- Email: [email protected]
Urban nocturnal temperatures, street geometry and land use
Atmospheric Environmenr Vol. 30, No. 3, pp. 379-392, 1996 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352-2310/96 S15.00 + 0.00
Pergamon
1352-2310(95)00033-X
URBAN NOCTURNAL GEOMETRY
TEMPERATURES, AND LAND USE
STREET
INGEGliRD ELIASSON Department of Physical Geography, Earth Sciences Centre, University of Giiteborg, S-413 81 Giiteborg, Sweden (First received 15 November 1992 and
in final form 10 December 1994)
Abstract-The climate in the city of GBteborg, Sweden, was investigated for a three year period. In this paper the nocturnal temperature distribution is analysed in relation to differences in street geometry and land use. The seasonal and monthly air temperature difference between a street canyon and a nearby open area, as well as case studies of the vertical and horizontal temperature distribution within and between different urban units and urban districts are discussed. In spite of a good relationship between the local surface temperature and the sky view-factor, both continuous measurements and case studies show small variations in air temperature within the city centre. Air temperature variations between urban districts of different land use are greater. The average horizontal temperature decrease of 4°C in the transition zone from the city centre to a large park southwest of the centre are, in fact, of the same order as the average urban-rural air temperature difference. Key word index: City climate, urban heat island, street canyon, urban structure, sky view-factor.
1.
and Nakamura and Oke (1988). The urban temperature pattern has also been simulated in hardware canyon models (e.g. Oke, 1981) and in numerical urban canyon models (e.g. Arnfield, 1982, 1990). An important objection to many of the existing studies of urban temperatures is the lack of a clear distinction between surface and air temperature. For example, model studies of surface temperature are compared with field studies of air temperature and the urban (screen level) heat island is often inferred from satellite or airborne ground radiation data. Field studies of both surface and air temperature have shown that in spite of the fact that the street surface is influenced by the canyon geometry, the local standard level air temperature is governed by more complex and regional factors (BPrring et al., 1985). The lack of simple coupling between surface and air temperatures in the urban system has been discussed by e.g. Goldreich (1985) and Roth et al. (1989). Recent studies of the surface/air temperature relationship have shown the importance of microadvection in the urban canopy layer (e.g. Roth et al., 1989; Stoll and Brazel, 1992). The aim of the present paper is to analyse the temperature climate at sites with different geometries and land use within the city. Great importance is attached to separating the surface and air temperatures. In order to elucidate the geometric effects on the sky view-factor at street level and, hence, on the cooling rate by net longwave radiation exchange, only nocturnal temperature data are considered in this
INTRODUCTION
With special interest focused on temperature variations related to difrerences in street geometry, or land use, the present parper analyses 3yr of temperature data from a high lartitude city. Urban geometry has for a long time been identified as an important fal:tor in explaining the urban heat island. However, most studies have been made on the local scale, which means that the urban-rural temperature difference (Al-,-,) has been compared with the geometry of city centres. Oke (1981) showed the relationship between the maximum heat island intensity and the sky view-factor (SVF) for some Australian, European and North American cities, a relation which also proved to be valid for Japanese and Korean cities (Park, 1!)87). Urban-rural temperature differences are of course of interest for comparison between cities, but might, as noted by Oke 1[1981), be difficult to use within a single city. A good knowledge of the influence of street geometry on temperature over very short distances, within cities, would be of great value, not least for urban planning. Field measurements of the influence of urban geometry on temperature on the microscale, i.e. within a city, are sparse. Most of them are case studies, with a measuring period of one or a few days only (e.g. Tuller, 1973; Blrring et al., 1985; Yamashita, 1988). Measurements within separate street canyons have been made by, for example, Nunez and Oke (1977) 379
I. ELIASSON
380
paper. The analysis is based on a measuring period of 3 yr, which includes continuous measurements at two climatic stations and case studies in the cold season by three different methods. The following data are discussed in the paper: (a) the seasonal and monthly air temperature differences between a street canyon and a nearby open area, (b) the vertical and horizontal air and surface temperature variation in a street canyon, (c) the horizontal air and surface temperature pattern in a central city area, (d) horizontal air and surface temperature gradients between different urban districts.
2. THE STRUCTURE
OF GGTEBORG
Gateborg is a city of approximately half-a-million inhabitants. The city is situated on the Swedish west coast in a fissure valley landscape. The central areas of the city lie on the southern shore of the Giita river. With reference to the urban structure, the inner city could be divided into two parts (Fig. 1). (A) The northern part is the oldest part of the city. It is located between the river and the canals. In this area, there is a mixture of old and new buildings, but the old dense structure remains. The streets are, with some exceptions, oriented in an ENE-WSW and NNW-SSE direction. The building heights vary between 10 and 15 m and the average street width is 7 m.
THE NORTH CITY SVF OF CANYONS THE SVF
SOUTH CITY 0~ CANYONS
0.25-045
0.5-0.9
,:. OPEN
AREA
PERMANENT ??
POINT
0
Fig. 1. Map of central GBteborg.
STATION
OF MEASUREMENT
500
m
.
Urban nocturnal temperatures, street geometry and land use
The sky view-factors (SVF) of the canyons vary between 0.25 and 0.45. (B) The southern part, south of the canals, is a part of the city which has a more heterogeneous geometry. Some streets in this area are quite wide (45m) but others are narrower, about 13m. The heights of the buildings in this part of the city vary between 5 and 25m but the average height interval is 17-22m. The SVF of the canyons varies between 0.5 and 0.9 and the streets are, with salme exceptions, oriented in a N-S and E-W, or a NW-SE and NE-SW pattern. To the northeast is a large open area. Small parks are scattered within this southern part, which is bordered by a large park in the southwest.
3. METHODS 3.1. Climatic stations Two climatic stations were set up in the central city area. One was sited in a street canyon which is oriented with its long axis in a northwest-southeast direction, and the other station was located in a large open area, about 400m away from the canyon (Fig 1). Continuous measurements of temperature and wind were carried out during a 3-yr period at these two stations. The following is a description of the station sites and instrumentation. The street canyon is formed by symmetrical buildings 120 m long and 20 m high. The width of the street is 14 m and thus the height/width (H/W) ratio is 1.4 and the sky viewfactor is 0.5. To the northwest, the canyon is interrupted by a cross street, but continues in the next block. To the southeast, the canyon ends; in a T-junction. The buildings in the canyon are of brick and are used for housing. The east side of the canyon consists of a rather new building and, on the west side, the buildings are older but renovated. Figure 2 shows the instrumentation in the canyon. The air temperature was measured at four points within the canyon. For practical reasons, these points were located between 4 and 10 m above the street surface. The wind speed and direction were
\
\ L.
--
T,W -\
---
I’
22/12
\
T= Temp.
\
W=Wind
\
\
XT
20n
\
\ XT +
I
XT
2216
I WEST
XT
\
I--
14 m-
EAST
Fig. 2. Instrumentation in the northwest-southeast oriented street canyon and the penetration of the solar radiation on 22 June and 22 December at 1400 h, when the solar azimuth angle is perpendicular to the orientation of the canyon. T= temperature, W = wind speed and direction.
381
monitored within the canyon at a point 4 m away from the east wall and 5 m above the street surface. On the roof of the east building, temperature and wind (speed and direction) were monitored at 2 m above the roof surface. The penetration of the solar radiation into the canyon during summer and winter at 1400 h when the solar azimuth angle is perpendicular to the orientation of the canyon is also marked in Fig. 2. On 22 June solar altitude is 48”; thus, at this time almost all of the east wall is sunlit. On 22 December owing to the low solar altitude (3”),only the uppermost 2 m of the east wall is sunlit. The open area is approximately 250 m wide and 400 m long. The area is mainly covered by gravel and is used for sports. The area is crossed by a bicycle/foot path covered with asphalt. Air temperature was measured at 2 and 10m above the surface and the wind (speed and direction) was monitored at 5 m and 10 m above the surface. At both stations the temperature was recorded utilizing wire-wound resistance thermometers, Pt 100 R and Ni 1000 Q, protected by radiation shields. The wind speed and direction were recorded by Vaisala cup anemometers (VAAlS) and wind vanes (VAWlS). All sensors were calibrated in a climatic chamber before and after the measuring period and in situ at regular intervals during the measurements. Hourly mean and lO-min mean values were recorded with a computer. 3.2. Case studies Case studies of the horizontal and vertical urban temperature pattern were made during weather conditions with a wind speed less than 2 m/s and cloud cover less than 2 octas, utilizing automobile traverses, a mobile datalogger system and an infrared image system. 1. Automobile traverses were carried out to study the horizontal temperature pattern within the city. The vehicle used was equipped with a computer which stored measured data at every 5 m. The exact location of sites with certain interest for the analysis (e.g. canyons and intersections) were also stored in the computer. During the measurements, the speed of the vehicle did not exceed 50 km h-l. The surface temperature was measured with a Barnes precision radiation thermometer mounted on the vehicle. The air temperature was monitored at two levels, 0.2 and 2.0 m, utilizing wirewound resistance thermometers. 2. The mobile datalogger system was used to measure horizontal and vertical temperature profiles along street canyons and street intersections. The advantage of the system is that the datalogger and the set of instruments easily can be changed or relocated. The thermistors recording air temperature were equipped with simple radiation shields, and a heat-conducting paste was applied to thermistors attached to the surface of the street. The wind was recorded with a cup anemometer (Thies). In general, the measurements were carried out along selected profiles during a few hours up to a few days. In some case studies, two or three dataloggers were used for simultaneous measurements at different urban sites. 3. The infrared thermography system (Agema 870 system) was used to record the surface temperature in horizontal profiles through street canyons and street intersections. The system consists of a scanner, sensitive to radiation between 2 and 5 pm, a display unit and a video tape recorder. The scanner was carried by hand, while a box containing the display unit and video tape recorder was carried on a luggage carrier. The different parts of the canyon were identified by aluminium strips at selected points on the.street surface. Due to the low emissivity of aluminium foil these strips appeared as cold spots in the thermal image. The analyses were made with specially designed software which includes correction functions for the prevailing measuring conditions. Further descriptions of the system are given by Lindqvist (1987) and Eliasson (1992).
I. ELIASSON
382 3.3. Street geometry
The street geometry is described by the sky view-factor (SVF), which was calculated at the street surface on the midpoint of the urban unit (canyon, intersection or open area) under study. In a first phase the sky view-factor of 50 uniform street canyons was calculated by a graphical method (Watson and Johnson, 1987). This method is easy to use as it ‘only requires data fo; the difference in the azi&h angles and data for the two elevation angles of the ends of the buildings. A clinometer, with a precision of 0.5”, was used for measuring the elevation angles. The graphical method is, however, not suitable for street intersections, squares and non-uniform canyons and thus in a second phase a fish-eye photograph method proposed by Holmer (1992) was used as a complement to the graphical method. The fundamental theory is that of Steyn (1980), but Holmer has simplified the calculation by digitizing the border between sky and wall in the photograph. In this study, 18 street intersections and open areas and 15 canyons were photographed from the surface level, in the midpoint of the urban unit, with a fisheye lens (Nikkor 8 mm,f/2.8, picture angle 180”).The projection formula of the lens is almost equidistant and thus an approximately linear relation exists between the zenith angle and the distance of point from image centre. The photographs were enlarged to a radius of 1Ocm and finally digitized for every 2” sector. The number of annuli chosen was 100. The two methods were compared at 10 street canyons and the calculated values showed a good correspondence.
