Urban Geometry, surface temperature and air temperature

Energy and Buildings, 15 - 16 (1990/91) 141 - 145

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Urban Geometry, Surface Temperature and Air Temperature INGEG~RD ELIASSON Department of Physical Geography, University of G6teborg, Reutersgatan 2C, S-41320 Go'teborg (Sweden)

ABSTRACT

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This paper presents a local climatological investigation of urban climate and urban structure carried out in G6teborg, Sweden. The surface and air temperature pattern are studied on a city scale and on a canyon scale. Data from ordinary weather stations and three specially designed weather stations are used. Recordings of temperature, humidity and wind are also made by an infrared thermography system (Thermovision 870), measurement cars and dataloggers. The geometry of the city is described by the sky view factor (SVF). In general the results show a lesser temperature variation than expected. The surface temperature varies a few degrees between different urban geometries, but the air temperature variation is very small. Further studies are planned to determine the temperature variation within the city and between the city and the suburbs.

INTRODUCTION In GSteborg, Sweden, u r b a n local climatological s u r v e y i n g for p l a n n i n g and building has b e e n c a r r i e d out d u r i n g a long time. This p a p e r p r e s e n t s some results from the p r e s e n t project, an i n v e s t i g a t i o n of u r b a n local climate and u r b a n s t r u c t u r e . T h e p r o j e c t s t a r t e d in 1987 a n d it is a c o n t i n u a t i o n of a n e a r l i e r p r o j e c t c a r r i e d o u t in MalmS, Sweden. The Malm5 p r o j e c t was a c o l l a b o r a t i o n w i t h the c l i m a t o l o g y g r o u p at the D e p a r t m e n t of P h y s i c a l G e o g r a p h y in Lund. T h e results from Malm6 indicated, a m o n g o t h e r things, t h a t the surface t e m p e r a t u r e p a t t e r n of the city is d e p e n d e n t on the s t r e e t g e o m e t r y and its r e g i o n a l d i s t r i b u t i o n w i t h i n the city. F i g u r e 1 was p r e s e n t e d by B~irring, Mattsson a n d L i n d q v i s t [1]. It shows the relationship b e t w e e n the s t r e e t surface t e m p e r a t u r e 0378-7788/91/$3.50

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PURPOSE, METHODS AND RESULTS T h e main p u r p o s e of the p r e s e n t investigation is: • to study the u r b a n g e o m e t r y and its relat i o n to the u r b a n surface t e m p e r a t u r e p a t t e r n ; • to s t u d y the influence of g e o m e t r y and surface t e m p e r a t u r e e x e r t e d on the u r b a n air temperature; • to try new t e c h n i q u e s and methods; • to w o r k out a model of u r b a n s t r u c t u r e and climate for use in u r b a n p l a n n i n g and design, and to test already-existing models. T h e climatic studies are m a d e in v a r i o u s u r b a n e n v i r o n m e n t s on a city scale and on a c a n y o n scale. W e a t h e r d a t a from s t a t i o n s perm a n e n t l y r u n in the G 5 t e b o r g a r e a a n d from t h r e e specially designed s t a t i o n s are used as a climatic base. T h e t h r e e s t a t i o n s A, B, and C are designed for c o n t i n u o u s r e c o r d i n g s of data. H o u r l y m e a n v a l u e s and t e n - m i n u t e m e a n v a l u e s are saved e v e r y hour. S t a t i o n s A a n d B are l o c a t e d in the city (Fig. 2); s t a t i o n A in a s t r e e t c a n y o n w i t h a sky view f a c t o r of 0.51 and s t a t i o n B on a sports g r o u n d a b o u t 500 m e t r e s from s t a t i o n A. © Elsevier Sequoia/Printed in The Netherlands

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At station A (Fig. 3) the wind, temperature and humidity are measured on and beneath wall surfaces, in different points inside the canyon and on the roof of one of the buildings. At stations B and C (Fig. 4) the temperature, wind and humidity are measured at 2, 5 and 10 metres. Station C is located in the western suburban area of G6teborg. The tower is placed on a yard of four two-storey buildings which are one unit of a series of buildings.

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Now the study is concentrated on three different parts. The first is a part study of the relation between the urban geometry, described by the sky view factor (SVF) and the urban surface and air temperatures. The surface temperature is studied by an infrared thermography system (Thermovision 870) from the air and from the ground. Thermovision 870 is a realtime thermal imaging system for temperature measurement and analysis of dynamic and static thermal patterns. The basic system consists of a scanner and display unit. The scanner records in the 2 - 5 #m wavelengths. A thermal image computer, based on an IBM PC with an associated software package, is used for the analysis of the thermal images. Figure 5 is an infrared image from an airborne recording made on November 18, 1987. In the Figure, warm surfaces are white and cold ones are black. Station A is located in the small street to the right. The surface temperature difference between streets of different width and between street canyons and street intersections can be seen in the image. Figure 6 shows the relation between the SVF of 58 street canyons and the surface temperature measured by the Thermovision 870 system

