Street Design and Urban Canopy Layer Climate

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Ene rgy and Bu i ld ings, 11 (1988) 103 - 113 103

Street Des ign and Urb an an op y Layer lim ate

T. R. OKE

D e p a r tm en t o f G eo g ra p h y , T h e U n ivers i t y o f B r i t i sh C o lu m b ia , V a n co u ver , B . C . V 6 T 1 W 5 (C a na d a)

SUMMARY

Planning is always involved in making

choices between alternatives. In the case of

designing for street climate the objectives may

be mutually exclusive. For example, whilst

open geometry is conducive to air pollution

dispersion and solar access, a more densely

clustered arrangement is favourable ~or shelter

and energy conservation.

This dilemma is investigated by reviewing

the results of recent urban canyon field

studies and of scale and mathematical model-

ling. By concentrating on quantifiable rela-

tions it appears that it may be possible to find

a range of canyon geometries that are compa-

tible with the apparently conflicting design

objectives of mid-latitude cities. If this is

correct, traditional European urban forms are

climatically more favourable than more mod-

ern, especially North American, ones.

THE PROBLEM

In a previous paper we have argued for

urban climatology to become a more predic-

tive science so that its findings can be of

direct value in urban planning and design [1].

The present paper seeks to show by example

how this can be achieved. It poses the simple

but very fundamental question Does urban

climate research have quantitative guidelines

to offer regarding street geo met ry? It at-

tempts to provide some answers by reviewing

relevant research findings and interpreting

them in terms that are readily understood,

and easily controlled, by design professionals,

viz., street dimensions and building density.

When starting to consider such a question

it is easy to beco me overwhelmed with the

vast range of possibilities and special cases.

These are associated with the almost infinite

combination of different climatic contexts,

urban geometries, climate variables and design

objectives. Obviously there is no single solu-

tion, i.e., ther e is no universally opti mum

geometry. However, this should not stop us

seeking general guidelines as long as they are

flexible enough to cater to special needs and

situations. We certainly do not want a rigid

'solution' whose blind application leads to

furth er problems.

The essence of planning and design is mak-

ing choices between alternatives. This is not

an easy task, especially when it involves social

and economic values. Even here, where we are

mainly concerned with physical outcomes,

the choices are not obvious. We can illustrate

this by considering four of the most basic

objectives confronting those charged with

designing for street climate. For a mid- or

high-latitude cit y these goals may be:

(1) to maximize shelter. The obvious needs

are t o ensure the safety and comfo rt of pedes-

trians by not exposing them to high winds

which might impede their progress or even

blow them over, or lead to high wind chill or

driving rain. Shelter may also avoid disruptive

snowdrifting and contribu te to energy conser-

vation by reducing turbul ent heat losses.

(2) to maximize

dispersion

of pollutants.

In the case of streets the most important

source of pollution is probably that from

vehicle traffic on the canyon floor and the

primary concern is to minimize negative

impacts on receptors such as people (pedes-

trians, occupants of vehicles and houses) or

vegetation.

(3) to maximize urban warmth. By creating

a street structure which enhances the urban

heat island effect the climate will help reduce

pedestrian discomfort and the need for space

heating in the buildings. These benefit s will be

most obvious at night and/or in the cold sea-

son. It is assumed that heat stress is not a

major problem in such climates.

(4) to maximize solar access. In order to

make best use of solar energy, either passively

0378-7788/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands



104

o r v i a a c t i v e c o l l ec t o r s, t h e g e o m e t r i e s a s s o -

c i a t e d w i t h b e a m r a d i a t i o n a n d t h e u r b a n

s t r u ct u r e m u s t b e f a v o u m b l e . S i m i l ar l y s h a d -

i n g o f c a n y o n s m u s t n o t b e s o g r e a t a s t o

h i n d e r d a y l i g h t i n g o f b u i l d i n g i n t e ri o r s o r t o

c a s t a g en e r a l g l o o m w i t h i n c a n y o n s .

T h e o b j e c t iv e s a n d t h e s t r u c tu r e s t h e y d i c -

t a t e a r e i n c o n fl i c t. T h e f ir st w o g o a l s r e q u i r e

opposite structures; shelter is best provided

by narrow streets and compactness, whereas

dispersion demands separation and low build-

ing density. Similar dichotomies exist between

the other two objectives: warm th is promot ed

by compactness but access by openness. Thus

full compatibility is possible only if the de-

signer is concerned to provide either shelter

a n d w a r m t h o r d is p e rs i o n a n d a c ce s s. I n g e n-

e r a l t h i s h a p p e n s t a n c e i s u n l i k e l y t o o c c u r .

A c i t y is m o r e l i k e ly t o w i s h t o f o s t e r a m i x

o f o b j e c t iv e s. [ A s a n a s i d e w e s h o u l d n o t e

t h a t if s o m e o b j e c t iv e m e a n s o f r a n k i n g t h e s e

g o a l s w e r e a v a i l a b le , s u c h a s a c o s t - b e n e f i t

a n al y si s o r h a z a r d a s s e s s me n t , i t w o u l d g r e at l y

h e l p i n t h e o v e r a l l e v a l u a t i o n a n d s e l e c t i o n o f

t h e c o m p r o m i s e s t ru c tu r e. ]

I n t h e f o l l o w in g w e w i l l r e v i e w o u r k n o w l -

e d g e o f re l at i on s hi p s b e t w e e n u r b a n g e o m e t r y

a n d s t re e t c l i m a t e a s t h e y r e l at e t o e a c h o f t h e

p r e c e d in g f o u r g o a l s. T h e n , a s s u m i n g m o s t

c it ie s s e e k t o m e e t e a c h o f t h e s e g o a l s a t le a st

m i n i m a ll y , w e w i l l i n v es t i ga t e w h e t h e r t h e r e

i s a z o n e o f c o m p a ti b i l i ty i .e ., r a n g e o f

c a n y o n g e o m e t ri e s a n d b u i l di n g d e n s i ti e s

w h i c h a v o id t h e w o r s t a s p e c t s o f n o t p r o v i d -

i n g s h el t er , i s p e r s i o n , w a r m t h o r a cc e s s) .

O u r c o n s i d e r a t i o n i s r e s t r i c t e d t o t h e f o l -

l o w i n g l i m i t e d e x a m p l e :

- - a m i d - l a t i t u d e a p p r o x i m a t e l y 4 5 °) c i t y

w h e r e w i n t e r s p a c e h e a t i n g is n e c e s sa r y ,

v e h i c l e t r a f f i c i s s u f f i c i e n t o p r o v i d e a s ig ni fi -

c a n t s o u r c e o f p o ll u t a n t s, a n d t h e w e a t h e r

i n c lu d e s w i d e r a n g e s o f w i n d s p e e d a n d s o la r

i r r a d i a n c e .

- - t h e s t r ee t c a n y o n is t h e b a s i c g e o m e t r i c

u n i t. I t c a n b e a p p r o x i m a t e d r e a so n a b l y b y a

t w o < l i m e n s i o n a l c r o s s - se c t i o n i . e. , w e w i l l

n e g l e c t st r ee t j u n c t i on s a n d a s s u m e t h e b u i ld -

i n g s

flanking the canyon are semi-infinite in

length.

-- the urban cross-section is approximated by

a simple repetiti on o f these street canyon units.

--the predominant direction of airflow is

approximately normal (say +30°) to the long

axis of the street c anyon.

Most of these restrictions can be relaxed or

changed to accommodate other circumstances

such as different latitudes, climates, cubic

structures, winds parallel to the street, etc.

Given the configuration we have specified

we can limit the geometric descriptors to two

simple measures. These are the ratio

H / W ,

where H is the average height of the ca nyon

walls and W is the canyon width (see Fig. 1);

and the building densi ty, ~ = At~A1 where Ar

is the plan or roo f area of the average building

and A1 is the 'lot' area or unit ground area

occupied by each building (see Fig. 3). H / W

is sometimes referred to as the aspect ratio.