4. RESULTS
4.1. Canyon/open area, seasonal and monthly air temperature differences The nocturnal air temperature differences between the street canyon and the open area (permanent stations in Fig. 1) have been analysed for the 3-yr measuring period. Hourly mean values were used in the analysis and the frequency of nocturnal hours with a canyon-open area temperature difference greater than OYC (AT,-, > O.YC) were calculated. The limit of OYC was chosen, as the specifications from the manufacturers, and the calibration procedure described above, have shown that the variation among the temperature probes is well below 0S”C. The “canyon temperature value” used in the analysis is a mean value of the four measurements inside the canyon (Fig. 2) as the temperature variation within the canyon was small, see Section 4.2. By analogy with the canyon data, the “open area temperature value” used is a mean value of the measurements at 2 and 10 m. Owing to the high latitude (N57”42’) of Giiteborg, the length of the night varies from 5 h in the middle of June to 16 h in December. With regard to this great variation the data were first divided into data sets of 10d. For each 10-d period the percentage of night hours with a canyon-open area temperature difference greater than 0.5”C were calculated. Secondly, the 10-d periods were arranged into monthly and seasonal data sets. In Fig. 3, the seasonal frequency of night hours with a canyon-open area temperature difference of > 0.5 or < - 0.5 are shown for 3 yr: 1988, 1989 and 1990.
The four seasons represent the following months: spring (March, April and May), summer (June, July, August), autumn (September, October, November) and winter (December, January and February). The diagrams show a marked variation between the different seasons. The highest percentages occur in the summer season; in 1988, on nearly all of the night hours, the AT,-, was greater than 0.5”C, and on 40% of the night hours the temperature difference was greater than 1°C. On the other hand, in the winter season a temperature difference of 0.5”C occurred on only l&15% of the total night hours. The spring and autumn are in an intermediate position with about 40% of the total night hours having a temperature difference greater than 0.5”C. However, it is important to note that the greatest variation in temperature between the two urban sites is similar for all seasons, that is, maximum AT,-, is about 2.7”C. The frequency of hours when the air temperature is higher at the open area than in the canyon (AT,-, < - O.S’C) is very low in all seasons. The grouping of data into seasons is convenient for a compact presentation of the 3-yr data set, but it might hide the variation between the different months in each season. Figure 4 shows a monthly classification of the data of 1 yr. It is evident from the diagrams that a monthly variation within each season exists. The highest percentages occur in May, June and July and the lowest percentages were recorded in January, February and March. The greatest temperature differences recorded were 2.5”C, in July 1989 and, 2.9”C, in December 1988. It must be pointed out that the diagrams shown in Figs 3 and 4 are a result of an analysis of all night hours during a 3-yr measuring period and thus cover all types of weather situations. Clear and calm weather favours high-temperature differences. The frequency of AT,-, > 0.5”C is undoubtedly dependent on the weather conditions, i.e. the occurrence of clear and calm weather. The monthly frequency of clear and calm nights and clear 24-h periods for the period September 1988 to August 1989 are shown in comparison with the percentage of AT,-, > 0.5”C (results from Fig. 4) in Fig. 5. Hourly data from the synoptic weather station at SIve airport, located 9 km NW of the city centre, formed the database from which nights with light winds and clear skies were selected. On a clear, calm night the sum of the cloud amount observed from 1900 h until 0700 h was required to be less than 23% of the maximum amount possible and the wind speed less than 2 m s- ‘. This definition follows the selection rules used in meteorological statistics, published by the Swedish Meteorological and Hydrological Institute (Taesler, 1972). During a clear 24-h period, the sum of the cloud amount should correspondingly be less than 23% of the maximum amount possible. Two peaks of clear weather, one in October, November and December and the other in May, June and July are recognised in
383
Urban nocturnal temperatures, street geometry and land use SPRING h ("1.1
1'3RR
h lo/.1
1989
h (O/o)
100
100
80
80
60
60
. ATc_o( C)
c-o.5
SUMMER h I'/.) 100
1990
h I%)
1!388
100
80
80
60
60
LO
. I-
100 1
1
1.
I
h I'/.)
1989
i
LO -
20
20 -
1990
WMT-TTTO ' ' ' ' ' ' :'3'
.
c-05
AT,_,l'Cl
AT,_,1Cl
AUTUMN hl'l.1
I'388
h ('1.)
1989
h P/o)
100
100
80 i
80
80
60
60
60
100
I990
WINTER h ("I.)
h ("I.1
h ('/.I
1001
100
100
80
80
60 r -
60
1989
I990
LO I
Fig. 3. Seasonal frequencies of night hours with a canyon-open area temperature difference greater than OST, or less than - 0.5”C for 1988, 1989 and 1990. Vertical axis shows the percentage of the total night hours(h) in %. Horizontal axis shows the temperature difference (AT,_,) in “C. Positive values indicate the canyon air to be warmer.
Fig. 5 and the frelquency of AT,-, > OS’C corresponds to the occurrence of clear and calm weather. However, in spite of similar values of clear weather in the percentages of summer and late autumn
AT,-, > O.S”C found during May, June and July are much higher than during October, November and December. This discrepancy is probably due to the higher sun elevation in the summer as shown in Fig. 2.
I. ELIASSON
384 AUTUMN h I’/. )
SEP.
1988
OCT.
h I’/.)
198E
80
80 I
80
60
60
LO
NOV.
1988
FEB.
1989
LO
.
20i
I
<- 0.5
h 1%) 100
100
100
2 . AT,.,1 C)
>3
. AT,.,( C)
c-o.5
<
WINTER DEC. 1988
h I’/.)
JAN.
h I’/.)
1989
h I*/.) 100
100
100
80
80
80
60
60
60
LO
LO
LO
zo-
zo-
c-o.5
AT c-0(C)
SPRING h I% )
MAR.
1989
h (% )
APR.
1989
80
80
60
60
h I %I
MAY
1989
100,
1001
1001
SUMMER h f”/o, 100
1
JUN.1989
h (“/.I
JU.L. 1989
100
80-
h(‘/e)
AUG.
1989
too&360. 40-
Fig. 4. Monthly frequencies of night hours with a canyon-open area temperature difference greater than 0.5”C, or less than - O.S”C, for 1 yr (September 1988 to August 1989). Vertical axis shows the percentage of the total night hours (h) in %. Horizontal axis shows the temperature difference (AT,_,,) in “C. Positive values indicate the canyon air to be warmer.
This gives a greater penetration of solar radiation in the canyon and, thus, a greater daytime uptake by the sunlit walls. 4.2. Temperature variations within a canyon Additional sensors were set up in the northwest southeast-oriented canyon (Figs 1 and 2) on several
occasions during the 3-yr measuring period. Usually, the air temperature variation within the canyon was less than OYC. The example shown in Fig. 6, a September night with light winds and no clouds, is one of the studies with the greatest temperature variation. The diagrams in Fig. 6 show the climate condition in the canyon every 2 h from 1900 to 0500 h. Despite
Urban nocturnal temperatures, street geometry and land use
SONDJFMAMJJA 1988 . Y
0
1989 Number of clear Z&-hour pernds Number of clear ?nd calm ntghts ‘/. ATC_O > 0,5 c
Fig. 5. The monthly frequency of clear and calm nights and clear 24-h periolds for the period September 1988 to August 1989 in comparison with the percentage of AT,_, > 0.5”C as shown in Fig. 4.
calm conditions, a canyon-roof temperature difference of l.o”C and a canyon-open area temperature difference of 1.5”C in the early evening, a clear temper-
ature pattern within the canyon was not obvious until 0100 h. At that time the air was warmer near the walls than in the canyon. However, the difference was still only 0S”C. The cold centre remained through the night and early morning until the sun rose. Several case studies were also carried out in different canyon/intersection transition zones with the mobile datalogger system. Examples of representative case studies of the horizontal surface and air temperature variation in the canyon/intersection transition zone are shown in Fig. 7. Diagram 7A shows a surface and an air temperature profile from the middle of the canyon (SVF = 0.5) to the intersection (SVF = 0.7), measured by the mobile datalogger system. The measurements were made during an October evening in 1988. The air ‘temperature variation was small throughout the whlole evening. The diagram shows the situation of 2300 h when the difference was less than 0.2”C. The wind at that time was 0.2 m s- ’ in the canyon and 1.0 m s- ’ in the intersection. The surface temperature was rather uniform in the canyon but an abrupt change occurred between the canyon edge and 16 m inside the canyon and the surface temperature was about 2°C lower at the intersection. The profile, however, is made up of separate measuring points and the actual temperature between the points is unknown. Spotwise information could be avoided by using the infrared image isystem as this method enables the recording of a surface area. Diagram 7B shows the surface temperature in the canyon measured with the infrared scanner during an October evening in 1990 at 2000 h. The distance between the surface and the scanner was 1 m and the scanned surface area was thus 0.5 m2. The mean surface temperature of the recorded surface area was calculated from the infrared
385
images using a box method (Eliasson, 1992). The results show a uniform street surface temperature in the canyon, until 10 m before the canyon edge. At this distance the temperature starts to fall rapidly, the gradient is very steep and at the edge the temperature has decreased by 1.8”C. After that, the decrease is reduced and in the middle of the street intersection, 10 m from the edge, the temperature is 2°C lower as compared to the canyon. Diagram 7C illustrates the cooling rates in the middle of the canyon and at the intersection during a September evening in 1989. The temperatures at the 0.1 and 1.Om levels were more or less the same at both sites during the evening. Four hours after sunset, the difference between surface and air temperature was 2S”C in the canyon and 1.5”C in the intersection. The measurements started at 1800 h, 1 h and 40 min before sunset. At that time, the surface cooling had already started. Four hours after sunset the decrease in temperature was 1°C h-’ for the air, 0.75”C h-l for the intersection surface and 0.62”C h- ’for the canyon surface. These diagrams clearly show large surface temperature differences but small air temperature variations between adjacent urban units (see also Eliasson, 1990/91, 1992). The measurements also show the air temperature to be more or less uniform above 10 cm. 4.3. Temperature patterns in the central city area In order to analyse the horizontal temperature distribution within the inner city, in relation to the urban structure, automobile traverses were made through the different urban units (point of measurements in Fig. 1). The SVF of these urban units range from 0.33 to 1.0. The traverses were made in the late evening, 3 h or more after sunset and each traverse was standardized to one point of time. The maximum time of a traverse was 1 h. During the traverses the wind speed at the open area station was less than 2 ms-’ and, at the synoptic weather station, SHve airport, the mean values of the wind speed and cloud cover from the 6 h preceding the traverse were less than 3 m s- ’ and 2 octas, respectively. Figure 8 shows the results of one traverse from the northern part of the city (see inset map). The analysis was made in 3 steps for all urban units marked in Fig. 8. Firstly, the selected unit, canyon, street intersection or open area were identified on the diagram. Secondly, the position of the SVF measurements, the midpoint of the urban unit, was fixed and, thirdly, a mean value of the recorded data in this middle sector, about 20 m for canyons, was calculated. The canyons, the intersections, a park and an open area, as well as the sky view-factors of these urban sites are indicated in Fig. 8. It is evident, from the diagram, how the surface temperature is influenced by the city structure, while the air temperature is not. Minor surface irregularities were easily identified as scattered data in the diagrams (for example, Fig. 8, area 5) and were excluded from the analysis. Major surface variations, such as differences in bed material
I. ELIASSON
386
‘3.0 ‘c !.3 mls
1933 r-
A’,_,:
