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Transcript of 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
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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-
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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
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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 .
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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.
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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
8/19/2019 Street Design and Urban Canopy Layer Climate
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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 .
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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
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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.
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