 0.13

BoonTatSt
Low-rise development with street aspect ratio ≈1 (high sky view factor), and NW-SE orientation. Shadow during the afternoon. Farthest site from the shore line.

McCallumSt
High-rise development with street aspect ratio ≈2.5 (low sky view factor), and NW-SE orientation. Shadow of high-rise building in the afternoon.

Analysis of Climate Variables
At each site, 10-min mean climatic values were calculated from 1-min records stored in the data logger (see Section 3.1).
With the aim of analyzing the impact of high-rise buildings' shadow and cloud cover on Tmrt, diurnal cycles for different time periods (e.g., climatic seasons) were calculated. In addition, average values during the midday/afternoon period (11:10 to 16:00) were extracted to compare the daily highest Tmrt values between different sites. The analysis was carried out for different climatic seasons as well as for specific days.
In Pinnacle1 and Pinnacle2 (Duxton Park area), two days (13 July and 11 October 2019) with clear skies were selected. At each site, WS, Ta and e mean afternoon values were similar during both days, although Pinnacle1 always registered higher WS and lower Ta (1) Calculations were carried out with Rayman software [48,49].

CBD Area
The high-rise CBD of Singapore is different to Duxton Park (Section 3.2.1) not only because of the type of urban development but also the activities carried out. It is the heart of Singapore's financial activities and is currently mostly used by office worker commuters. The area where the sensors were deployed is less than 0.3 km 2 (Figure 4), around Robinson Road, and between Boon Tat Street and McCallum Street. It includes streets with different aspect ratios, street orientations (NE-SW and NW-SE), and building height asymmetry (e.g., BoonTat711 site), as described in Table 2.
At four outdoor sites (BoonTat711, BoonTatSt, RobinsonRd, and McCallumSt), measurements were carried out simultaneously between 15 November 2019 and 13 January 2020 (2 months), which includes the last part of the wet Inter-Monsoon season and the wet NE-Monsoon season (see Section 2). A description of the specific sites is presented in Table  2.

Analysis of Climate Variables
At each site, 10-min mean climatic values were calculated from 1-min records stored in the data logger (see Section 3.1).
With the aim of analyzing the impact of high-rise buildings' shadow and cloud cover on T mrt , diurnal cycles for different time periods (e.g., climatic seasons) were calculated. In addition, average values during the midday/afternoon period (11:10 to 16:00) were extracted to compare the daily highest T mrt values between different sites. The analysis was carried out for different climatic seasons as well as for specific days.
In Pinnacle1 and Pinnacle2 (Duxton Park area), two days (13 July and 11 October 2019) with clear skies were selected. At each site, WS, T a and e mean afternoon values were similar during both days, although Pinnacle1 always registered higher WS and lower T a values (Table 3). When building shadow started to affect both sites, global radiation was 720 Watt/m 2 and 1002 Watt/m 2 on 13 July and 11 October, respectively. To analyze the influence of cloudiness and solar radiation in T mrt , hourly values were evaluated against the clearness index defined as: where GHI is the global horizontal irradiance and GHI CS is the clear skies global horizontal irradiance. Thus, K t represents a dimensionless number between 0 and 1, which is defined as the surface global radiation divided by the surface global radiation when conditions of clear skies occur (i.e., maximum transmission of solar radiation through the atmosphere).
For Singapore, Dazhi et al. [50] defined the GHI CS as: where E 0 is an eccentricity correction factor of Earth, I sc is the solar constant (1366.1 Watt/m 2 ), and θ z is the solar zenith angle. Correlation analysis between T mrt and K t have been undertaken for August 2019 at 10:30 and 17:00 for different sites (shaded and non-shaded).
Additionally, the diurnal cycle of T mrt was analyzed on a clear skies day and a cloudy day. Two days in the SW-Monsoon season (25 July and 1 August) were selected. These selected days were close to one another to avoid significant difference in the solar elevation (and the timing of T mrt peak values). In addition, no precipitation occurred during these days. On 25 July, the sky was covered with clouds throughout the day, with different levels of incoming solar radiation. On the contrary, 1 August presented during most of the day clear skies (high solar radiation), although some periods of clouds occurred before midday. Table 4 shows the mean and maximum solar irradiance registered in the area during different periods of the day. The mean wind conditions were very similar throughout the day, but the temperature was higher on 1 August (clear sky day).