I .5
ATC-T
I.0 r
0 m/s
8.9 ‘ c 3 m/s
2300 AT,.,.
1.7
AT,_,=
‘,3
IC, 6
'0.L
52 ox3
xi 9:l
Ii.hZ/ 9.:
E
x ,
.:;‘, ~’
cm/s
10,l ,3 ‘c 7115
6.0
‘c
Omls
0500
0300
AT c-o= I,7
AT c_o=2.5
ATc_r-
AT c-,
I,L
= I,2
?4
W
17.2 3 m/s
7.$ x I.4
I \
“/ 7 c X-id 7.0 x 7,l
17.; * 6.9
E
G m/s
* 7,3
Fig. 6. The vertical air temperature distribution every 2 h from 1900 to 0500 in a canyon with a sky view-factor of 0.5. The measurements were carried out during a night with light winds and no clouds. Inside the canyon the wind speed was zero during all night The values of the air temperature and the wind speed at the roof of the canyon are shown at the east top of the canyon symbol. AT,_, = Temperature difference between the canyon and the nearby open area, AT,_, = temperature difference between the canyon and the roof. See also Fig. 2.
and in heated street sections, may of course influence the temperature recording. Gustavsson and Bogren (1991) discussed the influence of road construction on road surface temperature. On a test road, they had found a nocturnal maximum difference of 1S”C between beds consisting of blast furnace slag and those consisting of gravel. Such extreme differences in bed material, however, are not found in central Goteborg. Street intersections and canyons affected by tram lines, heat or moisture, have been excluded.
A regression analysis and hypothesis test with the null hypothesis that the temperature is not related to the geometry were carried out on the data recorded by the traverses (Table 1). The temperatures were presumed to follow a Gaussian normal distribution. The results presented in Table 1 shows that the probability values (Prob > t) for the surface temperature and SVF relation are significant on the 5% level for all occasions. This means that the surface temperature is correlated to the sky view-factor.
387
Urban nocturnal temperatures, street geometry and land use
surface
0
lo
20
30
LO 50
60
70
60
temperature
air 0.15m
”
air
”
1.0 m
m
surface
0
10
20 30
LO 50 60
70
80
temperature
90 m
16 15 lk 13 12
11 10 gt
I 18
-.-
__..-
I
19
20
canyon N *
I
I
I
21
22
23
intersection
I
2L HOUR
surface
temp.
surface temp. 0.1 m air *I
_--_-
,/
0.1 mair
*
,.Omair
. . . . . ..I.
”
l.Omair
”
,,
Fig. 7. Hori.zontal temperature variations within a canyon during clear and calm evenings and nights. The map at the top of the figure refers to all diagrams but the distance scale refers only to diagram A and B. (A) Horizontal profiles of surface temperature and air temperature at two levels measured by the mobile datalogger system; (B) horizontal profile of surface temperature measured by the infrared image system; (C) S:ooling rates in the canyon and in the adjoining intersection (mobile datalogger system).
On the other hanId, Table 1 shows high probability values for both the 0.2 and 2.0 m level (except for the 2 m level on 4 October 1989). This means that the air temperature could not be statistically related to the sky view-factor. The air temperature/sky view-factor
relation was further analysed by calculating the power of the hypothesis test. In this calculation a non-central t-distribution with n - 2 degrees of freedom was used. The presumption for the calculation is that an air temperature difference less than 1°C within the city
388
I. ELIASSON A
B
I
I 1
1
C
3
2
B
A
L
5
6
7
PARK
~0.33-~X~O.3L--fX~O.3L+Y&O.33(Xt_O334
IXj--O.LO+0.85 065
.
'c I
b
100
. .....
SURFACE
---
AIR
t-1
STREET
CANYON
IXI
STREET
INTERSECTION
0.33
SVF
A
LOO
560
m
TEMPERATURE
TEMPERATURE
OF
360
200
8
THE
URBAN
IDENTIFICATION
“,:;;
m
BUILDINGS
(:::
PARK
UNIT MARK
O-
200m
J
Fig. 8. Automobile traverse in the northern part of the city centre. Profiles of surface temperatures and air temperatures at 0.2 and 2.0 m.
Table 1. The results of regression analysis and hypothesis test including calculation of the “power of the test” on the relationship between temperature (surface and air) and sky view-factor within Giiteborg. Data from automobile traverses under clear and calm weather conditions in the cold season Date
18 November 1987
12 December 1988
4 October 1989
7 December 1989
1 February 1991
Number of observations
centre is of minor importance.
Slope
Prob > t 5%
Significance of test
Power of test
16 16 16
0
0.2 2.0
- 3.4 -0.1 - 0.4
0.003 0.872 0.376
Yes No No
0.8 1.0
30 30 30
0 0.2 2.0
- 3.2 0.4 0.7
0.000 0.317 0.121
Yes No No
0.9 0.9
30 30 30 30 30 30 10 10
0 0.2 2.0
- 1.7 - 1.0 - 1.8
0.011 0.054 0.000
Yes No Yes
0 0.2 2.0 0 0.2 2.0 0 0.2 2.0
-
0.000 0.092 0.116
Yes No No
0.000 0.474 0.324 0.002 0.647 0.158
Yes No No Yes No No
10
25 March 1991
Level (m) above
12 12 12
The results shown in the last column of Table 1 indicate that if the air temperature difference between the most dense and most open area in Giiteborg (SVF interval 0.33-1.0) was greater than 1°C there would be a 40-100%
3.9 1.0 1.1 6.4 0.8 - 0.4 - 2.8 8:f
0.8
0.6 0.6 0.4 0.9 0.4 0.7
chance of finding it with the hypothesis test. Thus, the conclusion is that the hypothesis test is strong and that there is no evidence for a statistically significant relationship between the air temperature and sky view-factor in central Giiteborg.
Urban nocturnal temperatures, street geometry and land use In infrared images, from airborne recordings, large street intersections often appear as colder spots (Biirring et al., 1985; Eliasson, 1990/91, 1992). However, it is more uncertain as to what extent smaller open areas show a lower surface temperature than adjoining canyons. The results from the measurements with the mobile datalogger :system and infrared thermography system presented above clearly show differences in surface temperatures within the city. A closer examination of the air and surface temperatures at street intersections and in open areas was made with data from the automobile traverses. The intersections and/or open areas was combined with the adjoining canyon, giving 18 combinations altogether. Data from three occasions were used to calculate mean values of the temperature differences between street intersections and canyons (ATi-,). These mean values of ATi-, were then plotted vs the sky view-factor of the street intersection/open area (Fig. 9). The difference in surface temperatures is shown in Fig. 9A. A trend is distinguishable in the diagram, as higher SVF show
‘SURFACE
Al 2 1
389
larger negative values. The air temperature differences, 0.2 and 2.0 m, are close to zero for all SVF except for SVF = 1.0 which is about 1°C colder than the adjoining canyon (Fig. 9B). The great difference in air temperature between the areas with sky view-factors around 0.9 and the area with SVF = 1.0 could be explained by different size and by different surface material. The sites with a sky view-factor around 0.9 are open areas of approximately 80 m x 80 m, covered by stone and asphalt. The SVF = 1.0 is represented by the relatively large open area 250 m x400 m (discussed earlier in Section 4.1) which is covered by gravel and asphalt. Microadvection between the smaller open areas and their adjoining canyons is probably important to level out the air temperature difference on small open areas. On the other hand is the open area of SVF = 1.0, probably large enough to develop its own microclimate compared to the adjoining canyons. The difference in surface material is also important. Oke et al. (1991), for example, argues that differences in thermal admittance and street geometry are primary causes equal to one another in importance at least, for the surface heat island. According to Oke (1987) the thermal admittance (J m-2s-1’2 K-l) for sandy soils is 620 and for asphalt 1200 and stone 2220. This indicates that the open area of SVF = 1.0 which is covered with gravel has a tendency to cool more rapidly than asphalt-stone covered areas. 4.4. Temperature gradients between d#erent districts
0
0.2 ??
0.4
0.6
0.8
1 SVF
Surface
AIR
BJ
-3+--l--r 0
1 0.L
0.2
0
Air 0.2m
x
Air
I
I 0.6
I
I 0.8
I
I 1
I SVF
2.0m
Fig. 9. Relationship between the temperature difference between street intersection/open area and adjoining canyon (ATi_c in “C) and the sky view-factor of the intersection/open area (SVF): (A) surface temperature; (B) air temperature (0.2 and 2.0m). Data from automobile traverses.
All the temperature data discussed in the above sections were measured within the city centre, an area of approximately 4 km’ (Fig. 1). The results from these measurements show a small variation (< 3°C) in air temperature. Large differences in air temperature between builtup areas and a large park about 3 km southwest of the north central city (Fig. 1) have been reported by Lindqvist (1992). On some occasions the air temperature increased 6°C from 100 m inside the park to a point within the built up areas 150 m outside the park. However, more frequently, the air temperature gradient in the transition zone was 0.330).4”C per 100 m and the cool park climate could be traced about 1000 m outside the park (Lindqvist, 1992). The park is bordered in the north by the central built-up areas (Fig. 1) and in the south by suburban areas, mainly multi-storey buildings. Figure 10 is an example of a temperature traverse from the central areas through the large park and into the suburban areas during a clear and calm night in March. The diagram also shows the variation in building density (sky view-factor) from the city centre to the southern suburbs. The park is surrounded by warmer built-up areas as shown by the curves of 0.2 and 2.0 m air temperature in Fig. 10. The air temperature first decreases from the city centre into the park and then increases in the suburban area. The determination of sky view-factors in the park represents a special problem because of the partial transmission through the
I. ELIASSON
390
‘c I
City
I
centre
I
Park
Suburban
lo-:’
. ... 0.’ ..;
..‘...’
. ...
??
??
??
...* ‘...
. ,,...
1SVF
area
12. ..‘.... .. . . . . . .. .. . .
.....
-1 -0.8 -0.6
0
1 .... ...
Surface
-
Air
----
Air 0.2 m
2
3
1
5
6
7 .
8 km
SVF
2m
Fig. 10. Automobile traverse from the city centre through a large park and into the suburban districts, showing the variation in temperature and sky view-factor vs distance from the city centre.