These geometric measures do not explicitly

include compass orientation although this

will be mentioned in relation to solar radia-

tion. Any final design 'solution' regarding

the l ayout of the street pattern in relation to

urban canopy layer climate must also address

this factor.

SHELTER AND URBAN GEOMETRY

T h e w i n d c l i m a t e a r o u n d a n i s o la t e d o b -

s t ac l e s u c h a s a b u i l d i n g is w e l l d o c u m e n t e d

[ 2, 3 ] . T h e f l o w i n a n e n v e l o p e s u r r o u n d i n g

the building is perturbed. There are three

main zones of disturbance: ahead there is a

bolster eddy vortex due to flow down the

windward face, behind there is a lee eddy

drawn into the cavity of low pressure due to

flow separation from the sharp edges of the

building top and sides, and further down-

stream is the building wake characterized by

increased turbulence but lower horizontal

speeds than the undisturbed flow. Only the

first two zones are illustrated in Fig. l(a).

The flow over arrays of buildings is less

well understood. If the buildings are well

apart

( H / W

> 0.05) their flow fields do not

interact. At closer spacings, such as that in

Fig. l(a), the wakes are disturbed. When the

height, spacing and density of the array

combine to disturb the bolster and cavity

eddies, this

isolated roughness f low

regime

changes to one referred to as

wake interfer-

ence flow (Fig. l(b)) . This is characterized by

secondary flows in the canyon space where

the downward flow of the cavity eddy is re-

inforced by deflection down the windward

face of the next building downstream. At

even greater H / W and density, a stable circula-



105

(a) Iso lated roughness f low

I~ W =I

( /3) Wake nter ference f low (C) Skimming f low

l

, ~ ~s ,~ '~~~ '~ , . . .w

Fig. 1. The flow regimes aseochited with air flow over build ing arrays of Lncreasing H/W.

t o r y v o r t e x i s e s t a b l i sh e d i n t h e c a n y o n a n d

t r a n s i t i o n t o a

skimming flow

r e g i m e o c c u r s

w h e r e t h e b u ~ o f t h e f l ow d o e s n o t e n t er t h e

c a n y o n ( F i g . 1 ( c ) ) . T h e t ra n s i t i o n s b e t w e e n

t h e s e t h r e e r e g i m e s o c c u r a t c r i ti c a l c o m b i n a -

t i o n s o f H/W and L/W ( w h e r e L i s t h e l e n g t h

o f t h e b u i l d in g n o rm ~ d t o t h e f l o w ) a s g i ve n

i n F i g . 2 .

I f w e a s s u m e t h a t t h e s e w i n d t u n n e l r e s u lt s

a p p l y t o a r r a y s o f b u i l d i n g s i n c i ti e s, w e h a v e

t h e b a s is f o r a f ~ s t ~ ) r d e r v ie w o f t h e e f f e c t o f

c a n y o n g e o m e t r y a n d b u i l d in g d e n s i ty o n

s h e l te r . W e d o n o t , h o w e v e r , h a v e a fu l l r e ~ -

t i o n sh i p b e t w e e n w i n d s p e e d r e d u c t i o n a n d

t h e s e m e a s u r e s o f g e o m e t r y . T o g iv e so m e

~ t ea w e c a n n o t e t h a t N a k a m u r a a n d O k e [ 5 ]

s u g g e s t t h e s i m p l e l i n e a r f o r m :

U c a n y o n ----Pfiroof

w h e r e ~ i s t h e m e a n h o r i z o n t a l w i n d s p e e d

a n d p is a d i m i n u t i o n f a c t o r w h i c h d e p e n d s

o n H/W a n d t h e m e a s u r e m e n t le v els . T h e y

s h o w f o r w i n d s p e e d s u p t o 5 m s 1 , w i t h

H/W ~ 1 , a n d c a n y o n c e n t r e a n d a b o v e - r o o f

m e a s u r e m e n t a t h e ig h ts o f a b o u t 0 . 0 6 / / a n d

1 . 2 H r e s p e c t i v e l y , t h a t p ~ 2 / 3 . P r e s u m a b l y

a t s m a l l e r

H/W, p

a p p r o a c h e s u n i t y a n d

s h e l te r i s lo s t H o w e v e r , w e s h o u l d r e i t e r a te

t h a t o u r c o n c e r n is w i t h t h e c o m f o r t a n d

s a f e t y o f p e d e s t r i a n s a n d h e a t l o s s f r o m b u i ld -

in g w a l k . B o t h t e n d t o b e c o n c e n t r a t e d a t t h e

s id e s o f t h e c a n y o n , w h e r e w e c a n a n t i c i p a te

g r e a t e r s h e l t e r t h a n a t t h e c a n y o n c e n t r e .

T h e r e a p p e a r s t o b e l i tt le u s e f u l e m p ~ i c a l

i n f o r m a t i o n a v a i l a b l e o n t h i s p o i n t . S h ~ i l~ r l y ,

0 20

0 25

0 . 3 3

0 . 5 0

1

2

4

~ ~H Iso la ted roughness f l ow

.222;-

i

S k i m m i n g

Cube

1 Canvon ~

I I I I I I I

1 2 3 4 5 6 7

L/H

Fig. 2. Threshold lines dividing flow into three re-

gimes as functions of the building

L/H)

and canyon

H/W),

geometry. Modified from a diagram in ref. 3

based on the wind tu nne l restflts of ref. 4.

t h e r e i s l it tl e k n o w l e d g e o n w h i c h t o b a s e

j u d g e m e n t s r e g a r d i n g t u r b u l e n c e c o n d i t i on s

a c r o s s a s t r e e t . G u s t i n e s s is a s i m p o r t a n t a s

m e a n w i n d s p e e d f o r m a n y a p p l ic a t io n s [ 6 ] .

I t i~ v e r y d i f f i c u l t t o c h o o s e a n o b j e c t i v e

c r i t e ri o n f o r t h e m i n i m u m a c c e p t a b l e a m o u n t

o f s h e lt e r. T h e c r i te r i o n , o r m o r e p r o b a b l y

c r i te r m , s h o u l d b e b a s e d o n t h e o b j e c t w e ( s )

i n v ol v e d . I f c o n c e r n i~ f o r p e d e s t r m n c o m f o r t

a n d s a f e t y , r e h i t i o n s h i p s a r e a v f l a b le t o c a lc u -

~ t e t h e e f f e c t o f w i n d s o n t h e t h e r m a l c o r n ,

f o r t a n d m e c h a n i c a l b u f f e t i n g o f p e r s o n s ,

e . g . [ 7 -

9 ] . T h e s e , t o g e t h e r w i t h a k n o w l e d g e

o f t h e g e n e r a l w i n d a n d te m p e r a t u r e c l im a -

t o l o g y a n d t h e e f f e c t s o f g e o m e t r y , c o u l d b e

• u s e d t o s e t t h e t h r e s h o l d o f a c c e p t a b l e c o n d i -

t i o n s a t a g i v e n l o c a t i o n . T h e m o s t p r e s s i n g

n e e d i s t o m o r e a c c u r a t e l y a n d f u l l y e s t a b li s h



106

wind and turbulence diminution factors as a

function of H / W and position within the

canyon. This could be achieved via field ob-

servation or models (e.g., wind tunnel [2 - 4,

6] or numerical [10] models). Before use it

is essential that such models have been vali-

dated using field data.

Given the present state of u ncerta inty it

seems pruden t to suggest no more than a very

general threshold based on Fig. 2. We presume

that some shelter is provided by wake inter-

ference and that the transition to skimming

flow bestows significant shelter. Using Fig. 2

for L / H values typical of canyons, we suggest

that H / W ~ 0.65 should ensure considerable

protection and, therefore, a minimum accept-

able value may be somewhere in the middle of

the wake interference regime at a bout 0.4.