Impact of Building Shadow
In this section, the influence of the shadow of a high-rise building on T mrt is presented. Results are analyzed at the pedestrian level. The analysis is presented for different climatic seasons (mean values) and for two clear skies days when the impact of building shadow on T mrt is highest. Data registered both in Duxton Park and CBD (Section 3.2) are used to discuss the findings. The influence of the nearby high-rise buildings' shadow ( Figure 3) is clearly noticed in Pinnacle1 and Pinnacle2 in the afternoon. In the Inter-Monsoon season, Tmrt drops sharply at 12:40 in Pinnacle 2 and at 13:10 in Pinnacle1, with reductions mean Tmrt of ≈19 °C and ≈11 °C, respectively. Time differences between sites are attributed to different orientation (21°) of the buildings providing shadow. However, during the SW-Monsoon, the effect of building shadow is delayed ≈1.5 h with respect to the Inter-Monsoon in both sites due to the variation of the sun elevation and sun path between seasons.

Differences between Climatic Seasons
The effect of building shadow (i.e., reduction of Tmrt) is much higher in Pinnacle2 than in Pinnacle1. Pinnacle2 drops ≈8 °C more than Pinnacle1 in the Inter-Monsoon season. The reason for this difference between sites is that the Pinnacle1 measuring site is already (before the shadow of the building) under the influence of trees that partly block solar radiation. After the sharp drop in mean Tmrt, under the building shadow, Tmrt levels remain higher in Pinnacle2. This is best shown in the Inter-Monsoon season. During the afternoon (14:00 to 16:00), the difference between both sites can reach 2.5 °C. During this period of the day (both sites are under building shadow), the Tmrt difference is due to lower levels of radiation in Pinnacle1. As commented previously, the latter is partly covered by trees that limit the access of diffuse solar radiation but also the surrounding surfaces are colder (e.g., leaves), reducing longwave irradiance in Pinnacle1. The influence of the nearby high-rise buildings' shadow ( Figure 3) is clearly noticed in Pinnacle1 and Pinnacle2 in the afternoon. In the Inter-Monsoon season, T mrt drops sharply at 12:40 in Pinnacle 2 and at 13:10 in Pinnacle1, with reductions mean T mrt of ≈19 • C and ≈11 • C, respectively. Time differences between sites are attributed to different orientation (21 • ) of the buildings providing shadow. However, during the SW-Monsoon, the effect of building shadow is delayed ≈1.5 h with respect to the Inter-Monsoon in both sites due to the variation of the sun elevation and sun path between seasons.
The effect of building shadow (i.e., reduction of T mrt ) is much higher in Pinnacle2 than in Pinnacle1. Pinnacle2 drops ≈8 • C more than Pinnacle1 in the Inter-Monsoon season. The reason for this difference between sites is that the Pinnacle1 measuring site is already (before the shadow of the building) under the influence of trees that partly block solar radiation.
After the sharp drop in mean T mrt , under the building shadow, T mrt levels remain higher in Pinnacle2. This is best shown in the Inter-Monsoon season. During the afternoon (14:00 to 16:00), the difference between both sites can reach 2.5 • C. During this period of the day (both sites are under building shadow), the T mrt difference is due to lower levels of radiation in Pinnacle1. As commented previously, the latter is partly covered by trees that limit the access of diffuse solar radiation but also the surrounding surfaces are colder (e.g., leaves), reducing longwave irradiance in Pinnacle1.
Similar effects are observed in McCallumSt, BoonTatSt, BoonTat711, and RobinsonRd (CBD area). Figure 6 shows the different mean T mrt diurnal profile for a two-week period in the wet Inter-Monsoon (last half of November 2019) and in the wet NE-Monsoon (first half of January 2020). In this case, the timing of the peak values in each site is conditioned by the street orientation. McCallumSt, BoonTatSt, and BoonTat711 are all NW-oriented streets and have the peak at ≈10:10 in the Wet Inter-Monsoon season, while for RobinsonRd (a NE-oriented street), the peak happens 3.5 h later (at 13:40). In the wet NE-Monsoon period, the time lag between the two street orientations increases to 4 h due to the variation in the sun path (elevation and azimuth) with respect to the wet Inter-Monsoon. Similar effects are observed in McCallumSt, BoonTatSt, BoonTat711, and Robin-sonRd (CBD area). Figure 6 shows the different mean Tmrt diurnal profile for a two-week period in the wet Inter-Monsoon (last half of November 2019) and in the wet NE-Monsoon (first half of January 2020). In this case, the timing of the peak values in each site is conditioned by the street orientation. McCallumSt, BoonTatSt, and BoonTat711 are all NW-oriented streets and have the peak at ≈10:10 in the Wet Inter-Monsoon season, while for RobinsonRd (a NE-oriented street), the peak happens 3.5 h later (at 13:40). In the wet NE-Monsoon period, the time lag between the two street orientations increases to 4 h due to the variation in the sun path (elevation and azimuth) with respect to the wet Inter-Monsoon.
Peak mean values in these two periods (wet Inter-Monsson and wet NE-Monsoon) are significantly lower than the ones measured in Duxton Park during the SW-Monsoon season, which was mainly due to a lower precipitation (less cloud cover) in the SW-monsoon season and especially in August 2019. In addition, RobinsonRd shows lower mean peak values than the other three sites in the CBD, which can be explained by higher cloud cover and showers during the afternoon that reduce solar incoming radiation at the time when Tmrt peak occurs.

