[Original]
Annual variation of air temperature and thermal environment in a three-year
data set obtained in urbanized areas of Japan
Yosuke HORIE1, Masahide AIKAWA2
1

Atmospheric Environmental Division, Hyogo Prefectural Institute of Environmental Sciences,3-1-27 Yukihira-cho, Suma-ku, Kobe, Hyogo, 654-0037 Japan

2

Center for Global Environmental Research, National Institute for Environmental Studies,
16-2 Onogawa, Tsukuba, Ibaraki, 305-8506 Japan

阪神・播磨地域における３年間の気温観測結果に見る気温および熱環境の年較差
堀江

1

2

洋佑 1,

藍川

昌秀 2

兵庫県環境研究センター 大気環境科（〒654-0037 神戸市須磨区行平町 3-1-27），

国立環境研究所

地球環境研究センター（〒305-8506

茨城県つくば市小野川 16-2）

The air temperature was measured at 27 sites located in an urbanized area for 3 years to evaluate theannual/seasonal variation of the air temperature. The air temperature showed a larger variation in the winter season
than in the summer season. However, the annual variation was observed even in the summer season when it wa s
evaluated using some indexes to represent the living environment. The air temperature should be evaluated from theviewpoint of the living environment in addition to the air temperature itself, especially when the heat island
phenomenon is considered.

Keywords: Air temperature, Thermal environment, Urban heat island, Intensely hot night, Hot day, Extremely hotday

I

Government of Japan settled on an action plan for

INTRODUCTION

the heat island phenomenon and constructed a
The thermal environment in urbanized areas

monitoring

networktomeasuretheair

has become severe for humans due to the urban

temperature for the verification of the effects ofheat island phenomenon. The urban heat island

the action plan. The action plan aims to mitigate

phenomenon has been studied worldwide with thethe heat island phenomenon by measures such as

objective of limiting thermal pollution in urban

decreasing the artificial waste heat, improving

areas 1-6) (e.g., Oke, 1973; Oke and Maxwell, 1975;

the urban earth’s surface, reshaping the urban

Gotoh, 1993; Saitoh et al., 1996; Yamashita, 1996;

form, and changing people’s lifestyle. Aikawa etOkeal.7) thoroughly analyzed the data sets obtained in

et

al.,

1999).

The

Hyogo

Prefectural1

the new monitoring network and clearly defined

airtemperaturewasthe growing heat island phenomenon in the area.

thermometer

The(Thermo

RecorderFurthermore, Aikawa et al. 8-10) showed the air

Corporation,

Nagano,Japan)

temperature distribution and clarified the severer

thermostat bath at two temperatures of 5 and 35C.

thermal condition in inland areas than in coastal

The

areas. The monitoring network was constructed inmeasurement accuracy was ± 0.3C. The thermometer2005. The annual variation of the air temperature

was installed in a naturally ventilated thermometeras well as the seasonal variation was analyzed in

shelter (about 1.5 m above the ground). The air

the present study. The findings are reported

temperature was measured at the survey site every 15

below.

minutes, and hourly data measured on the hour were

measurement

resolution

measuredTR-52,

calibrated
wasusing aTandD
with

0.1C.

aThe

used for the evaluation. The air temperature measured

II

METHODS

from July 2005 to April 2008 was analyzed in the
study.

1.

Air temperature measurement
The air temperature was measured at elementary

2.

Survey site characteristics

and junior high schools located within the 10 x 15-km

The survey area can be classified into three(eastern area) and 20 x 15-km (western area) regionscategories: (1) the highly urbanized area along thein Hyogo Prefecture. The air temperature wascoast, (2) the suburban area, primarily in the southernmeasured at 27 sites. The location of the site is shownpart of the study areas, and (3) the residential areain Fig. 1. The eastern area is between Osaka City

being developed as satellite cities, primarily in the2(population density: 2,634,000/ 222 km ) and Kobe

northern part of the study areas. This categorization is2

reported in detail elsewhere 10-11) .

City (population density: 1,520,000/ 551 km ), andthe western area is in the west of Kobe City. The
southern

areas

are

characterized

by

III

intensive

RESULTS AND DISCUSSION

industrial development and dense populations, which
categorize them as urban areas. In contrast, the1.

Trend of monthly air temperatureurbanization has been progressing rapidly in theFig. 2 shows the trend of the averaged monthly

northern areas, which mainly include satellite cities ofmean air temperature of all sites and the highest and11)

Osaka and Kobe .

the lowest monthly means among all sites. Here, theHyogo Prefecture

averaged monthly mean air temperature is the averageof the monthly mean air temperature of the 27 sites,JAPANand the highest and the lowest monthly means are thehighest and the lowest values among the monthly
mean air temperature of the 27 sites, respectively. In
Fig. 2, the highest air temperature was always
observed in August for 3 years. In contrast, the large
variation was observed at the lowest air temperature:
the lowest air temperature was observed in December,
January, and February in the 2005, 2006, and 2007
winter season, respectively. Fig. 3 shows the trend of
the difference between the highest and the lowest
monthly means. The large difference was observed in
the winter season, while the difference was small inthe summer season. Taking Fig. 1 and Fig. 2 into

Fig. 1account, the annual variation of the air temperature

Location map of the survey sites.

was large in the winter season and small in the2summer season. (Aikawa and Hiraki12); Horie andnumber of hours in August of 2006 was large for all

Hiraki 13)) analyzed the annual variation in August andsites, with a smaller variation, while the number ofDecember between 2005 and 2006 in detail. The

hours in August of 2005 and 2007 was smaller thanconclusions of their detailed analysis were coincident

that in August of 2006. Furthermore, the number ofwith the finding in the present study.

hours at some sites in August of 2005 and 2007 wasnotably small.
The annual variation of the monthly air temperature

35
Highest monthly meano

Air temperature / C

30

in the summer was smaller than that in the winter, as

Averaged monthly mean
Lowest monthly mean

25

shown in Section III -1. However, the thermally20

severe condition showed the annual variation when it15

was evaluated by the number of hours during which
the air temperature was higher than a threshold value.

