The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, JapanUrban Cool Island in Daytime－Analysis by Using Thermal Image and Air Temperature Measurements－Yoshinori SHIGETA*, Yukitaka OHASHI** and Osamu TSUKAMOTO** Graduate school Okayama University，Okayama，Japan** Okayama University of science，Okayama，JapanAbstract
We found that an urban heat island appeared clearly on night In Okayama City. The heat island intensity (i.e., thetemperature difference between urban and rural areas) was 3.0-4.0 degree C. during the summer. Especially, theheat island appears clearly for 2100-2400 JST. On the other hand, during the daytime, the air temperature ofurban central area was 1.0-2.0 degree C. lower than that of residential areas. This phenomenon can beconsidered as the “urban cool island”. At the same time, the urban surface temperature was lower than theresidential surface temperature. The horizontal scale was 2-3km.
Key words: Urban Heat Island, Urban Cool Island, Surface Temperature,
1. Introduction
In recent years, the urban heat island becomes an object of public concern. Its feature changes greatly with theurban scale and structure. Urban heat island phenomena, which are known as high temperature occurrences in
the central part of city, would be estimated to become in a large city (e.g.,Robert, 1968). A large number ofinvestigations have been theoretically and observationally given on the problems of urban heat island. We
constructed the weather observation network in Okayama City, Japan. Therefore, the comparison among ourobservation results helps to clarify the mechanism of urban climate.
2. Meteorological observations
We constructed an observation network in Okayama City (700,000 people), Japan (Fig.1). The observation
network data were obtained during the following period: the surface temperature measurements by the helicopter
on Aug.5 and 7; air temperature (2.0m) from Jul.30-Sep.10 in 2008. The Air temperature and sky view factor (Fig.
2) are measured in the 60 observation points.
3. Methods
The values measured temperature sensors were corrected an error. Temperature value was accurate to ±0.2℃.
Temperatures were averaged to 20 min. The heat island intensity and cool island intensity (ΔTu-r) is defined asurban-rural maximum temperature difference as shown by as follows: (Oke, 1973 ).

ΔTu-r ＝ Tu － Tr

(1)

Tu explain the maximum temperature in urban area. Tr explain the maximum temperature in rural area. Our study
uses the difference between mean temperature of the inside points, A-C (X-Z), and the outside points, a-c (x-y). In
this paper, Japanese Standard Time (JST) is used unless otherwise noted.
Japan Sea

●N●

a●

x●

●
●

●

●

●

●

●

(a) ●

Pacific Oceanc●●●(b) Residential space
8 points

●

BC●

●
●
●

●

Industrialarea
工業街区●

●

z ● 5kmCommercial
商業街区area

●

●

●

●

●

●(b)

●

●

A

●y(a) Commercial space
8 points (X,Y,Z)●●OKAYAMAb●●

住宅地 area
Residential

Asahi River●●Kojima Bay

Paddy
field
池・河川
Mountainous
region
山地

Sea
area
池・河川

Fig. 1 Map showing the locations of the observation points in Okayama City, Japan ●: observation points.
Urban area points (A-C) and rural area points (a-c) are used for the estimation of the heat island intensity.
Urban area points (X-Z) and rural area points (x-z) are used for the estimation of the cool island intensity.

The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan

4. Results
4.1. Typical temperature distribution and Temporal change of heat island intensity
Figure 2(a) shows temporal change of heat island intensity. The heat island intensity of Okayama City indicate
tendency to be large from 1800 to 0600 JST and to be small from 0900 to 1500. Especially, a heat island clearly
appears about 3.0-4.0 degree C. for 2200-0100 JST (Fig.3a). The heat island intensity of Okayama City issignificantly small in the daytime, while that clearly appeared clearly in nighttime.
4.2. Typical temperature distribution and Temporal change of cool island intensity
Figure 2(b) shows temporal change of cool island intensity. The cool island intensity of Okayama City indicate
tendency to be large from 0800 to 1800 JST and to be small from 2100 to 0600. Especially, a cool island clearly
appears about 1.0-2.0 degree C. for 1000-1200 JST (Fig.3b). The cool island intensity of Okayama City issignificantly small in the nighttime, while that clearly appeared clearly in daytime. In the daytime, the urban
temperature was often lower than the rural temperature in Okayama City (i.e., the urban cool island).
4.3. Temporal change of temperature in commercial and residential space
We constructed an observation network in commercial and residential space (Fig.1). The Air temperature and skyview factor (Fig. 4a,b) are measured in the 8 observation points. Figure 5 shows temporal variation of the spatiallyaveraged (a) air temperature, (b) standard deviation, from observations at the sites.
The Standard deviation of commercial space indicate tendency to be large from 1000 to 0800 JST and to besmall from 2100 to 0600.
(a) Heat Island intensity
Urban＞Rural

3.0
2.0

3.3℃

1.0
0.0
-1.0

Urban＜Rural
12:00

15:00

18:00

21:00

0:00

3:00

6:00

(b) Cool Island intensity

1.5
Cool Island intensity(℃)

Heat Island intensity(℃)

4.0

9:00

Urban＞Rural

1.0
0.5
0.0

1.3℃

-0.5
-1.0
-1.5

Urban＜Rural
0:00

3:00

Time (JST)

6:00

9:00
12:00 15:00
Time (JST)

18:00

21:00

Fig. 2 Temporal change of (a) heat island intensity, Aug.13-14 , (b) cool island intensity, Aug.11.