The measurement point at 3.8 km is surrounded by trees which are shown as a semitransparent zone in the fish-eye photograph. The analysis of the photograph was made in two steps, the first for the border between open sky and the semitransparent zone and the second for the border to the totally obstructed horizon. For this measurement point, a transmission factor of 75% was used, which gave a SVF of 0.75. The general pattern with distance from the city centre is a decreasing air temperature and an increasing sky view-factor. However, a closer examination of Fig. 10 reveals that sites of equal SVF-value show different air temperatures. For example, at the distances 0.8,4.2 and 7.6 km the SVF are 0.88,0.92 and 0.95, respectively, which implies a more or less equal air temperature. However, the corresponding temperatures are 7.3, 4.5 and 5.o”C. These results show that sites of similar density may show different air temperature if the sites are located in different urban regions. The conclusion is that the sky view-factor alone is inadequate to explain the air temperature variation from the city centre to the suburban areas as this variation is governed by more complex factors. The lower temperature in the suburban areas could be explained, for example, by advection from the rural areas (Eliasson and Holmer, 1990). Urban-rural temperature traverses carried out during the case studies discussed in this paper show a well-developed urban heat island of magnitude 5°C for both surface and air. Also, comparisons of data recorded at the permanent stations in the central area and at the airport 9 km northwest of the city show an urban heat island (mean values) ranging from 3.5”C in winter and 6°C in summer (Eliasson, 1994). Urban-rural traverses carried out in Gijteborg (unpublished data) show how the air temperatures at both 0.2 and 2.0 m successively decrease from the central city to the rural areas in all points of the compass. A warm-air central core is discernible, but is not very vegetation.
distinct. The air temperature pattern is very much influenced by land use, landscape morphology, topography and distance to the sea. The surface temperature profiles, on the other hand, are characterized by a distinct central core (approximately 3 x 6 km). The temperature decreases rapidly at the margin of this warm-surface core and in the suburbs the temperature is nearly constant.
5. DISCUSSION AND CONCLUSION
This paper shows that air temperature differences at night are small within the central city of Giiteborg. The difference in the greatest mean air temperatures between the canyon and the open area is less than 3°C and the frequency of night hours with AT, - 0 > 0.5”C are unexpectedly small. The frequency varied over the year with the highest frequencies occurring during the summer and the lowest during the winter. Years with a higher frequency of clear and calm nights will probably show higher frequencies of hours with a canyon-open area air temperature difference greater than 0.5”C. Available statistics for the period between 1931 and 1960 shows that the “normal number” of clear days during a year is 48.8 for Goteborg (Taesler, 1972) which is somewhat higher than during 1988-1990. Comparable studies of air temperature and geometry within a city are few; the results from Giiteborg, however, agree with those from the city of Malmo in the south of Sweden (Barring et al., 1985). Small air temperature variations, less than 1°C during the night within an urban canyon (SVF = 0.43) have been reported from Japan, although measurements were carried out during a hot summer day and night (Nakamura and Oke, 1988). Yoshida et al. (1990/1991) also reported a daily small horizontal and vertical air temperature difference, less than 0.3”C, within an urban canyon in summer.
Urban nocturnal temperatures, street geometry and land use Numerical models simulating the urban temperature/geometry relationship mainly predict the surface temperature. A simulation of nocturnal cooling of the surface temperature in the urban canopy under calm and cloudless conditions, made by Arnfield (1990), showed a canyon geometry capable of causing variations in surface heat island intensity within a city. Material differences between the walls and floors of the canyon and heated building interiors enhanced the differences among the cooling rates exhibited by different canyon geometries in the numerical experiments. Arnfield (19190,Experiment 2, p. 228) reported a surface temperature difference of 4°C (after 3 h cooling) between urban sites of different density (H/W ratio of 0.5 and 2.0, respectively). Comparable data from Giiteborg (winter automobile traverses about 3 h after sunset) showed similar results, i.e. the maximum surface temperature difference observed between urban sites of H/W = 0.5 and Hj W = 2.0 was 35°C. These two :;tudies clearly show variations in surface temperature between urban sites of different geometry. The results from a simulation of the effect of geometry on u.rban energy budgets (Todhunter, 1990), however, are not in complete correspondence with this, as it was shown that only a small percent of the daily variation in longwave emittance and only lO-15% of the daily variation in longwave irradiance were due to geometry. Instead, Todhunter (1990) found that the daily totals of net solar radiation, net radiation and turbulent sensible heat flux were particularly sensitive to urban geometry. There is, undoubtedly, an important difference between surface and a.ir thermal patterns in urban areas. The relationship between surface temperature and SVF was statistically significant for the city centre; this was expected, as the surface temperature is controlled by the surface energy balance. Oke and Maxwell (1975) argued that the surface loss of longwave radiation is balanced by an upward flux of stored heat under calm and cboudless weather conditions (summer). Arnfield (19’90) suggested that this may be equally applicable also during limited input of anthropogenic heat. Observed values of net long-wave radiation show a strong correlation to calculated values of sky view-factors at different locations within an urban canyon (Nunez, 1974). Holmer (1992) showed a good agreement between the SVF and the ratio of the outgoing radiation. and atmospheric counterradiation, respectively. The air temperature is, on the other hand, dependent upon the flux divergence in an air volume including that due to horizontal transport (Roth et al., 1989). Microadvection is most likely the reason for the uniform air temperature observed at night in the city of Giiteborg. Independent of cause and effect, the weak connection between geometry and air temperature in the central areas of Giiteborg suggests that the role of geometry should not be overestimated, at least not during the night. It might be argued that the structure
391
of the city centre of Goteborg, with its densely builtup areas interrupted by parks, open areas and water, is not homogeneous enough, and, compared to the large metropolises in the world (for example, New York, London and Tokyo) the structure of Giiteborg is quite different. However, most cities in Sweden and in Europe have a structure very similar to that of Giiteborg. Only mean values of the temperatures were used in this study in order to reflect the average situations. The results show that individual maximum values of temperature must be used with caution, as these might give an exaggerated picture of the temperature-geometry relationship. The major findings and conclusions from the study presented above may be summarized as follows. 1. The absence of great air temperature differences between the canyon and the open area throughout the 3-yr measuring period as well as the small air temperature variation found during case studies shows that urban geometry has a small influence on the air thermal pattern, at least in the city centre. 2. In spite of the uniform air temperature within the city centre the air temperature decreases from the city centre towards the rural areas during nights with light winds and clear skies. The warm central core consists of smaller heat islands separated from each other. It is also interesting to note that the air temperature difference observed between the large park and the city centre (4°C on an average) is of the same order as the average urban-rural air temperature difference of 3.5-6°C. 3. The study shows that the surface and air intraurban temperature pattern is very different. Thus, the use of infrared thermography for determination of the air temperature pattern within cities should be avoided. Acknowledgements-Financial support was received from the Swedish Council for Building Research. Thanks are due to Professor Sven Lindqvist and to Dr Bjiirn Holmer for giving advice during the study. Thanks also go to the reviewers of Urban Atmosphere for helpful suggestions on the manuscript.
REFERENCES Arnfield A. J. (1982) An approach to the estimation of the surface radiative properties and radiation budgets of cities. Phys. Geography 3,97-122.
Amfield A. J. (1990) Canyon geometry, the urban fabric and nocturnal cooling: a simulation approach. Phys. Geography 11, 209-239.
BIrring L., Mattsson J.O. and Lindqvist S. (1985) Canyon geometry, street temperatures and urban heat island in Malmii, Sweden. J. Climatol. 5, 433-444. Eliasson I. (1990/91) Urban geometry, surface temperature and air temperature. Energy Buildings 15, 141-145. Eliasson I. (1992) Infrared thermography and urban temperature patterns. ht. J. Remote Sensing 13, 869-879. Eliasson I. (1994) Urban-suburban-rural air temperature differences related to street geometry. Phys. Geography 15, l-22.
392
I. ELIASSON
Eliasson I. and Holmer B. (1990) Urban heat island circulation in Giiteborg, Sweden. Theoret. Appl. Climatol. 42, 187-196. Goldreich Y. (1985) The structure of the ground-level heat island in a central business district. J. Climate Appl. Met. 24, 1237-1244. Gustavsson T. and Bogren J. (1991) Infrared thermography in applied road climatological studies. Int. J. Remote Sensing 12, 1811-1828. Holmer B. (1992) A simple operative method for determination of sky view-factors in complex urban canyons from fish-eye photographs. Met. Zeit. Neue Folge 1, 236-239. Lindqvist S. (1987) Sensing road surface temperatures with infrared techniques. Guni Rapport 23. University of GBteborg, Department of Physical Geography, 22 pp. Lindqvist S. (1992) Local climatological modelling for road stretches and urban areas. Geograjska Annaler 14A, 265-274. Nakamura Y. and Oke T. R. (1988) Wind, temperature and stability conditions in an east-west oriented urban canyon. Atmospheric Environment 22, 2691-2700. Nunez M. (1974) The energy balance of an urban canvas. Ph.D. Thesis, Univ. Brit.Columbia, Vancouver. Nunez M. and Oke T. R. (19771 The enerev balance of an urban canyon. J. Appl. Met. i6, 11-19. -I Oke T. R. (1981) Canyon geometry and the nocturnal urban heat island: Comparison of scale model and field observations. J. Climatol. 1, 237-254. Oke T. R. (1987) Boundary Layer Climates, 435 pp. Methuen, London and New York. Oke T. R and Maxwell G. B. (1975) Urban heat island dynamics in Montreal and Vancouver. Atmospheric Enuironment 9, 191-200.
Oke T. R., Johnson G. T., Steyn D. G. and Watson I. D. (1991) Simulation of surface heat islands under ideal conditions at night. 2. Diagnosis of causation. BoundaryLayer Met. 56, 339-358. Park H.-S. (1987) City size and urban heat island intensity for Japanese and Korean cities. Geograph. Rev. Japan 60, 238-250. Roth M., Oke T. R. and Emery W. J. (1989) Satellite-derived urban heat islands from three coastal cities and the utility of such data in urban climatology. Int. J. Remote Sensing 10, 1699-1720. Steyn D. G. (1980) The calculation of view-factors from fisheye lens photographs. Atmos. Ocean l&254-258. Stoll M. J. and Braze1 A. J. (1992) Surface-air temperature relationships in the urban environments of Phoenix, Arizona, Phys. Geography 13, 160-179. Taesler R. (1972) Himatdata f% Suerige. 672 pp. K L Beckmans, Tryckerier AB. Todhunter P. E. (1990) Microclimatic variations attributable to urban-canyon asymmetry and orientation. Phys. Geography 11, 131-141. Tuller S. E. (1973) Microclimatic variations in a downtown urban environment. Geograjiska Annaler 54A, 123-134. Watson I. D. and Johnson G. T. (1987) Graphical estimation of sky view-factors in urban environments. J. Climatol. 7, 193-197. Yamashita S. (1988) Some studies of heat island in Japan with special emphasis of the climatological aspects. Geograph. Rev. Japan 61, l-13. Yoshida A., Tominaga K. and Watatani S. (1990/1991) Field measurements on energy balance of an urban canyon in the summer season. Energy Buildings 1516, 417-423.