This mi nimum is based on ful ly open canyons;

obviously if additional protection such as

trees or shop window awnings are provided

this limit becomes more conservative.

DISPERSION AND URBAN GEOMETRY

Urban geometry affects the capacity to

disperse pollutants in a city on at least two

scales. The t ota l array of roughness elements

affects the production of mechanical turbu-

lence, the form of the vertical wind profile

and the depth of the urban mixing layer.

These are local or meso'scale effects. The

wake shed by each building and the circula-

tion and turbulence associated with street

canyons also produce micro-scale effects in

amongst, and just above the buildings. The

relevance of the latter scale to the present

discussion is obvious, that of the former per-

haps needs elaboration. It is beneficial to

canopy-layer air quality to have the lowest

possible concentration in the boundary layer

a b o v e

for two reasons. Firstly, this provides a

relatively clean source of air to be entrained

down into the canyons, and secondly, the

lower the upper-level concentration, the

greater is the vertical concentration gradient

for upward turbule nt diffusion.

The critical surface parameter governing

the production of turbulence in the urban

boundary layer is th e roughness length (z0).

The larger the value of z0 the greater is the

intensity of turbulence [11, 12] and the

greater is the depth of frictional influence.

The roughness length is related to t he surface

geom etr y in the following way. As large rough-

ness elements (here buildings and trees) are

added to an area of less background rough-

ness, the total roughness is increased. The

taller the elements, and the greater their num-

ber, the greater is the resulting value of z0 up

to a certain density. Beyond this the addition

of elements serves to reduce z0. This is be-

cause interference between individual wakes

serves to smother their role in producing tur-

bulence. The progression is related to that

shown in the

( a ) -

(c) sequence of flow re-

gimes in Fig. 1.

Both

f i e l d a n d

wind tunnel studies confirm

that the relative roughness length, z o / h (where

h is the average height of the elements),

increases with element density to a single

peak and then declines [4, 13]. In these

studies the element density is described using

either the roughness density (~) or the build-

ing densi ty (~') -- see Fig. 3. For building-like

elements the peak of zo /h i s typic ally 0.2 - 0.3

and occurs at densities of about 0.25 (range

0.13 - 0.32) [4, 12 - 14]. The max imu m

roughness effect may approximately corres-

pond t o the transition betwee n wake inter-

ference and skimming flow [14] which for

canyons implies H / W ~ 0.65.

Within the canopy layer, pollutant trans-

port and diffusion is strongly dependent

upon the above-roof wind and turbulence and

the urban geometry [15]. For small H / W ,

dispersion is good and is closely linked to the

horizontal wind speed. Although the spatial

distribution of pollution may show localized

areas of high concentration (e.g., in the lee

cavity of isolated buildings) there is good

Ar

As

F i g . 3 . D i m e n s i o n s o f a n a v e ra g e u r b a n r o u g h n e s s

e l e m e n t ( e .g . , a b u i l d i n g ) a n d i t s l o t , w h e r e h i s i t s

h e i g h t , A s i s i t s s i l h o u e t t e a r e a ( t h e v e r t i c a l a r e a o f

t h e e l e m e n t s e en b y t h e a p p r o a c h in g w i n d ) , A l i s i t s

l o t a r e a ( u n i t g r o u n d a r e a o c c u p i e d b y e a c h e l e m e n t ) ,

a n d A r i s th e e l e m e n t o r r o o f a r e a . T h e s e m e a s u r e s

c a n b e u s e d t o d e f i n e a r o u g h n e s s d e n s i t y )~ =

As/A1

a n d a b u i l d i n g d e n s i t y ~ =

A r I A .



e x c h a n g e b e t w e e n g r o u n d - l e ve l s o u r c e s a n d

t h e c l e a n e r ai r i n t h e m i x i n g l a y e r a b o v e . T h i s

i s n o t t h e c a s e f o r

H / W

b e y o n d t h e t h r e s h o l d

f o r s k i m m i n g f l o w . T h e d e v e l o p m e n t o f t h e

s t a bl e c r o s s ~ a n y o n v o r t e x c i r cu l a t io n i s a s s o-

c i a t e d w i t h r e d u c e d e x c h a n g e b e t w e e n t h e

c a n o p y a n d b o u n d a r y l a y e r s . T h e v o r t e x i s

t a n g en t i a ll y d r i v e n b y t h e a b o v e - r o o f f l o w

b u t t h e c o u p l i ng b e c o m e s l e s s e f f e ct i v e a s

H/W is increased.

F i e l d s t u d i e s s h o w t h e v o r t e x is a s s o c i at e d

w i t h p o l l u t a n t c o n c e n t r a t i o n d i f f e r e n c e s w i t h -

i n t h e c a n y o n , e s p e ci a l ly n e a r t h e f l o o r [ 1 6 -

1 8 ] . F l o w d o w n t h e w i n d w a r d w a l l o f t h e

c a n y o n h a s r e la t iv e ly c l e a n ai r n e a r t h e t o p

b u t b e c o m e s m o r e c o n t a m i n a t e d a s it e n t r a i n s

c a n y o n p o l l u t a n t s t o w a r d s t h e f l o o r . T h e

r e t u r n f l o w a c r o s s t h e c a n y o n f l o o r t r a ve r s es

t h e s o u r c e o f v e h i c ul a r p o l l u t io n . C o n c e n t r a -

t i o n s a r e h i g h e s t a t t h e b a s e o f t h e l e e w a r d

w a l l a n d d e c re a s e w i t h h e i g h t . I n d e e p c a n y o n s

a s e c o n d a r y v o r t e x m a y d e v e l o p g i vi n g w o v o r -

t i c e s

r o t a t i n g i n o p p o s i t e d i r e c t i o n s [ 1 8 ] . T h e

c r o s s- s t re e t c o n c e n t r a t i o n s a r e t h e n t h e r e-

v e r se o f t h a t w i t h t h e s in g le v o r t e x , a n d t h e

b a se o f t h e c a n y o n b e c o m e s m o r e h e a v il y

p o l l u t e d b e c a u s e t h e ' h a n d o v e r ' o f m a t e r i a l s

b e t w e e n t h e t w o v o r t i c e s i s i n e f f e c t iv e . W i t h

w e a k a b o v e - r o o f w i n d s t h e v o r t e x d o e s n o t

f o r m , t h e c a n y o n a i r m a y b e c o m e d e c o u p l e d

f r o m t h a t a b o v e a n d s t a g n a t i o n o c c u r s . T h i s

h a s p o t e n t i a l l y s e r i o u s a i r < l u a l i t y c o n s e -

q u e n c e s n e a r t h e f l o o r u n l e s s t h e r m a l d i f f e r -

e n c e s a r e c a p a b l e o f g e n e r a t i n g a c i r c u la t i o n .

Q u a n t i t at i v e e s t im a t e s o f t h e r a t e o f r e m o v a l

o f p o l lu t a n t s f r o m t h e c a n y o n a n d t h e i r

r e p l a c e m e n t w i t h a ir f r o m a b o v e is n o t y e t

p o s s i b l e [ 3 , 1 8 ] . T h e r e is e v i d e n c e t o s h o w

t h a t t h e v e r t ic a l v o r t ic e s s h e d f r o m t h e

c o r n e r s o f b u i l d i n g - f a c e s a t i n t e r s e c t i o n s , a n d

t h e t u r b u l e n c e g e n e r a t e d f r o m a f e w t al l e r

t h a n a v e r a g e s t r u c t u r e s s c a t t e r e d w i t h i n m o r e

u n i f o r m a r r a y s o f b u i l d i n g s , a r e c a p a b l e o f

e n h a n c i n g b e l o w - a n d a b o v e - r o o f e x c h a n g e

t o t h e a d v a n t a g e o f c a n y o n a ir q u a l i ty [ 3 ,

19 , 20] .