(a) Urban heat IslandN(b) Urban cool Island

27.5

32.0

31.5

28.0

30.5
30.0

31.0
31.5

29.0
Asahi River

28.5

29.5
5km

N

Kojima Bay

Asahi River

31.0
30.5

32.0

33.0
5km

Kojima Bay

Fig. 3 Temperature distribution. (a) Urban heat island in nighttime, 2300 JST, Aug. 13 , (b) Urban cool island
in daytime, 1000 JST, Aug. 11. Numerals indicate air temperature (℃).

The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan(a) Commercial space
OkayamaStation③ Street
0.47(b) Residential space① Park
0.58② Road
0.48 ③ Park
0.77

②Crossing
0.58④ Crossing
0.32Crossing ⑤0.71⑦ Street
0.27① Street
0.72④ Street
0.42Road ⑤0.45⑥ Road0.50⑧ Park 0.60

⑧Park0.84⑦ Crossing 0.79

Road ⑥
0.88

Fig. 4 Map of the observation points in (a) commercial and (b) residential spaces〇: observation points. Numerals indicate sky view factor.

(b) Standard deviation

35.0

1.0

33.0

0.8

31.0
29.0

Commercial space27.0
25.0

Residential space
0:00

3:00

6:00

9:00 12:00 15:00 18:00 21:00 0:00
Time (JST)Standard deviation(℃)

Air temperature(℃)(a) Air temperature

Commercial space

Residential space

0.6
0.4
0.2
0.0

0:00

3:00

6:00

9:00 12:00 15:00 18:00 21:00
Time (JST)0:00

Fig. 5 Temporal variation of the spatially averaged (a) air temperature, (b) standard deviation, from observations
at the sites.

4.4. Relationship between sky view factor and air temperature in commercial and residential space
The spatial distribution of the mean minimum air temperature well corresponds to that of the sky view factor. Thesky view factor was small in the inside of all commercial space, and become to be large in residential. Scatter
diagram of the minimum, maximum and mean air temperature to the sky view factor is shown in Figure 6, 7.
The minimum air temperature the negative high correlation appears in commercial space (r=-0.69). On theother hand, residential space has not a negative high correlation (r=-0.02) at the minimum air temperature. Thecommercial and residential space has not a negative high correlation (r=-0.11, r=-0.09) at the maximum airtemperature.
4.5. Relationship between surface temperature and air temperature
The urban surface temperature was lower than the residential surface temperature. We are analyze therelationship between surface temperature and air temperature in Okayama City now.

The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan(a) Minimum air temperature(b) Maximum air temperature

Sky view factor

0.50

Sky view factor① Park0.60

r =－0.69

0.400.300.20

0.70

0.60

0.60

0.50

r =－0.11

0.40④ Crossing⑦ Street

0.30⑦ Street
25.5

26.0
26.5
27.0
Air temperature (℃)

0.20

27.5

(c) Mean air temperature

0.70

Sky view factor

0.70

32.5

33.0

33.5
34.0
34.5
Air temperature (℃)

0.50

r =－0.40

0.40④ Crossing
0.30
0.20

35.0

⑦ Street

29.5

30.0
30.5
Air temperature (℃)

31.0

Fig. 6 Scatter diagrams of the sky-view factor and the (a) minimum air temperature, (b) maximum air temperature,(c) mean air temperature, in commercial space. The value of symbol r represents the correlation coefficient.

(a) Minimum air temperature(b) Maximum air temperature
1.00

r =－0.02

0.60

0.40

0.20

26.0
26.5
27.0
Air temperature (℃)①Streetr =－0.09

0.60

27.5

0.20

③ Park0.80

0.40④ Street

25.5③ Park0.80① Street

Sky view factor

Sky view factor

0.80

1.00

Sky view factor

1.00(c) Mean air temperature

0.60① Streetr =－0.24

0.40

32.5

33.0

33.5
34.0
34.5
Air temperature (℃)

35.0

0.20

29.5

30.030.5
Air temperature (℃)

31.0Fig. 7 Scatter diagrams of the sky-view factor and the (a) minimum air temperature, (b) maximum air temperature,(c) mean air temperature, in residential space. The value of symbol r represents the correlation coefficient.

5. Conclusions
We constructed an observation network in Okayama City (700,000 people), Japan. We found that an urban heatisland appeared clearly on nighttime In Okayama City. The heat island intensity (i.e., the temperature difference
between urban and rural areas) was 3.0-4.0 degree C. during the summer. Especially, the heat island appears
clearly for 2200-0100 JST.
On the other hand, during the daytime, the air temperature of urban central area was 1.0-2.0 degree C. lower
than that of rural areas. This phenomenon can be considered as the “urban cool island”. At the same time, theurban surface temperature was lower than the residential surface temperature. The horizontal scale was 2-3km.
References
Bornstein, R.D., 1968. Observations of the Urban Heat Island Effect in New York City. Applied Meteorology, 7,575–582.
Oke, T.R. 1981. Canyon geometry and the nocturnal urban heat island. Comparison of scale model and fieldobservation, J. Climatology, 1, 237-254.