Tmrt Evaluation on Clear Skies Conditions
Two specific days were selected to quantify the impact of building shadow on clear skies days: 13 July and 11 October 2019 in the SW-Monsoon and Inter-Monsoon, respectively (Section 3.3). On 13 July, there are some clouds during the morning period, but in the afternoon, clear skies are present (Figure 7). In contrast, 11 October was a clear sky day for Singapore. Both days show similar Ta, WS, and e mean afternoon values (Table 3), but incoming solar radiation differs by 280 Watt/m 2 (at the time of the building shadow), which is mostly due to the different time (due to the season) at which the building shadow reaches the sites.
On 11 October, the maximum Tmrt values reach 65.6 °C at 12:40. Similar values were reported by Tan et al. [28] in Singapore when Tmrt was estimated on a roof top from six individual measurements/directions of the shortwave radiation and longwave radiation fluxes on a clear sky day. Values were ≈69.5 °C when the shortwave radiation from the sky was 1150 Watt/m 2 . Peak mean values in these two periods (wet Inter-Monsson and wet NE-Monsoon) are significantly lower than the ones measured in Duxton Park during the SW-Monsoon season, which was mainly due to a lower precipitation (less cloud cover) in the SW-monsoon season and especially in August 2019. In addition, RobinsonRd shows lower mean peak values than the other three sites in the CBD, which can be explained by higher cloud cover and showers during the afternoon that reduce solar incoming radiation at the time when T mrt peak occurs.