10
5
0

0803

0801

0711

0709

0707

0705

0703

0701

0611

0609

0607

0605

0603

0601

0511

0509

0507

800

(a)

Fig. 2

Number of hour higher than 25oCTime (month)

Trend of the averaged monthly mean air
temperature of all sites, and the highest andthe lowest monthly mea ns among all sites.

600

400

200

8

0803

0801

0711

0709

0707

0705

0703

0701

0611

0609

0607

0605

0603

0601

0511

Time (month)oHighest - Lowest / C

0509

0507

0

6

4
400

(b)

Number of hour higher than 30

o

C

2

0803

0801

0711

0709

0707

0705

0703

0701

0611

0609

0607

0605

0603

0601

0511

0509

0507

0

Time (month)Fig. 3

Trend of the difference between the highest

300

200

100

and the lowest monthly means.
0803

0801

0711

0709

0707

0705

0703

0701

0611

0609

0607

0605

0603

0601

0511

0509

0507

0

Time (month)2.

Annual variation of thermally severe condition

Fig. 4

Miyazaki et al. 14) introduced an index for an urban

Trend of the index for an urban heatisland introduced by Miyazaki etheat island that shows the total number of hours

al. for 25C (a) and 30C (b).

during which the air temperature was higher than athreshold value. Miyazaki et al. 14) adopted the3.

threshold values of 25C and 30C. Fig. 4(a) and (b)Characteristic differences between two indexes
that indicate a living environmentshows the trend of the index for an urban heat island

Some indexes to indicate a living environment are

introduced by Miyazaki et al. for 25C and 30C,

frequently used in Japan: i) an intensely hot night

respectively, where air temperature data, which are(Nettaiya in Japanese); ii) a summer day (Natsubi in

not corrected by the altitude of the sites, are used. Fig.

Japanese); iii) a hot day (Manatsubi in Japanese), and4(a) and (b) shows that the number of hours duringiv) an extremely hot day (Mousyobi in Japanese).

which the air temperature was higher than 25C andEach index is defined as follows:

30C was the largest in August at all sites. Thei) An intensely hot night is one in which the lowest air
3

IV

temperature is 25C or higher during the nighttime.

CONCLUSIONSii) A summer day is one in which the highest air
The air temperature was measured at 27 sites intemperature is 25C or higher.
iii) A hot day is one in which the highest air

urbanized areas of Japan from July 2005 to April 2008

temperature is 30C or higher.

to understand the annual/seasonal variation of the airiv) An extremely hot day is one in which the highest

temperature.

air temperature is 35C or higher.

evaluated not only by the air temperature itself but

The above four indexes can be classified into two

also by the thermal conditions. The number of days

categories. An intensely hot night is typified by

and/or number of hours during which the air

sleeplessness; on the other hand, a summer day, a hottemperature was higher than a threshold value wasday, and an extremely hot day are characterized byused to evaluate the thermal condition. The findings

severe heat during the daytime.

are summarized as follows:

Fig. 5(a) shows the number of days with intensely hot

1) The highest monthly mean air temperature wasnights at each site. Variation was observed among theobserved in August in all summer seasons, whilesites regarding the number of days with intensely hot

the lowest mean air temperature was observed innights. Fig. 5(b), (c), and (d) shows the number ofDecember 2005, January 2007, and Februarysummer days, hot days, and extremely hot days,2008 in the 2005, 2006, and 2007 winter seasons,respectively. All days in August were categorized as

Theannual/seasonalvariationwasrespectively.

summer days (Fig. 5(b)). Either no variation or a2) The annual variation in the summer was small,small one was observed in the number of summer (Fig.

while that in the winter was large.

5(b)) and hot days (Fig. 5(c)). In contrast, some3) Although the annual variation was small when it

variation was observed in the number of extremely hot

was evaluated by the air temperature itself, a clear

days (Fig. 5(d)). Furthermore, the number of summer

annual variation was revealed when it wasand hot days in 2006 exceeded those in 2005 andevaluated by the thermal conditions.

2007; this was particularly notable in the case of hot

In conclusion, the thermal conditions should bedays (Fig. 5(c)). There was no specific correlation

evaluated in detail using the above-described indexes

between the number of summer, hot, and extremely

to represent the living environment as well as the air

hot days and that of intensely hot nights among thetemperature itself.

sites, which suggests that the nighttime living

ACKNOWLEDGEMENTS

environment should be evaluated separately from theliving environment in the daytime.
0508

0608

The authors are grateful to Professor Masakazu

0708

20
10

(a)

Number of day

Number of day

30

0

30

Moriyama of Setsunan University and Dr. Hideki

20

Takebayashi of Kobe University for his support with
(b)

0
Site

Site

30

20
10

(c)
0

Number of day

Number of day

30

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5