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from the air. F u r t h e r studies from the air are planned. The SVF of the s t r e e t c a n y o n s is c a l c u l a t e d a c c o r d i n g to a g r a p h i c a l m e t h o d p r e s e n t e d by W a t s o n and J o h n s o n [2]. The s u r f a c e t e m p e r a t u r e is also r e c o r d e d by m e a s u r e m e n t cars. R e c o r d i n g s of the air temp e r a t u r e at two levels, 0.2 m e t r e s and 2 metres, and the h u m i d i t y at 2 m e t r e s are made with the car. A precision r a d i o m e t e r m o u n t e d in the b o t t o m of the car m e a s u r e s the surface t e m p e r a t u r e . T h e car is also equipped with a c o m p u t e r and the t e m p e r a t u r e is r e c o r d e d every fifth metre. F i g u r e 7 shows the results from a measurem e n t trip on D e c e m b e r 2, 1988, (17:00-20:00) with one of the cars. T h e line is s t r a i g h t for sky view factors less t h a n a b o u t 0.9. At SVF = 0.9 "the line is curved. This is an effect of a d e c r e a s i n g t e m p e r a t u r e at open areas and

the limit of SVF at 1.0. T h e air t e m p e r a t u r e at 0.2 and 2 m e t r e s above the surface v a r i e d v e r y little ( < 1.0 °C) w i t h i n the city. T h e second p a r t is a s t u d y of the temperat u r e v a r i a t i o n in the t r a n s i t i o n zone b e t w e e n a s t r e e t c a n y o n and a n open area. T h e Thermovision system, the m e a s u r e m e n t cars and d a t a l o g g e r s are used for this purpose. In Fig. 8 the m e a s u r e m e n t points in a case study of the air and surface t e m p e r a t u r e patt e r n in a s t r e e t c a n y o n and on a s q u a r e are shown.

Fig. 8, Recording points in a study of the temperature variation between a street canyon and a square on October 11, 1988, 16:00- 23:00.

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Fig. 10. Temperature differences between a street canyon (station A) and a sporting ground (station B) within the city on October 11 and 12, 1988. Positive values indicate higher temperatures in the canyon.

street canyon (A, B, C, D) and on the square (E, F) on October 11, 1988; see also Fig. 8.

T h e m e a s u r e m e n t s were made on O c t o b e r 11, 1988, in c e n t r a l G 6 t e b o r g b e t w e e n 16:00 and 23:00. T h e w e a t h e r c o n d i t i o n s d u r i n g the m e a s u r e m e n t s w h e r e c l e a r (zero to one octas) and calm, less t h a n 1 m/s in the city and 1 - 3 m/s at the S/ive a i r p o r t (a r u r a l w e a t h e r station). F i g u r e 9 shows some results from this case study. T h e surface t e m p e r a t u r e inside the c a n y o n was a b o u t 1.5 °C h i g h e r t h a n at the square. The air t e m p e r a t u r e inside the c a n y o n was a b o u t 1 °C h i g h e r t h a n at the square. The high p e a k v a l u e at the surface at the end of the c a n y o n is p r o b a b l y due to an a d d i t i o n a l h e a t i n g at this point, H o w e v e r , a difference in surface t e m p e r a t u r e exists b e t w e e n the c a n y o n and the square. The t h i r d p a r t is an analysis of the tempera t u r e v a r i a t i o n b e t w e e n s t r e e t c a n y o n s and open spaces in the city and b e t w e e n the city and the c o u n t r y s i d e . In Fig. 10 an example of the t e m p e r a t u r e differences b e t w e e n a s t r e e t c a n y o n ( s t a t i o n A) and an open a r e a w i t h i n the city ( s t a t i o n B) is s h o w n for O c t o b e r 11 and 12, 1988. T h e air t e m p e r a t u r e was a b o u t 1.0 °C h i g h e r in the s t r e e t c a n y o n t h a n at the sporting ground. In Fig. 11 the r e l a t i o n bet w e e n the city and the r u r a l w e a t h e r s t a t i o n (S~ive a i r p o r t ) is shown. T h e city h a d an excess t e m p e r a t u r e of b e t w e e n 1 °C and 5 °C from D e c e m b e r 11, at 14:00, to D e c e m b e r 12 at 06:00.

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CONCLUSION

So far, the studies in general show a smaller t e m p e r a t u r e difference b e t w e e n different city u r b a n e n v i r o n m e n t s t h a n expected. T h e surface t e m p e r a t u r e varies some degrees w i t h i n the city b u t the air t e m p e r a t u r e variation is v e r y small.

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With the purpose of determining the variation between urban environments inside the city and between the city and the suburbs, further studies of the surface and air temperatures are planned. These measurements will be made by the measurement cars, by the thermovision system from the ground and from the air and by dataloggers. To study the influence of geometry on the temperature, small-scale models will be built in different shapes. Already-existing models of urban climate and structure will be tested and it is hoped that the results from this investigation find an application in the area of planning and design.

ACKNOWLEDGEMENTS

The study was carried out under the supervision of Professor Sven Lindqvist and with grants from the Swedish Council for Building Research.

REFERENCES 1 L. B~rring, J. O. Mattsson and S. Lindqvist, Canyon geometry, street temperatures and urban heat island in MalmS, Sweden, J. Clirnatol., 5 (1985) 433- 444. 2 I. D. Watson and G. T. Johnson, Graphical estimation of sky view factors in urban environments, J. Climatol., 7 (1987) 193- 197.