T mrt Evaluation on Clear Skies Conditions
Two specific days were selected to quantify the impact of building shadow on clear skies days: 13 July and 11 October 2019 in the SW-Monsoon and Inter-Monsoon, respectively (Section 3.3). On 13 July, there are some clouds during the morning period, but in the afternoon, clear skies are present (Figure 7). In contrast, 11 October was a clear sky day for Singapore. Both days show similar T a , WS, and e mean afternoon values (Table 3), but incoming solar radiation differs by 280 Watt/m 2 (at the time of the building shadow), which is mostly due to the different time (due to the season) at which the building shadow reaches the sites.
Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 19 Section 4.1.1, Pinnacle 1 is partly affected by tree shadowing before the building effect starts. While in Pinnacle2 Tmrt is reduced ≈25 °C, Pinnacle1′s drop is ≈16°C (for the case of 13 July). However, higher incoming solar radiation (as on 11 October) causes higher reductions on Tmrt due to building shadow: ≈30 °C and ≈20 °C for Pinnacle2 and Pinnacle1, respectively. The solar trajectory again affects the time when the building shadow reaches each measuring site. A sharp reduction in Tmrt happens at ≈12:45 on 11 October (Inter-Monsoon) and at ≈15:15 h on 13 July (SW-Monsoon). This two-hour difference is 30 min more than the mean difference between August and October presented in Section 4.1.1.
These results are important to gain an understanding of the benefit of shadowing under clear skies conditions.

Tmrt Versus the Clearness Index
Tmrt values were analyzed based on the levels of cloudiness. Figure 8 shows the relation between the Tmrt and the clearness index (Kt) as defined in (2). Two different hours (10:30 and 17:00) are used to represent the morning and late afternoon period in Duxton3 and Pinnacle2. Duxton3 is non-shaded at both hours, while Pinnacle2 only at 10:30. At 17:00, Pinnacle2 is shaded by the near high-rise building (Figure 3).
Under non-shaded conditions, Tmrt and Kt show a good linear relationship (e.g., in Pinnacle2 at 10:30, r = 0.924 (p < 0.001)). The effect of cloudiness on Tmrt has been presented in other studies (Kántor et al., 2015;Thorsson et al., 2007). However, in the shaded site (17:00 in Pinnacle2), Kt and Tmrt have low correlation (r = 0.558 (p < 0.001)) (Figure 8), and thus, the level of cloudiness produces low variation in Tmrt values. In this sense, results highlight the importance of direct solar radiation to govern the variability of Tmrt levels in Singapore. On 11 October, the maximum T mrt values reach 65.6 • C at 12:40. Similar values were reported by Tan et al. [28] in Singapore when T mrt was estimated on a roof top from six individual measurements/directions of the shortwave radiation and longwave radiation fluxes on a clear sky day. Values were ≈69.5 • C when the shortwave radiation from the sky was 1150 Watt/m 2 . Figure 7 shows the T mrt diurnal profile for both days. A common result in both days is that the T mrt drop due to building shadow is higher in Pinnacle2 since, as commented in Section 4.1.1, Pinnacle 1 is partly affected by tree shadowing before the building effect starts. While in Pinnacle2 T mrt is reduced ≈25 • C, Pinnacle1 s drop is ≈16 • C (for the case of 13 July). However, higher incoming solar radiation (as on 11 October) causes higher reductions on T mrt due to building shadow: ≈30 • C and ≈20 • C for Pinnacle2 and Pinnacle1, respectively.
The solar trajectory again affects the time when the building shadow reaches each measuring site. A sharp reduction in T mrt happens at ≈12:45 on 11 October (Inter-Monsoon) and at ≈15:15 h on 13 July (SW-Monsoon). This two-hour difference is 30 min more than the mean difference between August and October presented in Section 4.1.1.
These results are important to gain an understanding of the benefit of shadowing under clear skies conditions.