F r o m t h e f o r e g o i n g it s e e m s t h a t t h e o n s e t

o f s k i m m i n g f l o w r o u g h l y c o i n c i d e s w i t h a

m a r k e d r e d u c t i o n i n t h e d i sp e r s iv e c a p a c i t y

o f c a n y o n a i r . F r o m F i g . 2 t h i s s u g g e st s a n

H/W l i m i t o f a b o u t 0 . 6 5 . T h e r e i s a l so s o m e

i n d i c a t i o n t h a t t h e V o r t e x c i r c u l a t i o n i s

s t r o n g e s t w i t h H/W ~ 1 . 0 [ 2 1 ] . H e r e w e o p t

f o r t h e m o r e c o n s e r v a t i v e v a l u e o f 0 . 6 5 .

107

I t i s v e r y d i f f i c u lt , a n d p r o b a b l y n o t p r u -

d e n t , t o s e t a r i gi d c r i t e r io n f o r t h e c a n y o n

g e o m e t r y w h i c h p r o v id e s t h e m a x i m u m a c -

c e p t a b l e d e g r a d a t i o n o f a i r q u a l i t y ( o r m i n i-

m u m a c c e p t a b l e c a n y o n d is p e r s io n ) . P r e s u m -

a b l y a i r q u a l i t y s t a n d a r d s p r o v i d e t h e t h r e s h -

o l d s c o m p a t i b l e w i t h a g i v e n j u r i s d ic t i o n ' s

q u a l i t y - o f - l i f e o b je c t i v e s , b u t i n a g i v e n c a n y o n

t h e c h a n c e s o f c o m p l y i n g w i t h t h e s t a nd a r d s

d e p e n d s o n t h e s t r e n g t h o f e m i s s i o n s a n d

c l i m a t e a s w e l l as t h e g e o m e t r y . H e r e a ga i n,

t h e d e v e l o p m e n t o f p h y s ic a l m o d e l s t o s i m u -

l a t e t h e l i k e l y c o n c e n t r a t i o n s a r i s i n g f r o m

d i f f e r e n t e m i s s i o n s c e n a r i o s s h o u l d b e e n -

c o u r a g e d .

T h u s , i n s u m m a r y , it i s i n t er e s ti n g t o n o t e

t h a t H / W ~ 0 . 6 5 w i t h a b u i l d i n g d e n s i t y o f

~ 0 . 2 5 m a y f o r t u i to u s l y p r o v i d e b o t h a m a x i -

m u m r o u g h n e s s e f f e ct f o r a b o v e - r o o f u r b a n

a i r f l ow a n d a n u p p e r l i m i t t o s a t is f a ct o r y

d i s p e r si o n f r o m s t re e t c a n y o n s .

U R B A N W A R M T H A N D G E O M E T R Y

A c o m p a c t c i t y f o r m p r o m o t e s u r b a n

w a r m t h i .e ., h e u r b a n h e a t i sl a n d) , e s p e c ia l l y

a t n i g h t . I n m i d - l a t i t u d e a r e a s t h i s i s g e n e r a n y

r e g a r d e d a s f a v o ur a b l e i n t h e c o l d s e a s o n

b e c a u s e i t r e d u c e s t h e n e e d f o r s p ac e h e a t i n g

i n h o u s e s a n d t h e r e b y p r o m o t e s e n e r g y c o n -

s e rv a ti o n. I t s h o u l d a l s o p r o v i d e a m e a s u r e o f

p e d e s t ri a n c o m f o r t , b e n e f i t s t o p l a n t a n d

a n i m a l s y s t e m s a n d e n h a n c e d p o l lu t i o n d is -

p e r s i on v i a t h e r m a l t u r b u l e n ce , r e d u c e d

s ta bi li ty a n d t h e r m a l b r e e ze s ) . T h e s e p r o b a b l y

o u t w e i g h s u c h d i s ad v a n t a ge s a s t h e a d d e d

h e a t s t r e s s i n t h e s u m m e r , p o l l u ta n t f u m i g a -

t i o n a n d i n c r e a se d c h e m i c a l w e a th e r i n g , b u t

h e r e a g a i n a f u l l c o s t - be n e f i t a n a l y s is w o u l d

b e v e r y i n s t r uc t i v e.

T h r e e o f t h e m o s t o b v i o u s c a u s e s o f t h e

c a n o p y l a y e r h e a t i s l a n d t h a t ar e g o v e r n e d

b y u r b a n g e o m e t r y a r e t h e in c r ea s e d a b s o r p -

t i o n o f s o l a r r a d i a t i o n c a u s e d b y m u l t i p le

r e f le c t io n , t h e r e d u c t i o n o f t u r b u l e n t s e n s i bl e

h e a t tr a n s fe r u t o f t h e c a n y o n s d u e t o s h e lt e r

a n d t h e r e d u c t i o n o f l o n g - w a v e r a d i a t io n l o ss

f r o m w i t h i n t h e c a n y o n s d u e t o t h e s c r e e n i n g

b y t h e f l a n k i n g b u i l d i n g s . F o r a f u l l l i st o f

h e a t i sl a n d c a u s e s s e e r ef . 2 . ) W e h a v e a l r e a d y

c o v e r e d t h e r e l a t i o ns h i p b e t w e e n g e o m e t r y

a n d s h e lt e r , a n d t h e f o l l o w i n g s e c t i o n d e a l s

w i t h t ha t b e t w e e n g e o m e t r y a n d s o l ar a d i a t i o n

r e ce i pt ; h e r e w e c o n s i d er l o n g - w a v e r a di a ti o n.





i i i i i i

12 e ~

10 0 e

• o

o.O+

oo ÷

0 E u r o p e

N . A m e r i c a

+ A u s t r a l a s i a

0 i

H/W

Fig. 6. The empirical relationship bet ween urban

canyon geometry ( H / W i n the c ity centre) and the

ma x i m u m heat island intensity ATu-r(max)). Based

on data in Ok e [25 ]. u = urban, = rural.

8

o

< 3

4

s ki es a n d c a l m t h e h e a t b R 1 e n c e i s d o m i n a t e d

b y t h e i n t e rp l ay b e t w e e n t h e n e t l o n g - w a v e

e n e r g y d r a i n a n d t h e h e a t s t o r a ge i n t h e u r b a n

f a b r i c , w i t h t u r b u l e n t i n f l u e n c e s b e i n g n e g li -

g i b l e . T h e a g r e e m e n t w i t h t h e c o n t r o l s u g -

g e s t e d b y F i g . 5 i s v e r y s u g g e s t i v e t h a t l o n g -

w a v e s c r e e n i n g i s t h e k e y . W i t h g r e a t e r c l o u d

a n d w i n d t h e r o l e o f t h is t e r m s h o u l d d i m i n -

i sh , a s d o e s t h e h e a t i s la n d i n te n s i t y . H o w e v e r ,

i t is a l so t o b e e x p e c t e d t h a t H/W is r e l a t e d

t o o t h e r p h y s i c a l f e a t u r e s a s s o c i a t e d w i t h t h e

d e n s i t y o f b u i ld i n g d e v e l o p m e n t . G r e a t e r d e n -

s it ie s c a n b e e x p e c t e d t o b e a c c o m p a n i e d b y

g r e a te r a m o u n t s o f i m p e r m e a b l e c o v e r, m a t e -

r i a l s w i t h h i g h e r t h e r m a l a d m i t t a n c e , a n d i n -

c r e a s e d a n t h r o p o g e n i c h e a t f l u x d e n s i t y . I n

t u r n t h e s e f e a t u r e s w i l l f a v o u r s e n s i b l e r a t h e r

t h a n l a t e n t h e a t , h e a t s t o r a g e r a t h e r t h a n h e a t

e x c h a n g e a n d e x t r a e n e r g y a v a i l ab i li ty .