Pergamon
1352-2310(95)00033-X
URBAN NOCTURNAL GEOMETRY
TEMPERATURES, AND LAND USE
STREET
INGEGliRD ELIASSON Department of Physical Geography, Earth Sciences Centre, University of Giiteborg, S-413 81 Giiteborg, Sweden (First received 15 November 1992 and
in final form 10 December 1994)
Abstract-The climate in the city of GBteborg, Sweden, was investigated for a three year period. In this paper the nocturnal temperature distribution is analysed in relation to differences in street geometry and land use. The seasonal and monthly air temperature difference between a street canyon and a nearby open area, as well as case studies of the vertical and horizontal temperature distribution within and between different urban units and urban districts are discussed. In spite of a good relationship between the local surface temperature and the sky view-factor, both continuous measurements and case studies show small variations in air temperature within the city centre. Air temperature variations between urban districts of different land use are greater. The average horizontal temperature decrease of 4°C in the transition zone from the city centre to a large park southwest of the centre are, in fact, of the same order as the average urban-rural air temperature difference. Key word index: City climate, urban heat island, street canyon, urban structure, sky view-factor.
1.
and Nakamura and Oke (1988). The urban temperature pattern has also been simulated in hardware canyon models (e.g. Oke, 1981) and in numerical urban canyon models (e.g. Arnfield, 1982, 1990). An important objection to many of the existing studies of urban temperatures is the lack of a clear distinction between surface and air temperature. For example, model studies of surface temperature are compared with field studies of air temperature and the urban (screen level) heat island is often inferred from satellite or airborne ground radiation data. Field studies of both surface and air temperature have shown that in spite of the fact that the street surface is influenced by the canyon geometry, the local standard level air temperature is governed by more complex and regional factors (BPrring et al., 1985). The lack of simple coupling between surface and air temperatures in the urban system has been discussed by e.g. Goldreich (1985) and Roth et al. (1989). Recent studies of the surface/air temperature relationship have shown the importance of microadvection in the urban canopy layer (e.g. Roth et al., 1989; Stoll and Brazel, 1992). The aim of the present paper is to analyse the temperature climate at sites with different geometries and land use within the city. Great importance is attached to separating the surface and air temperatures. In order to elucidate the geometric effects on the sky view-factor at street level and, hence, on the cooling rate by net longwave radiation exchange, only nocturnal temperature data are considered in this
INTRODUCTION
With special interest focused on temperature variations related to difrerences in street geometry, or land use, the present parper analyses 3yr of temperature data from a high lartitude city. Urban geometry has for a long time been identified as an important fal:tor in explaining the urban heat island. However, most studies have been made on the local scale, which means that the urban-rural temperature difference (Al-,-,) has been compared with the geometry of city centres. Oke (1981) showed the relationship between the maximum heat island intensity and the sky view-factor (SVF) for some Australian, European and North American cities, a relation which also proved to be valid for Japanese and Korean cities (Park, 1!)87). Urban-rural temperature differences are of course of interest for comparison between cities, but might, as noted by Oke 1[1981), be difficult to use within a single city. A good knowledge of the influence of street geometry on temperature over very short distances, within cities, would be of great value, not least for urban planning. Field measurements of the influence of urban geometry on temperature on the microscale, i.e. within a city, are sparse. Most of them are case studies, with a measuring period of one or a few days only (e.g. Tuller, 1973; Blrring et al., 1985; Yamashita, 1988). Measurements within separate street canyons have been made by, for example, Nunez and Oke (1977) 379
I. ELIASSON
380
paper. The analysis is based on a measuring period of 3 yr, which includes continuous measurements at two climatic stations and case studies in the cold season by three different methods. The following data are discussed in the paper: (a) the seasonal and monthly air temperature differences between a street canyon and a nearby open area, (b) the vertical and horizontal air and surface temperature variation in a street canyon, (c) the horizontal air and surface temperature pattern in a central city area, (d) horizontal air and surface temperature gradients between different urban districts.
2. THE STRUCTURE
OF GGTEBORG
Gateborg is a city of approximately half-a-million inhabitants. The city is situated on the Swedish west coast in a fissure valley landscape. The central areas of the city lie on the southern shore of the Giita river. With reference to the urban structure, the inner city could be divided into two parts (Fig. 1). (A) The northern part is the oldest part of the city. It is located between the river and the canals. In this area, there is a mixture of old and new buildings, but the old dense structure remains. The streets are, with some exceptions, oriented in an ENE-WSW and NNW-SSE direction. The building heights vary between 10 and 15 m and the average street width is 7 m.
THE NORTH CITY SVF OF CANYONS THE SVF
SOUTH CITY 0~ CANYONS
0.25-045
0.5-0.9
,:. OPEN
AREA
PERMANENT ??
POINT
0
Fig. 1. Map of central GBteborg.
STATION
OF MEASUREMENT
500
m
.
Urban nocturnal temperatures, street geometry and land use
The sky view-factors (SVF) of the canyons vary between 0.25 and 0.45. (B) The southern part, south of the canals, is a part of the city which has a more heterogeneous geometry. Some streets in this area are quite wide (45m) but others are narrower, about 13m. The heights of the buildings in this part of the city vary between 5 and 25m but the average height interval is 17-22m. The SVF of the canyons varies between 0.5 and 0.9 and the streets are, with salme exceptions, oriented in a N-S and E-W, or a NW-SE and NE-SW pattern. To the northeast is a large open area. Small parks are scattered within this southern part, which is bordered by a large park in the southwest.
3. METHODS 3.1. Climatic stations Two climatic stations were set up in the central city area. One was sited in a street canyon which is oriented with its long axis in a northwest-southeast direction, and the other station was located in a large open area, about 400m away from the canyon (Fig 1). Continuous measurements of temperature and wind were carried out during a 3-yr period at these two stations. The following is a description of the station sites and instrumentation. The street canyon is formed by symmetrical buildings 120 m long and 20 m high. The width of the street is 14 m and thus the height/width (H/W) ratio is 1.4 and the sky viewfactor is 0.5. To the northwest, the canyon is interrupted by a cross street, but continues in the next block. To the southeast, the canyon ends; in a T-junction. The buildings in the canyon are of brick and are used for housing. The east side of the canyon consists of a rather new building and, on the west side, the buildings are older but renovated. Figure 2 shows the instrumentation in the canyon. The air temperature was measured at four points within the canyon. For practical reasons, these points were located between 4 and 10 m above the street surface. The wind speed and direction were
\
\ L.
--
T,W -\
---
I’
22/12
\
T= Temp.
\
W=Wind
\
\
XT
20n
\
\ XT +
I
XT
2216
I WEST
XT
\
I--
14 m-
EAST
Fig. 2. Instrumentation in the northwest-southeast oriented street canyon and the penetration of the solar radiation on 22 June and 22 December at 1400 h, when the solar azimuth angle is perpendicular to the orientation of the canyon. T= temperature, W = wind speed and direction.
381
monitored within the canyon at a point 4 m away from the east wall and 5 m above the street surface. On the roof of the east building, temperature and wind (speed and direction) were monitored at 2 m above the roof surface. The penetration of the solar radiation into the canyon during summer and winter at 1400 h when the solar azimuth angle is perpendicular to the orientation of the canyon is also marked in Fig. 2. On 22 June solar altitude is 48”; thus, at this time almost all of the east wall is sunlit. On 22 December owing to the low solar altitude (3”),only the uppermost 2 m of the east wall is sunlit. The open area is approximately 250 m wide and 400 m long. The area is mainly covered by gravel and is used for sports. The area is crossed by a bicycle/foot path covered with asphalt. Air temperature was measured at 2 and 10m above the surface and the wind (speed and direction) was monitored at 5 m and 10 m above the surface. At both stations the temperature was recorded utilizing wire-wound resistance thermometers, Pt 100 R and Ni 1000 Q, protected by radiation shields. The wind speed and direction were recorded by Vaisala cup anemometers (VAAlS) and wind vanes (VAWlS). All sensors were calibrated in a climatic chamber before and after the measuring period and in situ at regular intervals during the measurements. Hourly mean and lO-min mean values were recorded with a computer. 3.2. Case studies Case studies of the horizontal and vertical urban temperature pattern were made during weather conditions with a wind speed less than 2 m/s and cloud cover less than 2 octas, utilizing automobile traverses, a mobile datalogger system and an infrared image system. 1. Automobile traverses were carried out to study the horizontal temperature pattern within the city. The vehicle used was equipped with a computer which stored measured data at every 5 m. The exact location of sites with certain interest for the analysis (e.g. canyons and intersections) were also stored in the computer. During the measurements, the speed of the vehicle did not exceed 50 km h-l. The surface temperature was measured with a Barnes precision radiation thermometer mounted on the vehicle. The air temperature was monitored at two levels, 0.2 and 2.0 m, utilizing wirewound resistance thermometers. 2. The mobile datalogger system was used to measure horizontal and vertical temperature profiles along street canyons and street intersections. The advantage of the system is that the datalogger and the set of instruments easily can be changed or relocated. The thermistors recording air temperature were equipped with simple radiation shields, and a heat-conducting paste was applied to thermistors attached to the surface of the street. The wind was recorded with a cup anemometer (Thies). In general, the measurements were carried out along selected profiles during a few hours up to a few days. In some case studies, two or three dataloggers were used for simultaneous measurements at different urban sites. 3. The infrared thermography system (Agema 870 system) was used to record the surface temperature in horizontal profiles through street canyons and street intersections. The system consists of a scanner, sensitive to radiation between 2 and 5 pm, a display unit and a video tape recorder. The scanner was carried by hand, while a box containing the display unit and video tape recorder was carried on a luggage carrier. The different parts of the canyon were identified by aluminium strips at selected points on the.street surface. Due to the low emissivity of aluminium foil these strips appeared as cold spots in the thermal image. The analyses were made with specially designed software which includes correction functions for the prevailing measuring conditions. Further descriptions of the system are given by Lindqvist (1987) and Eliasson (1992).
I. ELIASSON
382 3.3. Street geometry
The street geometry is described by the sky view-factor (SVF), which was calculated at the street surface on the midpoint of the urban unit (canyon, intersection or open area) under study. In a first phase the sky view-factor of 50 uniform street canyons was calculated by a graphical method (Watson and Johnson, 1987). This method is easy to use as it ‘only requires data fo; the difference in the azi&h angles and data for the two elevation angles of the ends of the buildings. A clinometer, with a precision of 0.5”, was used for measuring the elevation angles. The graphical method is, however, not suitable for street intersections, squares and non-uniform canyons and thus in a second phase a fish-eye photograph method proposed by Holmer (1992) was used as a complement to the graphical method. The fundamental theory is that of Steyn (1980), but Holmer has simplified the calculation by digitizing the border between sky and wall in the photograph. In this study, 18 street intersections and open areas and 15 canyons were photographed from the surface level, in the midpoint of the urban unit, with a fisheye lens (Nikkor 8 mm,f/2.8, picture angle 180”).The projection formula of the lens is almost equidistant and thus an approximately linear relation exists between the zenith angle and the distance of point from image centre. The photographs were enlarged to a radius of 1Ocm and finally digitized for every 2” sector. The number of annuli chosen was 100. The two methods were compared at 10 street canyons and the calculated values showed a good correspondence.