T mrt Versus the Clearness Index
T mrt values were analyzed based on the levels of cloudiness. Figure 8 shows the relation between the T mrt and the clearness index (K t ) as defined in (2). Two different hours (10:30 and 17:00) are used to represent the morning and late afternoon period in Duxton3 and Pinnacle2. Duxton3 is non-shaded at both hours, while Pinnacle2 only at 10:30. At 17:00, Pinnacle2 is shaded by the near high-rise building (Figure 3). Further analysis on the influence of diffuse solar radiation on Tmrt shows a low relationship between these two variables in shaded sites (Pinnacle2 and Pinnacle1 at 17:00 have r = 0.041 (p < 0.001) and r = 0.297 (p < 0.001) respectively) (Figure 9), suggesting the contribution to Tmrt of other radiation components (i.e., longwave radiation). The importance of longwave radiation (especially lateral fluxes) to Tmrt has already been reported in the literature [22,34], which in shaded sites should be higher. On the contrary, in nonshaded sites (Duxton3) at the same hour, diffuse solar radiation and Tmrt show a negative correlation (r = −0.520 (p < 0.001)). Since higher diffuse radiation is associated with lower direct solar radiation, the results again highlight the role of direct solar radiation in the variability of Tmrt levels in non-shaded sites. Under non-shaded conditions, T mrt and K t show a good linear relationship (e.g., in Pinnacle2 at 10:30, r = 0.924 (p < 0.001)). The effect of cloudiness on T mrt has been presented in other studies (Kántor et al., 2015;Thorsson et al., 2007). However, in the shaded site (17:00 in Pinnacle2), K t and T mrt have low correlation (r = 0.558 (p < 0.001)) (Figure 8), and thus, the level of cloudiness produces low variation in T mrt values. In this sense, results highlight the importance of direct solar radiation to govern the variability of T mrt levels in Singapore.
Further analysis on the influence of diffuse solar radiation on T mrt shows a low relationship between these two variables in shaded sites (Pinnacle2 and Pinnacle1 at 17:00 have r = 0.041 (p < 0.001) and r = 0.297 (p < 0.001) respectively) (Figure 9), suggesting the contribution to T mrt of other radiation components (i.e., longwave radiation). The importance of longwave radiation (especially lateral fluxes) to T mrt has already been reported in the literature [22,34], which in shaded sites should be higher. On the contrary, in non-shaded sites (Duxton3) at the same hour, diffuse solar radiation and T mrt show a negative correlation (r = −0.520 (p < 0.001)). Since higher diffuse radiation is associated with lower direct solar radiation, the results again highlight the role of direct solar radiation in the variability of T mrt levels in non-shaded sites. Atmosphere 2021, 12, x FOR PEER REVIEW 13 of 19  shadow affected Pinnacle1 differed by 217 Watt/m 2 (mean value). During this period (before the building shadow effect), Tmrt values were lower in the cloudy day (aligned with outcomes in Section 4.1). Tmrt differences between the two days (cloudy vs. clear day) were 7.8 °C, 11.1 °C, and 12.5 °C in Duxton3, Pinnacle 1, and Pinnacle2 respectively. The lower difference in Duxton3 is associated with a higher sky view factor of the site that allows a higher impact of diffuse radiation under cloudy days.
As expected, a higher decrease in incoming solar radiation causes a higher Tmrt reduction. In Pinnacle2, before noon (11:00), the Tmrt difference between the two days (cloudy vs. clear day) reaches 14.2 °C due to a 400 Watt/m 2 difference in the incoming solar radiation.

Urban Development and Diffuse Radiation
As commented in the previous section, during cloudy conditions, sites with a higher sky view factor were associated with higher Tmrt values due to higher levels of solar diffuse radiation. This was the case in low-rise areas such as Duxton3. Figure 11 shows  In general, clouds significantly change the T mrt diurnal cycle. During the afternoon, cloud cover can reduce T mrt values significantly in sites exposed to direct solar radiation. Between 25 July and 1 August, incoming solar radiation in the 30 min before the building shadow affected Pinnacle1 differed by 217 Watt/m 2 (mean value). During this period (before the building shadow effect), T mrt values were lower in the cloudy day (aligned with outcomes in Section 4.1). T mrt differences between the two days (cloudy vs. clear day) were 7.8 • C, 11.1 • C, and 12.5 • C in Duxton3, Pinnacle 1, and Pinnacle2 respectively. The lower difference in Duxton3 is associated with a higher sky view factor of the site that allows a higher impact of diffuse radiation under cloudy days.
As expected, a higher decrease in incoming solar radiation causes a higher T mrt reduction. In Pinnacle2, before noon (11:00), the T mrt difference between the two days (cloudy vs. clear day) reaches 14.2 • C due to a 400 Watt/m 2 difference in the incoming solar radiation.