I t i s v e r y d i f f ic u l t t o q u a n t i f y t h e v a l u e o f

t h e u r b a n h e a t i sl a n d in r e la t i o n t o e n e r g y

c o n s e r v a ti o n o r h u m a n c o m f o r t . T h e r e a r e

m o d e l s t o c a l c u l a t e e n e r g y l o s s f r o m b u i l d in g s

w h i c h i n c l u d e t h e i m p o r t a n c e o f si te f a c t o r s

s u c h a s t h e u r b a n h e a t i s l an d [ 2 6 ] . T h e im -

p o r t a n c e o f b o t h w i n d ( s h e l t er ) a n d t e m p e r a -

t u r e

( h e a t is l an d ) c a n t h e n b e a s s es s e d in r e la -

t io n t o t h e t h e r m a l c h a ra c t er is tic s o f t h e b u ild -

i n gs a n d t h e s y n o p t i c c l i m a t o l o gy . E m p i r i c a l

s t u die s s h o w t h a t it is p o s sib l e t o s a v e 5 - . 5

o f s p a c e h e a t in g c o s ts p e r o n e C e l siu s d e g r e e

increase of m e a n d a i l y t e m p e r a t u r e [ 2 7 , 2 8 ] .

109

S i m i la r l y t h e e n e r g y b a l a n c e a n d b i o - c li m a t e

o f p e d e s t r i a n s c a n b e m o d e l l e d [ 2 9 ] .

A g a i n w e f a c e t h e q u e s t i o n o f c h o o s i n g a

t h r e sh o l d . T h i s t i m e w e n e e d t o k n o w t h e

g e o m e t r y w h i c h w i ll r e t a in a d e s i ra b l e p r o p o r -

t i o n o f t h e h e a t i s la n d . I f a s a f i r s t -o r d e r a s -

s u m p t i o n w e a c c e p t t h a t t h e f o r m o f F ig . 6

o n l y d e p e n d s o n t h e c o n f i g ur a t io n o f u r b a n

s t r u c t u r e s , w e s e e t h a t b e c a u s e o f t h e l o ga r it h -

m i c s h a p e w e c a n a c h i e v e c o n s i d e r a b l e c o n t r o l

o v e r t h e h e a t i s l a n d w i t h s m a l l c h a n g e s i n

c a n y o n g e o m e t r y a t l o w va l ue s o f H/W. F o r

e x a m p l e , u n d e r i d e al h e a t i sl a nd c o n d i t i o n s ,

o n e t h i rd o f t h e m a x i m u m p o s s i b l e i n t e n s i ty

i s g a i n e d w i t h H/W o f 0 . 4 , o n e h a l f w i t h 0 . 7

a n d t w o t h ir d s w i t h 1 .0 . W e d o n o t k n o w t h e

A T u . r v s . H/W r e l a t i o n s h i p f o r l e s s t h a n i d e a l

m e t e o r o l o g i c a l c o n d i t i o n s b u t i t se e m s re a -

s o n a b l e t o e x p e c t i t t o b e o f s im i l ar f o r m . I f

t h i s i s c o r r e c t t h e n w e m i g h t a r b i t r a r i l y s a y

t h a t t h e m i n i m u m

H/W

a c c e p t a b l e i s a b o u t

0 . 4 , t h e r e b y m a i n t a in i n g a b o u t o n e th i r d o f

t h e h e a t i s l a n d p o t e n t i a l f o r a g i v e n c i t y .

SOLA R RADIATION AND URBAN GEOMETRY

T h e r e a r e t w o a s p e c t s o f d ir e c t re l e v a n c e

u n d e r t h i s h e a d in g : s u r f a c e a l b e d o a n d s o l ar

a c c e s s .

T h e t o t a l a l b e d o o f a n u r b a n s y s t e m , a nd

t h e r e f o r e i t s a b i l i t y t o a b s o r b s o l a r r a d i a t i o n ,

d e p e n d s u p o n t h e a l be d o o f t h e c o m p o n e n t

m a t e r i a l s a n d t h e i r g e o m e t r i c a l a r r a n g e m e n t .

T h e i m p o r t a n c e o f g e o m e t r y h as b e e n d e m o n -

s t r a t e d u s i n g o b s e r v a t i o n s [ 2 4 ] a n d b o t h

n u m e r i c a l a n d s c a le m o d e l s o f c a n y o n r ad i a-

t iv e e x c h a n g e [ 3 0 - 3 4 ] . A l l a p p r o a c h e s s h o w

t h a t f o r b u i l d i n g s o f e q u a l h e i g h t t h e a l b e d o

o f a c r e n e l l a t e d s u r f a c e is l o w e r t h a n t h a t o f

a f l a t p l a n e c o m p o s e d o f th e s a m e m a t e ri a ls .

I t a l s o a p p e a r s t h a t t h e e f f e c t i n c re a s e s w i t h

l a t i tu d e , a n d i s m o r e p r o n o u n c e d i n t h e l o w -

s u n s e a s o n [ 3 1 , 3 3 ] . F u r t h e r , i t i n c re a s e s w i t h

H/W [ 3 1 3 3 ] a n d is g r e a t e r i n E - W r a t h e r

t h a n N - S o r i e n t e d c a n y o n s [ 3 1 ] . U s i ng d i f-

f e r e n t s c a l e m o d e l a r r a y s , A i d a [ 3 3 ] f o u n d

a b s o r p t i o n i n c r e m e n t s o f 1 3 - 2 7 % f o r

H/W in

t h e r a n g e 0 . 5 - 2 . 0 w h e n t h e f l a t p l a n e a l b e d o

w a s 0 . 4 0 . H e a l s o s u g g e s t e d t h a t t h i s in c r e -

m e n t is m a i n ly d e p e n d e n t o n t h e p l a n d e n s it y .

I n a n u m e r i c a l s im u l a t i o n A i d a a n d G o t o h

[ 3 4 ] s t u d i e d t h e a l b e d o o f d i f f e r e n t c a n y o n

a r r a n g e m e n t s a s a f u n c t i o n o f t h e r a t i o W I / W 2



1 1 0

S L l f l

b ~ - - - W l r - l ~ W 2 - - - ~

W 1 / W 2

0 . 0 6 0 . 1 3 0 . 2 5 0 . 5 2 4 8 1 6

= = i i ; , i i

0 4

. . ,% .. . . .. . ~'

0.3 o . . . .

< 0 . 2

0 0 8 = 0 °

: ~ ~: ~- - - - ~ 0 =20 °

Min. Z~ . . . . . . . . ~

8

= 40 °

a l b e d o

O. ... . .0 0 = 60 °

0 . 1

0 - - - - - 0 8

=80 °

I

I I I

0 , 0 - 1 . 2 - 0 1 .9 - 0 1 6 - 0 1 .3 0 . 0 0 1 .3 0 1 .6 0 . 9 1 . 2

l o g W 1 / W 2 )

F ig . 7 . T h e d e p e n d e n c y o f t h e t o t a l s y s t e m a l b e d o

u p o n t h e s y s t e m g e o m e t r y a s d e s c r i b e d b y t h e r a ti o

o f b l o c k - t o - c a n y o n w i d t h ( g r l / w 2 s e e t o p s c h e m a t i c ) .

A l s o i n c l u d e s t h e i n f l u e n c e o f s o l a r z e n i t h a n g l e ( 0 ) .

B a s e d o n n u m e r i c a l s i m u l a t i o n s w i t h

H/W2

= 1 . 0 .