4. RESULTS
4.1. Canyon/open area, seasonal and monthly air temperature differences The nocturnal air temperature differences between the street canyon and the open area (permanent stations in Fig. 1) have been analysed for the 3-yr measuring period. Hourly mean values were used in the analysis and the frequency of nocturnal hours with a canyon-open area temperature difference greater than OYC (AT,-, > O.YC) were calculated. The limit of OYC was chosen, as the specifications from the manufacturers, and the calibration procedure described above, have shown that the variation among the temperature probes is well below 0S”C. The “canyon temperature value” used in the analysis is a mean value of the four measurements inside the canyon (Fig. 2) as the temperature variation within the canyon was small, see Section 4.2. By analogy with the canyon data, the “open area temperature value” used is a mean value of the measurements at 2 and 10 m. Owing to the high latitude (N57”42’) of Giiteborg, the length of the night varies from 5 h in the middle of June to 16 h in December. With regard to this great variation the data were first divided into data sets of 10d. For each 10-d period the percentage of night hours with a canyon-open area temperature difference greater than 0.5”C were calculated. Secondly, the 10-d periods were arranged into monthly and seasonal data sets. In Fig. 3, the seasonal frequency of night hours with a canyon-open area temperature difference of > 0.5 or < - 0.5 are shown for 3 yr: 1988, 1989 and 1990.
The four seasons represent the following months: spring (March, April and May), summer (June, July, August), autumn (September, October, November) and winter (December, January and February). The diagrams show a marked variation between the different seasons. The highest percentages occur in the summer season; in 1988, on nearly all of the night hours, the AT,-, was greater than 0.5”C, and on 40% of the night hours the temperature difference was greater than 1°C. On the other hand, in the winter season a temperature difference of 0.5”C occurred on only l&15% of the total night hours. The spring and autumn are in an intermediate position with about 40% of the total night hours having a temperature difference greater than 0.5”C. However, it is important to note that the greatest variation in temperature between the two urban sites is similar for all seasons, that is, maximum AT,-, is about 2.7”C. The frequency of hours when the air temperature is higher at the open area than in the canyon (AT,-, < - O.S’C) is very low in all seasons. The grouping of data into seasons is convenient for a compact presentation of the 3-yr data set, but it might hide the variation between the different months in each season. Figure 4 shows a monthly classification of the data of 1 yr. It is evident from the diagrams that a monthly variation within each season exists. The highest percentages occur in May, June and July and the lowest percentages were recorded in January, February and March. The greatest temperature differences recorded were 2.5”C, in July 1989 and, 2.9”C, in December 1988. It must be pointed out that the diagrams shown in Figs 3 and 4 are a result of an analysis of all night hours during a 3-yr measuring period and thus cover all types of weather situations. Clear and calm weather favours high-temperature differences. The frequency of AT,-, > 0.5”C is undoubtedly dependent on the weather conditions, i.e. the occurrence of clear and calm weather. The monthly frequency of clear and calm nights and clear 24-h periods for the period September 1988 to August 1989 are shown in comparison with the percentage of AT,-, > 0.5”C (results from Fig. 4) in Fig. 5. Hourly data from the synoptic weather station at SIve airport, located 9 km NW of the city centre, formed the database from which nights with light winds and clear skies were selected. On a clear, calm night the sum of the cloud amount observed from 1900 h until 0700 h was required to be less than 23% of the maximum amount possible and the wind speed less than 2 m s- ‘. This definition follows the selection rules used in meteorological statistics, published by the Swedish Meteorological and Hydrological Institute (Taesler, 1972). During a clear 24-h period, the sum of the cloud amount should correspondingly be less than 23% of the maximum amount possible. Two peaks of clear weather, one in October, November and December and the other in May, June and July are recognised in
383
Urban nocturnal temperatures, street geometry and land use SPRING h ("1.1
1'3RR
h lo/.1
1989
h (O/o)
100
100
80
80
60
60
. ATc_o( C)
c-o.5
SUMMER h I'/.) 100
1990
h I%)
1!388
100
80
80
60
60
LO
. I-
100 1
1
1.
I
h I'/.)
1989
i
LO -
20
20 -
1990
WMT-TTTO ' ' ' ' ' ' :'3'
.
c-05
AT,_,l'Cl
AT,_,1Cl
AUTUMN hl'l.1
I'388
h ('1.)
1989
h P/o)
100
100
80 i
80
80
60
60
60
100
I990
WINTER h ("I.)
h ("I.1
h ('/.I
1001
100
100
80
80
60 r -
60
1989
I990
LO I
Fig. 3. Seasonal frequencies of night hours with a canyon-open area temperature difference greater than OST, or less than - 0.5”C for 1988, 1989 and 1990. Vertical axis shows the percentage of the total night hours(h) in %. Horizontal axis shows the temperature difference (AT,_,) in “C. Positive values indicate the canyon air to be warmer.
Fig. 5 and the frelquency of AT,-, > OS’C corresponds to the occurrence of clear and calm weather. However, in spite of similar values of clear weather in the percentages of summer and late autumn
AT,-, > O.S”C found during May, June and July are much higher than during October, November and December. This discrepancy is probably due to the higher sun elevation in the summer as shown in Fig. 2.
I. ELIASSON
384 AUTUMN h I’/. )
SEP.
1988
OCT.
h I’/.)
198E
80
80 I
80
60
60
LO
NOV.
1988
FEB.
1989
LO
.
20i
I
<- 0.5
h 1%) 100
100
100
2 . AT,.,1 C)
>3
. AT,.,( C)
c-o.5
<
WINTER DEC. 1988
h I’/.)
JAN.
h I’/.)
1989
h I*/.) 100
100
100
80
80
80
60
60
60
LO
LO
LO
zo-
zo-
c-o.5
AT c-0(C)
SPRING h I% )
MAR.
1989
h (% )
APR.
1989
80
80
60
60
h I %I
MAY
1989
100,
1001
1001
SUMMER h f”/o, 100
1
JUN.1989
h (“/.I
JU.L. 1989
100
80-
h(‘/e)
AUG.
1989
too&360. 40-
Fig. 4. Monthly frequencies of night hours with a canyon-open area temperature difference greater than 0.5”C, or less than - O.S”C, for 1 yr (September 1988 to August 1989). Vertical axis shows the percentage of the total night hours (h) in %. Horizontal axis shows the temperature difference (AT,_,,) in “C. Positive values indicate the canyon air to be warmer.
This gives a greater penetration of solar radiation in the canyon and, thus, a greater daytime uptake by the sunlit walls. 4.2. Temperature variations within a canyon Additional sensors were set up in the northwest southeast-oriented canyon (Figs 1 and 2) on several
occasions during the 3-yr measuring period. Usually, the air temperature variation within the canyon was less than OYC. The example shown in Fig. 6, a September night with light winds and no clouds, is one of the studies with the greatest temperature variation. The diagrams in Fig. 6 show the climate condition in the canyon every 2 h from 1900 to 0500 h. Despite
Urban nocturnal temperatures, street geometry and land use
SONDJFMAMJJA 1988 . Y
0
1989 Number of clear Z&-hour pernds Number of clear ?nd calm ntghts ‘/. ATC_O > 0,5 c
Fig. 5. The monthly frequency of clear and calm nights and clear 24-h periolds for the period September 1988 to August 1989 in comparison with the percentage of AT,_, > 0.5”C as shown in Fig. 4.
calm conditions, a canyon-roof temperature difference of l.o”C and a canyon-open area temperature difference of 1.5”C in the early evening, a clear temper-
ature pattern within the canyon was not obvious until 0100 h. At that time the air was warmer near the walls than in the canyon. However, the difference was still only 0S”C. The cold centre remained through the night and early morning until the sun rose. Several case studies were also carried out in different canyon/intersection transition zones with the mobile datalogger system. Examples of representative case studies of the horizontal surface and air temperature variation in the canyon/intersection transition zone are shown in Fig. 7. Diagram 7A shows a surface and an air temperature profile from the middle of the canyon (SVF = 0.5) to the intersection (SVF = 0.7), measured by the mobile datalogger system. The measurements were made during an October evening in 1988. The air ‘temperature variation was small throughout the whlole evening. The diagram shows the situation of 2300 h when the difference was less than 0.2”C. The wind at that time was 0.2 m s- ’ in the canyon and 1.0 m s- ’ in the intersection. The surface temperature was rather uniform in the canyon but an abrupt change occurred between the canyon edge and 16 m inside the canyon and the surface temperature was about 2°C lower at the intersection. The profile, however, is made up of separate measuring points and the actual temperature between the points is unknown. Spotwise information could be avoided by using the infrared image isystem as this method enables the recording of a surface area. Diagram 7B shows the surface temperature in the canyon measured with the infrared scanner during an October evening in 1990 at 2000 h. The distance between the surface and the scanner was 1 m and the scanned surface area was thus 0.5 m2. The mean surface temperature of the recorded surface area was calculated from the infrared
385
images using a box method (Eliasson, 1992). The results show a uniform street surface temperature in the canyon, until 10 m before the canyon edge. At this distance the temperature starts to fall rapidly, the gradient is very steep and at the edge the temperature has decreased by 1.8”C. After that, the decrease is reduced and in the middle of the street intersection, 10 m from the edge, the temperature is 2°C lower as compared to the canyon. Diagram 7C illustrates the cooling rates in the middle of the canyon and at the intersection during a September evening in 1989. The temperatures at the 0.1 and 1.Om levels were more or less the same at both sites during the evening. Four hours after sunset, the difference between surface and air temperature was 2S”C in the canyon and 1.5”C in the intersection. The measurements started at 1800 h, 1 h and 40 min before sunset. At that time, the surface cooling had already started. Four hours after sunset the decrease in temperature was 1°C h-’ for the air, 0.75”C h-l for the intersection surface and 0.62”C h- ’for the canyon surface. These diagrams clearly show large surface temperature differences but small air temperature variations between adjacent urban units (see also Eliasson, 1990/91, 1992). The measurements also show the air temperature to be more or less uniform above 10 cm. 4.3. Temperature patterns in the central city area In order to analyse the horizontal temperature distribution within the inner city, in relation to the urban structure, automobile traverses were made through the different urban units (point of measurements in Fig. 1). The SVF of these urban units range from 0.33 to 1.0. The traverses were made in the late evening, 3 h or more after sunset and each traverse was standardized to one point of time. The maximum time of a traverse was 1 h. During the traverses the wind speed at the open area station was less than 2 ms-’ and, at the synoptic weather station, SHve airport, the mean values of the wind speed and cloud cover from the 6 h preceding the traverse were less than 3 m s- ’ and 2 octas, respectively. Figure 8 shows the results of one traverse from the northern part of the city (see inset map). The analysis was made in 3 steps for all urban units marked in Fig. 8. Firstly, the selected unit, canyon, street intersection or open area were identified on the diagram. Secondly, the position of the SVF measurements, the midpoint of the urban unit, was fixed and, thirdly, a mean value of the recorded data in this middle sector, about 20 m for canyons, was calculated. The canyons, the intersections, a park and an open area, as well as the sky view-factors of these urban sites are indicated in Fig. 8. It is evident, from the diagram, how the surface temperature is influenced by the city structure, while the air temperature is not. Minor surface irregularities were easily identified as scattered data in the diagrams (for example, Fig. 8, area 5) and were excluded from the analysis. Major surface variations, such as differences in bed material
I. ELIASSON
386
‘3.0 ‘c !.3 mls
1933 r-
A’,_,:
I .5
ATC-T
I.0 r
0 m/s
8.9 ‘ c 3 m/s
2300 AT,.,.
1.7
AT,_,=
‘,3
IC, 6
'0.L
52 ox3
xi 9:l
Ii.hZ/ 9.:
E
x ,
.:;‘, ~’
cm/s
10,l ,3 ‘c 7115
6.0
‘c
Omls
0500
0300
AT c-o= I,7
AT c_o=2.5
ATc_r-
AT c-,
I,L
= I,2
?4
W
17.2 3 m/s
7.$ x I.4
I \
“/ 7 c X-id 7.0 x 7,l
17.; * 6.9
E
G m/s
* 7,3
Fig. 6. The vertical air temperature distribution every 2 h from 1900 to 0500 in a canyon with a sky view-factor of 0.5. The measurements were carried out during a night with light winds and no clouds. Inside the canyon the wind speed was zero during all night The values of the air temperature and the wind speed at the roof of the canyon are shown at the east top of the canyon symbol. AT,_, = Temperature difference between the canyon and the nearby open area, AT,_, = temperature difference between the canyon and the roof. See also Fig. 2.
and in heated street sections, may of course influence the temperature recording. Gustavsson and Bogren (1991) discussed the influence of road construction on road surface temperature. On a test road, they had found a nocturnal maximum difference of 1S”C between beds consisting of blast furnace slag and those consisting of gravel. Such extreme differences in bed material, however, are not found in central Goteborg. Street intersections and canyons affected by tram lines, heat or moisture, have been excluded.
A regression analysis and hypothesis test with the null hypothesis that the temperature is not related to the geometry were carried out on the data recorded by the traverses (Table 1). The temperatures were presumed to follow a Gaussian normal distribution. The results presented in Table 1 shows that the probability values (Prob > t) for the surface temperature and SVF relation are significant on the 5% level for all occasions. This means that the surface temperature is correlated to the sky view-factor.
387
Urban nocturnal temperatures, street geometry and land use
surface
0
lo
20
30
LO 50
60
70
60
temperature
air 0.15m
”
air
”
1.0 m
m
surface
0
10
20 30
LO 50 60
70
80
temperature
90 m
16 15 lk 13 12
11 10 gt
I 18
-.-
__..-
I
19
20
canyon N *
I
I
I
21
22
23
intersection
I
2L HOUR
surface
temp.
surface temp. 0.1 m air *I
_--_-
,/
0.1 mair
*
,.Omair
. . . . . ..I.
”
l.Omair
”
,,
Fig. 7. Hori.zontal temperature variations within a canyon during clear and calm evenings and nights. The map at the top of the figure refers to all diagrams but the distance scale refers only to diagram A and B. (A) Horizontal profiles of surface temperature and air temperature at two levels measured by the mobile datalogger system; (B) horizontal profile of surface temperature measured by the infrared image system; (C) S:ooling rates in the canyon and in the adjoining intersection (mobile datalogger system).
On the other hanId, Table 1 shows high probability values for both the 0.2 and 2.0 m level (except for the 2 m level on 4 October 1989). This means that the air temperature could not be statistically related to the sky view-factor. The air temperature/sky view-factor
relation was further analysed by calculating the power of the hypothesis test. In this calculation a non-central t-distribution with n - 2 degrees of freedom was used. The presumption for the calculation is that an air temperature difference less than 1°C within the city
388
I. ELIASSON A
B
I
I 1
1
C
3
2
B
A
L
5
6
7
PARK
~0.33-~X~O.3L--fX~O.3L+Y&O.33(Xt_O334
IXj--O.LO+0.85 065
.
'c I
b
100
. .....
SURFACE
---
AIR
t-1
STREET
CANYON
IXI
STREET
INTERSECTION
0.33
SVF
A
LOO
560
m
TEMPERATURE
TEMPERATURE
OF
360
200
8
THE
URBAN
IDENTIFICATION
“,:;;
m
BUILDINGS
(:::
PARK
UNIT MARK
O-
200m
J
Fig. 8. Automobile traverse in the northern part of the city centre. Profiles of surface temperatures and air temperatures at 0.2 and 2.0 m.
Table 1. The results of regression analysis and hypothesis test including calculation of the “power of the test” on the relationship between temperature (surface and air) and sky view-factor within Giiteborg. Data from automobile traverses under clear and calm weather conditions in the cold season Date
18 November 1987
12 December 1988
4 October 1989
7 December 1989
1 February 1991
Number of observations
centre is of minor importance.
Slope
Prob > t 5%
Significance of test
Power of test
16 16 16
0
0.2 2.0
- 3.4 -0.1 - 0.4
0.003 0.872 0.376
Yes No No
0.8 1.0
30 30 30
0 0.2 2.0
- 3.2 0.4 0.7
0.000 0.317 0.121
Yes No No
0.9 0.9
30 30 30 30 30 30 10 10
0 0.2 2.0
- 1.7 - 1.0 - 1.8
0.011 0.054 0.000
Yes No Yes
0 0.2 2.0 0 0.2 2.0 0 0.2 2.0
-
0.000 0.092 0.116
Yes No No
0.000 0.474 0.324 0.002 0.647 0.158
Yes No No Yes No No
10
25 March 1991
Level (m) above
12 12 12
The results shown in the last column of Table 1 indicate that if the air temperature difference between the most dense and most open area in Giiteborg (SVF interval 0.33-1.0) was greater than 1°C there would be a 40-100%
3.9 1.0 1.1 6.4 0.8 - 0.4 - 2.8 8:f
0.8
0.6 0.6 0.4 0.9 0.4 0.7
chance of finding it with the hypothesis test. Thus, the conclusion is that the hypothesis test is strong and that there is no evidence for a statistically significant relationship between the air temperature and sky view-factor in central Giiteborg.
Urban nocturnal temperatures, street geometry and land use In infrared images, from airborne recordings, large street intersections often appear as colder spots (Biirring et al., 1985; Eliasson, 1990/91, 1992). However, it is more uncertain as to what extent smaller open areas show a lower surface temperature than adjoining canyons. The results from the measurements with the mobile datalogger :system and infrared thermography system presented above clearly show differences in surface temperatures within the city. A closer examination of the air and surface temperatures at street intersections and in open areas was made with data from the automobile traverses. The intersections and/or open areas was combined with the adjoining canyon, giving 18 combinations altogether. Data from three occasions were used to calculate mean values of the temperature differences between street intersections and canyons (ATi-,). These mean values of ATi-, were then plotted vs the sky view-factor of the street intersection/open area (Fig. 9). The difference in surface temperatures is shown in Fig. 9A. A trend is distinguishable in the diagram, as higher SVF show
‘SURFACE
Al 2 1
389
larger negative values. The air temperature differences, 0.2 and 2.0 m, are close to zero for all SVF except for SVF = 1.0 which is about 1°C colder than the adjoining canyon (Fig. 9B). The great difference in air temperature between the areas with sky view-factors around 0.9 and the area with SVF = 1.0 could be explained by different size and by different surface material. The sites with a sky view-factor around 0.9 are open areas of approximately 80 m x 80 m, covered by stone and asphalt. The SVF = 1.0 is represented by the relatively large open area 250 m x400 m (discussed earlier in Section 4.1) which is covered by gravel and asphalt. Microadvection between the smaller open areas and their adjoining canyons is probably important to level out the air temperature difference on small open areas. On the other hand is the open area of SVF = 1.0, probably large enough to develop its own microclimate compared to the adjoining canyons. The difference in surface material is also important. Oke et al. (1991), for example, argues that differences in thermal admittance and street geometry are primary causes equal to one another in importance at least, for the surface heat island. According to Oke (1987) the thermal admittance (J m-2s-1’2 K-l) for sandy soils is 620 and for asphalt 1200 and stone 2220. This indicates that the open area of SVF = 1.0 which is covered with gravel has a tendency to cool more rapidly than asphalt-stone covered areas. 4.4. Temperature gradients between d#erent districts
0
0.2 ??
0.4
0.6
0.8
1 SVF
Surface
AIR
BJ
-3+--l--r 0
1 0.L
0.2
0
Air 0.2m
x
Air
I
I 0.6
I
I 0.8
I
I 1
I SVF
2.0m
Fig. 9. Relationship between the temperature difference between street intersection/open area and adjoining canyon (ATi_c in “C) and the sky view-factor of the intersection/open area (SVF): (A) surface temperature; (B) air temperature (0.2 and 2.0m). Data from automobile traverses.
All the temperature data discussed in the above sections were measured within the city centre, an area of approximately 4 km’ (Fig. 1). The results from these measurements show a small variation (< 3°C) in air temperature. Large differences in air temperature between builtup areas and a large park about 3 km southwest of the north central city (Fig. 1) have been reported by Lindqvist (1992). On some occasions the air temperature increased 6°C from 100 m inside the park to a point within the built up areas 150 m outside the park. However, more frequently, the air temperature gradient in the transition zone was 0.330).4”C per 100 m and the cool park climate could be traced about 1000 m outside the park (Lindqvist, 1992). The park is bordered in the north by the central built-up areas (Fig. 1) and in the south by suburban areas, mainly multi-storey buildings. Figure 10 is an example of a temperature traverse from the central areas through the large park and into the suburban areas during a clear and calm night in March. The diagram also shows the variation in building density (sky view-factor) from the city centre to the southern suburbs. The park is surrounded by warmer built-up areas as shown by the curves of 0.2 and 2.0 m air temperature in Fig. 10. The air temperature first decreases from the city centre into the park and then increases in the suburban area. The determination of sky view-factors in the park represents a special problem because of the partial transmission through the
I. ELIASSON
390
‘c I
City
I
centre
I
Park
Suburban
lo-:’
. ... 0.’ ..;
..‘...’
. ...
??
??
??
...* ‘...
. ,,...
1SVF
area
12. ..‘.... .. . . . . . .. .. . .
.....
-1 -0.8 -0.6
0
1 .... ...
Surface
-
Air
----
Air 0.2 m
2
3
1
5
6
7 .
8 km
SVF
2m
Fig. 10. Automobile traverse from the city centre through a large park and into the suburban districts, showing the variation in temperature and sky view-factor vs distance from the city centre.