Urban Development and Diffuse Radiation
As commented in the previous section, during cloudy conditions, sites with a higher sky view factor were associated with higher T mrt values due to higher levels of solar diffuse radiation. This was the case in low-rise areas such as Duxton3. Figure 11 shows the mean T mrt values during most of the wet NE-Monsoon season (1 December 2019 to 13 January 2020), when precipitation and cloudiness is highest in Singapore. All sites considered (BoonTatSt, BoonTat711, and McCallumSt) are in NW oriented streets, which causes maximum T mrt values to occur at ≈10:00 am (see Section 4.1.1). During the morning period (until ≈9:30am), T mrt values are highest in BoonTat711 (partly open urban area and asymmetrical aspect ratio) due to high exposure to solar radiation. At this time of the day, the lowest values are in McCallumSt due to lowest SVF (higher in BoonTat711 and BoonTatSt). After the morning peak (≈10:00am), T mrt values in all three sites drop ≈8 • C. In the midday/afternoon period (11:00 to 16:00), without the direct solar radiation, BoonTatSt and BoonTat711 register similar results. The difference of these two sites with McCallumSt remains quite stable throughout the afternoon. Mean T mrt values in McCallumSt are 3.4 • C and 2.9 • C lower than BoonTatSt and BoonTat711, respectively. These results are influenced by a lower SVF in McCallum that reduces solar radiation (at this time of the day, only reflected and diffuse radiation) reaching the site in comparison to BoonTatSt and BoonTat711 and are in agreement with previous studies that reported higher radiant fluxes inside street canyons with higher SVF. Du et al. [51] showed a positive correlation between SVF and both shortwave and longwave radiation in a severely cold region (Dwa climate type). In a hot and dry region, Middel and Krayenhoff [34] reported that access to solar radiation dominated on T mrt together with a positive effect of impervious overheated surfaces on longwave contributions. Although a positive correlation between SVF and T mrt during the daytime under direct solar radiation is expected to be found [22,28,52], on shaded sites and with clear skies, SVF can have a weak explanatory power for variations in shortwave fluxes, and T mrt can show a negative correlation with SVF; i.e., higher SVF can facilitate a release of longwave radiation and thus decrease T mrt in shaded sites [53]. This is not the case in our study area where under building shadow conditions, lower SVF showed lower T mrt values. Differences with the study of Lai et al. [53] could be attributed to the frequent cloudy conditions in Singapore that increase the diffuse solar radiation component and thus the solar radiation fluxes in shaded areas. This interpretation is supported by a study from Chatzipoulka et al. [54] analyzing solar radiation in different façade orientations, which found a consistent strong relationship between the SVF and diffuse irradiance and that higher urban openness could increase average diffuse and reflected solar radiation.
higher radiant fluxes inside street canyons with higher SVF. Du et al. [51] sho tive correlation between SVF and both shortwave and longwave radiation in cold region (Dwa climate type). In a hot and dry region, Middel and Krayenh ported that access to solar radiation dominated on Tmrt together with a positi impervious overheated surfaces on longwave contributions. Although a posit tion between SVF and Tmrt during the daytime under direct solar radiation is be found [22,28,52], on shaded sites and with clear skies, SVF can have a weak e power for variations in shortwave fluxes, and Tmrt can show a negative corre SVF; i.e., higher SVF can facilitate a release of longwave radiation and thus d in shaded sites [53]. This is not the case in our study area where under buildi conditions, lower SVF showed lower Tmrt values. Differences with the study [53] could be attributed to the frequent cloudy conditions in Singapore that i diffuse solar radiation component and thus the solar radiation fluxes in shaded interpretation is supported by a study from Chatzipoulka et al. [54] analyzing ation in different façade orientations, which found a consistent strong relat tween the SVF and diffuse irradiance and that higher urban openness could i erage diffuse and reflected solar radiation.
During the evening, Tmrt difference between BoonTat sites and McCallum duces. By sunset (≈19:00), when incoming solar radiation is negligible, all thre ister similar Tmrt values.  Figure 12 represents the effect of increasing the diffuse solar radiation (ab ban canopy) on the Tmrt difference between a high SVF site (BoonTatSt/BoonTa low SVF site (McCallumSt) in the absence of direct solar radiation. The diffuse ation is measured at a nearby roof top. Results show that under higher incom radiation, the Tmrt difference between a low SVF and a high SVF site also tends which is influenced by higher levels of diffuse radiation reaching the pedestr the high SVF site. This outcome is aligned with the result presented in a pre graph showing that under shadow conditions, low SVF street canyons reduc fluxes at the pedestrian level and thus show lower Tmrt. During the evening, T mrt difference between BoonTat sites and McCallum slowly reduces. By sunset (≈19:00), when incoming solar radiation is negligible, all three sites register similar T mrt values. Figure 12 represents the effect of increasing the diffuse solar radiation (above the urban canopy) on the T mrt difference between a high SVF site (BoonTatSt/BoonTat711) and a low SVF site (McCallumSt) in the absence of direct solar radiation. The diffuse solar radiation is measured at a nearby roof top. Results show that under higher incoming diffuse radiation, the T mrt difference between a low SVF and a high SVF site also tends to increase, which is influenced by higher levels of diffuse radiation reaching the pedestrian level in the high SVF site. This outcome is aligned with the result presented in a previous paragraph showing that under shadow conditions, low SVF street canyons reduce radiative fluxes at the pedestrian level and thus show lower T mrt .