A f t e r A i d a a n d G o t o h [ 3 4 ] .

w h e r e W 1 i s t h e w i d t h o f t h e b l o c k e l e m e n t s

a n d W 2 i s t h e i n t e r - b l o c k o r s t r e e t w i d t h ( s e e

s c h e m a t i c i n F i g . 7 ) . O b v i o u s l y W1/W2 is

p r o p o r t i o n a l t o

W1/ W1 +

W 2 ) , a n d t h e r e f o r e

~ , f o r r e g u l a r a r r a y s . T h e i n t e r e s t i n g r e s u l t

s h o w n i n F ig . 7 is t h a t , w h e n

H/W

i s h e l d c o n -

s t a n t a t u n i t y , t h e r e i s a m i n i m u m a l b e d o f o r

t h e s y s t e m a t log W1/W2) = - - 0 . 3 t o - - 0 . 6 .

T h i s m e a n s t h a t i n c a s es w h e r e w e w i s h t o

m a x i m i z e s o l a r a b s o r p t i o n , s u c h a s m i d - l a ti -

t u d e c i t ie s i n t h e c o l d s e a s o n , t h e i d e a l W1/W2

is a p p r o x i m a t e l y 0 . 5 . W h e n t ra n s l a t e d i n t o

b l o c k s o f s e m i - i n f in i t e l e n g t h t h i s g i v e s ~

0 . 3 3 .

A l t h o u g h m i n i m i z in g t h e a l b e d o i n c r e a s e s

t h e a b s o r p t a n c e o f t h e t o t a l s y s t e m f o r so l a r

r a d i a t i o n , i t d o e s n o t n e c e s s a r i l y b e n e f i t p o s i -

t io n s n e a r th e c a n y o n f l o o r w h i c h m a y b e i n

s h a d e . I n d e e d c e r t a i n g e o m e t r i c a r r a n g e m e n t s

c a n c r e a t e a b s o r p t a n c e t h a t is le s s t h a n t h a t

o f a f l a t p l a n e [ 3 1 ] . H e r e w e a r e o n l y c o n s i d -

e r in g t h e c a s e o f c a n y o n s w i t h f l a n k in g b u i l d-

i ng s o f a p p r o x i m a t e l y e q u a l h e i g h t, o b v i o u s l y

a n o m a l o u s l y l a r g e s t r u c t u r e s c a n c r e a t e e n e r -

g y d e f i c i e n t a r e a s in th e i r s h a d o w .

T h e r e l e v a n t l i m i t f o r s o l a r a c c e s s t o s t r e e t

c a n y o n s d e p e n d s u p o n t h e d e g re e o f p e n et ra -

t i o n a t t h e w i n t e r s o ls t i c e . P e n e t r a t i o n i s

n e e d e d t o f a c i l i t a t e s o l a r e n e r g y g a i n b y

e q u a t o r - f a c i n g w a l l s , t o p r o v i d e s u f f i c i e n t

d a y l i g h t f o r b u i l d i n g i n t e r i o r s a n d t o a i d i n

t h e c o m f o r t a n d p s y c h o l o g ic a l a t t i tu d e o f

p e d e s t r i a n s .

T a b l e 1 g i v es s o m e i d e a o f p e n e t r a t i o n i n t o

a n E - W c a n y o n b y i n di c at in g t h e a m o u n t o f

a n e q u a t o r - f a c i n g w a l l t h a t i s o p e n t o d i r e c t -

b e a m i r ra d i a ti o n a t n o o n o n t h e w i n t e r so ls -

t ic e . F o r s u c h a c a n y o n a t a l a t i t u d e o f 4 5 °

w i t h a n

H/W

o f 1 . 0 w e s e e t h a t o n l y t h e u p p e r

3 9 % i s d i r e c t l y s u n l i t ( F i g . 8 ( a ) ) . A n H/W o f

a b o u t 0 . 4 i s r e q u i r e d f o r th e w h o l e w a l l t o b e

d i r e c t l y l i t ( F ig . 8 ( b ) ) . F u l l i r r a d i a t io n a t t h e

s o l s t i c e i s p r o b a b l y t o o r e s t r i c t i v e a r e q u i r e -

m e n t . A r e a s o n a bl e c o m p r o m i s e m a y b e an

H/W o f 0 . 6 w h i c h w o u l d r e s u l t i n a b o u t t w o

t h i r d s o f t h e w a l l b e i n g l i t.

T A B L E 1

T h e a r e a o f a n e q u a t o r - fa c i n g w a l l in a n E - W o r i e n t e d

s t r e e t c a n y o n t h a t p o t e n t i a l l y r e c e i v es d i r e c t - b e a m

s o la r r a d i a t i o n a t n o o n o n t h e w i n t e r s o l st i c e a s f u n c -

t i o n s o f l a t it u d e a n d

H/W. The

s u n l i t a r e a i s e x p r e s s e d

a s a p e r c e n t a g e f r a c t i o n o f t h e t o t a l w a l l a r e a

Lati tude

H/W

0.3 0.4 0.5 0.6 0.8 1.0 2.0

40 ° 100 100 100 83 62 50 25

45 ° 100 98 79 66 49 39 20

50 ° 99 74 59 49 37 30 15

S i n c e m a n y m i d - l a t i tu d e c i t ie s e x p e r i e n c e

h i g h f r e q u e n c i e s o f c l o u d i n w i n t e r , t h e

a m o u n t o f d i f f u s e r a d i a t i o n f o r d a y l ig h t in g i s

a l so i m p o r t a n t . T h e c o n v e n t i o n a l c r it e r io n o f

d a y l i g h t i n g s p e c i a l is t s is t h a t a n H/W o f 0 . 5 8

i s a p p r o p r i a t e a t a l a t i t u d e o f 4 5 ° [ 3 5 , p . 7 4 ] .

T h e c o r r e s p o n d i n g r a t i o s a t 4 0 ° a n d 5 0 ° a r e

0 . 7 0 a n d 0 . 4 6 , r e s p e c t i v e l y .

T h e r e f o r e w e m a y t e n t a ti v e l y c o n c l u d e

t h a t a n H/W o f a b o u t 0 . 6 s e e m s t o b e a s ui t-

a b l e u p p e r l i m i t t o m a i n t a i n s o l a r a c c e s s in a

c i t y a t a l a t i t u d e o f 4 5 °. O b v i o u s l y t h e r e i s

n e e d t o r e f in e t h i s c o n c l u s i o n o n t h e b a s i s o f

a f u l l a n a l y s i s o f t h e p a s s i v e s o l a r e n e r g y g a i n

a n d i l lu m i n a t i o n o f t h e t o t a l u r b a n s y s t e m .



1 1 1

/ •

. , . ~ ~ ~ W i n t e r

N ~ / / / / / i / ~ j , i / / / / / , / . S

(a)

~ W i n t e r

s

t , , / / / / , / j f / / , / / / / / , / / / / / / / / / / / / / / / / / / / / / / / / / / /

I~ W ~ ,

(b)

F i g. 8 . A n g l e s o f i n c i d e n c e o f d l r e c t - b e a m s o l a r r a d m -

t i o n a t n o o n i n a n E - W c a n y o n , i n a c i t y a t 4 5 °N .

( a ) I n a c a n y o n w i t h . / - / / W = 1 . 0 ; ( b ) tt/W = 0 . 4 0 .

T h i s s h o u l d i n c l u d e a r e c o g n i t i o n o f t h e s o l ar

r a d i a t i o n c l i m a t o l o g y o f t h e c i t y i n q u e s t i o n

i n c lu d i n g t h e f r e q u e n c i e s o f d ir e c t a n d d i f f u s e

i n s o l a t i o n . S i m i l a r l y , t h e r e l a t i v e i m p o r t a n c e

o f th e e f f e c t o f g e o m e t r y o n s o la r a b s o rp -

t a n c e v e r s u s t h a t o n s o la r a c c e s s t o t h e c a n -

y o n s c a n n o t b e s i m p ly d e c i d e d , b u t o n e s us -

p e c t s t h a t t h e s o l a r a c c e s s c r i t e r i o n i s t h e

m o s t c r i t i c a l .