The measurement point at 3.8 km is surrounded by trees which are shown as a semitransparent zone in the fish-eye photograph. The analysis of the photograph was made in two steps, the first for the border between open sky and the semitransparent zone and the second for the border to the totally obstructed horizon. For this measurement point, a transmission factor of 75% was used, which gave a SVF of 0.75. The general pattern with distance from the city centre is a decreasing air temperature and an increasing sky view-factor. However, a closer examination of Fig. 10 reveals that sites of equal SVF-value show different air temperatures. For example, at the distances 0.8,4.2 and 7.6 km the SVF are 0.88,0.92 and 0.95, respectively, which implies a more or less equal air temperature. However, the corresponding temperatures are 7.3, 4.5 and 5.o”C. These results show that sites of similar density may show different air temperature if the sites are located in different urban regions. The conclusion is that the sky view-factor alone is inadequate to explain the air temperature variation from the city centre to the suburban areas as this variation is governed by more complex factors. The lower temperature in the suburban areas could be explained, for example, by advection from the rural areas (Eliasson and Holmer, 1990). Urban-rural temperature traverses carried out during the case studies discussed in this paper show a well-developed urban heat island of magnitude 5°C for both surface and air. Also, comparisons of data recorded at the permanent stations in the central area and at the airport 9 km northwest of the city show an urban heat island (mean values) ranging from 3.5”C in winter and 6°C in summer (Eliasson, 1994). Urban-rural traverses carried out in Gijteborg (unpublished data) show how the air temperatures at both 0.2 and 2.0 m successively decrease from the central city to the rural areas in all points of the compass. A warm-air central core is discernible, but is not very vegetation.
distinct. The air temperature pattern is very much influenced by land use, landscape morphology, topography and distance to the sea. The surface temperature profiles, on the other hand, are characterized by a distinct central core (approximately 3 x 6 km). The temperature decreases rapidly at the margin of this warm-surface core and in the suburbs the temperature is nearly constant.
5. DISCUSSION AND CONCLUSION
This paper shows that air temperature differences at night are small within the central city of Giiteborg. The difference in the greatest mean air temperatures between the canyon and the open area is less than 3°C and the frequency of night hours with AT, - 0 > 0.5”C are unexpectedly small. The frequency varied over the year with the highest frequencies occurring during the summer and the lowest during the winter. Years with a higher frequency of clear and calm nights will probably show higher frequencies of hours with a canyon-open area air temperature difference greater than 0.5”C. Available statistics for the period between 1931 and 1960 shows that the “normal number” of clear days during a year is 48.8 for Goteborg (Taesler, 1972) which is somewhat higher than during 1988-1990. Comparable studies of air temperature and geometry within a city are few; the results from Giiteborg, however, agree with those from the city of Malmo in the south of Sweden (Barring et al., 1985). Small air temperature variations, less than 1°C during the night within an urban canyon (SVF = 0.43) have been reported from Japan, although measurements were carried out during a hot summer day and night (Nakamura and Oke, 1988). Yoshida et al. (1990/1991) also reported a daily small horizontal and vertical air temperature difference, less than 0.3”C, within an urban canyon in summer.
Urban nocturnal temperatures, street geometry and land use Numerical models simulating the urban temperature/geometry relationship mainly predict the surface temperature. A simulation of nocturnal cooling of the surface temperature in the urban canopy under calm and cloudless conditions, made by Arnfield (1990), showed a canyon geometry capable of causing variations in surface heat island intensity within a city. Material differences between the walls and floors of the canyon and heated building interiors enhanced the differences among the cooling rates exhibited by different canyon geometries in the numerical experiments. Arnfield (19190,Experiment 2, p. 228) reported a surface temperature difference of 4°C (after 3 h cooling) between urban sites of different density (H/W ratio of 0.5 and 2.0, respectively). Comparable data from Giiteborg (winter automobile traverses about 3 h after sunset) showed similar results, i.e. the maximum surface temperature difference observed between urban sites of H/W = 0.5 and Hj W = 2.0 was 35°C. These two :;tudies clearly show variations in surface temperature between urban sites of different geometry. The results from a simulation of the effect of geometry on u.rban energy budgets (Todhunter, 1990), however, are not in complete correspondence with this, as it was shown that only a small percent of the daily variation in longwave emittance and only lO-15% of the daily variation in longwave irradiance were due to geometry. Instead, Todhunter (1990) found that the daily totals of net solar radiation, net radiation and turbulent sensible heat flux were particularly sensitive to urban geometry. There is, undoubtedly, an important difference between surface and a.ir thermal patterns in urban areas. The relationship between surface temperature and SVF was statistically significant for the city centre; this was expected, as the surface temperature is controlled by the surface energy balance. Oke and Maxwell (1975) argued that the surface loss of longwave radiation is balanced by an upward flux of stored heat under calm and cboudless weather conditions (summer). Arnfield (19’90) suggested that this may be equally applicable also during limited input of anthropogenic heat. Observed values of net long-wave radiation show a strong correlation to calculated values of sky view-factors at different locations within an urban canyon (Nunez, 1974). Holmer (1992) showed a good agreement between the SVF and the ratio of the outgoing radiation. and atmospheric counterradiation, respectively. The air temperature is, on the other hand, dependent upon the flux divergence in an air volume including that due to horizontal transport (Roth et al., 1989). Microadvection is most likely the reason for the uniform air temperature observed at night in the city of Giiteborg. Independent of cause and effect, the weak connection between geometry and air temperature in the central areas of Giiteborg suggests that the role of geometry should not be overestimated, at least not during the night. It might be argued that the structure
391
of the city centre of Goteborg, with its densely builtup areas interrupted by parks, open areas and water, is not homogeneous enough, and, compared to the large metropolises in the world (for example, New York, London and Tokyo) the structure of Giiteborg is quite different. However, most cities in Sweden and in Europe have a structure very similar to that of Giiteborg. Only mean values of the temperatures were used in this study in order to reflect the average situations. The results show that individual maximum values of temperature must be used with caution, as these might give an exaggerated picture of the temperature-geometry relationship. The major findings and conclusions from the study presented above may be summarized as follows. 1. The absence of great air temperature differences between the canyon and the open area throughout the 3-yr measuring period as well as the small air temperature variation found during case studies shows that urban geometry has a small influence on the air thermal pattern, at least in the city centre. 2. In spite of the uniform air temperature within the city centre the air temperature decreases from the city centre towards the rural areas during nights with light winds and clear skies. The warm central core consists of smaller heat islands separated from each other. It is also interesting to note that the air temperature difference observed between the large park and the city centre (4°C on an average) is of the same order as the average urban-rural air temperature difference of 3.5-6°C. 3. The study shows that the surface and air intraurban temperature pattern is very different. Thus, the use of infrared thermography for determination of the air temperature pattern within cities should be avoided. Acknowledgements-Financial support was received from the Swedish Council for Building Research. Thanks are due to Professor Sven Lindqvist and to Dr Bjiirn Holmer for giving advice during the study. Thanks also go to the reviewers of Urban Atmosphere for helpful suggestions on the manuscript.
REFERENCES Arnfield A. J. (1982) An approach to the estimation of the surface radiative properties and radiation budgets of cities. Phys. Geography 3,97-122.
Amfield A. J. (1990) Canyon geometry, the urban fabric and nocturnal cooling: a simulation approach. Phys. Geography 11, 209-239.
BIrring L., Mattsson J.O. and Lindqvist S. (1985) Canyon geometry, street temperatures and urban heat island in Malmii, Sweden. J. Climatol. 5, 433-444. Eliasson I. (1990/91) Urban geometry, surface temperature and air temperature. Energy Buildings 15, 141-145. Eliasson I. (1992) Infrared thermography and urban temperature patterns. ht. J. Remote Sensing 13, 869-879. Eliasson I. (1994) Urban-suburban-rural air temperature differences related to street geometry. Phys. Geography 15, l-22.
392
I. ELIASSON
Eliasson I. and Holmer B. (1990) Urban heat island circulation in Giiteborg, Sweden. Theoret. Appl. Climatol. 42, 187-196. Goldreich Y. (1985) The structure of the ground-level heat island in a central business district. J. Climate Appl. Met. 24, 1237-1244. Gustavsson T. and Bogren J. (1991) Infrared thermography in applied road climatological studies. Int. J. Remote Sensing 12, 1811-1828. Holmer B. (1992) A simple operative method for determination of sky view-factors in complex urban canyons from fish-eye photographs. Met. Zeit. Neue Folge 1, 236-239. Lindqvist S. (1987) Sensing road surface temperatures with infrared techniques. Guni Rapport 23. University of GBteborg, Department of Physical Geography, 22 pp. Lindqvist S. (1992) Local climatological modelling for road stretches and urban areas. Geograjska Annaler 14A, 265-274. Nakamura Y. and Oke T. R. (1988) Wind, temperature and stability conditions in an east-west oriented urban canyon. Atmospheric Environment 22, 2691-2700. Nunez M. (1974) The energy balance of an urban canvas. Ph.D. Thesis, Univ. Brit.Columbia, Vancouver. Nunez M. and Oke T. R. (19771 The enerev balance of an urban canyon. J. Appl. Met. i6, 11-19. -I Oke T. R. (1981) Canyon geometry and the nocturnal urban heat island: Comparison of scale model and field observations. J. Climatol. 1, 237-254. Oke T. R. (1987) Boundary Layer Climates, 435 pp. Methuen, London and New York. Oke T. R and Maxwell G. B. (1975) Urban heat island dynamics in Montreal and Vancouver. Atmospheric Enuironment 9, 191-200.
Oke T. R., Johnson G. T., Steyn D. G. and Watson I. D. (1991) Simulation of surface heat islands under ideal conditions at night. 2. Diagnosis of causation. BoundaryLayer Met. 56, 339-358. Park H.-S. (1987) City size and urban heat island intensity for Japanese and Korean cities. Geograph. Rev. Japan 60, 238-250. Roth M., Oke T. R. and Emery W. J. (1989) Satellite-derived urban heat islands from three coastal cities and the utility of such data in urban climatology. Int. J. Remote Sensing 10, 1699-1720. Steyn D. G. (1980) The calculation of view-factors from fisheye lens photographs. Atmos. Ocean l&254-258. Stoll M. J. and Braze1 A. J. (1992) Surface-air temperature relationships in the urban environments of Phoenix, Arizona, Phys. Geography 13, 160-179. Taesler R. (1972) Himatdata f% Suerige. 672 pp. K L Beckmans, Tryckerier AB. Todhunter P. E. (1990) Microclimatic variations attributable to urban-canyon asymmetry and orientation. Phys. Geography 11, 131-141. Tuller S. E. (1973) Microclimatic variations in a downtown urban environment. Geograjiska Annaler 54A, 123-134. Watson I. D. and Johnson G. T. (1987) Graphical estimation of sky view-factors in urban environments. J. Climatol. 7, 193-197. Yamashita S. (1988) Some studies of heat island in Japan with special emphasis of the climatological aspects. Geograph. Rev. Japan 61, l-13. Yoshida A., Tominaga K. and Watatani S. (1990/1991) Field measurements on energy balance of an urban canyon in the summer season. Energy Buildings 1516, 417-423.