Conclusions
In this study, we have analyzed the impact of the shadow of high-rise buildings on Tmrt as well as the effect of cloudiness and sky view factor in the hot and humid tropical city of Singapore.
Based on measurement campaigns with sensors deployed simultaneously in different sites, the results are presented for different seasons and for specific days (with different level of cloudiness), providing representative Tmrt values in the selected sites.
Street orientation and aspect ratio have shown an important impact on Tmrt due to their influence in solar access at pedestrian level. The street orientation conditions the time when Tmrt peak values occur (up to 4 h difference between a NE and a NW-oriented street in the wet NE-Monsoon). The street aspect ratio affects the width of Tmrt peak (i.e., period of time with highest Tmrt levels). Compared to high-rise developments, wider (in time) Tmrt peaks are expected in low-rise and partly open urban areas due to higher exposure to solar radiation. Additionally, Tmrt diurnal cycles at each site have shown a different pattern throughout the year. Not only can levels of cloudiness vary between seasons but also the change in solar elevation is responsible for Tmrt peaks occurring at different times and with different intensity throughout the year.
Under clear skies days, the effect of high-rise building shadow produced a Tmrt reduction of ≈25 °C and ≈30 °C in the SW-Monsoon and wet Inter-Monsoon seasons, respectively. Differences between seasons were conditioned by the solar elevation and thus the time when the building produced shadow on the analyzed site.
On the other hand, cloudiness has a clear effect on Tmrt in Singapore. On non-shaded sites (after midday), when incoming solar radiation (above the urban canopy) is reduced ≈217 Watt/m 2 , Tmrt levels can be reduced between 7.8 and 12.5 °C (depending on the sky view factor and surrounding elements).
On shaded sites, the level of diffuse solar radiation (above the urban canopy) does not show a relevant impact on Tmrt, suggesting the influence of longwave radiative fluxes. However, under shaded conditions, the influence of SVF can still be noticed in Tmrt values at the pedestrian level. In these conditions, a low sky view factor can reduce diffuse the solar radiation reaching the pedestrian level, and thus, Tmrt is expected to be lower than in sites with higher sky view factors. This effect is emphasized for high levels of diffuse solar Figure 12. Relation between diffuse solar radiation (above the urban canopy) and the T mrt difference (∆T mrt in • C) between a low SVF site (BoonTatSt/BoonTat711) and a high SVF site (McCallumSt) in the period 12:00 to 13:00 during the wet NE-Monsoon season (1 December 2019 to 13 January 2020) and under dry conditions.