A Z O N E O F C O M P A T I B I L I T Y ?

T h i s p a p e r d e m o n s t r a t e s t h a t a n u m b e r o f

u s e f u l r e l a t io n s h i p s e x i s t b e t w e e n t h e g e o m -

e t r y a n d t h e m i c r o c l i m a t e o f u r b a n s t r e e t

c a n y o n s . T h e y a r e p o t e n t i a l l y h e l p f u l t o t h e

e s t a b l i s h m e n t o f g u id e l in e s g o v e r n in g s t r e e t

d i m e n s i o n s f o r u s e b y u r b a n d e s i gn e r s . T h e

r e l a t io n s h i p s h a v e t h e s p e c ia l m e r i ts o f b e i n g

q u a n t i t a t i v e a n d o n l y d e p e n d i n g o n s im p l e

m e a s u I ~ s s .

U n f o r t u n a t e l y t h e r e i s l e s s b a s i s f o r l in k i n g

c a n y o n c l i m a t e c h a r a c t e r i s t i c s t o s o c i o -

e c o n o m i c , c o m f o r t o r s a f e t y o b j e c t i v e s . I n

m a n y c a s e s t h e c h o i c e o f t h r e s h o l d s i s a r bi -

t r a r y o r l a r g e ly s u b j e c t i v e . T h e r e i s a n e e d t o

r e f in e t h e s e t h r o u g h f u r t h e r r e s e a r c h b u t

t h e r e w i ll a l w a y s b e a n e l e m e n t o f v a l u e

j u d g e m e n t i n v o l v e d i n s e t t i n g t h e p r i o r i t ie s

a n d a c c e p t a b l e l i m i t s i n a g i v e n c i t y .

D e s p i t e t h i s p ro b l e m w e c a n s u m m a r iz e

t h e f in d i n g s o f t h e p r e s e n t e x p l o r a t o r y s t u d y

a n d s u g g e st s o m e g e n e r a l i z a ti o n s f o r o u r

h y p o t h e t i c a l m i d - l a t i t u d e c i t y .

( 1 ) I n t e r m s o f

H / W

w e f i n d s o m e d eg r e e

o f a g r e e m e n t . A l o w e r l i m i t o f a b o u t 0 . 4 i s s e t

b y t h e n e e d t o p r o v i d e s o m e d e g r e e o f s h e lt e r

a n d t o r e t a in a r e a s o n a b l e p r o p o r t i o n o f t h e

h e a t i sl a n d w a r m t h . A n u p p e r l i m i t o f 0 . 6 0 -

0 . 6 5 e n s u r e s t h a t b o t h a t m o s p h e r i c d i s p e r s i o n

a n d s o l a r a c c e s s is m a i n t a i n e d w i t h i n t h e s t r e e t

c a n y o n s . T h e r e f o r e , i f a ll f o u r g o a l s a r e t o b e

s a t i s fi e d , t o a t l e a s t a m i n i m a l e x t e n t , a c o m -

p a t i b l e r a n g e o f

H / W

i s 0 . 4 - 0 . 6 .

( 2 ) T h e r e a r e r a t h e r f e w r e l a t io n s h i p s a v a il -

a b l e w h i c h u s e t h e b u i l d i n g d e n s i t y , n e v e r t h e -

l es s i t s e e m s t h a t r o u g h n e s s m a y b e m a x i m i z e d

a t ~ ~ 0 . 2 5 a n d a b s o r p t a n c e a t ~ 0 . 3 3 . T e n t a -

t i v e ly w e m a y s u g g e st t h a t a r a n g e o f ~ b e -

t w e e n 0 . 2 0 a n d 0 . 4 0 i s p r o b a b l y s u i ta b l e .

I f w e a p p l y t h e r a n g e o f c o m p a t i b i l i ty t o

e x i s t i n g E u r o p e a n a n d N o r t h A m e r i c a n c i t y

f o r m s w e f in d t h a t n e i t h e r c o n f o r m i n t h e i r

c o r e a r e a s f o r l a rg e c i t i e s ( > 1 0 s i n h a b i t a n t s ) .

I n E u r o p e t h e t y p i c a l c e n t r a l a r e a h a s H/W in

t h e r a n g e o f 0 . 7 5 - 1 . 7 , a n d i n N o r t h A m e r i c a

i t i s 1 . 1 5 - 3 . 3 [ 2 5 ] . G i v e n t h e l o w e r v a l u e s ,

a n d t h e f a c t t h a t t h e E u r o p e a n d a t a i n c l u d e

c i ti e s w i t h v e r y m u c h l a r g er p o p u l a t i o n s , t h e

E u r o p e a n r e s u l t s a p p e a r t o b e m o r e fa v o u r -

a b l e . C e r t a i n l y t h e ' s k y s c r a p e r ' i s a p o o r c l i-

m a t i c f o r m o n t h e g r o u n d s o f d is p e r si o n a n d

s o la r a c c e s s n e a r t h e g r o u n d . W e m i g h t a l s o

n o t e t h a t b e c a u s e s u c h ta l l b u i ld i n g s j u t a b o v e

t h e g e n e r a l r o o f - l e v e l t h e y a l s o c r e a t e s p e c i a l

h i g h -w i n d s p e e d p r o b l e m s i n l o c a l iz e d a re a s

n e a r t h e i r b a s e , s o t h e y d o n o t p r o v i d e u n i-

f o r m s h e l te r e i th e r [ 6 ] . A l t h o u g h a s u r v e y o f

H / W

a n d ~ f o r E u r o p e a n a n d N o r t h A m e r i c a n

c i ti e s i s n o t a v a il a b le , m o s t o b s e r v e r s w o u l d

a g r e e t h a t t h e c o m p a c t f o r m o f r e s i d e n ti a l /

s u b u r b a n a r e a s in t h e f o r m e r a r e m o r e l i k e ly

t o c o n f o r m w i t h t h e s u g g e s te d c o m p a t i b l e

r a n g e s t h a n t h e s p r a w l in g s u b u r b s o f t h e l a tt e r .

T h u s t h e m o d e m ( e sp e c ia l ly ) N o r t h A m e r i c an

c i t y f o r m c o m p r i s e d o f t w o e x t r e m e s - - a v e r y

d e n s e c o r e a n d s c a t t e r e d s u b u r b s - - i s a p o o r

d e s i g n c l i m a t i c a l l y .

I t i s h o p e d t h a t t h e r e a d e r o f t h i s p a p e r

w i l l r e c o g n i z e t h e s p i r i t o f i t s i n t e n t . I t s e e k s

t o d e m o n s t r a t e b y e x a m p l e h o w u r b a n c li m a-

t o l o g i s ts m a y h e lp i n t h e f o r m u l a t i o n o f u r b a n



112

design guidelines tha t are bot h quantitative

and easily understood. Clearly there are gaps

i n k n o w l e d g e a n d m e t h o d s t h a t m u s t b e

im p r ov e d , a n d t h e r e is t o o m u c h h a n d -w a v in g

a n d s p e c ul a t io n h e r e in t h a t m u s t b e r e f in e d

and verified. Bu t if this paper encourages

o t he r s t o a s k s i m i ]Ar s i m p l e ques t i ons r ega rd -

i ng c l i m a t i c des i gn and t o t r an s l a t e c l i m a t i c

res ea rch i n t o u s e fu l gene ra l gu i de l i nes i t w i l l

h a v e a c h i e v e d t h e a u t h o r ' s a i m .

ACKNOWLEDGEMENTS

T h i s w o r k w a s s u pp o rt e d b y f u nd s f r o m

the Natural Sciences an d Engineering Research

C o u n cil o f C a n a d a . T h e d ia g r a m s w e r e d r a w n

by P. Jance.

REFERENCES

1 T. R. Oke, Towards a prescription for the greater

use of climatic principles in settleme nt planning,

Energy Build., 7 (1984) 1 - 10.

2 R. N. Meroney, Turbulent diffusion near build-

ings, in E. Plate (ed.), Engineering Meteorology,

Elsevier Science Publications, Amsterdam, 1982,

pp. 481 - 525.

3 R. P. Hosker, Jr., Flow around isolated structures

and building clusters: A review, AS HR AE Trans.,

91

2 B ) F e b . )

( 1 9 8 5 ) 1 6 7 1 - 1 6 92 .

4 M . H u s s a i n a n d B . E . L e e , A n Investigation f

Wind Forces on Three-Dimensional Roug hnes s

Elements in a Simulated Atmospheric Boundary

Layer Flow: Part II Flow over Large Arrays of

Identical Roughness Elements and the Effect of

Frontal and Side Aspect Ratio Variations, Report

BS 56, Dept. Building Science, University of

Sheffield, 1980.

5 Y. Nakamura and T. R. Oke, Wind, tempera ture

and stability conditions in an E -W oriented urban

canyon, Atmos. Environ., submitted for publi-

cation.

6 A. D. Penwarden and A. F. E. Wise, Wind Envi-

ronment Around Buildings, Building Research

Establishment Report, Dept. of the Environment,

HMSO, London, 1975.

7 A. D. Penwarden, Acceptable wind speeds in

towns, Build. 8ci., 8 (1973) 259 - 267.

8 T. V. Lawson, The Wind Environment of Build-

ings: a Logical Approach to the Establishment of

Criteria, Report No. TVL 7321, Dept. Aeronauti-

cal Engineering, University of Bristol, 1973.

9 A. G. Davenport, An approach to human comfort

criteria for environmenta l wind conditions, Proc.

CIB/WMO Colloquium Teaching the Teachers,

26

Swed ish Natio nal Building Institute, Stoc khol m,

1972.

1 0 U . S i e v er s a n d W . G . Z d u n k o w s k i , A m i c r o se a l e

u r b a n c l i m a t e m o d e l , Contrib. Atmos . Phy ,

59 (1986) 13 - 40.

11 J. Counihan, Adiabatic atmospheric boundary

layers: A review and analysis of data from the

period 1880 - 1972, Atmos. Enviror~, 9 (1975)

871 - 905.

12 J. F. Clarke, J. K. S. Ching and J. M. Godow itch,

An Experimental Study of Turbulence in an

Urban Environment, Environmental Science

Research Laboratory, USEPA, Research Triangle

Park, 1982.

] 3 I. Seginer, Ae rodyna mic roughness of vegetated

surfaces, Bound.-Layer MeteoroL, 5 (1974)

383 - 393.

14 J. Counihan, Wind tunne l determination of the

roughness length as a function of the fetch and

the roughness density of three- dimensional rough-

ness elements, Atmos. Environ., 5 (1971) 637 -

642.

15 R. A. McCormick, Air pollut ion in the locality of

buildings, Phil Trans. Roy. Soc. London), A26 9

(1971) 515 - 526.

16 H.-W. Georgii , E. Busch and E. Weber, Investiga-

tion of the Temporal and Spatial Distribution o f

the Emission Concentration .of Carbon Monoxide

in Frankfurt/Main, Report No. 11, Institute of

Meteorology and Geophysics, University of

Frankfurt]Main, 1967.

17 F. L. Ludwig and W. F. Dabbe rdt, Evaluation of

the APRAC-IA Urban Diffusion Model for Car-

bon Monoxide, Final Report CAPA-3-68 1-69),

Stanford Research Institu te, Menlo Park, 1972.

18 F. T. DePaul and C. M. Shieh, Measuremen ts of

wind velocity in a street canyon, Atmos. Envi-

rons, 20 (1986) 455 - 459.

19 W. G. Hoydysh , R. A. Grif fiths and Y. Ogawa, A

scale model study of the dispersion of pollution

in street canyons, ]>reprint No. 74-157, Proc. 6 7th

Annual Meeting, Air Pollution Control Assoc.,

1974.

20 J. B. Wedding, D. J. Lombardi and J. E. Cermak,

A wind tunnel study of gaseous pollutants in city

street canyons, J. Air Poll. Control Assoc., 27

(1977) 557 - 566.

21 I. Tanl, M. Iuchi and H. Komoda, Experimental

Investigation of Flow Associated with a Step or

Groove, Report No. 364,

Aeronautical Research

Institu te, University of Toky o, 1961.

22 T. R. Oke, The energetic basis of the u rban heat

island, Q. J. Roy. MeteoroL Soc., 108 (1982)

1 -24.

23 T. R. Oke, Boundary Layer Climates, Methuen

and Co. Ltd., London, 2nd edn., 1987.

24 M. Nunez, The energy balance of a n urban can-

yon, Ph.D. Thesis, Dept. Geography, University

of British Columbia, Vancouver, 1974.

25 T. R. Oke, Canyon geometry and the nocturna l

urban heat island: Comparison of scale model

and field observations, J. ClimatoL, 1 (1981)

237 - 254.

R. Taesier, Urban climatological methods and

data, in T. R. Oke (ed.), Urban Climatology and



113

i t s A p p l i ca t io n s w i th S p e c ia l R eg a rd to T ro p ica l

Areas, Re por t No. 652, WMO, Geneva, 1986,

pp. 199 - 236.

27 G. A. McKay and T. Allsopp, The role of climate

in affecting energy supply/demand, in W. Bach,

J. Pankrat h a nd J. Williams (eds.), I n t e r a c ti o n s o f

E n erg y a n d C l im a te ,

D. Reidal Publishing Co.,

Dordrecht, Holland, 1980, pp. 53 - 72.

28 R. Taealer, C l im a te , E n erg y D em a n d s a n d E n erg y

C o n s u m p t i o n w i t h S p e ci a l R e g a r d t o H u m a n

S e t t l em en t~ , R ep o r t N o . 2, Energy Special Appli-

cations Programmes, WMO, Geneva, 1981,

pp. 199 - 236.

29 G. Jen dritz ky and W. Niibler, A model analysing

the urban thermal e nvironme nt in physiologically

significant terms, A r c h l y . M e t e o r o L G e o p h y ~

B io k l im . , S er . B , 2 9 (1981) 313 - 326.

30 C. D. Craig and W. P. Lowry, R efle ction s on the

u r b a n a l be d o , P r ep r . C o n f o n U r b a n E n v ir o n. a n d

S e c o n d C o n f . o n B i o m e t e o r o L , A m e r i c a n M e t e o -

r o l og i c al S o c i e t y , B o s t o n , 1 9 7 2 , p p . 1 5 9 - 1 6 4 .

3 1 W . H . T e r j u n g a n d S . S . - F . L o u i e , S o l a r r a d i a t i on

a n d u r b a n h e a t i s la n ds , n n a l s A s so c . A m . G e og . ,

6 3 (1973) 181 - 207.

32 A. J. Arnfield, An approach to the e stimatio n of

the surface radiative properties and radiation

budgets of cities, Phys.

Geog., 3

(1982) 97 - 122.

33 M. Aida, Urban albedo as a func tion of the urban

structure -- a model experiment, B o u n d . - L a y e r

M eteo ro L , 2 3 (1982) 405 414.

34 M. Aida and K. Got oh, Urba n albedo as a func-

tion of the urban structure -- a two-dimensional

numerical simulation,

B o u n d . - L a y e r M e t e o r o L , 2 3

(1982) 415- 424.

35 M. Evans,

Ho u s in g , C l im a te a n d C o m fo r t ,

The

Architectu ral Press, Londo n, 1980.

Documents

Street Design and Urban Canopy Layer Climate