Conclusions
In this study, we have analyzed the impact of the shadow of high-rise buildings on T mrt as well as the effect of cloudiness and sky view factor in the hot and humid tropical city of Singapore.
Based on measurement campaigns with sensors deployed simultaneously in different sites, the results are presented for different seasons and for specific days (with different level of cloudiness), providing representative T mrt values in the selected sites.
Street orientation and aspect ratio have shown an important impact on T mrt due to their influence in solar access at pedestrian level. The street orientation conditions the time when T mrt peak values occur (up to 4 h difference between a NE and a NW-oriented street in the wet NE-Monsoon). The street aspect ratio affects the width of T mrt peak (i.e., period of time with highest T mrt levels). Compared to high-rise developments, wider (in time) T mrt peaks are expected in low-rise and partly open urban areas due to higher exposure to solar radiation. Additionally, T mrt diurnal cycles at each site have shown a different pattern throughout the year. Not only can levels of cloudiness vary between seasons but also the change in solar elevation is responsible for T mrt peaks occurring at different times and with different intensity throughout the year.
Under clear skies days, the effect of high-rise building shadow produced a T mrt reduction of ≈25 • C and ≈30 • C in the SW-Monsoon and wet Inter-Monsoon seasons, respectively. Differences between seasons were conditioned by the solar elevation and thus the time when the building produced shadow on the analyzed site.
On the other hand, cloudiness has a clear effect on T mrt in Singapore. On non-shaded sites (after midday), when incoming solar radiation (above the urban canopy) is reduced ≈217 Watt/m 2 , T mrt levels can be reduced between 7.8 and 12.5 • C (depending on the sky view factor and surrounding elements). On shaded sites, the level of diffuse solar radiation (above the urban canopy) does not show a relevant impact on T mrt , suggesting the influence of longwave radiative fluxes. However, under shaded conditions, the influence of SVF can still be noticed in T mrt values at the pedestrian level. In these conditions, a low sky view factor can reduce diffuse the solar radiation reaching the pedestrian level, and thus, T mrt is expected to be lower than in sites with higher sky view factors. This effect is emphasized for high levels of diffuse solar radiation (above the urban canopy), which are frequent in Singapore. For the same street orientation and during the wet NE-Monsoon, mean afternoon T mrt values were ≈3.4 • C higher in low-rise/partly open urban sites than in high-rise development sites.
We consider that these results bring relevant information on T mrt values regarding the interaction of incoming radiative fluxes and urban development, and we justify the necessity of a detailed analysis and proper quantification of the effects that govern T mrt in different climate contexts as well as the reduction that certain urban features can achieve. This is especially useful for urban planning due to the influence of T mrt in outdoor thermal comfort levels.
The measurements presented in this work will serve as a reliable database to evaluate the performance of microscale models to estimate T mrt . Further work should be focused on the analysis of the contribution of different radiative components (shortwave, longwave) on the T mrt levels in different urban developments both on shaded and non-shaded sites. Integral radiation measurements at the pedestrian level would allow improving the radiation schemes of models and thus the estimations and detailed spatial distribution of T mrt in the urban area.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to an approval required from the funding organization.